Indian Journal of ChemistryVol. 30A, April 1991, pp. 328-334

Phase behaviour and physicochemical properties of a microemulsionobtained from an edible oil (saffola)

B K Paul+, M L Das, DC Mukherjee] & S P Moulik*Department of Chemistry, Jadavpur University, Calcutta 700 032

Received 28 March 1990; revised and accepted 16 November 1990

A microemulsion obtained from edible oil saffola, Triton x 100, n-butanol and water (or brine) hasbeen prepared and its phase behaviour and physicochemical properties have been studied. The ternaryphase diagram shows an unusually large three-phase region. The single phase (l~) microemulsion zoneincreases with increase in temperature and decreases in presence of brine, urea and cholesterol. The ef-fect of the additives on the 1~ microemulsion zone varies in the order: urea - cholesterol < brine, upto50% (w/w) oil as per phase diagram; thereafter, the order is cholesterol < brine < urea. The viscosity ofthe microemulsion (S+ CoS 85%, oil 7.5% and brine 7.5% w/w) at different temperatures in presence ofadditives, viz.,cholesterol, esters of cholesterol (acetate, benzoate and stearate), a crown ether (18 crown6) and urea exhibits a systematic decrease, whereas the conductance varies non-systematically. Threedistinct trends in conductance are witnessed in the range 100-60°C at higher ratios of brine/oil. At lowerratios as well as in presence of additives two distinct zones are exhibited. A mild percolation phenomen-on is observed at temperatures above 20°C which turns into a moderate phenomenon at 50°e. While in-crease in temperature imparts fluidity to the microemulsions, the conductance behaviour advocates stagewise changes in their internal (microscopic) structure.

Microemulsions for industrial use and fundamentalstudies primarily contain hydrocarbons (C7-Cd,benzene, toluene, xylene, carbon tetrachloride, etc.,as the oil constituent, whereas the pharmaceuticalpreparations are based on paraffin and vegetableoils. Fundamental studies'<" regarding the structureand dynamics have been much more numerous onthe former class. In recent years, microemulsionshave been used as liquid membranes, and transportof both electrolytes and non-electrolytes acrossthem has been studieds-7. Microemulsions thus canmimic biological membranes as regards the trans-port of proteins and steroid bodies in the lipid ma-trix. Investigations on the formation and characteri-sation of such microemulsions are, therefore, ofgreat importance. Recently, we have initiated stud-ies on such systems, termed as 'biological microem-ulsions'v", using cholesterol, esters of cholesterol,proteins and polymers as the constituents. The hy-drocarbon oils so far used in the emulsion prepar-ations are biologically noncompatible.

In the present paper we report the preparation,phase behaviour and physical properties of an emul-sion system obtained from saffola (an edible oil and

tGeological Studies Unit, Indian Statistical Institute,Calcutta 700035.fDepartment of Chemistry, Calcutta University,Calcutta 700009

328

excellent source of pure linoleic acid), Triton x 100,n-butanol and brine. The effects of temperature andadditives, viz., cholesterol, esters of cholesterol(acetate, benzoate and stearate), a crown ether (18crown 6) and urea on the phase behaviour, viscosityand conductance of. the system are reported. It ishoped that results of the present study will be usefulin assessing the potential of vegetable oils as thebase of biological microemulsions. Cholesterol andseveral of its esters have been used as additives inthe system studied since sterol and its derivativesoccur in cell membrane and blood 10. The choice ofcrown ether and urea is arbitrary; the latter com-pound, of course, affects the lipid association andmembrane structure.

Materials and MethodsThe oil used, saffola, containing 73.3% linoleic

acid, was a purified product obtained from MisBombay Oil Industries Pvt. Ltd., Bombay. Its esti-mated acid value was 0.40 mg of KOH per g of oil.Triton x 100 was a Sigma (USA) product with poly-dispersity in the ethylene oxide group (9.5 EOgroups per molecule on the average). n-Butanol wasa BDH (England) product (b.p, 117.3°C). These ma-terials were used without further purification. Theadditives cholesterol (C), cholesteryl acetate (CA),cholesteryl benzoate (CB) and cholesteryl stearate(CS) had. the specifications and quality reported

PAUL et al.: PHASE BEHAVIOUR OF MICROEMULSION OBTAINED FROM SAFFOLA

elsewhere!'. The crown ether (18 crown 6) was ob-tamed from Aldrich (USA). Sodium chloride (G R,S. Merck, India) and urea (ExcelaR grade, BDH, In-dia) were used as received. Doubly distilled conduc-tivity water of specific conductance 1-2 !LS cm - I at303 K was used for all the preparations.

Conductance of the microemulsions was mea-sured with a Jenway (England) conductometer in acell of cell constant 0.75 ern - I. Viscosity measure-ments were made using a calibrated Ostwald vis-cometer of flowtime 54.6 see for water at 30°C as-suming newtonian behaviour of the microemul-sions. Densities were measured with a calibratedpycnometer. Phase changes of microemulsions werestudied with a SICa (India) polarising microscope.

All the measurements were made in a constanttemperature water bath accurate to ± 0.1"C.

Preparation of microemulsionMicroemulsions were prepared by taking known

amounts (by weight) of saffola, Triton x 100 and n-butanol in a series of stoppered and graduated glasstest tubes which were placed in the thermostatedbath at the desired temperature. Water was thenprogressively added from a microburette with con-stant stirring until each of the mixtures became justbiphasic when viewed against crossed polarisedlight. Two ratios of the surfactant/co-surfactant (S/CoS), 1:5 and 1:8 (w/w), were used. The solutionswith 1~ microemulsion were allowed to stand forconsiderable length of time for stability testing. Ran-dom check of samples stored for several weeks toseveral months revealed excellent stability. Theyneither became turbid nor separated into oil andwater phases. We did not check the chemical stabil-ity of the .polyunsarurated oil, saffola, It may bementioned that its chemical instability did not affectthe time dependenr stability of the prepared micro-emulsions. The prepared microemulsions used forphysicochemical studies at the temperatures 10°, 20°30°, 40°, 50° and 60°C, fell inside the single phasezones and were, therefore, of Winsor IV type!'. Un-less mentioned otherwise, the preparations hadS/CoS ratio of 1:5, but proportions of saffola andwater varied. Addition of either extra oil or wateryielded biphasic and triphasic preparations. In thetriphasic system, the bottom phase was normallyaqueous micellar solution of T x 100 with a low con-centration of butanol followed by the middle iso-tropic microemulsion phase and the top phase ofmainly oil with low concentrations of the surfactantand co-surfactant. To study the effects of the addi-tives (brine, urea, cholesterol) on the phase behav-iour, aqueous solutions of the first two and oil solu-tion of the third were used as titrants.

Results and DiscussionPhase diagram

The pseudo-ternary phase diagrams of the saffo-la/Tx 100/butanol/water system under differentconditions were constructed. A representative dia-gram is depicted in Fig. 1. At 20°C and atS/CoS = 1:5, the single phase and the three phasezones are larger in comparison to those atS/CoS = 1:8. Such a large three phase zone is un-usual. The extent of a phase was affected by the tem-perature. With increase in temperature from 30°Cto 50°C, the single phase microemulsion zone (Win-sor IV) increased; a larger three phase zone was ob-served at 40°C. The additives, NaCI (0.2 equiv.dm<1), cholesterol (2.5% w/v) and urea (3 mol.dm - .1) at S/CoS = 1:5 decreased the single phasemieroemulsion zone as well as the three phase zoneas per the phase diagram. The efficacy in decreasingthe Winsor IV microemulsion zone followed the or-der: urea - cholesterol < NaCl up to 50% (w/w] oilcontent; thereafter, the order was: cholesterol <NaCl <urea.

In the quaternary multiphasic system usually it isvery difficult to estimate the actual compositions ofthe different phases; only an overall estimate of theirtypes is possible. For the present system, in the twophase region, the water rich samples were Winsor I(o/w) type and the oil rich samples were Winsor II(w/o) type. The middle phase of the triphasic sam-ples was bicontinuous; the top and. bottom phaseswere dilute solutions of Triton x 100 and n-hutanolin oil and water respectively. The proportions of theindividual phases varied with the concentrations ofthe components. The middle region of the triphasiccompositions contained more of the bicontinuousphase than the end regions. It IS noteworthy that inthe phase diagrams a triphasic area is not centrallyflanked touching three single phase regions; instead,it is touched by two single phase regions. The for-mer pattern is the ideal situation, which is hardly (ifever) achieved in practice, particularly with multi-component systems containing amphiphiles I'.l~

\~~

.'-~80 100

Oil

Fig. I-Triangular phase diagram of saftolaiT x IOO/butanol!water system at 20°C lA, S/CoS = 1:8; B. S/CoS = 1:5].

INDIAN J CHEM, SEe. A, APRIL 1991

Trials to produce the ideal situation were unsuccess-ful.

Viscosity behaviourThe, viscosity of the microemulsion system was

measured at different compositions with S+CoSvarying in the range 48%-85% (w/w), oil in therange 2%-50% (w/w)and NaCI in the range 2%-15% (w/w), The viscosity was found to decrease asthe brine/oil ratio and temperature increased (Fig.2). The viscosity of the. system at brine/oil ratio = 1in presence of different additives, e.g., cholesterol(C, 0.5%-2.5%, w/v), cholesteryl acetate (CA, 2.5%,w/v), cholesteryl benzoate (CB, 1.60%, w/v), cho-lesteryl stearate (CS, 0.8%, w/v), urea (3 mol. dm - 3),crown ether (CE, 3%, w/v), crown ether + choles-terol (CE 3% +C 2.5%, w/v) was found to decreasegradually with increasing temperature. The curvesfor the additives are close together (Figs 2B & C). Atbrine/oil ratio 1 and above the viscosity remained

c,u

-4s:

1 4

u,u

- 3~

12

10

unchagned at all temperatures. The plots of log l] VS

T-I were fairly linear and yielded the energy of acti-vation, ll.E~isfor the viscous flow of the microemul-sions. The.ll.E~is values are presented in Tables 1and 2. The values remain practically unaffected inpresence of the additives as well as for the varyingbrine/oil ratios. Only 2.5% C and 1.6% CB loweredll.E~is; the flowing microemulsion crossed a lowerbarrier during transport. This was in contrast to thebehaviour in presence of 3% CE, where the barrierwas higher.

Conductance behaviourThe conductance-temperature profiles of the

microernulsions at different brine/oil ratios withoutadditives exhibited three distinct regions at higherratios (Fig. 3). In the lower temperature range (Hf-20°C), the conductance sharply declined. In themiddle range (200-S0°C), there was a mild increase,whereas a rapid increase was observed in the upper

A

c

10 20 30 40Iernpe rotur s tC)----+

B

10 20 3'0 40 50T.mp.ratur.l'C)~

50 60

60

Fig. 2- Temperature dependent viscosity of microemulsion [A: curves 1-4, brine/oil ratio, 0.05, 0.2, 1.0.7.5; B: brine/oil = I, curves1-4,2.5% C, 2.5% CA, 1.6% CB. 0.8% CS w/v; C: brine/oil = I, curves 1-2, (2.5% C + 3% CE), (2.5°;;,C + 3 mol dm - 1ureal.

330

PAUL etal.: PHASE BEHAVIOUR OF MlCROEMULSION OBTAINED FROM SAFFOLA

Table I-Activation energies for ionic conduction and viscous flow of saffola/T x 100/butanol/brine (0.2 equiv. dm - J)microemulsion)

~E.cond (kJmol- I)Composition(S + CoS/ oil/brine)

B/O ~E·.iS(kJmol-l)

Slope 1 Slope 2 Slope 3

55/43/2 (0.047) 27.4

48/50/2 (0.04) + 2.3

70/25/5 (0.2) 3.74 - 35.1 27.1

80/15/5 (0.33) 8.74 - 34.96

8517.517.5 (1.0) 55.9 1.3 -42.9 26.1

85/5/10 (2.0) 49.9 6.8 -44.5

83/2/15 (7.5) 17.7 3.5 -46.0 26.6

Table 2-Activation energies for ionic conduction and viscous flow of saffola/Tx 100/butanol/brine (0.2 equiv. dm-3) rnicroemul-sion of composition (8517.517.5 as (S + CoS/oil/brine) in presence of additives

Additives (0%, w/w) ~E·cond (kJmol-I) se«;(kJmol-I)

oC(0.5)C(1.0)

C(2.5)CA(2.5)CB (1.6)

CS(0.8)C (2.5) + urea (3 mol dm - 3)CE(3.0)C(2.5)+CE(3.0)

Slope 1

55.9 26.1

7.4

Slope 21.3

4.0

3.63.63.93.12.9

21.6

2.70.94

Slope 3

-42.91.1

2.2o1.31.8

- 31.7- 21.89

-28.6-29.7

24.525.925.4

26.025.627.526.9

range (50"-60°C). At brine/oil ratios of 0.33 and0.2, the third stage was absent.

The conductance of w/ 0 microemulsion may in-crease with temperature and may undergo a rapidchange after a threshold value; this phenomenon iscalled percolation 15 - 20. Above this threshold, themicro water droplets suffer sticky collisions formingclusters through which ions in the water pools effi-ciently migrate" - 25. In the present system, a moder-ate percolation appeared above 50°C. The rapiddecline in conductance between 10° and 20°C isvery unusual. In certain systems, only a mild initialdecrease in conductance has been observed". Theadditive cholesterol removed the first stage of dec-rease in conductance and shifted the initial percola-tion transition to higher temperature. A similar ef-fect was exhibited by CA and CB; CS, however, re-tained the features of the lower temperature rangeand removed the second transition (Fig. 4). Both Cand CE in combination, and CE alone exhibited ef-fects parallel to that of CS. In combination with C,urea showed the normal features with lowering of

the second transition (results are not presented).The above phenomena are striking. The internalstructure of microemulsion, controlling the con-ductance behaviour, seems to be significantly sus-ceptible to environmental modification. This wasnot at all reflected in the viscosity behaviour.

The activation energy for conduction was deter-mined from the log (J versus T - I plots; the estimat-ed Il.E~ond values are presented in Tables 1 and 2.Three stages of variation of conductance resulted inthree values of Il.E~ond for sets having brine/oil ra-tios 7.5, 2.0 and 1.0. Two other ratios (0.33 and 0.2)yielded two Il.E~ond values, and the lowest ratio(0.04) exhibited a single value. Graphical present-ations are not shown to save space. The Il.E~ond va-lues obtained from slope 1 in log (J vs T - I plotcorresponded to the phenomenon of second perco-lation. The values derived from slope 2 corre-sponded to the first stage of percolation and thosefrom slope 3 represented the initial stage of conduc-tion. For the additives C, CA, CB, CS and CE, thesecond percolation was absent and, therefore, the

331

INDIAN J CHEM, SEe. A, APRIL 1991

40.

i

)20

2"

-•~

a.

'"10

L_~Q~=~u==!:D==~.==~.=~. ,n u "T_,o'.,al't1-

Fig. 3-Specific conductance of microemulsions as a functionof temperature [Curves 1-6, brine/oil ratio 7.5, 2.0, 1.0, 0.33,

0.2 & 0.04 respectively].

results under siopes 2 and 3 are presented. Onlyurea in combination with C offered three distinct va-lues of liE~ond' It must be noted that in both the low-er and upper stages, the evaluation of liE~ond wasmainly based on two data points; the results, there-fore, were less accurate. But since the transitionswere sharp, their locations might be close to thoseshown in the figures. Since the differences in theslopes relative to the middle range were appreci-able, they could be indicative of the trend in the en-ergetics of conductance. The long time required forcompletion of duplicate sets of measurements(usually 8-10 hr for a set) was a constraint and thestudy could be carried out at a limited number oftemperatures only.

The earlier reported ":" activation energies forhydrocarbon oils are in the range 500-750 kJ mol- 1

in comparison to the present values of 2-50 kJmol- 1. The additives C, CA, CB and CS decreasedthe IiE~ondfor the first stage (slope 3) by nearly 50units (Table 2). The transport of ions, upon stickycollisions+r" among the micro brine droplets, be-came easier in presence of the additives. The thresh-old percolation temperatures increased in the addi-tive environments as was also observed with hydro-

.B2

no

u

20

10 20 !O 40 51T•••.• r.h.'. (c l------.. "

Fig. 4-Specific conductance of microemulsions as a functionof temperature at brine/oil = 1 [Curves 1-4,2.5% CA, 1.6% ca,

O.S%CS & 0% additive w/v]

carbon oils25•27. The effects of urea and crown etheralone and in combination were distinct. Urea in-creased the liE~ondfor the second percolation pro-cess, which was absent in presence of crown ether.The activation energy for conduction for the firstpercolation in presence of CE was, however, com-parable to that in presence of other additives. Thehigher positive liE !nd in presence of urea may beascribed to the hindrance offered by urea to theclustering of Water droplets by interaction (urea re-duces water structure or its association+="),

The plot of conductance (a) versus weight percent of surfactant plus brine (S + B) revealed occurr-ence of conductance maximum and minimum at14% and 16% (w/w) of (S +B) respectively in thetemperature range 10°-60°C (Fig. 5). Except at10°C, the trends at other temperatures were syste-matic; the conductance maximum decreased withdecreasing temperature whereas all the minimacrowded in a narrow region. The above phenomenasuggested transitions in the internal structure of themicroemulsion occurring at 14% and 16% (S +B).The viscosity decreased up to 19% (S +B) and re-mained unchanged thereafter. The higher the tem-perature, the lower was the effect. The weight per

PAUL et al.: PHASE BEHAVIOUR OF MICROEMULSION OBTAINED FROM SAFFOLA

l" 12

10

-·s j.b 2••··c 0.

i u

~c 5· ,v

"

4DD

A

12 2.Wt.% (SAtrf .• B,iuJ--+

Fig. 5-(A) Specific conductance & (B) viscosity of microemulsions as a function of weight % of surfactant + brine at different tem-peratures [A: curves 1-6,60·,10·,50·,40·,30· & 20·C respectively].

B

,>CT..

N

EOJ

60__-----------------950~~:=====~~~;;g4030

20,.,-

$? 4'"<

A

o 2 4 6Bri~/Oil--

Fig. 6-(A) Equivalent conductance & (B) Walden product ofmicroemulsion as a function of brine/oil ratio at different tem-

fperatures.

cents of (S + B) for conductance minima more orless corresponded to the viscosity minima. Theequivalent conductance (A) varied nonsystematical-ly with increase in the brine/oil ratio (Fig. 6). The ef-fect was pronounced up to the brine/oil ratio 2. At

60°C, there was a systematic increase, and a declinethereafter. The Walden product All sharply dec-reased upto brine/oil = 1, beyond which (upto 7.5) itremained almost constant. An initial increase wasobserved at 60°C. The Walden product, on thewhole, decreased with temperature, the change inconductance was not compensated by the change inviscosity upto brine/oil = 1; afterwards it was exact.Variable Walden products of microemulsions havebeen observed earlier also".

AcknowledgementPartial financial support for this work was ren-

dered by the Indian Statistical Institute, Calcutta.Thanks are due to Mr A K Das (G S U) for prepar-ing illustrations and to Mr D K Saha (G S U) for typ-ing the manuscript.

8

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