chapter 2 review of literature -...

Chapter 2

Review of Literature

P. Bora Chapter 2 [Ph.D. Thesis, 2015]

Review of Literature 8

Chapter 2

2. Review of literature

The concept of microemulsion was put forward by Prof. Jack H. Shulman & his co-

workers of Columbia University in 1959 [1]. Ever since their onset, it has been an area

of intense scientific and industrial focus. Microemulsion literally holds significant

promise in areas of fuels, cosmetics, drug delivery, enhanced oil recovery, food

processing, pharmaceuticals, petrochemicals, detergent industry etc. on account of

their numerous advantages like high thermodynamic stability, in situ production, ultra

low interfacial tension and targeted production (no byproduct generation).

Innovations of microemulsions in biofuels have revolutionized bio-energy research.

Although, past few decades have witnessed major emphasis on the production of

emulsion fuels mainly from diesel, microemulsion fuels (petrodiesel based) have

gained new impetus owing to its superior thermodynamic stability and interesting

structural features. However, with scientific and industrial research entering the new

millennium, microemulsion based hybrid biofuels are gaining wider acceptance on

account of their renewable nature. The present review of literature section aims to

provide a broader overview of both emulsion and microemulsion fuels (with varied

types) in terms of past and present developments.

2.1. Water-Diesel based systems

2.1.1. Emulsion fuel (Water-Diesel type)

The concept of using water-in-diesel emulsions was drawn from the fact that the

micrometer-sized water droplets exert some positive effects on fuel combustion.

Formulations of water-in-oil emulsions have been evaluated for different types of

fuels, ranging from light hydrocarbons to triglycerides. However, the experimental


Review of Literature Page 29

findings and theoretical verifications carried out almost 40 years back proved that the

high combustion temperature and the high pressure exist in diesel engine is

particularly favorable for this concept, and, therefore, emulsification of water in diesel

fuel has received much interest among the researchers in comparison to water in

gasoline emulsion. Use of such emulsions in diesel reduces NOx emission, decrease

soot and particulate contents in the exhaust, and improves combustion efficiency [2,3].

However, increase in emission of CO and hydrocarbons may be observed [4,5].

A reduction of 35% NOx emission was evident from a diesel-water emulsion system

comprising 15% of water under regular conditions [6]. It was also reported that NO

emission decreased from 1034 ppm with base diesel to 645 ppm under full load with

emulsion at water and diesel mass ratio of 0.4:1 [7]. However, for fuels with higher

nitrogen contents (e.g. heavier hydrocarbons), the NOx emitted in the exhaust was

mainly from oxidation of the fuel components containing nitrogen. The emulsified

water might have a small effect on the level of NOx emission in such cases. Moreover,

presence of high fraction of water in the fuel resulted in poor stability of the system.

Additionally, the emission of particulate matter (PM) also decreased considerably for

this type of fuels, especially for the fuels with high nitrogen contents. The

emulsification of water in diesel also reduced the amount of polycyclic aromatic

hydrocarbons (PAHs) in the flame, as well as in exhaust gas emission. The occurrence

of microexplosions during combustion possibly promoted oxidation of hydrocarbons

which resulted in the decrease in PAHs formation. The reduction in the amount of

PAHs formed was probably be one of the reasons for the lower amount of soot

obtained with emulsion fuels as PAHs act as precursors for soot particles [2,8-11].

Nazha et al [12] developed a simplified model capable of predicting concentration of

soot and gaseous species in a burning water-in-liquid fuel emulsion. Even though

effect of water content was found to have little effect on CO and hydrocarbon

emission, their emissions may increase with increasing water content of the emulsion

[2,13]. However, a sorbitan monooleate surfactant based system and a gemini

surfactant (1,2-ethane bis(dimethyl alkyl (CnH2n + 1) ammonium bromide) (n = 10))

based system showed reduction in CO emission along with NOx, PM and SOx [8].



Fahd et al. [13] showed increase in CO emission at low load and low speed condition

which eventually reduced significant amount of CO at higher engine speed. But, the

amount of CO2 emission increased with engine speed and decreased with the increase

in water content for a system prepared in presence of Tween 20 surfactant [13]. The

increasing fraction of water in Tween 20 surfactant based system also increased the

amount of oxygen emitted to the atmosphere [14]. Report of Texas diesel fuels project

showed decrease in fuel efficiency along with reduction in both NOx and PM.

Moreover, increased levels of oxidation products such as acetaldehyde, formaldehyde,

methyl ethyl ketone and acrolein were obtained [15]. Bidita et al. [16] prepared water-

in-diesel emulsion fuels through an ultrasonic processor by using high energy

emulsification method. Decrease in exhaust temperature during combustion at high

surfactant (iso-octylphenoxypolyethoxy ethanol) to water ratio was observed from the

study. Emulsification of water in diesel reduced the amount of emission of exhaust

gases such as CO2, CO, NH3 from the internal combustion engine. Reduction in

viscosity was also evident from the study.

The size of water droplets in an emulsion fuel system has great influence on properties,

engine performance and emission characteristics of the fuel. Sjogren [17] reported that

an emulsion system formulated with continuous heavy fuel oil containing smaller

droplet of water (or finer water dispersion) resulted in reduced emission of coke. It

reveals that smaller the droplets of water (or finer the water dispersion) in water-in-

fuel emulsion, the smaller the amount of water needed to obtain good results in terms

of reduced coke emission. Therefore, it also suggests that the efficiency in reducing the

emission depends on the oil–water interfacial area and is independent of the quantity

of water incorporated in the fuel. Moreover, the type of surface active agent used for

the formulation of emulsion greatly influences the size of dispersed water, whereas

investigation based on effect of surface active agent on the structure, performance and

emission characteristics of emulsion is also very limited. Ali et al. [18] reported

reduction in NOx emission up to 25% for a diesel-water emulsion system (formulated

using surfactant Tween 60 and Span 80) with larger size of water droplets. On the

other hand, the emulsions with finer droplets reduced engine smoke and unburned



hydrocarbons up to 80% and 35% respectively, as well as increased engine effective

efficiency up to 20%. Moreover, the viscosity of the emulsion system increased with

the decreasing droplet size of water and increasing water fraction.

Incorporation of water causes additional rupture and increases evaporation surface of

the drops. Better mixing of fuel and air also occurs in emulsion fuel due to the larger

air–fuel interfacial layer in comparison to that of neat diesel fuel [2]. Abu-Zaid [19]

investigated the effect of water emulsification on engine performance and exhaust gas

temperature in a single cylinder, direct injection diesel engine, operating at 1200 –

3300 rpm. The addition of water improved combustion efficiency, increased engine

torque, power and brake thermal efficiency (BTE) and decreased the gases exhaust

temperature [19]. Similar experimental results were also reported in some other

previous studies [3,5,6,10,13,14,16,19-23].

The results of investigation carried out in a constant volume chamber by Ghojel et al.

[24] and Gong et al. [25] showed that water content and the initial temperature of the

chamber have strong effects on ignition delay and lift-off length of combusting diesel-

water emulsion sprays. On the other hand, effect of injection pressure (up to 100 MPa)

on ignition delay was very little, while effect on the lift-off length was significant

[24,25]. An investigation with alumina nanoparticles blended (at mass fractions 25, 50,

and 100 ppm) water–diesel emulsion fuel system comprising a surfactant mixture of

Span 80 and Tween 80 also showed improvement in engine performance which were

concordant with previous studies. The emulsification of 83% diesel, 15% water, and

2% surfactants by volume formed creamy white emulsion in the study [26].

A recent study by Califano et al. [27] showed that emulsions with a narrow size

distribution with diameters around 1 µm resulted in the simultaneous evaporation of a

great number of droplets in the dispersed phase and was responsible for the occurrence

of a ‘cooperative’ microexplosion [27]. The effect of variables such as temperature,

drop size and parent fuel properties on microexplosion phenomenon of water-in-

hydrocarbon emulsions (for diesel, gasoline and hydrocarbons from hexane to

hexadecane) had been also investigated by developing mathematical model [28]. Huo



et al. [29] investigated the behavior of microexplosion of diesel-water based systems in

a burning spray. The occurrence of microexplosion in a burning spray flame was

confirmed from the puffing and disruptive droplet combustion which was observed at

high ambient temperature in the central lift-off region with emulsified diesel.

Morozumi et al. [30] investigated the effect of emulsifier content on microexplosion

characteristics of water - n-hexadecane (oil) based emulsion. The emulsifiers used in

the study were Solgen 40 (HLB = 4.3) and Noigen TDS-30 (HLB = 8). The study

confirmed that interfacial tension had negligible influence on droplet diameter,

regardless of emulsifier conditions. However, microemulsion temperature and waiting

time were affected by emulsifier type. Generally, increasing content of emulsifier was

reported to cause negative influence on microexplosion occurrence and viscosity of the

fuel system. The microexplosion characteristics observed in the investigation were

most likely due to the thermal decomposition of the emulsifiers.

Morozumi et al. [30] further reported that reduction in heat flux, the metal

temperatures and thermal loading of combustion chamber components of diesel engine

were greatly influenced by the addition of water to diesel fuel. Lif et al. [11]

investigated the sizes of water droplets by NMR diffusometry and light microscopy. In

this study, larger droplet size values were observed for NMR analysis than that of the

microscopy images. It might be due to two effects: molecular diffusion of water

between drops, i.e., through the continuous oil phase and diffusion of the drops.

Wenzel et al. [31] in US. Pat. No. 4,083,698, prepared stable water-in-oil emulsions

comprising a hydrocarbon fuel, water, an alcohol, and a multicomponent surfactant

system comprising: (1) a long-chain fatty acid salt, or, more preferably, an ammonium

or sodium long-chain fatty acid salt, or mixture thereof as the detergent; (2) a free

unsaturated long-chain fatty acid, or a mixture of a free unsaturated organic acid and a

free saturated long-chain fatty acid; and (3) a nonionic surfactant typified by ethylene

oxide condensation products and esterification products of a fatty acid with ethylene

oxide. Another water-emulsified type motor fuel was reported to be formulated in US.

Pat. No. 2,892,694 by means of a detergent-type emulsifier comprising the reaction

product of alkyl-4-sulfophthalate and ammonia or an amine [32,33].



Al-Amrousi [34] formulated fuel oil-water emulsions with calorific values ranging

from 33.9 – 42.3 MJ/kg and fuel oil-water-charcoal emulsions with calorific values

ranging from 30.6 – 37.8 MJ/kg using the l,l’-(Laurylamido)propyl ammonium

chloride as a cationic surfactant, dodecylbenzene sulfonic acid sodium salt as an

anionic surfactant, and dodecylphenol ethoxylate as a non-ionic surfactant. The

physicochemical properties of the fuels were comparable to boiler fuels. The emulsion

fuel systems resulted in low sulfur and wax contents, underwent the water gas shift

reaction during combustion and make use of solid charcoal. The introduction of

surfactants resulted in increasing corrosion resistance of the fuels. The system

incorporating non-ionic surfactant showed higher efficiency than the cationic and

anionic surfactant in this respect. The fuel systems remained stable (during storage and

handling) at higher values of zeta potential, dynamic viscosity and density, while at

lower values fuel samples were unstable as they got separated into two phases.

Wamankar et al. [35] investigated emulsion systems formulated from carbon black (a

solid waste obtained from the pyrolysis of waste automobile tyres exhibiting a

considerable heating value in it), diesel, Tween-20 (nonionic surfactant) and water in a

single cylinder compression ignition (CI) engine. The emulsions exhibited longer

ignition delay, higher BSFC compared to that of diesel at full load. Moreover, at full

load NO emission was decreased by about 16–42%, while CO and smoke emissions

were increased respectively by about 29% to 51% and 13% to 42.2%. The number of

droplets present in the emulsions was higher due to incorporation of carbon black in

the system with maximum of the droplets within the size range of less than 15 μm and

minimum in the size range of 60 – 70 μm.

Chen et al. [36] reported the methane hydrate formation and dissociation in a emulsion

comprising 5 vol % water dispersed in 95 vol % diesel in presence of surfactant

mixture (or combination of anti-agglomerant) of sorbitan monolaurate (Span 20) and

esters. The concentrations of these two surfactants were 1 wt % and 2 wt %,

respectively, relative to the weight fraction of water [36]. The performance and

emission characteristics of water-diesel systems blended with ‘green’ energy resource

Acetone–Butanol–Ethanol (ABE) had also been reported [37].



However, one of the commonly known demerits pertaining to emulsion systems is

their poor physical stability. They are separated into two (or sometimes three) phases.

There is paucity of scientific literature available which deals with physical stability of

diesel-water emulsion system. The phase separation of the alumina nanoparticles

blended water–diesel emulsion fuel systems were observed after two days of their

preparation [26]. The diesel-water emulsion comprising 30% water remained stable up

to one week and 20% water in diesel for four weeks. These results were obtained for

the systems prepared by using nonionic surfactant iso-octylphenoxypolyethoxyethanol

(in the concentration range of 0 – 5% by volume) when the mixing speed over the rpm

range of 5000 – 26000 up to 30 min was applied [21]. Ghannam et al. [38] reported

that that emulsion with 10% and 20% water with diesel formulated using iso-

octylphenoxypolyethoxyethanol exhibited stability of 4 weeks and 10 days,

respectively, under the conditions of 0.2% surfactant, 15000 rpm mixing speed, and 2

min of mixing time. However, the stability period reduced to 5 h for emulsions with

water fraction higher than 20% under the same conditions.

Ghannam et al. [38] investigated the optimum surfactant concentration for a water-

diesel emulsion and found it to be 2% for a system comprising 40% water. In this

study, droplet size distribution and average diameter decreased gradually with water

concentration of 10%, 20%, and 30% in the emulsions due to the increase of the total

number of mixing revolutions. Besides, gradual increase in droplet size distribution

and average diameter were observed for systems with water concentration of 30%,

40%, and 50% due to the application of the same total number of mixing revolutions.

The density, decrease in viscosity and surface tension increased with increase in water

content in the system. Moreover, the increase in temperature decreased both viscosity

and density.

The INSEDELF method (a thermoanalytic technique) of measuring solid– liquid phase

separation was extended to water-diesel emulsion fuel systems for studying liquid–

liquid phase separation, i.e. for characterizing the stability of these fuels. The method

was found suitable for determining the optimal content of antifreeze in the aqueous

phase, which would also indicate the cold flow property of the emulsion fuel. The



study also suggested that this optimization can be extended to studies relating to diesel

fuel phase (in the emulsion system) like the role of the diesel fuel used (sulphur

content, freezing point depressants) and its subsequent modification due to addition of

vegetable oil methyl ester [39].

Samec et al. [40] conducted numerical investigations of combustion’s chemical kinetic

aspects by simulation of water/n-heptane mixture combustion assuming a model of a

homogenous reactor’s concentric shells. The injection and fuel spray characteristics

were also analyzed numerically in order to investigate indirectly the physical effects of

water present in diesel fuel during the combustion process. Reduction in pollutant

emission (NOx by nearly 20% and soot by up to 50% for fuel comprising 10% and

15% of water) with no worsening of specific fuel consumption was evident from the

study a for broad range of engine loads and speeds. Emulsification also attributed to

increase the ignition delay by about 10% and the gradient of rate-of-heat release during

premixed burning up to 26%, in comparison to that of neat diesel under the same

engine-operating conditions.

Yilmaz et al. [41] studied the performance and exhaust emissions of a three phase

O/W/O (diesel-in-water-in-diesel) emulsion system comprising 5% (E5) and 10%

(E10) water, which was formulated using Span 80 and Tween 80 as surfactants and

mono ethylene glycol as an auxiliary emulsifier (cosurfactant). In this study, reduction

in subsidence rate (due to introduction of mono ethylene glycol into the system), NOx

and soot levels were observed. Lin et al. [42] conducted a comparative study of engine

performance and emission characteristics between ASTM No. 2D diesel fuel, W/O

(water-in-oil) emulsion, and O/W/O (oil-in-water-in-oil) emulsion. Emulsions were

formulated using diglyme (diethylene glycol dimethyl ether) as surfactant. The study

reported improvement in engine performance and emission characteristics for W/O

and O/W/O emulsion in comparison to neat ASTM No. 2D diesel fuel. Another study

carried out by the same researchers showed decrease in drop size of the O/W/O

emulsion (comprising surfactants Tween 80 and Span 80) with increasing

homogenizing machine revolution speed. The increase in inner phase fraction (water)

of the O/W/O emulsion resulted in increasing the emulsion viscosity. Moreover, the



viscosity of O/W/O emulsion was greater than that for W/O emulsion for the same

water content. The presence of greater fraction of water led to form a larger number of

liquid droplets for three phase emulsion system. It also resulted in a faster formation

rate and greater emulsion turbidity at the beginning and consequently a faster

descending rate of emulsion turbidity afterwards [43]. A comparison of emulsion

properties, engine performance, and engine emission characteristics between two-

phase W/O and three phase O/W/O emulsions, prepared by a mechanical homogenizer

and an ultrasonic vibrator, respectively, were also reported by Lin et al. [44]. The

findings showed that the O/W/O emulsions prepared by ultrasonic vibrator appeared to

have more favorable emulsification characteristics such as smaller dispersed water

droplets that were distributed more uniformly in the continuous oil phase, lower rate of

coalescence of droplets and thus a lower separating rate of the dispersed water droplets

from the emulsion, larger emulsion stability, and larger emulsion viscosity than the

two phase emulsions produced using a mechanical homogenizer. In addition, a larger

quantity of water was emulsified when the emulsion was prepared using the ultrasonic

vibrator than the mechanical homogenizer. The engine performance and engine

emission characteristics were also in agreement with their previous studies [44].

Similar results were obtained for performance and emission characteristics of another

multi-phase O/W/O emulsion system formulated by Armas et al. [45].

2.1.2. Microemulsion fuel (Water-Diesel type)

Lack of long-term stability limits the commercialization of emulsion fuels. Emulsion

fuels may undergo phase separation during engine applications and storage owing to

their poor thermodynamic stability. As a result of insufficient lubrication by the water

phase that resides in the bottom of the tank, severe damage to the engine may also

occur. Due to these limitations, microemulsion based fuels are gaining importance

even though the quantity of surface active agents required for the formulation of

microemulsion systems are higher than that of emulsion systems.

Gillberg and Friberg [46] reported the use of water-in-diesel microemulsions as fuel

for the first time in 1976. Adiga et al. [47] investigated the vaporization behavior of



water-diesel microemulsion systems under dynamic heating conditions. They

highlighted combustion characteristics of microemulsions based on varying chain

lengths of hydrocarbons. Their findings indicated that the microemulsion droplets

played a significant role in the physical effects related to microexplosion, and, thus the

combustion process [47].

In recent years, water-diesel based microemulsion systems are mostly studied based on

their phase behavior (physical/thermodynamic stability) and the effect of surfactant

types on the fuel characteristics of the systems. Nonionic surfactants are mostly

preferred for formulating microemulsion fuels in such studies [48].

Fan et al. [49] investigated the phase behavior of microemulsion fuel comprising 82%

diesel oil, 10% water, and 8% emulsifier and co-emulsifier. The emulsifier was

synthesized with oleic acid and amine, while methanol was introduced as co-

emulsifier. Introduction of methanol into the formulated system led to the formation of

associated molecule by the hydrogen bonding with water molecule. The fuel

incorporating methanol exhibited more physical stability with better combustion effect

than that of water-diesel emulsion. The increase in concentration of emulsifying agent

suggested improvement in stability of the system and the optimum ratio of emulsifier

to co-emulsifier was obtained as 5:1. The viscosity of microemulsion diesel oil

comprising less than 10% water was obtained ≤ 8 mm2/s.

Wang et al. [50] investigated the interfacial and thermodynamic properties of water-

cetyltrimethylammonium chlorine (CTAC)-alkanol-diesel oil W/O microemulsion

systems prepared by dilution method. Alkanols with different carbon numbers (n-

butanol, n-pentanol, iso-pentanol, n-hexanol, n-octanol) were used to study the effect

of cosurfactant on the formation of diesel oil microemulsion. The composition of the

cosurfactant and the surfactant in the interfacial region, the distribution of the

cosurfactant between the interfacial region and the continuous oil phase, and the

energetics of transfer of the cosurfactant from the oil to the interface were estimated in

the study. The Gibbs free energies of all alkanols transfer process were found to be

negative and dependent on the alkanol carbon number. It was most negative for n-



octanol among the alkanols and except for n-octanol the enthalpy of the process was

mostly endothermic for other alkanol. The entropy change involved in the transfer

process was also negative for n-octanol. These signified that the transfer of n-octanol

from oil phase to the interfacial region is the most spontaneous and n-octanol was

found to be most suitable cosurfactant for formulating microemulsion system

comprising water, CTAC and diesel.

Lif et al. [51] investigated the stability of the water-diesel microemulsion formulations

by varying amounts of water and temperature in the interval 17 – 50 ˚C. In this

investigation, microemulsions were formed with a mixture of two nonionic surfactants:

alcohol ethoxylate (C11E5) and sorbitan monooloeate (mixing at 1:1 weight ratio).

Internal structure of the microemulsions was analyzed by NMR diffusometry at 25 ˚C.

The one-dimensional and diffusion NMR spectra suggested that the presence of

surfactants stabilized the water-diesel system by dispersing water droplets in diesel

phase. In presence of lower quantity of water, the domains were reported to be

spherical and rather small. Furthermore, the domains started swelling with increasing

content of water and it increased in number as the content reached 10% water. The

diffusion dependence of C11E5 on water content was found to be much stronger than

the diffusion dependence of sorbitan monooloeate, which might be due to effective

obstruction by the large nonspherical aggregates. The diffusion of C11E5 was slower

than the diffusion of water, which indicated formation of aggregates of C11E5 [51]. Lif

et al. [11] also suggested that microemulsions based on small surfactant molecules are

generally more dynamic in character than microemulsion formulated with larger

surfactants like sorbitan monooleate and long-chain alcohol ethoxylates. The NMR

investigation of the microemulsions prepared from Fischer–Tropsch (FT) diesel using

a polymeric stabilizer along with sorbitan monooleate and sorbitan monooleate

ethoxylates showed increase in diffusion coefficient of the dispersed component, i.e.,

water. It was due to the incorporation of small size of the surfactant molecules in the

system.

Nguyen et al. [52] investigated the phase behavior of microemulsion system with

surfactant mixtures containing soy methyl ester ethoxylate, rhamnolipid and oleyl



alcohol with limonene oil. The solubilization parameter of rhamnolipid based water-

diesel-limonene oil microemulsion were found almost double than that of systems with

sodium bis(2-ethyl)dihexylsulfosuccinate, sodium dihexyl sulfosuccinate, sodium

mono- and dimethyl naphthalene sulfonate at the same total molar concentration [52].

Kayali et al. [48] investigated the phase behavior at temperatures between 0 ˚C – 50 ˚C

for bicontinuous and water-in-diesel microemulsions formulated using single nonionic

alkyl poly glycol ethers (CiEj, where i is the number of carbon atoms in the alkyl chain

and j the number of ethoxy units in the head group) surfactant combined with

hydrophilic alcohol ethoxylates (cosurfactant). This investigation reported that

increasing the hydrophobic chain length of the alcohol ethoxylates led to a wider range

of temperature stability at lower surfactant concentration, whereas, increasing the

ethylene oxide units in the head group by two units led to a phase diagram dominated

by lyotropic liquid crystal. Moreover, improvement in efficiency and thermodynamic

stability was observed at lower surfactant concentration when the length of the

hydrophobic chain of surfactant was increased maintaining the length of the head

group, i.e. with C14E3. The surfactant to cosurfactant ratio was maintained at 10:1 and

combustion experiments were also performed on water-diesel microemulsion using a

diesel engine.

Jankowski [53] investigated the thermodynamic stability of water-diesel

microemulsion systems comprising a fatty acid (lipophilic) and ionized fatty acid

(hydrophilic) surfactants at 4 ˚C by varying the percentages of water. In his study,

stable self organized microemulsions were formed for systems comprising up to 25%

of water and without the application of mixers. However, systems with 30% and 40%

water content showed cloudiness after 60 min although they were subjected to mixing

for 30 min during preparation. The viscosity of the formulated microemulsion system

was much higher (9.457 mm2/s and 11.48 mm2/s at 40 ˚C for systems with 10% and

20% water, respectively) and the heating value was very low (30.72 MJ/kg) in

comparison to diesel fuel. But, reduction in specific fuel consumption and emissions of

NOx and soots were evident in the investigation [53]. Systems with 15% ethoxylated

nonylphenol surfactant exhibited density and viscosity greater than those obtained for



neat diesel, where as cloud point and corrosiveness were not significantly affected by

the water and surfactant [54]. Microemulsion with surfactant Tween-80 and

cosurfactant n-butanol at ratio 1:1 showed clouding phenomenon at high temperature

and system was separated into three different phases at 93 ˚C. The cloud point

decreased with decrease in diesel content and increase in total surface active agent

content [55].

Jiang et al. [56] investigated the effect of preparation methods on the stability and

physicochemical properties of microemulsion for system comprising 10.4 wt% water,

4.95 wt% surfactants and 0.25 wt% n-amyl alcohol (cosurfactant). The methods

employed for preparation of this microemulsion fuel were spontaneous emulsifying

method and ultrasonic dispersing method and surfactants consisted of Tween, Span,

OP surfactants, and ammonium oleate. The microemulsion prepared by ultrasonic

dispersing method exhibited higher closed-cup flash point (66 ˚C), lower freezing point

(-17 ˚C) and lower cold filter plugging point (-1 ˚C) than that by spontaneous

emulsifying method. The values for spontaneous emulsifying method were 63 ˚C, -15

˚C and -9 ˚C, respectively. The storage stability of microemulsion fuel prepared by

spontaneous emulsifying method was better than that by ultrasonic dispersing method

at normal and elevated temperature. The average droplet sizes for spontaneous

emulsifying method and ultrasonic emulsifying method were obtained as 13.32 nm and

13.44 nm, respectively.

Neto et al. [54] reported that the presence of small water droplets improved the

combustion of the microemulsion fuel comprising 15% ethoxylated nonylphenol

surfactant, even though specific fuel consumption for the fuel system was greater than

that of diesel. This microemulsion formulation with up to 6% water satisfied the

Brazilian diesel/biodiesel fuel regulations and specifications. An increase in emission

of CO and NOx and a decrease in emissions of black smoke compared to that of diesel

were obtained [54]. Lower amount of soot formation, longer period of burning

compared to neat diesel were also observed for the fuel comprising Tween 80 and n-

butanol [55].



2.2. Ethanol-diesel system

2.2.1. Emulsion fuel (Ethanol-Diesel type)

The solubility of ethanol in diesel is greatly influenced by the types of hydrocarbon in

diesel and temperature. Lei et al. [57] formulated an ethanol-diesel emulsion system

using biofuel and castor oil based mixed surface active agents (or emulsifier), named

‘‘CLZ’’. The emulsification performance and stability of the emulsions were studied

with single and mixed surface active agents and the effects of temperature on

emulsification properties were investigated based on the theory of HLB and principle

of emulsification. The fuel system E10 (system comprising 90 ml diesel + 10 ml

ethanol + 1 ml emulsifier) exhibited physical stability at a wide range of temperatures.

Stable E10 systems could be formulated at temperature about 25 ˚C only by using

single surfactant. The HLB values in the range 4.1 – 4.4 for CLZ were found to be

most suitable for formulating stable emulsion. Moreover, improvement in engine

performance and emission characteristics was also observed from the study.

Ashok and Saravanan [58-61] studied the engine performance and emission

characteristics of ethanol-diesel emulsion system formulated using different types of

surfactant. Increase in BTE and decrease in BSFC, ignition delay and emission levels

of NOx, smoke density, particulate matter and hydrocarbons were evident in these

studies. Ashok [62] also reported that out of three types of nonionic additives (surface

active agents) viz: diethyl ether, dimethyl ether (DME) and hydrogen peroxide, DME

was the best surfactant for formulating a ethanol–diesel based emulsified fuel in terms

of performance and emission characteristics of the fuel. However, these systems

exhibited very poor physical stability. The homogeneous system formulated using

diethyl ether and Span 80 exhibited a very low stability (1.5 days) [58], while for the

ethyl acetate based system the physical stability was 41 h and 30 m [59].

Reyes et al. [63] formulated ethanol in diesel (oil) emulsion using membrane

emulsification. The ABE (acetone–butanol–ethanol)–diesel blends with 20 vol % ABE

(at A:B:E::3:6:1) showed increase in combustion efficiency, longer ignition delay and

soot lift-off length compared to that of pure diesel under all circumstances [64].



A glycerol−diesel emulsions formulated at 10 and 20 vol % glycerol using different

mixtures of nonionic surfactants (hydrophilic and lipophilic) produced a narrow

droplet size distribution with a mean size of approximately 3.3 μm. However, the

water content of glycerol tended to increase the droplet size distribution. Moreover,

increase in fuel consumption corresponding to an increased glycerol concentration due

to reduction in the emulsion energy density and improvement in thermal efficiency,

NOx emissions and PM emissions were observed [65].

2.2.2. Microemulsion fuel (Ethanol-Diesel type)

Till now there are only a handful of studies pertaining to ethanol-diesel microemusion

system. Most of the fuel characteristics including engine performance and emissions of

this type of fuel were found similar to that of water-diesel microemulsions. Boruff et

al. [66] prepared microemulsion based hybrid fuels by mixing diesel and ethanol using

ionic surfactant (prepared from equimolar amounts of soybean fatty acids and of an

amino alcohol) and 1-butanol (cosolvent/cosurfactant). They also formulated a

detergentless microemulsion without using the ionic surfactants (formulated only by

using 1-butanol). Both the systems exhibited physical stability at temperatures as low

as -15.5 ˚C. However, detergentless microemulsion was observed to be superior to that

of ionic microemulsion in terms of fuel properties and performance characteristics.

Chandra et al. [67] investigated the feasibility of ethanol–ethyl acetate–diesel

microemulsion fuel by studying their physical properties and engine performance. The

results obtained showed decrease in kinematic viscosity and gross heat of combustion

in comparison to that of pertodiesel. This fuel exhibited thermodynamic stability

within the temperature range of 5 – 45 ˚C. The flash points, fire points, and pour points

of the microemulsions were in the range of 8.3 – 16.7 ˚C, 11.5 – 20.5 ˚C, and 6.7 to –

2.3 ˚C, respectively. The use of this microemulsion fuel type replaced 19 – 49% of the

diesel in low brake horse power (bhp) constant speed CI engines. The fuel exhibited

almost similar power producing capability with reduced exhaust emission than that of

diesel fuel [67]. The results of this study are also in accordance with another

investigation on ethanol–ethyl acetate–diesel microemulsion fuel [68].



Mehta et al. [69] reported the fuel quality of ethanol-diesel microemulsion prepared

using surfactants Span 80 and jatropha based biodiesel. The blends were most stable

for systems with 25% biodiesel and 1.5% Span 80. The stability of microemulsions

increased with increasing concentration of surfactant in the system.

Silva et al. [70] investigated the influence of surfactant type and ethanol composition

on the structure of droplets present in ethanol-diesel microemulsion by introducing

surfactant like oleic acid and dodecylamine. Light scattering measurements conducted

on these fuel systems confirmed the presence of microemulsion droplets with average

radius in the range 0.7 – 1.7 nm. The droplet size increased with the increase in ethanol

content and decrease in surfactant content and temperature.

2.3. Vegetable oil based system

2.3.1. Emulsion fuel from vegetable oil

Vegetable oil based emulsion fuels were also reported to exhibit engine performance

and combustion characteristics comparable to diesel fuel. Moreover, improvement in

emission characteristics was also evident from these studies [71-73].

Lujaji et al. [71] prepared a emulsion fuel from croton mogalocarpus oil (CRO) using

butanol (BU) and diesel (D2) at CRO-BU-D2 ratio of 15:5:80 and 10:10:80. The

investigated fuel systems satisfied the requirement for ASTM standards of biodiesel.

Xu et al. [74] investigated the stability of emulsion at high-oleic sunflower oil (oleic

acid content > 90%) to ethanol (95%) in the ratio of 8:2 w/w using 13 different type of

nonionic surfactants at concentration of 5 wt %. Among these surfactants, the system

with decaglycerol mono-oleate (MO750) showed best results with stability of

appox.150 days. Although, the systems with 0.1, 0.5, and 1 wt % MO750 were stable,

the stability duration decreased with increasing content of MO750. Simultaneously,

systems were separated into two phases with the formation of two different turbid

layers when ethanol content exceeded 20 wt %. The average droplet size of the

systems was observed in the range of 2.2 – 2.7 µm. Moreover, droplet coalescence

phenomenon was not significant even after 20 days of fuel preparation.



Stable methanol-in-canola oil emulsions could also be formulated by using a

combination of Span 80 and Tween 80 surfactants. Introduction of methanol improved

the fuel quality of the systems in comparison to that of pure canola oil. Furthermore,

emulsification was reported to cause decrease in viscosity, higher heating value,

stability and emissions of NOx, CO and unburned hydrocarbon than that of pure

canola oil [75]. Dunn [76] studied the cold flow properties and phase equilibrium

behavior associated with emulsion of soybean oil (SBO) and methanol. From the

investigation, a nearly linear correlation was found between phase separation

temperature and cloud point. A correlation of cold filter plugging point and low-

temperature flow test with phase separation temperature was also observed from

statistical analysis. The study also suggested that peak temperatures obtained from

heating and cooling DSC curves can be employed to predict cold flow properties and

phase separation temperature behavior for SBO-cosolvent blends.

Emulsification is also emerging as a new pathway for improving the fuel quality of

biodiesel. Basha et al. [77] reported that the BTE of Jatropha Methyl Esters (JME) was

increased from 24.80% – 26.34% by formulating JME emulsion fuel (comprising 93%

of JME, 5% of water and 2% of surfactants by volume), which was further increased to

28.45% by introducing Carbon Nanotubes (CNT) into the system (by 25%, 50% and

100%). At the full load, the level of NOx emission and smoke opacity for the neat

JME was 1282 ppm and 69%, while it was 910 ppm and 49% for the emulsion fuel

incorporating 100% CNT respectively. The increase in CNT content in the fuel

resulted in increase in BTE and reduction in NOx and smoke opacity [77].

Cheenkachorn et al. [78] investigated the stability of biodiesel based emulsion for a

system comprising diesel, biodiesel, octanol, hydrous ethanol and anhydrous ethanol

(99.5% pure). The surfactant behavior of biodiesel was confirmed from the study and

the stability of the system in the fuel increased with increasing content of biodiesel and

octanol (cosurfactant). Stable systems could be formed only by using biodiesel (in

absence of octanol) when the combined amount of hydrous and anhydrous ethanols

was less than 0.8%. However, the introduction of octanol resulted in formation of

single phase emulsion for ethanol concentration higher than 6.4%. Among different



formulations investigated in the study, only the blend comprising 84% (vol.) diesel,

0.25% hydrous ethanol, 4.75% anhydrous ethanol, and 11% biodiesel met the specific

requirements of standard diesel. Shahir et al. [79] also reported that a maximum of

25% biodiesel and 5% of ethanol/bioethanol can be blended with diesel effectively.

The emulsion comprising rapeseed methyl ester, anhydrous ethanol (containing less

than 0.1% of water) and diesel at ratio 40:20:40 also showed improvement in fuel

quality like blend stability, cetane number, flash point and emission characteristics

[80]. However, the formulations resulted in reduction of lower heating value of pure

diesel from 42.9 – 37.7 MJ/kg, although the addition of iso-pentane increased it up to

39.1 MJ/kg [80].

Husnawan et al. [81] investigated the thermodynamic behavior of biodiesel based

emulsion using TG-DSC for systems comprising four different volume percentages

(0%, 5%, 10% and 15%) of water and 20% of palm oil methyl ester. The

thermogravimetric analysis showed that each of the deposited sample derived from the

investigated fuel with varying amount of water exhibited unique thermal-oxidative

characteristics, by virtue of the specific proportions of the fractions present in it. The

increase in water content in the fuel resulted in the formation of less aromatic and less

reactive deposits. It also attributed to increase the quantity of deposit formed. Bampi et

al. [82] investigated the average droplet size and water content in water-biodiesel

emulsions prepared using industrial biodiesel obtained from soybean oil (85 wt %) and

animal fat (15 wt %) by the method of near infrared spectroscopy (NIR). The average

droplet size for different systems was within the range 3 – 5 µm.

Emulsion fuels were also formulated from bio-oil produced by fast pyrolysis.

Although, bio-oil has certain demerits such as high viscosity, high acid value and poor

burning characteristics; emulsification with diesel caused improvement in fuel quality.

In a bio-oil–diesel formulation prepared using surfactants Hypermers and CANMET

(commercial name), the very heavy fractions of bio-oil were removed by

centrifugation prior to emulsication. The heating value of centrifuged bio-oil was

approximately one third of that of No. 2 diesel and cetane no. was 5.6. However, for

each 10% increase in bio-oil concentration, cetane number decreased by 4. The



viscosities of the systems incorporating 10–20% bio-oil were lower than the viscosity

of bio-oil itself. The stability of the system increased with increase in surfactant

content and decrease in bio-oil content in the system. Most of the formulated systems

(comprising maximum 30%) of bio-oil exhibited stability of < 21 days [83]. According

to another study, bio-oil based emulsion showed higher physical stability than pure

bio-oil. However, the emulsion remained stable only for 3 days even at 70 ◦C. The

study indicated increase in viscosity with the increase in bio-oil and emulsifier content

[84].

Prakash et al. [85] investigated the feasibility of emulsion system obtained from wood

pyrolysis oil and karanja methyl ester as fuels in a direct injection diesel engine. The

formulated systems exhibited lower BSFC and ignition delay, where as higher peak

cylinder pressures and exhaust gas temperatures compared to diesel. Moreover, the NO

and smoke emission levels were higher and HC emission level was lower for these

systems than that of pure karanja methyl ester. Lin et al. [86] investigated the effect of

aqueous ammonia by introducing the emulsion containing aqueous ammonia (NOx-

inhibitor agent) into a three-phase biodiesel emulsion of oil-in-water drops-in oil

(O/W/O). It was observed that incorporation of aqueous ammonia reduced emulsion

stability, specific gravity, kinematic viscosity and average droplet size and increased

amount of carbon residue formed due to combustion of the fuel.

2.3.2. Microemulsion fuel from vegetable oil

Vegetable oil based microemulsion system shows interesting phase behavior and as

such phase behavior of these systems is of significant importance.

Balcan et al. [87] studied the water solubilization in diesel and diesel–colza oil

mixtures (1:1 w/w) at ambient temperature (i.e., 25 ± 1 ˚C) by pseudo-ternary phase

diagrams. Brij 30 and AOT were employed as surfactants and iso-butanol as

cosurfactant. The single phase area was found to be smaller for Brij 30 than for AOT

(area decreased with hydrophobicity of surfactant) but larger for anionic–nonionic

surfactant and cosurfactant mixture. Moreover, the area decreased in presence of

electrolyte (NaCl) and the increase in electrolyte concentration caused conversion of



Winsor I system into Winsor II through Winsor III system. The Winsor III systems

exhibited lowest interfacial tension and the highest oil and water solubilization

parameters. The size of droplets present in Winsor III system was also higher than

other three types of Winsor system. Besides, the increase in anionic–nonionic

surfactant and cosurfactant mixture resulted in the formation of monophasic Winsor IV

system.

Zhu et al. [88] investigated the physicochemical properties, phase behavior of the

microemulsion and the solubilization mechanism of water and castor oil in diesel for a

system comprising rhamnolipid (surfactant) and n-octanol (cosurfactant). The

cosurfactant/surfactant (C/S) mass ratio (w/w) was maintained at 0.60 for the study

and formulation with castor oil/diesel (V/D) volume ratio (v/v) of 0.18 showed fuel

characteristics (density, viscosity, cloud point, pour point, HHV) comparable to diesel.

However, the microemulsion system remained stable only for small amounts of water

content (1.2 − 1.5 vol %). Moreover, fuel quality degraded for formulations with

higher V/D ratio. On the other hand, excessive amounts of cosurfactant were reported

to increase the distance between rhamnolipid molecules, resulting in an unstable

palisades layer.

Do et al. [89] studied the phase behavior of triglyceride (vegetable) oils namely: olive,

peanut, soybean, canola and sunflower by introducing extended surfactants (linear

alkyl polypropoxylated sulfate and linear alkyl polypropoxylated ethoxylated sulfate),

lipophilic linkers (oleyl alcohol) and hydrophilic linkers (sodium mono and dimethyl

naphthalene sulfonate and polyglucoside).

Polizelli et al. [90] also investigated the phase behavior for the system comprising soy

bean oil (SBO), surfactant and water at 25 ˚C. Surfactants used for the study were

sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and nonionic monoolein (MO). The

area of microemulsion formation in the phase diagrams was found to be dependent on

the relative amount of surfactants, being larger for MO:AOT at 2:1. The viscosity of

the water in oil phase exhibited two different behaviors depending on composition

which was confirmed from rheological and DLS studies. The viscosity of “dry”



(waterless) microemulsion initially decreased with increasing water content and

attained a minimum value following a dilution line in the phase diagram, i.e., a

constant surfactant:SBO percentage ratio. Moreover, increasing water content above 5

wt % caused increase in relative viscosity. The size of the droplets was obtained in the

range 3.6 – 16.5 nm, depending on composition of SBO, surfactant and water. Small

angle X-ray scattering (SAXS) also indicated the existence of structures with different

characteristics.

Wang et al. [91] studied the phase behavior of castor, jatropha and soybean oil based

ionic liquid microemulsions by varying the surfactant to cosurfactant mass ratio (Km)

and temperature. The ionic liquid [BMIM][BF4], surfactant TX-100 and cosurfactant

n-butanol were used for the preparation of microemulsion in this study. The maximum

single phase region area of jatropha and soybean oil-based systems were observed for

Km = 2:1. Moreover, dehydration of the oxyethylene group of nonionic surfactant TX-

100 at higher temperatures increased single-phase region areas of each vegetable oil-

based system with increasing temperature.

Kibbey et al. [92] analyzed the dependency of vegetable oil-diesel microemulsion fuel

viscosity with temperature by the Chevron model coupled with temperature-dependent

relationships for density and viscosity of the reverse microemulsion fuel components.

Reverse microemulsion formulated with vegetable oil (canola, algae or palm oil), No.

2 diesel, ethyl alcohol and oleylamine (surfactant) and octyl alcohol (cosurfactant)

were used as reference systems for the study. The measurements were conducted for

the single phase formulations at 5, 10, 25 and 40 ˚C. The trends of viscosity variation

with changing temperature and the increase in viscosity with increasing surfactant

concentrations were accurately predicted and captured. However, the mixing model

was successful in predicting observed viscosities only for the samples with high water

content (i.e.4%).

Dantas et al. [93] formulated microemulsion systems containing diesel and different

percentages of vegetable oils (soy, palm and ricin). The main parameters that can



influence the microemulsion area like nature of surfactant (T) and cosurfactant (C),

composition of the oil phase and C/T ratio were studied in this investigation.

Schwab et al [32] formulated a detergentless microemulsion in which water and a C1-

C3 alcohol were dispersed in the oil (like soybean, corn, rapeseed, sesame, safflower

and cottonseed) by means of l-butanol (nonionic surface active agent), which exhibited

viscosity in the range 2 – 9 cSt (at 37.8 ˚C) and showed engine performance

comparable to that of No. 2 diesel. Aqueous ethanol containing 5 – 20% of water was

particularly preferred for formulating these systems [32]. They further reported the

improvement in fuel properties of the fuel system by formulating two types of systems:

one by introducing straight-chain octanol serving as a single-component and other by

introducing N,N-dimethylethanolamine and a long-chain fatty acid [33,94]. Although

the formulated hybrid fuels showed decrease in cetane number and GCV,

improvement of cold flow properties was also evident [32,33,94].

Ethanol is also gaining importance as a polar solvent in formulating vegetable oil

based microemulsion. Do et al. [95] prepared reverse micellar microemulsions of

vegetable oils (canola, palm, and algae oils) with ethanol using different combinations

of surfactants (oleyl alcohol and oleyl amine) and co-surfactants (2-ethylhexylnitrate,

2-ethylhexanol, 1-octanol and EGBE). Palm-diesel microemulsion was solidified at 6–

6.5 ˚C due to high saturated triglyceride content. The viscosity of microemulsion fuel

comprising 50 vol % of canola or palm oils in diesel fuel and 24 vol % of ethanol met

the requirement of ASTM standard for No. 2 diesel, while for the algae-diesel based

microemulsion fuel it could be achieved by introducing only 15 vol % of ethanol. The

microemulsion fuels exhibited heating values comparable to that of biodiesel and 10%

less than that of No. 2 diesel. The investigated systems were formulated at surfactant

to cosurfactant mole ratios 4:1, 1:1, 1:4, 1:8, and 1:16. The presence of droplets of size

6-8 nm was evident for systems comprising surface active agents.

Attaphong et al. [96] investigated a canola oil-diesel-ethanol based system formulated

using carboxylate-based extended surfactant. Carboxylate-based extended surfactant at

surfactant/cosurfactant ratio of 1:16 with canola oil-diesel ratio of 50:50 and



approximately 24% volume of ethanol also met the ASTM standard for No. 2 diesel

(4.1 cSt) at 40 ˚C. The study confirmed that branching and the number of ethylene

oxide (EO) groups moderately affected the hydrophobicity of the systems, as because

branching of surfactants increases the water solubility and EO groups increase the

polarity of surfactants [96]. This research group also formulated microemulsion fuels

comprising vegetable oil, water and ethanol by using an alcohol ethoxylate surfactant.

Furthermore, nonedible oil mixtures (algae mixed with castor) based system exhibited

properties comparable to edible oil (canola) based microemulsion fuels at 0 ˚C

formulated with ethanol. The fuel properties were even better at 25 ˚C in presence of

bioethanol. Salt-free microemulsion was formulated by using single and mixed

surfactants even in absence of cosurfactants. However, the systems with cosurfactants

showed better physical stability (avoid phase separation up to -5 ˚C) than the system

without cosurfactant (phase separation occurred at 0 ˚C). Although, oleyl alcohol had

the highest solubilization capacity (required in lowest concentration to achieve single

phase microemulsion), linear alcohol ethoxylate surfactant based systems also showed

comparable fuel quality with oleyl alcohol based systems. The kinematic viscosity (at

40 ˚C) and lower heating value of the microemulsion fuels (3.9 – 4.6 mm2/s and 35 –

38 MJ/kg, respectively) were also comparable to that of canola oil biodiesel (4.5

mm2/s and 37.4 MJ/kg, respectively) [97].

The effects of surfactant saturation, unsaturation, EO groups on the phase behavior,

kinematic viscosity, and microemulsion-droplet size were also investigated with the

goal of formulating optimized surfactant based fuel comprising anhydrous ethanol (of

25 vol %) as polar solvent and palm oil–diesel blends (at 1:1 v/v). Four different types

of nonionic surfactants namely: stearyl alcohol (saturated), oleyl alcohol (unsaturated),

methyl oleate (unsaturated with ester group), and Brij-010 (EO groups) and

cosurfactants 1-butanol (99% purity), 1-octanol (99% purity), 1-decanol (99% purity),

and 2-ethyl-hexanol (≥ 99.6% purity) were used for the study. The presence of methyl

oleate unsaturated surfactant greatly reduced the bulk viscosity and produced uniform

size of microemulsion droplets (of size 21.86 nm) while using the least amount of

surfactant for solubilizing ethanol-in-oil in the system. However, the average droplet



size of the system formulated with oleyl alcohol, stearyl alcohol and Brij-010 were

much smaller (1.65, 1.71 and 2.39 nm, respectively). The kinematic viscosities of

systems with surfactant to cosurfactant molar ratio 1:8 decreased with increase in

temperature. Even though the surfactant concentration in the formulated systems was

much lower than that of cosurfactant, the former had greater effect on kinematic

viscosity than the later. The increase in cosurfactant chain length reduced the droplet

size of the microemulsion besides increasing the viscosity [98].

The engine performance characteristics of vegetable oil based microemulsion systems

comprising ethanol were found to be comparable with neat diesel fuel. Moreover,

improvement in emission characteristics had been also observed in these studies

[95,99-104].

Formulation of microemulsion system also proved to be beneficial for improving fuel

quality of biodiesel. Ethanol-biodiesel-diesel (EB-diesel) fuel blend microemulsions

were stable well below sub-zero temperatures and showed comparable or superior fuel

properties to petrodiesel in terms of heat of combustion and cetane numbers. However,

reduction of flash point by few degrees was also observed for EB-diesel fuels. The

phase stability of EB-diesel fuel blends using ultralow-sulfur diesel had better low-

temperature phase stabilities than those with low-sulfur diesel, which confirmed the

effect of sulfur content of diesel fuels on the phase transition. Moreover, introduction

of 0.05% water into the system resulted in phase separation of the formulated systems

[105]. Alander et al. [106] investigated thermodynamic stability of the microemulsion

system comprising fatty acid esters (biodiesel) and triglycerides as lipophilic

components. Improvement in brake specific energy consumption (BSEC) along with

emission characteristics for emulsion fuel comprising biodiesel (derived from waste

cooking palm oil and soybean oil) were evident from the studies of Kannan et al. [107]

and Qi et al. [108]. Qi et al. [108] further reported that the maximum water content to

maintain the physical stability of the formulated microemulsions was increased with

increasing quantity of Span 80.



2.4. Non-energy and other type of energy aspects of emulsion and

microemulsion system

The non-energy utilities of microemulsification are vast and lucrative. The feasibility

of vegetable oil and waste-material based microemulsion product to use as detergent,

medicine, food, cosmetics, organic solvent (in certain analytical studies, e.g. ICP-MS)

etc. have been reported [109-114]. Moreover, microemulsification could also be used

successfully in oil extraction and removal and in triglyceride based enhance oil

recovery techniques [115-119]. Formulation of a middle phase microemulsion with a

weathered jet fuel waste (using a mixture of nonionic and ionic surface active

compounds) is one of the recent applications of microemulsification [120]. Few

reports concerning formulation of emulsion and microemulsion fuel from gasoline are

also available [121-123].

2.5. Nanoemulsion fuel

In the years 2013 and 2014, few publications relating to formulation and

characterization of nanoemulsion based fuels have been published. The basic

difference involved in the preparation of nanoemulsion and microemulsion is that

nanoemulsion is formed by mechanical shear where as microemulsion is ‘self

assembled’ or ‘spontaneously formed’ systems. Although nanoemulsions have

structural similarity with microemulsion in terms of droplets and long-term physical

stability, unlike microemulsions (which are also transparent or translucent and

thermodynamically stable), nanoemulsions are thermodynamically unstable. In

contrast to microemulsion, these systems require external energy for the formation

which is the main reason for their thermodynamic instability. There are two main

preparation methods [124-129];

i. The dispersion or high-energy methods, that consists on the application of high

mechanical energy during emulsification. The high-energy emulsification is

generally achieved by using high-pressure homogenizers, high shear stirring

and ultrasound generators.



ii. The condensation or low-energy method, in which change of curvature or

phase transition takes place during emulsification. In this method either the

temperature (Emulsion Inversion Point method, EIP) or the composition (Phase

Inversion Temperature method, PIT) can be kept constant.

El-Din et al. [130] reported that the water (5, 6, 7, 8 and 9 wt.%) -in-diesel

nanoemulsions stabilized by mixed nonionic surfactants (10% concentration of a

mixture of 80% polyoxyethylene sorbitan monooleate and 20% sorbitan monooleate)

and higher speed stirring (at 30,000 rpm) using high speed homogenizer for 5 min

contained droplets of size in the range 49 – 140 nm. The increase in water (dispersed

phase) content in the system increased the water droplet size and viscosity of

nanoemulsion fuel. In addition, the time influence on the rheology of the

nanoemulsions led to evolution of water droplet size due to Ostwald ripening. A slight

decrease in viscosity was observed which indicated the change of rheological character

of the system into time-dependent non-Newtonian character as a result of interfacial

relaxation stress by time [130]. The results of this study are in agreement with another

previous study by the same research group [131]. Furthermore, Bidita et al. [132] and

Koc et al. [133] reported improvement in engine performance and emission

characteristics for nanoemulsion fuels in comparison to petro diesel and biodiesel.

The fastest growing interest on emulsified fuels (emulsion, microemulsion and

nanoemulsion fuels) signifies the wide scope of this domain in ensuing future.

Microemulsion based fuel from vegetable oil is sprouting out as the most sustainable

type of emulsified fuel as it may address the issue of renewable fuel production and

thermodynamic stability of fuel systems simultaneously. Therefore, in-depth and

systematic research activities to investigate the characteristics of the fuel systems,

which would lay down the basis for commercial scale production and study of process

economics, are imperative.



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