Journal of the Korean Physical Society, Vol. 47, September 2005, pp. S255∼S258

Dependence of Salt in Water-in-Crude Oil Emulsion System

Jong-Ho Park and Nam Gwon Back

Department of Science Education, Chinju National University of Education, Jinju 660-756

Ho-Soon Yang

Department of Physics, Pusan National University, Busan 609-735

Byung-Chun Choi

Department of Physics, Pukyong National University, Busan 608-737

Jeong-Bae Kim

School of Computer Aided Science, Inje University of Education, Kimhae 621-749

K. S. Hong∗

Busan Center, Korea Basic Science Institute, Busan 609-735

(Received 13 February 2005)

A cylindrical-shape sample cell was prepared to investigate the dependence of salt amounts ina water-in-crude oil emulsion system by measuring the dielectric properties in the frequency rangefrom 10−2 Hz to 107 Hz with an impedance analyzer. High-sensitivity complex dielectric constantmeasurements were obtained after calibration with several fluids having known dielectric constants.From complex dielectric spectra, we observed two regions for frequency characteristics: conductionrelaxation in the low-frequency region due to diffusion by charge transport caused by impuritiessuch as resins and asphaltenes in the continuous phase, and the dielectric-relaxation mechanism inthe high-frequency region due to the modified Debye type where the relaxation time was in linearproportion to the salt content in the disperse phase of the water-in-crude oil emulsion system.

PACS numbers: 77.22.-d, 77.84.NhKeywords: Water-oil emulsion, Dielectric constants, Dielectric relaxation, Emulsion stability

I. INTRODUCTION

The systems commonly called ‘emulsions’ are complexsystems relevant to applications in the oil, food, or paint-ing industries, and their properties are studied empiri-cally in relation to various fields of science and engineer-ing [1]. The behavior of emulsions depends on the natureof the disperse phase [2], and the properties of obtainedemulsions strongly depend on the size of drops and theirconcentration.

Dielectric spectroscopy is a powerful technique in ana-lyzing heterogeneous systems of water-in-oil (W/O) andoil-in-water (O/W) emulsions. The conductive and di-electric properties of emulsions have been extensivelystudied theoretically and experimentally [1–6]. Dielec-tric relaxation was investigated in several materials re-cently [7–10], but this is not known in W/O and O/Wemulsions. We can divide an emulsion into continuous

∗E-mail: kyongsoo@kbsi.re.kr; Fax: +82-51-517-2497

phase (oil region) and disperse phase (water region).The main dielectric characteristics of a W/O systemare: conducting continuous and disperse phases whichdo not exhibit intrinsic dielectric relaxations in the high-frequency region in the case of non-polar phases; thedisperse phase exhibiting a Debye-type relaxation. Sys-tematic calculations proved that Hanai’s formula can betransformed into Cole-Cole-type dielectric relaxation fora non-conducting dispersive phase [11]. When the dis-perse phase is conducting, the system exhibits two kindsof dielectric relaxation: one connected with the disperse-phase Debye relaxation and the other arising from inter-facial polarization phenomena; as well as a conductingcontinuous phase where the system exhibits a steady con-ductivity caused by charge diffusion in the low-frequencyregion.

However, crude oil is a mixture of aliphatic and aro-matic hydrocarbons, oxygen, nitrogen, and sulfur. Usu-ally, it contains conducting compounds such as resins andasphaltenes. These are large polar molecules forming ag-

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gregates or inverse micelle structures. It is reasonable tobelieve that these polar compounds are the source of thedielectric dispersions measured in crude oil [12], and themeasured spectra show relaxation behavior in the mega-hertz region and a low-frequency conductivity. If theconductivity of the disperse phase is high enough, con-duction phenomena interfere with dipolar relaxation andthe total dielectric relaxation of the system results fromoverlapping of the dielectric relaxation of dipolar origin.

In this paper, we calibrated the sample cell by mea-suring dielectric-constants of known samples. We investi-gated dielectric relaxation behavior in salt-water/crude-oil emulsions and the relationship between salt contentand electrical parameters by using an impedance ana-lyzer.

II. EXPERIMENTAL DETAILS

1. The impedance cell and the estimation ofcomplex dielectric constant

The impedance cell was designed to be sensitive tosmall changes in the dielectric constant, which meansthat the geometric capacitance must be maximized. Inorder to minimize the influence of impurities in the liq-uid, a large sample volume is needed. Based on this, acell with geometric capacitance of approximately 11.60pF and mechanical length (lm) of 60 mm, with a = 4.5mm and b = 6.0 mm, was designed as shown in Figure1.

Since the development of an exact analytical modelfor the measurement cell is difficult, the system is basedupon a numerical calibration of the relationship betweenmeasured capacitance Cmeas and the real part of the com-plex dielectric constant of the fluid. The parameters Cp

and C0 are determined by linear regression on fluids withknown dielectric constants, where Cp is the stray capac-itance and C0 the geometric capacitance of the cell. Thedielectric constants of several fluids used in the calibra-tion are given in Table 1. We plotted the capacitance of

Fig. 1. Schematic diagram of cylindrical-shape sample cellfor the dielectric measurements.

Table 1. Dielectric constants of the standard samples at20 ◦C.

Medium Dielectric constant (ε′) References

Air 1.0005 ± 0.0002 [12]

n-heptane 1.924 ± 0.01 [13]

Carbon tetrachloride 2.238 ± 0.002 [14]

Chloroform 4.82 ± 0.02 [15]

Monochlorobenzene 5.699 ± 0.008 [14], [15]

1,2-dichloroethane 10.65 ± 0.05 [13], [14]

the samples as a function of frequency, and found thatcapacitances of the samples used were almost indepen-dent of measuring frequency. The numerical calibrationdepends on accurate data of the real and imaginary partsof the complex dielectric constant. The real part of thedielectric constant is determined from the measured ca-pacitance, Cmeas. The imaginary part of the dielectricconstant of low-loss fluids involves large uncertainties;hence, an accurate linear regression model is difficult toobtain.

2. Sample preparation and test of emulsion sta-bility

To make emulsion samples, 50 ml of distilled waterand crude oil are prepared. A small amount of salt ismelted in the distilled water(50 ml). Melted salt waterand crude oil are mixed for 5 minutes with an ultrasonichomogenizer. We divided salt-water/crude oil emulsionsamples into 5 classes (0.0, 0.2, 0.4, 0.6 and 0.8 PTB-crude oil/water, where 0.5 PTB is 1.427 mg/l = 1.427ppm). Sometimes, thermal reaction happens by super-sonic waves when the mixture is made into emulsion. Itis very difficult to obtain dielectric spectra in a stablestate because particles are moving until thermal equilib-rium is achieved. We checked the stable condition of theemulsion by measuring the time dependence of capaci-tance at 104 Hz for a 0.0 PTB-crude oil/water emulsionsample.

Figure 2 shows the time dependence of capacitanceand tan δ in 0.0 PTB-water/crude oil emulsion. We canobserve that the capacitance increases abruptly and sat-urates after 10 minutes. Although the time dependenceof tan δ was opposite to capacitance, it also saturates af-ter 20 minutes. This means that the emulsion becomesstable because of the decrease of particle mobility. Thus,we made emulsion samples and obtained dielectric spec-tra in the emulsion after a stabilizing time of 20 min-utes. The dielectric spectroscopy of salt-crude oil/wateremulsion is measured in a frequency domain (10−2 ∼ 107

Hz) with an impedance analyzer (Solatron, SI1260). Thedata were taken at 20 ◦C in atmosphere, and the tem-perature of the sample was measured with a platinum-rhodium thermocouple with stability ±0.1 ◦C.

Dependence of Salt in Water-in-Crude Oil Emulsion System – Jong-Ho Park et al. -S257-

Fig. 2. Time dependence of capacitance and tan δ in 0.0PTB-water/crude oil emulsion.

Fig. 3. Schematic diagram of continuous and dispersephases in salt-water/crude-oil emulsion system. S means salt,white circles are water drops, and small black circles are im-purities.

III. RESULTS AND DISCUSSION

Figure 3 shows a schematic diagram of the salt-crude-oil/water emulsion system, where we can distinguish thecrude-oil region (continuous phase) and the salt-waterregion (disperse phase). The continuous phase includesconducting materials such as asphaltenes, resin, and non-conducting materials. Water holds salt (conducting ma-terial) in the disperse phase.

When we consider water only, as for Debye-type idealmaterials, the measured complex dielectric constant as afunction of angular frequency can be described by

ε∗(ω) = ε∞ +εs − ε∞1 + iωτ

, (1)

where ε∞ is the high-frequency dielectric constant, εs thestatic dielectric constant of the liquid, and τ the relax-ation time of the Debye process. When we consider theorientation of polarizations caused by molecules and in-terfacial polarizations of the disperse phase at the bound-

Fig. 4. ε′ and ε′′ vs. frequency characteristics of salt-water/crude oil emulsion.

ary, the dielectric constant (1) can be rewritten as

ε∗(ω) = ε∞ +εs − ε∞

1 + (iωτ)1−α

+εp − εs

1 + (iωτp)1−αp, (2)

where τ and τp are the relaxation times and α and αp

are parameters describeing the distribution of relaxationtimes. A conductivity term was fitted to complex dielec-tric constant spectra for conducting materials; thus, theexpression for dielectric constant (2) becomes

ε∗(ω) = ε∞ +εs − ε∞

1 + (iωτ)1−α

+εp − εs

1 + (iωτp)1−αp− iσ

ε0ω. (3)

The first two terms on the right-hand side are the originalCole-Cole model, which is dielectric relaxation caused bydipole orientation polarization of polar molecules in thefrequency range 100 MHz ∼ 10 GHz. The third termaccounts for a low-frequency relaxation due to the inter-facial polarization effect at the grain boundary. The lastterm is purely empirical, and is the conductive loss inthe liquid, depending on the measurement cell, and σ isthe dc conductivity.

Figure 4 shows variations of the real (ε′) and imaginary(ε′′) parts of the dielectric constant versus frequency inlog-log scale for several salt contents. The real part ε′

decreases slowly in the frequency range from 10 to 106 Hzwith salt content. The characteristic peak in the imagi-nary part ε′′, which is shifted towards higher frequencieswhen salt content increases, indicates the presence of arelaxation process.

The values ε′ and ε′′ decrease rapidly with frequency.The absolute values of the slopes below 104 Hz are largerthan those above 104 Hz and seem to be independent ofthe salt content. These two tendencies in frequency de-pendence suggest that two dispersion mechanisms areinvolved in the dielectric-constant measurement. There-fore, the conductivity term in Eq. (3) should be im-proved, because the slopes of ε′ and ε′′ at low frequency

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are not unity. Since diffusion occurs in these emulsionsystems, two polarization mechanisms are possible: thedielectric relaxation due to continuous/disperse-phaseinterfacial polarizations which do not involve long-rangemobile droplets, characterized by τ1 and m, and the con-ductivity relaxation or carrier response associated withconducting charges moving in the continuous phase, de-scribed by τ2 and n, where τ1 and τ2 are the relaxationtimes in the order of µs and ms, respectively. Therefore,the total complex dielectric constant can be described as

ε∗(ω) = ε∞ +εs − ε∞

1 + (iωτ)m

+σ0

ε0ω[1 + (iωτ)n]. (4)

After evaluating all the frequency-dependent complex-dielectric-constant data fitted with Eq. (4), we foundthat the carrier polarization mechanism was weakly dis-persive, while the interfacial polarization mechanism wassomewhat more dispersive. From the calculated valuesof τ1 and τ2 in the emulsion system, we observed thatthe relaxation time τ1 was in linear proportion to saltcontent while τ2 was independent of the salt content inthe emulsion systems. These facts suggest that the high-frequency dielectric relaxation is not related to resinsand asphaltenes in the continuous phase, but is relatedto the salt in the disperse phase. Since τ1 and σ do notdepend on salt content, salt cannot move from dispersephase to continuous phase and can be trapped stronglyto droplets. A dielectric response relation, which is con-sidered as a generalization to the Cole-Cole dielectric ex-pression, has been proposed. The influence of the charge-carrier contribution on the complex dielectric constant issignificant, as is demonstrated when both interfacial andcarrier polarization mechanisms are simultaneously con-sidered.

IV. CONCLUSIONS

A cylindrical-shape sample cell was prepared to in-vestigate the dependence of salt amounts in a water-in-crude-oil emulsion system by measuring the dielec-tric properties in the frequency range from 10−2 Hz to107 Hz with an impedance analyzer. The characteris-tics of the measurement cell are optimized to give highsensitivity. High-sensitivity complex-dielectric-constantmeasurements were obtained after calibration with sev-eral fluids having known dielectric constants. From com-plex dielectric spectra, we observed two regions for fre-quency characteristics: conduction relaxation in the low-

frequency region due to diffusion by charge transportcaused by impurities such as resins and asphaltenes inthe continuous phase, and the dielectric-relaxation mech-anism in the high-frequency region due to the modifiedDebye type. In the high-frequency region, it was ob-served that the relaxation time, τ1, was in linear pro-portion to the salt content in the disperse phase of thewater-in-crude-oil emulsion system.

ACKNOWLEDGMENTS

This work was supported by Korea Science and Engi-neering Foundation Grant R05-2003-000-10451-0.

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