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2Surfactants Used in Formulation of Dispersions

Surface-active agents (usually called surfactants) are sometimes referred to asamphiphilic molecules, which implies that they consist of at least two parts, onepart which is soluble in a specific liquid (the lyophilic part) and one part which isinsoluble (the lyophobic part). If the fluid is water, then one refers to the hydrophilicand hydrophobic parts, respectively. In this case, the molecule consists of anonpolar hydrophobic portion; this is usually a straight or branched hydrocarbonor fluorocarbon chain containing between eight and 18 carbon atoms, which isattached to a polar or ionic portion (hydrophilic). The hydrophilic portion can,therefore, be nonionic, ionic, or zwitterionic, and is accompanied by counterionsin the last two cases. The hydrocarbon chain interacts weakly with the watermolecules in an aqueous environment, whereas the polar or ionic head groupinteracts strongly with water molecules via dipole or ion–dipole interactions. Itis this strong interaction with the water molecules which renders the surfactantsoluble in water. However, the cooperative action of dispersion and hydrogenbonding between the water molecules tends to squeeze the hydrocarbon chain outof the water, and hence these chains are referred to as hydrophobic. The balancebetween the hydrophobic and hydrophilic parts of the molecule gives these systemstheir special properties, for example their accumulation at various interfaces andtheir association in solution (to form micelles). In addition to the name surface-active agents, these molecules are often known by other names which includesurfactants, association colloids, colloidal electrolytes, amphipathic compounds,and tensides.

The driving force for surfactant adsorption is a lowering of the free energy of thephase boundary. The interfacial free energy per unit area is the amount of workrequired to expand the interface. This interfacial free energy, referred to as surfaceor interfacial tension, γ, is given in millijoules per metre square or millinewtonsper metre. The adsorption of surfactant molecules at the interface lowers γ, andthe higher the surfactant adsorption (i.e., the more dense the layer is) the largeris the reduction in γ. The degree of surfactant adsorption at the interface dependson surfactant structure and the nature of the two phases that meet the interface[1, 2]. Surface-active agents also aggregate in solution-forming micelles. The drivingforce of micelle formation (or micellisation) is a reduction of contact between thehydrocarbon chain and water, thereby reducing the free energy of the system. In

Formulation of Disperse Systems: Science and Technology, First Edition. Tharwat F. Tadros.c© 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

12 2 Surfactants Used in Formulation of Dispersions

the micelle, the surfactant hydrophobic groups are directed towards the interiorof the aggregate, while the polar head groups are directed towards the solvent.These micelles are in dynamic equilibrium, and the rate of exchange between asurfactant molecule and the micelle may vary by orders of magnitude, dependingon the structure of the surfactant molecule.

Surfactants find application in almost all disperse systems that are utilised inareas such as paints, dyestuffs, cosmetics, pharmaceuticals, agrochemicals, fibres,and plastics. Therefore, a fundamental understanding of the physical chemistryof surface-active agents, their unusual properties, and their phase behaviour isessential for most formulation chemists. In addition, an understanding of the basicphenomena involved in the application of surfactants, such as in the preparation ofemulsions and suspensions and their subsequent stabilisation, in microemulsions,in wetting, spreading and adhesion, is vitally important to arrive at the correctcomposition and control of the system involved [1, 2]. This is particularly the casewith many formulations in the chemical industry mentioned above.

It should be stated that commercially produced surfactants are not pure chem-icals, and within each chemical type there can be tremendous variation. This isunderstandable as surfactants are prepared from various feedstocks, namely petro-chemicals, natural vegetable oils and natural animal fats. It is important to realizethat, in every case, the hydrophobic group exists as a mixture of chains of differentlengths. The same can be applied to the polar head group, for example in the caseof polyethylene oxide (PEO) (the major component of nonionic surfactants) whichconsists of a distribution of ethylene oxide (EO) units. Hence, products that maybe given the same generic name might vary a great deal in their properties, and theformulation chemist should bear this in mind when choosing a surfactant from aparticular manufacturer. It is advisable to obtain as much information as possiblefrom the manufacturer about the properties of the surfactant chosen, such as itssuitability for the job, its batch-to-batch variation, and its toxicity. The manufacturerwill usually have available more information on the surfactant than is printed on theaccompanying data sheet, and in most cases such extra data will be given on request.

2.1General Classification of Surface-Active Agents

A simple classification of surfactants, based on the nature of the hydrophilic group,is commonly used, with four main classes being distinguished: anionic; cationic;amphoteric; and nonionic. A useful technical reference here is McCutcheon [3],which is produced annually to update the list of available surfactants. A recenttext by van Os et al. [4], listing the physico-chemical properties of selected anionic,cationic and nonionic surfactants, has been published by Elsevier.

Another useful text is the Handbook of Surfactants by Porter [5]. It should bementioned also that a fifth class of surfactants, usually referred to as polymericsurfactants, has been used for many years for the preparation of emulsions andsuspensions, and for their stabilisation.

2.1 General Classification of Surface-Active Agents 13

2.1.1Anionic Surfactants

These are the most widely used class of surfactants in industrial applications[5–7], due mainly to their relatively low cost of manufacture, and they are used inpractically every type of detergent. For optimum detergency the hydrophobic chainis a linear alkyl group with a chain length in the region of 12–16 C atoms, and thepolar head group should be at the end of the chain. Linear chains are preferred asthey are more effective and more degradable than are branched chains. The mostcommonly used hydrophilic groups are carboxylates, sulphates, sulphonates andphosphates. A general formula may be ascribed to anionic surfactants as follows:

Carboxylates: CnH2n+1 COO− X+

Sulphates: CnH2n+1 OSO3− X+

Sulphonates: CnH2n+1 SO3− X+

Phosphates: CnH2n+1 OPO(OH)O− X+.

where n is in the range of eight to 16 atoms and the counterion X+ is usually Na+.Several other anionic surfactants are commercially available, such as

sulphosuccinates, isethionates (esters of isothionic acid with the general formulaRCOOCH2-CH2-SO3Na) and taurates (derivatives of methyl taurine with thegeneral formula RCON(R′)CH2-CH2-SO3Na), sarcosinates (with the generalformula RCON(R′)COONa), and these are sometimes used for special applications.A brief description of the above-described anionic classes is provided below,together with some of their applications.

2.1.1.1 CarboxylatesThese are perhaps the earliest known surfactants, as they constitute the earliestsoaps, for example sodium or potassium stearate, C17H35COONa, or sodiummyristate, C14H29COONa. The alkyl group may contain unsaturated portions,for example sodium oleate, which contains one double bond in the C17 alkylchain. Most commercial soaps will be a mixture of fatty acids obtained frommaterials such as tallow, coconut oil and palm oil. The main attraction of thesesimple soaps is their low cost, their ready biodegradability, and low toxicity. Theirmain disadvantage is their ready precipitation in water containing bivalent ionssuch as Ca2+ and Mg2+. In order to avoid their precipitation in hard water,the carboxylates are modified by introducing hydrophilic chains, for exampleethoxy carboxylates with the general structure RO(CH2CH2O)nCH2COO−, estercarboxylates containing hydroxyl or multi COOH groups, and sarcosinates whichcontain an amide group with the general structure RCON(R′)COO−. Addition of theethoxylated groups results in an increased water solubility and enhanced chemicalstability (no hydrolysis). The modified ether carboxylates are more compatiblewith electrolytes, and are also compatible with other nonionic, amphoteric, andsometimes even cationic, surfactants. The ester carboxylates are very soluble inwater but suffer from the problem of hydrolysis. The sarcosinates are not verysoluble in acid or neutral solutions but are quite soluble in alkaline media, and are


compatible with other anionics, nonionics, and cationics. The phosphate esters havevery interesting properties, being intermediate between ethoxylated nonionics andsulphated derivatives; they also have good compatibility with inorganic buildersand they can be good emulsifiers. One specific salt of a fatty acid is lithium12-hydroxystearic acid, which forms the major constituent of greases.

2.1.1.2 SulphatesThese are the largest and most important class of synthetic surfactants, which wereproduced by the reaction of an alcohol with sulphuric acid; that is, they are estersof sulphuric acid. In practice, however, sulphuric acid is seldom used and chloro-sulphonic or sulphur dioxide/air mixtures are the most common methods used tosulphate the alcohol. Unfortunately, due to their chemical instability (hydrolysingto the alcohol, particularly in acid solutions) they have now been overtaken by thesulphonates, which are chemically stable. The properties of the sulphate surfactantsdepend on the nature of the alkyl chain and the sulphate group. The alkali metalsalts show good solubility in water, but tend to be affected by the presence ofelectrolytes. The most common sulphate surfactant is sodium dodecyl sulphate(SDS, sometimes also referred to as sodium lauryl sulphate), which is used exten-sively not only for fundamental studies but also for many applications in industry.At room temperature (∼25 ◦C) this surfactant is quite soluble, and 30% aqueoussolutions are fairly fluid (low viscosity). However, below 25 ◦C the surfactant mayseparate out as a soft paste as the temperature falls below its Krafft point (thetemperature above which the surfactant shows a rapid increase in solubility witha further increase of temperature). The latter depends on the distribution of chainlengths in the alkyl chain; that is, the wider the distribution the lower the Kraffttemperature. Thus, by controlling this distribution it is possible to achieve a Kraffttemperature of ∼10 ◦C. As the surfactant concentration is increased to 30–40%(depending on the distribution of chain length in the alkyl group), the viscosity ofthe solution increases very rapidly with the production of a gel; subsequently, asthe concentration reaches about 60–70% the solution becomes a pourable liquid,but above this concentration a gel is again formed. The concentration at which theminimum occurs varies according to the alcohol sulphate used, and also accordingto the presence of impurities such as unsaturated alcohol. The viscosity of the aque-ous solutions can be reduced by the addition of short-chain alcohols and glycols.The critical micelle concentration (cmc) of SDS (the concentration above which theproperties of the solution show abrupt changes) is 8× 10−3 mol dm−3 (0.24%). Thealkyl sulphates give good foaming properties with an optimum at C12 –C14. As withthe carboxylates, the sulphate surfactants are also chemically modified to changetheir properties; the most common modification is to introduce some EO unitsinto the chain, at which point the sulphates are usually referred to as alcohol ethersulphates. The latter are prepared by the sulphation of ethoxylated alcohols. Forexample, sodium dodecyl 3 mol ether sulphate is essentially dodecyl alcohol reactedwith 3 mol EO, and then sulphated and neutralised with NaOH. The presence ofPEO confers an improved solubility when compared to the straight-chain alcoholsulphates. In addition, the surfactant becomes more compatible with electrolytes


in aqueous solution. The ether sulphates are also more chemically stable thanthe alcohol sulphates. The cmc of the ether sulphates is also lower than thecorresponding surfactant without the EO units. The viscosity behaviours ofthe aqueous solutions are similar to those of the alcohol sulphates, giving gelsin the range of 30–60% ether sulphate concentration. The ether sulphates showa pronounced salt effect, with a significant increase in the viscosity of a dilutesolution on the addition of an electrolyte, such as NaCl. The ether sulphates arecommonly used in hand-dishwashing liquids and in shampoos, in combinationwith amphoteric surfactants.

2.1.1.3 SulphonatesWith sulphonates, the sulphur atom is directly attached to the carbon atom ofthe alkyl group, and this gives the molecule stability against hydrolysis whencompared to the sulphates (where the sulphur atom is linked indirectly to thecarbon of the hydrophobe via an oxygen atom). The alkyl aryl sulphonates are themost common type of these surfactants (e.g., sodium alkyl benzene sulphonate),and these are usually prepared by the reaction of sulphuric acid with alkyl arylhydrocarbons, for example dodecyl benzene. One special class of sulphonatesurfactants are the naphthalene and alkyl naphthalene sulphonates, which arecommonly used as dispersants. As with the sulphates, some chemical modificationis used by introducing EO units, for example sodium nonyl phenol 2 mol ethoxylateethane sulphonate C9H19C6H4(OCH2CH2)2SO3

−Na+. The paraffin sulphonatesare produced by the sulpho-oxidation of normal linear paraffins with sulphurdioxide and oxygen, and catalysed with ultraviolet or gamma radiation; the resultingalkane sulphonic acid is neutralised with NaOH. These surfactants have excellentwater solubility and biodegradability, and they are also compatible with manyaqueous ions. The linear alkyl benzene sulphonates (LABSs) are manufacturedfrom alkyl benzene, and the alkyl chain length can vary from C8 to C15; theirproperties are mainly influenced by the average molecular weight and the spreadof carbon number of the alkyl side chain. The cmc of sodium dodecyl benzenesulphonate is 5× 10−3 mol dm−3 (0.18%). The main disadvantages of LABS istheir effect on the skin, and consequently they cannot be used in personal careformulations.

Another class of sulphonates is the α-olefin sulphonates which are prepared byreacting linear α-olefin with sulphur trioxide, typically yielding a mixture of alkenesulphonates (60–70%), 3- and 4-hydroxyalkane sulphonates (∼30%), and somedisulphonates and other species. The two main α-olefin fractions used as startingmaterial are C12 –C16 and C16 –C18. Fatty acid and ester sulphonates are producedby the sulphonation of unsaturated fatty acids or esters, a good example beingsulphonated oleic acid:

CH3(CH2)7CH(CH2)8COOH

SO3H


A special class of sulphonates are sulphosuccinates which are esters of sulpho-succinic acid:

CH2COOH

HSO3 CH COOH

Both, monoesters and diesters are produced. A widely used diester in many for-mulations is sodium di(2-ethylhexyl)sulphosuccinate, which is sold commerciallyunder the trade name Aerosol OT. The cmc of the diesters is very low, in theregion of 0.06% for C6 –C8 sodium salts, and they give a minimum in the surfacetension of 26 mN m−1 for the C8 diester; thus, these molecules are excellent wettingagents. The diesters are soluble both in water and in many organic solvents, andare particularly useful for the preparation of water-in-oil (W/O) microemulsions.

2.1.1.4 Phosphate-Containing Anionic SurfactantsBoth, alkyl phosphates and alkyl ether phosphates are made by treating the fattyalcohol or alcohol ethoxylates with a phosphorylating agent, usually phosphorouspentoxide, P4O10. The reaction yields a mixture of monoesters and diesters ofphosphoric acid, with the ratio of the two esters being determined by the ratioof the reactants and the amount of water present in the reaction mixture. Thephysico-chemical properties of the alkyl phosphate surfactants depend on the ratioof the esters. Phosphate surfactants are used in the metal-working industry, due totheir anticorrosive properties.

2.1.2Cationic Surfactants

The most common cationic surfactants are the quaternary ammonium compounds[8, 9] with the general formula R′R′′R′′′R′′′′N+X−, where X− is usually chlorideion and R represents alkyl groups. These quaternaries are made by reacting anappropriate tertiary amine with an organic halide or organic sulphate. A commonclass of cationics is the alkyl trimethyl ammonium chloride, where R containsbetween eight and 18 C atoms, for example dodecyl trimethyl ammonium chloride,C12H25(CH3)3NCl. Another widely used cationic surfactant class is that containingtwo long-chain alkyl groups, that is dialkyl dimethyl ammonium chloride, with thealkyl groups having a chain length of between eight and 18 C atoms. These dialkylsurfactants are less soluble in water than the monoalkyl quaternary compounds, butthey are commonly used in detergents as fabric softeners. A widely used cationicsurfactant is alkyl dimethyl benzyl ammonium chloride (sometimes referred to asbenzalkonium chloride and widely used as bactericide), having the structure:

C12H25 CH3

+N Cl−

CH2 CH3


Imidazolines can also form quaternaries, the most common product being theditallow derivative quaternise with dimethyl sulphate:

CH3

[C17H35 C–N–CH2-CH2-NH-CO-C17H35]+

N CH CH3 SO4−

CH

Cationic surfactants can also be modified by incorporating PEO chains, forexample dodecyl methyl polyethylene oxide ammonium chloride having thestructure:

C12H25 (CH2CH2O)nH

+N Cl−

CH3 (CH2CH2O)nH

Cationic surfactants are generally water-soluble when there is only one long alkylgroup. When there are two or more long-chain hydrophobes the product becomesdispersible in water and soluble in organic solvents. They are generally compatiblewith most inorganic ions and hard water, but are incompatible with metasilicatesand highly condensed phosphates. They are also incompatible with protein-likematerials. Cationics are generally stable to pH changes, both acid and alkaline.They are also incompatible with most anionic surfactants, but are compatible withnonionics. These cationic surfactants are insoluble in hydrocarbon oils. In contrast,cationics with two or more long alkyl chains are soluble in hydrocarbon solvents,but they become only dispersible in water (sometimes forming bilayer vesicle-typestructures). They are generally chemically stable and can tolerate electrolytes. Thecmc of cationic surfactants is close to that of anionics with the same alkyl chainlength; for example, the cmc of benzalkonium chloride is 0.17%. The prime useof cationic surfactants is their tendency to adsorb at negatively charged surfaces;examples include anticorrosive agents for steel, flotation collectors for mineralores, dispersants for inorganic pigments, antistatic agents for plastics, antistaticagents and fabric softeners, hair conditioners, anticaking agents for fertilizers, andas bactericides.

2.1.3Amphoteric (Zwitterionic) Surfactants

These are surfactants containing both cationic and anionic groups [10]. The mostcommon amphoterics are the N-alkyl betaines, which are derivatives of trimethylglycine (CH3)3NCH2COOH (that was described as betaine). An example of a betaine


surfactant is lauryl amido propyl dimethyl betaine C12H25CON(CH3)2CH2COOH.These alkyl betaines are sometimes described as alkyl dimethyl glycinates. Themain characteristics of amphoteric surfactants is their dependence on the pH of thesolution in which they are dissolved. In acid pH solutions, the molecule acquiresa positive charge and behaves like a cationic, whereas in alkaline pH solutionsthey become negatively charged and behave like an anionic. A specific pH can bedefined at which both ionic groups show equal ionisation; this is the isoelectricpoint (i.e.p.) of the molecule, described by the following scheme:

N+ … COOH ↔ N+ … COO− ↔ NH … COO−

acid pH < 3 isoelectric pH > 6alkaline

Amphoteric surfactants are sometimes referred to as zwitterionic molecules.They are soluble in water, but their solubility shows a minimum at the i.e.p.Amphoterics show excellent compatibility with other surfactants, forming mixedmicelles; they are also chemically stable both in acids and alkalis. The surfaceactivity of amphoterics varies widely, and depends on the distance between thecharged groups. Amphoterics display a maximum in surface activity at the i.e.p.

Another class of amphoteric are the N-alkyl amino propionates having the struc-ture R-NHCH2CH2COOH. The NH group is reactive and can react with anotheracid molecule (e.g., acrylic) to form an amino dipropionate R-N(CH2CH2COOH)2.An alkyl imidazoline-based product can also be produced by reacting alkyl imidozo-line with a chloro acid, but the imidazoline ring will break down during formationof the amphoteric.

The change in charge with pH of amphoteric surfactants affects their properties,such as wetting, detergency, and foaming. At the i.e.p., the properties of amphotericsresemble those of nonionics very closely, but below and above the i.e.p. theproperties shift towards those of cationic and anionic surfactants, respectively.Zwitterionic surfactants have excellent dermatological properties, and also exhibitlow eye irritation; consequently, they are frequently used in shampoos and otherpersonal care products (e.g., cosmetics).

2.1.4Nonionic Surfactants

The most common nonionic surfactants are those based on EO, referred to asethoxylated surfactants [11–13]. Several classes can be distinguished, including alco-hol ethoxylates, alkyl phenol ethoxylates, fatty acid ethoxylates, monoalkaolamideethoxylates, sorbitan ester ethoxylates, fatty amine ethoxylates, and EO–propyleneoxide copolymers (sometimes referred to as polymeric surfactants). Another impor-tant group of nonionics are the multihydroxy products such as glycol esters, glycerol(and polyglycerol) esters, glucosides (and polyglucosides) and sucrose esters. Amineoxides and sulphinyl surfactants represent nonionics with a small head group.


2.1.4.1 Alcohol EthoxylatesThese are generally produced by the ethoxylation of a fatty chain alcohol suchas dodecanol. Several generic names have been given to this class of surfactants,including ethoxylated fatty alcohols, alkyl polyoxyethylene glycol, and monoalkylpolyethylene oxide glycol ethers. A typical example is dodecyl hexaoxyethyleneglycol monoether with the chemical formula C12H25(OCH2CH2O)6OH (sometimesabbreviated as C12E6). In practice, the starting alcohol will have a distribution ofalkyl chain lengths, and the resulting ethoxylate will have a distribution of EO chainlength. Thus, the numbers listed in the literature refer to average numbers.

The cmc of nonionic surfactants is about two orders of magnitude lower thanthe corresponding anionics with the same alkyl chain length. At a given alkyl chainlength, the cmc decreases with a decrease in the number of EO units. The solubilityof the alcohol ethoxylates depend both on the alkyl chain length and the numberof EO units in the molecule. Molecules with an average alkyl chain length of 12C atoms and containing more than five EO units are usually soluble in water atroom temperature; however, as the temperature of the solution is gradually raisedthe solution becomes cloudy (as a result of dehydration of the PEO chain and thechange in conformation of the PEO chain); the temperature at which this occurs isreferred to as the cloud point (CP) of the surfactant. At a given alkyl chain length,the CP increases with an increase in the EO chain of the molecule. The CP changeswith change of concentration of the surfactant solution, and the trade literatureusually quotes the CP of a 1% solution. The CP is also affected by the presenceof an electrolyte in the aqueous solution, as most electrolytes lower the CP of anonionic surfactant solution. Nonionics tend to have maximum surface activitynear to the CP, while the CP of most nonionics increases markedly on the additionof small quantities of anionic surfactants. The surface tension of alcohol ethoxylatesolutions decreases with increases in its concentration until it reaches its cmc; afterthis it remains constant with further increases in its concentration. The minimumsurface tension reached at and above the cmc decreases with a decrease in thenumber of EO units of the chain (at a given alkyl chain). The viscosity of a nonionicsurfactant solution increases gradually with increases in its concentration, but ata critical concentration (which depends on the alkyl and EO chain length) theviscosity show a rapid increase and, ultimately, a gel-like structure appears. Thisis due to the formation of a liquid crystalline structure of the hexagonal type. Inmany cases, the viscosity reaches a maximum, after which it shows a decrease dueto the formation of other structures (e.g. lamellar phases; see below).

2.1.4.2 Alkyl Phenol EthoxylatesThese are prepared by the reaction of EO with an appropriate alkyl phenol. Themost common surfactants of this type are those based on nonyl phenol; they arecheap to produce but suffer from problems of biodegradability and potential toxicity(the byproduct of degradation is nonyl phenol, which is considerably toxic in fishand mammals). Despite these problems, nonyl phenol ethoxylates are still used inmany industrial properties, due mainly to their advantageous properties such as


solubility both in aqueous and nonaqueous media, and their good emulsificationand dispersion properties.

2.1.4.3 Fatty Acid EthoxylatesThese are produced by the reaction of EO with a fatty acid or a polyglycol, andhave the general formula RCOO-(CH2CH2O)nH. When a polyglycol is used, amixture of monoesters and diesters (RCOO-(CH2CH2O)n-OCOR) is produced.These surfactants are generally soluble in water, provided that there are sufficientEO units present and the alkyl chain length of the acid is not too long. Themonoesters are much more soluble in water than are the diesters; in the latter casea longer EO chain is required to render the molecule soluble. The surfactants arecompatible with aqueous ions, provided that not too much unreacted acid is present.However, these surfactants undergo hydrolysis in highly alkaline solutions.

2.1.4.4 Sorbitan Esters and Their Ethoxylated Derivatives (Spans and Tweens)The fatty acid esters of sorbitan (which generally are referred to as Spans, an Atlascommercial trade name) and their ethoxylated derivatives (generally referred to asTweens) are perhaps one of the most commonly used nonionics. They were firstcommercialised in the USA by Atlas (which has since been purchased by ICI).The sorbitan esters are produced by the reaction of sorbitol with a fatty acid at ahigh temperature (>200 ◦C). The sorbitol dehydrates to 1,4-sorbitan, after whichesterification takes place. If 1 mol of fatty acid is reacted with 1 mol of sorbitol,a monoester is obtained (some diester is also produced as a byproduct). Thus,sorbitan monoester has the following general formula:

CH2

H – C – OH

HO – C – H O

H – C

H – C – OH

CH2OCOR

The free OH groups in the molecule can be esterified, producing diesters andtriesters. Several products are available, depending on the nature of the alkyl groupof the acid and whether the product is a monoester, diester, or triester. Someexamples are given below:

Sorbitan monolaurate – Span 20Sorbitan monopalmitate – Span 40Sorbitan monostearate – Span 60Sorbitan mono-oleate – Span 80Sorbitan tristearate – Span 65Sorbitan trioleate – Span 85.

The ethoxylated derivatives of Spans (Tweens) are produced by the reaction ofEO on any hydroxyl group remaining on the sorbitan ester group. Alternatively, the


sorbitol is first ethoxylated and then esterified, although the final product will havedifferent surfactant properties to the Tweens. Some examples of Tween surfactantsare given below:

Polyoxyethylene (20) sorbitan monolaurate – Tween 20Polyoxyethylene (20) sorbitan monopalmitate – Tween 40Polyoxyethylene (20) sorbitan monostearate – Tween 60Polyoxyethylene (20) sorbitan mono-oleate – Tween 80Polyoxyethylene (20) sorbitan tristearate – Tween 65Polyoxyethylene (20) sorbitan tri-oleate – Tween 85.

The sorbitan esters are insoluble in water, but are soluble in most organic solvents(low hydrophilic–lipophilic-balance (HLB) number surfactants). The ethoxylatedproducts are generally soluble in number, and have relatively high HLB numbers.One of the main advantages of the sorbitan esters and their ethoxylated derivativesis their approval as food additives. They are also used widely in cosmetics and somepharmaceutical preparations.

2.1.4.5 Ethoxylated Fats and OilsA number of natural fats and oils have been ethoxylated, for example linolin(wool fat) and castor oil ethoxylates. These products are useful for applications inpharmaceutical products, for example as solubilisers.

2.1.4.6 Amine EthoxylatesThese are prepared by the addition of EO to primary or secondary fatty amines.With primary amines, both hydrogen atoms on the amine group react with EO,and therefore the resulting surfactant has the structure:

(CH2CH2O)xH

R N

(CH2CH2O)yH

The above surfactants acquire a cationic character if the EO units are small innumber and if the pH is low; however, at high EO levels and neutral pH they behavevery similarly to nonionics. At low EO content the surfactants are not soluble inwater, but become soluble in an acid solution. At high pH, the amine ethoxylatesare water-soluble, provided that the alkyl chain length of the compound is not long(usually a C12 chain is adequate for reasonable solubility at sufficient EO content).

2.1.4.7 Amine OxidesThese are prepared by oxidizing a tertiary nitrogen group with aqueous hydrogenperoxide at temperatures in the region of 60–80 ◦C. Several examples can be quoted,including N-alkyl amidopropyl-dimethyl amine oxide, N-alkyl bis(2-hydroxyethyl)


amine oxide and N-alkyl dimethyl amine oxide. These have the general formula:

CH2CH2OH

Coco N → O Coco bis (2-hydroxyethyl) amine oxide

CH2CH2OH

CH3

CocoCONHCH2CH2CH2N → O Alkyl amidopropyl-dimethyl amine oxide

CH3

CH3

CH12 H25N → O Lauryl dimethyl amine oxide

CH3

In acid solutions, the amino group is protonated and acts as a cationic surfactant,whereas in neutral or alkaline solution the amine oxides are essentially nonionicin character. Alkyl dimethyl amine oxides are water-soluble up to C16 alkyl chain.Above pH 9, amine oxides are compatible with most anionics, but at pH 6.5 andbelow some anionics tend to interact and form precipitates. In combination withanionics, amine oxides can be used as foam boosters (e.g., in shampoos).

2.1.5Specialty Surfactants

2.1.5.1 Fluorocarbon and Silicone SurfactantsThese surfactants can lower the surface tension of water to values below 20 mN m−1.By comparison, most of the surfactants described above lower the surface tensionof water to values above 20 mN m−1, typically in the region of 25–27 mN m−1. Thefluorocarbon and silicone surfactants are sometimes referred to as ‘‘superwetters’’as they cause enhanced wetting and spreading of their aqueous solution. However,they are much more expensive than conventional surfactants and are only appliedfor specific applications whereby the low surface tension is a desirable property.Fluorocarbon surfactants have been prepared with various structures consistingof perfluoroalkyl chains and anionic, cationic, amphoteric and PEO polar groups.These surfactants have good thermal and chemical stabilities, and are excellentwetting agents for low-energy surfaces. Silicone surfactants, which sometimes arereferred to as organosilicones, are those with polydimethylsilixane backbone. Sili-cone surfactants are prepared by the incorporation of a water-soluble or hydrophilicgroup into a siloxane backbone; the latter can also be modified by the incorporation


of a paraffinic hydrophobic chain, either at the end or along the polysiloxaneback bone. The most common hydrophilic groups are EO/PO (propylene oxide),and the structures produced are rather complex; in fact, most manufacturersof silicone surfactants do not reveal the exact structure of their product. Themechanism by which these molecules reduce the surface tension of water to lowvalues is far from being well understood; nonetheless, these surfactants are widelyapplied as spreading agents on many hydrophobic surfaces. The incorporation oforganophilic groups into the backbone of the polydimethyl siloxane backbone canyield products that exhibit surface-active properties in organic solvents.

2.1.5.2 Gemini SurfactantsA gemini surfactant is a dimeric molecule consisting of two hydrophobic tails andtwo head groups, linked together with a short spacer [14]. This is illustrated belowfor a molecule containing two cationic head groups (separated by two methylenegroups) with two alkyl chains:

Br− Br−

H3C-N+-CH2- CH2- H3C-N+- CH3

R R

These surfactants show several interesting physico-chemical properties, such as avery high efficiency in lowering the surface tension and a very low cmc. For example,the cmc of a conventional cationic dodecyltrimethylammonium bromide is 16 mM,whereas that of the corresponding Gemini surfactant, having a two-carbon linkagebetween the head groups, is 0.9 mM. In addition, the surface tension reachedat and above the cmc is lower for gemini surfactants when compared to that oftheir corresponding conventional counterparts. Gemini surfactants are also moreeffective in lowering the dynamic surface tension (the time required to reach theequilibrium value is shorter). These effects are due to a better packing of the geminisurfactant molecules at the air/water interface.

2.1.5.3 Surfactants Derived from Monosaccharides and PolysaccharidesSeveral surfactants were synthesised starting from monosaccharides or oligosac-charides by reaction with the multifunctional hydroxyl groups, including alkylglucosides, alkyl polyglucosides (APGs) [15], sugar fatty acid esters, and sucroseesters [16]. The technical problem here is one of joining a hydrophobic group tothe multihydroxyl structure. Several surfactants were produced, an example beingthe esterification of sucrose with fatty acids or fatty glycerides to produce sucroseesters having the following structure:


O

O

O

CH2COOR

CH2OH H

HH

H

HH

H H

OHOH

OH

OH OH

CH2OH

The most interesting sugar surfactants are the APGs, which are synthesisedvia a two-stage transacetalisation process [15]. In the first stage, the carbohydratereacts with a short-chain alcohol such as butanol or propylene glycol, while inthe second stage the short-chain alkyl glucoside is transacetalised with a relativelylong-chain alcohol (C12–14-OH) to form the required APG. This process is appliedif oligoglucoses and polyglucoses such as starch or syrups with a low dextroseequivalent (DE) are used. In a simplified transacetalisation process, syrups with ahigh glucose content (DE> 96%) or solid glucose types can react with short-chainalcohols under normal pressure. The scheme for APG synthesis is shown below.Commercial APGs are complex mixtures of species that vary in their degree ofpolymerisation (DP, usually in the range 1.1–3) and also in the length of their alkylchains. When the latter is shorter than C14 the product is water-soluble. The cmcvalues of APGs are comparable to those of nonionic surfactants, and they decreaseas the alkyl chain length is increased.

APG surfactants have good solubility in water and have high CP values (>100 ◦C).Moreover, they are stable in neutral and alkaline solutions but are unstable in strongacid solutions. APG surfactants can tolerate high electrolyte concentrations, andare compatible with most types of surfactant. They are used in personal careproducts for cleansing formulations, for skin care and hair products, and alsoin hard-surface cleaners and laundry detergents. APG surfactants have also beenapplied in agrochemical formulations, to serve as wetting agents and penetratingagents for the active ingredient.

References

1. Tadros, T.F. (ed.) (1984) Surfactants, Aca-demic Press, London.

2. Holmberg, K., Jonsson, B., Kronberg, B.,and Lindman, B. (2003) Surfactants and

Polymers in Solution, 2nd edn, John Wiley& Sons, Ltd.

3. McCutcheon. Detergents and Emulsi-

fiers, Allied Publishing Co., New Jersey(published annually).

4. van Os, N.M., Haak, J.R., and Rupert,L.A.M. (1993) Physico-Chemical Properties

of Selected Anionic, Cationic and Nonionic

Surfactants, Elsevier Publishing Co.,Amsterdam.

5. Porter, M.R. (1994) Handbook of Surfac-

tants, Chapman & Hall, Blackie.6. Linfield, W.M. (ed.) (1967) Anionic

Surfactants, Marcel Dekker, New York.7. Lucasssen-Reynders, E.H. (1981)

Anionic Surfactants – Physical Chem-

istry of Surfactant Action, Marcel Dekker,New York.

8. Jungermana, E. (1970) Cationic Surfac-

tants, Marcel Dekker, New York.

References 25

9. Rubingh, N. and Holland, P.M. (eds)(1991) Cationic Surfactants – PhysicalChemistry, Marcel Dekker, New York.

10. Buestein, B.R. and Hiliton, C.L. (1982)Amphoteric Surfactants, Marcel Dekker,New York.

11. Schick, M.J. (ed.) (1966) Nonionic Surfac-tants, Marcel Dekker, New York.

12. Schick, M.J. (ed.) (1987) Nonionic Surfac-tants: Physical Chemistry, Marcel Dekker,New York.

13. Schonfeldt, N. (1970) Surface Active Ethy-lene Oxide Adducts, Pergamon Press,New York.

14. Zana, R.R. and Alami, E. (2003) Geminisurfactants, in Novel Surfactants, Chapter12 (ed. K. Holberg), Marcel Dekker, NewYork.

15. von Rybinsky, W. and Hill, K. (2003)in Novel Surfactants, Chapter 2 (ed. K.Holmberg), Marcel Dekker, New York.

16. Drummond, C.J., Fong, C., Krodkiewska,I., Boyd, B.J., and Baker, I.J.A. (2003)in Novel Surfactants, Chapter 3 (ed. K.Holmberg), Marcel Dekker, New York.

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