Indian Journal of ChemistryVol. 28A, July 1989, pp. 550-556

Phase behaviour and physicochemical properties of triton X 100 and aerosolOT induced micro emulsions of hydrocarbons and water in presence of

n-butanol

S R Bisal, P K Bhattacharya & S P MouIik*

Department of Chemistry, Jadavpur University, Calcutta 700 032 ~

Received 13 June 1988; accepted 30 September 1988

~hase diagrams of microemulsions composed of waterl oil! AOTI n-butanol and water/oiI!fX 1001n-butanol are presented. The oils used are: n-hexane, n-heptane, n-octane and n-decane. Small lamellarzones appear with AOT along with large isotropic microemulsion areas. The 1:1 (wt/wt) ratio of sur-factant-cosurfactant produces appreciable microemulsion area. At constant surfactant-cosurfactant ra-tio, the microemulsion areas decrease with the chain length of the alkanes. The minimum percentage ofAOT required for dissolving about 40-50 wt % of water in hexane is - 8 wt %, which increases from8-15 wt % for heptane, octane and decane. TX 100 imparts less microemulsion area than AOT with in-crease in alkane chain length. The conductances, viscosities, specific volumes and compressibilities ofboth the microemulsion systems at 1:1 (wt/wt) surfactant-cosurfactant ratio have been determined with-in no-water-to-no-oil limi The-viseosity is maximum-at -31 vol % water. The conductance-sharplyincreases after addition of 0 01 % water. Both the specific volumes and the compressibilities decreasewith increase in water content. The microemulsions become more compact with increase in water con-tent; the ion conductance increases due to the combined effects of increased dielectric constant andchannel formation. A comparison among the hydrocarbons on the basis of Walden product (A.'l), theproduct of specific volume and the compressibility (v~ -1), the excess specific volume (vex)and the ex-cess compressibility (~ex) at different stages of water addition, shows that the viscosities and conduct-ances of AOT based systems compensate each other at low and high percentages of water and not inthe intermediate range. The v~ - 1 and vexvalues for both the AOT and TX 100 based systems suggestswelled microemulsions with increase in alkane chain length, particularly at lower percentages of water;these even out at higher water levels. The internal degree of flexibility is more for AOT based than thatfor TX 100 based microemulsions.

Microemulsions 1- 4 are isotropic, thermodynami-cally stable low viscous solutions of oil and waterspontaneously formed in the presence of surfact-ants and cosurfactants. When the proportions ofoil and water are nearly equal and their combinedpercentage is relatively high, the microemulsion isbicontinuous, i.e. on its two extremes the emul-sions are either oil continuous (water-in-oil) or wa-ter continuous (oil-in-water )5.6. The preparation ofbicontinuous microemulsions has advantage for itcan solubilize a large proportion of water in oil.Normally both surfactants and cosurfactants(usually lower alkanols and arnines 7) are requiredfor the preparation of microemulsions. Aerosolorange T (AOT) however, can generate microem-ulsions without a cosurfactant":". Although non-ionic surfactants are amply used in microernulsionstudies!"!", Triton X 100 (TX 100) a useful sur-factant has been seldom employed. It is, therefore,of interest to study and compare the behaviour of

550

AOT and TX 100, two entirely different types ofsurfactants, under otherwise identical conditions.

The hydrocarbons employed were n-hexane, n-heptane, n-octane and n-decane, the cosurfactantbeing n-butanol. Several physicochemical propert-ies, viz., conductance, viscosity, specific volumeand compressibility have been studied at variouslevels of water addition covering the entire rangeof stability. Dependence of the phase behaviourand the physicochemical properties on the chainlength of the oil has also been investigated. Com-prehensive studies of similar kind are limited in li-terature":". Recently Khalweit et aL12-14 haveworked on phase behaviour of different microem-ulsions formed mainly using nonionic surfactants.

Materials and MethodsSodium di- 2-ethylhexyl sulphosuccinate (AOT)

and Triton X 100 were products of Sigma andBDH (England) respectively. The former was

BISAL et af.: PHASE BEHAVIOUR OF TRITON X 100 & AEROSOL OT INDUCED MICROEMULSIONS

more than 99% pure and was used as such. TX100 was purified and characterised as reported inan earlier publicatiorr'", n-Butanol (AR, BDH) wasused as such. Its boiling point and density agreedwith literature values within ± 0.2% and± 0.004% respectively. Doubly distilled conductiv-ity water of specific conductance 2-3 us em - I at303 K was used throughout the experiment.

The hydrocarbons, hexane, heptane, octane anddecane were of SD pure grade having densities of0.6605 (0.6603), 0.6835 (0.6837), 0.7025(0.7025) and 0.7300 (0.7300) respectively at293K (literature densities' are given in paren-theses).

All the experiments were performed at303 ± 0.5 K in a thermostatic bath.

Densities of the microemulsions were measuredin a calibrated pycnometer as reported earlier!".An ostwald viscometer of flow time 50 see for wa-ter was used for viscosity measurements. Conduct-ance was measured in a Jenway conductivitybridge (UK) in a cell of cell constant 1.1.

An ultrasonic interferometer (Mittal Enter-prises, India) was used for sound velocity mea-surements. Details of measurements have beengiven in our earlier publications 11•. 21.

The isotropicity and lamelJar phase of the mic-ro emulsions were determined with a polarizingmicroscope (SICO, India).

The microemulsions were prepared by titratingknown quantities (by weight) of mixtures of oil,surfactant and cosurfactant with water added froma microburette upto the point of turbidity deter-mined by observing against cross polarized light.For further details we refer to our previous publi-cation!". Several samples of the microemulsionsprepared were kept for prolonged time (2-4weeks) after preparation to judge their stability.There was hardly any phase separation or turbid-ity. They were also thermally stable in the temper-ature range of 29H-30H K.

Results and Discussions

.Phase behaviourphase diagrams of microemulsion systems wa-

ter /hydrocarbons/TX 100/ n-butanol and water /hydrocarbons/ AOT / n-butanol are depicted in Figs1-4. The results of water/hexane/AOT/n-butanolsystem are presented in Fig. 1 at AOT/n-butanolratios of 4: 1 and 1:4 (wt/wt). In the former a smallclear zone (C) is seen at the water corner. A lam-ellar zone (L) appears on the water-rich side inthe range 50 to HO wt 'X). The compositions havegiven nice bicontinuous microemulsions with sur-

AOT +n- BUTANOL

-,..••._-----

WATER HEXANE

Fig. 1- Phase diagrams of hexane-Afrf-n-butanol-water rnic-roemulsion system at 303K. [Solid line, AOT/n-butanol ratio4:1 (wt/wt); broken line AOT/n-butanol ratio .1:4 (wt/wt).L= lamellar phase; C = clear zone. Top portion of phase boun-

dary limit represents microemulsion zone]AOT + n- BUTANOL

)lE

WATER

Fig. 2- Phase diagram of hexane-AOT-n-butanol-water micro-emulsion system with AOT/n-butanol ratio 1:1 (wt/wt) at

303 K [L= lamellar phase; and C = clear zone] /AOT + n- BUTANOL

OIL

Fig. 3-Phase diagram of oil-AOT-n-butanol-water microem-ulsion system with AOT/n-butanol ratio 1:1 (wt/wt) at 303K[Line with single dot, heptane; broken line, octane; solid line.decane. L= lamellar phase; C = clear zone. Top portion of

phase boundary limit represents microemulsion zone]

551

INDIAN J CHEM, SEe. A, JULY 19119

TX 100+n - BUTANOL

WATER OIL

Fig. 4- Phase diagram of oil-TX 100-n-butanol-water micro-emulsion system at 303K [Thick solid line, TX 1OOln-butanolratio I: I (wt/wt) with hexane; line with cross, TX 1001n-butanol ratio I: I (wt/wt) with heptane; line with double dots,TX 1001n-butanol ratio I: I (wt/wt) with octane; broken line,TX 1OOln-butanol ratio 1:1 (wt/wt] with decane; line withsingle dot, TX 1OOln-butanol ratio 4:1 (wt/wt) with hexane:solid line, TX 100ln-butanol ratio 1:4 (wt/wt] with hexaneTop portion of phase boundary limit represents microemul-

sion zone]

factant and cosurfactant levels at approximately I()wt %. The microemulsion zone- hecomes signifi-cantly reduced at 1:4 (wt/wt) ratio; no lamellarzone appears. At 1:1 (wt/wt) surfactant/cosurfact-ant ratio, the microemulsion zone is mainly bicon-tinuous with a viscous lamellar zone (L) and a nar-row clear zone (C) again at the water corner (Fig.2). The other three oils, viz. heptane, octane anddecane at 1: 1 (wt/wt) ratio also show a tiny clearzone (C) and a viscous lamellar zone (L) (Fig. 3).A perusal of Figs 2 and 3 shows that the micro-emulsion range has decreased with increase in al-kane chain length. The areas (0;',) of the clear andthe lamellar zones, given in Table 1, show that theAOT/ n-hutanol ratios above I: I (wt/wt) have littleeffect on the area. At this ratio the area decreasesin the order of the alkane chain length. Lamellarzones are mainly exhibited by the hexane system.The per cent microemulsion area for I: 1 (wt/wt )AOT / n-butanol has been reported to be 47.4 'Yousing xylene as the oil"; hydrocarbons are bettermicroemulsion-forming oil than xylene.

Results of microemulsions formed with TX 100as surfactant are presented in Fig. 4 at surfactant/cosurfactant ratios (wt/wt) 1: 1, 1:4 and 4: I. Theorder of decrease in the microemulsion areas at1: I (wt/wt) is hexane < heptane < octane < decane.For hexane the 1:4 ratio formulation can be oil-continuous whereas the 4:1 ratio system can hewater-continuous. The microemulsion zones for1: I ratio in hexane, heptane, octane and decane,

552

Table I -Areas of clear and lamellar zones at different AOT-n-butanol ratios

AOT-n-butanol flE area Lamellar arearatio (%) %

Hexane1:1 6R.3 I.R

1:4 24.34;\ 64.2 5.7

Heptane

1:1 611.3 0.5Octane

1:1 65.1

Decarie

1:1 59.4

Table 2 - Volume per cent water and oil used for micro-emul-sion formation I: I surfactant-cosurfactaru ratio

Oil(Vol 0;',)

Oil(Vol %)

Water(Vol%)

Water(Vol 0;',)

AOT system

41.4 0.0

TX 100 system

40,() D.D

36.2 :'.2 35.:' :'.1

31.0 10.4 30.:' Ill.!

20.7 20.7 20.3 20.3

10.4 .j 1.0 10.1 3D.:'

n.o 41.4 D.D 4().r.

4: 1 ratio in hexane and 1:4 ratio in hexane are33.8%, 32.0%, 28.1 '/'0, 21.5%, 29.2% and 11.9%respectively.

The results presented in Figures 1-4 reveal thatAOT is a better microemulsion-forming surfactantthan T?<. 100 and may yield to give bicontinuouspreparations. The monophase zones are almostdouble for the former; the percentage of the sur-factant + cosurfactant content is also low.

Physicochemical propertiesPhysicochemical properties of microemulsions

were studied at 1:1 surfactant-cosurfactant ratio atnearly 60 vol % at 303 K. Volume percentages ofwater and oil were varied as shown in Table 2. Si-milar volume percentages of water were used forall the oils for comparison at equal addition levels.The constant volume of all the systems made itpossible to compare the properties at equimolarconcentration of the surfactant (AOT).

The viscosity-water profiles are shown in Fig. 5.For the TX 100 system the viscosity values passthrough a maximum around 35 vol % Water, theorder of viscosity being decane> octane> heptane> hexane. AOT systems show a mild maximum al-

BISAL et al: PHASE BEHAVIOUR OF TRITON X 100 & AEROSOL OT INDUCED MICROEMULSIONS

I70III

II

~='IIIJ~. r (T x 100)

70~........,.~ •••••.• II(AOT )

40o Hx/:, Hp

I. Dc

~~ Dc

10

10=---~------=,:1 ,-- -'-_o 25 50

volume 'I, of water

Fig. 5-Viscosity of microemulsion as a function of volume %water at a constant volume % of surfactant + cosurfactant

(58.6% for AOT and 59.4% for TX 100) at 303K.

4:6=0.:6=2·06:6=4·00:6=6·0

12·0

50

4·0---

25volume 0" of water

Fig. 6-Specific conductance of microemulsion as a functionof volume % of water at constant volume % (5R.6) ofAOT + n-butanol at 303K [Symbols as in Fig. 5. /!., magnitude

of scale shifts I

most at 35 vol 'y;, water; the viscosity increases al-most in the same order as for TX 100 system.

We have recently reported III that TX 100/ tlr

butanol/xylene/water and AOT/II-butanol/xylene/

II

100 0'

01,e

""-,"'E 80u

","~5!)(

CIl...

60

nIAOT)

I ITX 1001

25Volume % of water

Fig. 7-Compressibility of microemulsion as a function of vo-lume % of water at constant volume % of surfactant + co sur-factant (cf. Fig. 5) at 303K [Symbols as in Fig. 5, primed sym-

bols represent calculated values]

o 50

water systems exhibit viscosity maxima in therange 20-25°/', of water and the increase in viscos-ity is sharp. The present systems exhibit lessersharpness. The conductance-water diagram (Fig.(i) shows similar trends for all the oils. After 15%of water addition there is a radical change in con-ductance. Channel formation 11,.22 is efficient afterthis stage. With increase in water content the rnic-roemulsions show an increase in dielectric con-stam ". This acts in conjunction with the 'channeleffect' to offer rapid increase in conductance. Atequal level of water, the dielectric constants ofmicroemulsions have been reported to increasewith higher alkanes+'.

Conductance and viscosity are conventionallycoupled through the Walden product All. The vis-cosity and conductance profiles in Figs 5 and 6reveal that both the properties register an increasewith increase in percentage of water. This is con-trary to what is expected, since the Walden pro-duct approximately refers to a compensation be-tween the two properties. Increased viscosityshould decrease the ion transport.

The adiabatic compressibility of the microemul-sions is a linear function of water content, thecompressibility sequence of the systems being hex-

553

INDIAN J CHEM, SEe. A, JULY 1989

789CARBON NUMBER

Fig. 9-(a) Specific conductance-alkane chain length profiles for AOT systems at different volume % of water at 303K. (symbolsas in Fig. 5). (b) Viscosity-alkane chain length profiles for microemulsion systems at different volume % of water for Am andTX 100 microemulsion at 303 K [Symbols as in Fig. 5. Broken line for TX 100 systems and solid line for AOT systems. Curves1-5 represent 5.2, 10.4,20.7,31.0,41.4 volume %.:;f water in AOT system; curves 1'-5' represent 5.1, 10.1,20.3,30.5,40.6 vo-

lume % of water in TX 100 system]

II

1·24 .: A=O

• :6=0·02A: 6=0·04o :A=O·06

1·16

01<,

E 1·28i>~

1·08

',00

1·12

1·04·L- ..L-. __ ---' __ -'-_---'

o 25 50

Velume % of water

Fig. 8-Specific volume of microemulsion as a function of vo-lume % of water at constant volume % of surfactant + cosur-factant (cf. Fig. 5) at 303K [Symbols as in Fig. 5; primed sym-bols represent calculated values. l!., magnitude of scale shifts,

equivalent for both systems]

(0 )

c)6·2 % water

31·0% water:..

•41-

2 20-7% water...•..

~

10·4% water

0 7 8 9

CARBON NUMBER

10

554

ane> heptane> octane> decane (Fig. 7). The va-lues for both AOT and TX toO based systems aremore or less equivalent. The calculated compres-

sibilities are obtained by the relation I ~iVi' where

f3i and Vi are the compressibility and specific vo-lume fraction of the ith component, respectively.The observed compressibilities are always lessthan the calculated ones, the magnitude of differ-ence being more in the case of AOT than that forTX 100. The former system is more compact ororganized than the latter. The increased viscosityis complementary to the decreased compressibilityonly the curvelinearity of the former at higher per-centage of water is not observed for the latter.This accounts for some structural rearrangementof the microemulsion assembly with the applica-tion of pressure. Complementary plots of the spe-cific volumes of the microemulsions against thepercentage of water are presented in Fig. 8. Theobserved values are again lower than the calculat-ed 16 values suggesting volume contraction uponemulsification. Both the AOT and TX toO systemsshow deviations from the expected values. Unlikethe compressibility, the profiles of the specific vo-lume are curved.

The physicochemical properties of microemul-sion systems are not oil-independent. At constantwater percentage individual properties are found

n 1 (b)

I I4&.5 __ 41-

.s->::' !S6-0 7.0t:_=_==_==_~;;;:=~~;';'-44-=::=JFll

---~--------

(

...-J'f'..•.-..•.-J ." .•••.••..--

.J ---__.a--

2·03·u,s_---

s 10

BISAL et al: PHASE BEHAVIOUR OF TRITON X too & AEROSOL OT INDUCED MICROEMULSIONS

)6·2·/0 water60 -ee------~.~-----~.~--·----------~_~

31·0% water..•...';"~ .a..._--tr-----1..--------'8. 40·-- -

20·7 % water

•10·4 V. water

6 789CARBON NUMBER

Fig. 10- Waden product (A.T] )-alkane chain length profiles forAOT systems at different volume % of water at 303K. [Sym-

bols as in Fig. 5 J

10

to depend on the length of the molecule. The con-ductance and viscosity profiles with the alkanecarbon number are presented in Fig. 9 (a and b).At low percentage of water (10%) the conductanceof the AOT system is independent of the length ofthe alkanes. There is a slight increase at 20% wa-ter which is prominent at 30°;',. At 35% the con-ductance decreases with chain length. Higher al-kanes impart higher dielectric constant to the rnic-roernulsions ", resulting in better 'conduits' orchannels' upto a water content of 30%. The de-crease at 35% water means diminution in the oc-currence of conduit with higher alkane length at'low level of oil.

The viscosities (Fig. 9b) are also a positivefunction of the chain length of the hydrocarbons,which at higher percentage of water becomesweakly dependent.

The Walden products (1..1']) as a function of al-kane chain length at various percentages of waterare presented in Fig. 10 for the AOT system. Atlower water level the Walden products (1..1']) are al-most constant, i.e. viscosity and conductance rea-sonably compensate each other. At higher percen-tage of water, there is an increase in 1..1'] with thechain length which at 36.2% is practically invar-iant. At 41.4% water (where oil is absent) thecompensation is exact. This shows that at low andappreciable water levels the behaviour of the rnic-roemulsions is like normal solutions; in the in-termediate range increase in chain length makesthe microemulsions to conduct more than that isexpected from viscosity. This is considered to bedue to increase in dielectric constant of microem-

20 5~e~-----~-----.e-----------_-&/•...._----------- ..--~-----

o •, ,

4-G-Ql

;, 181JEu

'E01

Q 16"'~-

5 9

----~~-<~-~~----------~4-8 ~ __ ---.•.• /_--3....()""

/_-----3 ....",- _ •.• ---2~f!J _-/:?------...-1~.e~""

214

12~~6~----~7~----~8------~9-------1~O~

CARBON NUMBER

Fig. ll-Specific volume/compressibility ratio as a function ofalkane chain length at different volume % of water for AOTand TX 100 microemulsions at 303K [Symbols and curves as

in Fig. 5 and Fig. 9b respectively J

ulsions in the presence of higher alkanes at equallevels of water addition.

The compressibilities and the specific volumeshave also been shown to be considerably dependon the hydrocarbon chain length at lower watercontent. The dependence is linear. At higher per-centages of water, the properties level off. The re-sults are not presented. The dependence of vB - I

on the chain length is presented in Fig. 11 forboth AOT and TX 100 based systems. At higherwater content the vB - I values are almost invar-iant; the degree of looseness is equally reflectedon both v and B. With decrease in water contentvB - I values increase with the alkane chain length.The internal degree of rigidity becomes an in-creasing function of the chain length. This is maxi-mum in the presence of minimum water, i.e. in thepresence of maximum oil. Addition of water sof-tens the microemulsion, the emulsion swells" andbecomes more compressible as the hydrocarbonchain length increases. At comparable water levels,TX 100 based solutions have greater specific vo-lume and lower compressibility than those of theAOT-based solutions. The swelled microernulsionsdue to the former have more degree of rigiditythan that due to the latter. Increase in compressib-ility with increase in concentration of water hasbeen recently reported for some microemulsionsystems:".

In Table 3 the excess specific volume and ex-Ci~SS compressibilities are listed for all the systems.

555

INDIAN J CHEM. SEe. A. JULY 1989

Table 3-Excess compressibilities (Be,) and excess specific yolumes (vex) of microemulsions at different volume % water at303 K

-B« (-Vex)cm/dyne ml. g - I

Vol % Hx* Hp Ocwater

AOT based systems

41.40 8.63 (0.0085) 8.63 (0.0085) 8.63 (0.0085)36.20 9.99 (0.0076) 9.13 (0.0080) 9.01 (9.0079)31.00 11.65 (0.0097) 9.56 (0.0095) 8.66 (0.0084)20.70 14.06 (0.0122) 11.11 (0.0107) 9.08 (0.0091)10.40 15.20 (0.0065) 12.43 (0.0076) 9.47 (0.0066)o· 16.78 (0.0078) 13.87 (0.0061) 10.51 (0.0021)

TX 100 based systems

40.60 7.33 (0.0\80) 7.33 (0.0180) 7.33 (0.0\80)35.50 8.08 (0.0145) 7.17 (0.0137) 6.52 (0.0129)30.50 7.56 (0.0130) 6.69 (0.0120) 6.52 (0.0111)20.30 6.83 (0.0140) 5.60 (0.0133) 5.40 (0.0117)10.10 6.96 (0.0150) 5.32 (0.0130) 2.03 (0.0089)0 7.52 (0.0100) 5.71 (0.0075) 2.48 (0.0024)

*Hx = n-hexane; Hp = n-heptane; Oc = n-octane; and De = n-decane

Dc

8.63 (0.0085)7.95 (0.0067)8.64 (0.0096)8.55 (0.0078)9.20 (0.0092)9.05 (0.0005)

7.33 (0.0180)5.79 (0.0113)5.34 (0.0122)4.34 (0.0120)1.76 (0.0085)0.90 (0.0020)

Vexand Bexvalues are considered as the differencebetween the observed and the calculated values(Bex= Bobs- Bealeand vex= vohs- veale)'The negativesign implies microernulsions to be more compactthan that calculated on the basis of ideal mixing;there is volume contraction and component asso-ciation in solutions+-". For both the AOT and TX100 microemulsions, the negative excess specificvolumes increase with percentage of water, indi-cating overall contraction in volume upon emulsi-fication. But the excess compressibilities decreasefor the AOT based systems and increase for theTX 100 based systems with increase in percentageof water. The former system, therefore, has lessinternal degree of rigidity than the latter which isnot reflected in the excess specific volume. This isalso supported by the earlier works":": The inter-nal degree of flexibility is thus a specific propertyof the microemulsion system which can be under-stood from the excess compressibility value.

AcknowledgementWe thank the CSIR, New Delhi for financial as-

sistance.

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