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Microemulsions: Formation and stabilization
Microemulsions: Formation and Stabilization W I L L I A M G E R B A C I A AND H E N R I L. R O S A N O Department of Chemistry, The City College of the City University of New York, New York, New York 10031
Received June 5, 1972; accepted November 1, 1972 The influence on the the formation of water-in-oil microemulsions of the chain length and cation of the surfactant and the nature of the solvent were studied. From NMR and free energy of adsorption of the alcohol, it was concluded that the alcohol-surfactant interaction is weak. Measurement of the change in the water-oil interracial tension (-~) while alcohol was injected into one of the phases was recorded. It was found that ~ may be temporarily lowered to zero while the alcohol diffused through the interface. It would therefore, be possible for a dispersion to occur spontaneously (while ~,~ = 0). The role of the surfactant would be to lower "rs and stabilize the system against coalescence. INTRODUCTION Microemulsions are transparent emulsions of high stability. Like macroemulsions additional a m p h i p h a t i c components are needed to form these emulsions. Usually, two additives, a surfactant and a cosm'factant are required. Unlike macroemulsions they are optically clear, the spherical droplet diameters of the dispersed phase being less than 1400 A. The dispersed component can be an oil and the continuous phase water or an aqueous solution, in which case the system is described as an oil-in-water (o/w) microemulsion, or the oil and water can exchange roles, in which case one speaks of a water-in-oil (w/o) microemulsion. Microemulsions of other immiscible liquids are also possible, b u t only the w / o t y p e will be discussed here. Since the term microemulsion was first introduced b y Schulman (1), t h e y have been studied b y him and others using a wide v a r i e t y of techniques. Numerous explanations of their stability have been offered (1-9, 21) b u t as y e t no theory explaining their formation and s t a b i l i t y has proved completely satisfactory. This paper will consider one aspect of micro-
emulsion formation which has not been given much attention. I t was originally suggested b y Schulman t h a t microemnlsions would form when the suri a c t a n t and cosurfactant in the right ratio produced a mixed adsorbed film t h a t would reduce the interfacial tension (~'i) between the oil and water below zero. There would then be free energy in the a m o u n t - ' y i d A for dispersion, where A is the interfacial area. The interfacial tension in the presence of the mixed film is given b y "Yi = "Yo/w-- wi, where ~'o/, is the o / w interfacial tension without the film present and ~-i is the spreading pressure of the film. A t equilibrium 7i becomes zero. If the concept of zero interfacial tension is accepted, stabilization of the microemulsion is implied. However, this model does not seem conceptually valid since a ~,~ = 0 would not require the dispersed phase to be distributed in spherical droplets as is found in the systems under discussion (2). Another objection raised b y Prince (3), was the necessity of attaining
242 Journal of Colloidand Interface Science. Vol.44. No. 2, August 1973
Copyright ~ 1973 by AcademicPress. Inc. All rights of reproductionin any form reserved.
MICROEMULSIONS very high equilibrium, interfacial film pressures ( > 55 dyne/cm for alkanes; > 35 dyne/cm for benzene) in order to obtain a 71 = 0. To alleviate this difficulty Prince (3) maintained that it was not ~'ozw but (~'o/,~)a (i.e., the oil/water interfacial tension of the oil and water with the cosurfactant, e.g., pentanol present which should be taken into account. This would eliminate the necessity for such high film pressures. Prince (3, 4) therefore, attributed to the cosurfactant the role of lowering the initial interfacial tension to permit the formation of microemulsions. (%/w)~ is, however, an equilibrium expression but it does, nevertheless, express the role of the cosurfactant as more than just an association moiety of the suffactant. The dynamic role of the cosurfactant in lowering the interracial tension has not as yet been considered in trying to explain microemulsion formation, although the effect is mentioned in the literature (14). The transient lowering of ~'i due to transport of the cosuffactant through the interface and the lack of strong interfacial complexing between the cosurfactant and the surfactant in the process of microemulsion formation is the subject of this paper. MATERIAL The n-hexadecane and benzene were reagent grade (Eastman Organic Chemicals, Rochester, NY) and were used without further purification. Lauryl sulfates were prepared from sodium lauryl sulfate (A&S Corporation, NY) by an ion exchange process described by Rosano, et al. (5). The remaining long chain sulfates and carboxylic acids were practical grade (Eastman Organic Chemicals). The LiOH, NaOH, KOH, NH4OH and (CHa)4NOH were reagent grade (J. T. Baker Co., Phillipsburg, NJ). The CsOIt and RbOH were 99.5% pure (Amend Drug and Chemical Co. Inc., NY). The tetraethylammonium hydroxide was reagent grade (Matheson Coleman and Bell, Cincinnati, OH). The 2-amino-2-methyl1-propanol (AMP) was a commercial sample
243
(Commercial Solvents Corp., NY). Distilled water was used in all experiments. PREPARATION OF THE MICROEMULSIONS The microemulsions for Tables 1 and 2 were prepared by adding to 40.0 ml of oil and 0.0032 moles of the sulfate or carboxylic acid, 1.44 ml of water or 2.2 N base solution (in the case of the carboxylic acids) and titrating the mixture to clarity with alcohol at 30°C. Ten or twenty milliliters of oil was then added causing a transition from the clear microemulsion to a macroemulsion with a concomitant increase in turbidity. Clarity was reproduced in the sample by again titrating with alcohol. This procedure was repeated several times to get a sufficient number of points to plot. INTERFACIAL TENSION The interfacial tension of water against oil as a function of time was monitored by using an automated Wilhelmy slide method (11). A sandblasted Pt blade prewet with water was attached by a thread to a microforce transducer. The signal from the transducer was amplified by a transducer amplifier (transducer and amplifier were manufactured by HewlettPackard Corporation). The signal was then plotted as a function of time on a calibrated strip-chart recorder (Sargent-Welch Corporation). If the blade was drawn through an aqueous-oil interface, the signal was proportional to ~'i. No hysterysis was observed on drawing the blade through the interface from either side, therefore it was assumed that the contact angle was zero (14). Pentanol was injected into one of the phases by using an agla micrometer syringe accurate to -4-0.00005 ml (Burroughs-Welcome and Company, Durham, NC). Gentle stirring was maintained in the phase in which the alcohol was being injected by using a magnetic stirrer (aqueous phase) or a motorized glass stirrer (oil phase). Several injections were made into each phase so that the systems in Fig. 2 contain varying amounts of alcohol. Fifty ml each of the oil and aqueous phase were used. Sodium
Journal of Colloid and Interface Science, Vol. 44, No. 2, A u g u s t 1973
244
GERBACIA AND ROSANO TABLE I
TABLE I--Contlnued
WATER-n-HEXADECANE MICIZOEMULSIONS AT Chain length X~
X,
0.97 0.96 0.93 0.88 0.86
0.36 0.35 0.29 0.20 0.21
Chain length
k I Moles Moles penpentanol/ tano]/ Moles Moles oil soap 0.57 0.5 0.41 0.25 0.27
28.5 24.5 14.0 7.5 6.5
Xi
X,
0.78 0.88 0.88 0.83 0.78 0.78 0.92 0.93 0.90
0.22 0.22 0,22 0.24 0.22 0.22 0.28 0.31 0.28
Xi
X~
k Moles pentanol/ Moles oil
0.29 0.28 0.28 0.31 0.29 0.29 0.38 0.45 0.40
2" Moles pentanol/ Moles soap
3.5 7.5 7.5 5,0 3.5 3.5 11.5 13.5 8.5
Chain length
k Moles pentanol/ Moles oil
I Moles pentanol/ Moles soap
AG8 kcal/ mole
~G~ kcaI/ mole
C12 C14 Cls Cls C20
0.33 0.33
No microemulsification No microemulsification 0.14 0.16 0.5 --0.53 --0.64 0.12 0.13 0.5 No microemulsification
40.0 ml n-hexadecane; 1.44 mI water; 0.0032 moles surfactant. Ammonium carboxylates did not form microemulsions in hexadecane.
AG~ kcal/ mole
RESULTS
0.88 0.83 0.75 0.71
0.26 0.36 7.5 0.20 0.25 5.0 0.17 0.20 3.0 0.14 0.16 2.5 No microemulsification
--0.73 --0.86 --0.90 --0.99
0.91 0.83 0.83 0.85
0.27 0.21 0.18 0.15
0.38 0.27 0.23 0.17
10.0 5.0 5.0 5.5
--0.72 --0.82 --0.90 --1.05
0.31 0.23 0.21 0.18
0.44 0.30 0.26 0.22
7.0 2.5 2.5 2.5
--0.63 --0.68 --0.74 --0.83
0.88 0.71 0.71 0.71
I Moles pentanol/ Moles soap
--0.75 --0.84 --0.84 --0.76 --0.75 --0.75 --0.72 --0.66 --0.69
Rb carboxylate C12 C14 C16 Cls
k Moles pentanol/ Moles oil
dodecyl sulfate (SDS) solution was introduced into the aqueous phase b y injecting an appropriate a m o u n t of an SDS solution. Sufficient time was allowed for the interface to reach equilibrium before alcohol was injected. This was judged b y the stability of 3'~ as a function of time. All systems were m a i n t a i n e d at 30°C in a thermostated breaker 4.90 cm in diameter. The interface was cleaned initially before a n y injections were done b y using suction through a narrow pipet.
K carboxylate CI2 C14 C16 C1s
X,
AMP carboxylate --0.59 --0.60 --0.70 --0.89 --0.85
Long-chain carboxylates
Na carboxylate
Long-chain carboxylates Xi
zxa, kcal/ mole
Dodecyl sulfates
Counterion
C12 C14 CI6 Cls C20
Na carboxylate
Long-chain sodium sulfates
CG Cs Clo C~2 C14
Li+ Na+ K Rb + Cs+ NH4+ (CFI~)4N+ (C~Hs)2N+ AMP
30°Ca
The graphs of moles of alcohol per mole of surfactant versus moles of oil per mole of surfactant gave straight lines for small additions of oil to the original microemulsions (Fig. 1). The intercept (/) was taken to be the n u m b e r of moles of alcohol at the interface per mole of surfactant. The a m o u n t of pentanol dissolved in the dispersed phase was assumed negligible. The slope (k) gave the solubility of the alcohol in the continuous phase (5). From the data the free energy per mole for the absorption of alcohol into the interphase from the continuous phase was calculated using the formula
AG. = - - R T In (Xa~/Xa ~) where Xa; and Xa ~ are the mole fraction of pentanol ill the interphase and the continuous phase, respectively.
Journal of Colloid and Interface Science, Vol. 44, No. 2, August 1973
245
MICROEMULSIONS
The intercepts, slopes and AG~ are tabulated in Tables 1 and 2 for several oil-surfactant combinations. I t was noted that all 2xG~ are small negative values for these systems. When n-hexadecane was used as the oil phase, the requirement for pentanol diminished and the free energy for adsorption decreased slightly as the chain length of the sulfate surfactant was increased. The same trend was shown with the carboxylates although it was less marked. In the case of the value of I a leveling was noted after C12. The same trends were noted when benzene was substituted for n-hexadecane and carboxylates were used as the surfactants. However, no leveling in the value of I was observed except when rubidium was the counter ion and then it occurred at a higher value, Cls. The values of I were significantly higher with benzene than in n-hexadecane in the case of the carboxylates. In all cases the zXG, was lower in benzene than in n-hexadecane. The behavior of the long-chain sulfates in benzene was slightly different in that a minimum value was reached and C10 and Cs, C1~ and C14 demonstrated the same requirements for cosurfactant. The effect of changing the counterion is not as clear cut. The stericly larger counterions such as tetramethyl and tetraethylammonium ion required the largest amount of pentanol in the interface in hexadecane but these cations do not show any significant difference from Cs+, K +, Li +, Rb +, and NH4 +, in benzene when the dodecyl sulfate was the anion. The value I does not show any ordering in terms of the size of the counterion for the alkali cations and it is felt that this type of experiment is not sensitive enough to detect fine differences in very similar counterions. The effect of transport of pentanol through the oil-aqueous interface was studied to determine its relevance in microemulsion formation. Fifty milliliters of n-hexadecane was placed gently on 50 ml of water. The effect of diffusion of pentanol was measured for different volumes of pentanol injected into the aqueous or oil phase. Measurements were also made with SD S in the aqueous phase. The SDS was injected
TABLE II WATER-BENZENE MICROE~ULSlONS AT 30°C ~ Chain length
Long chain sodium sulfates X~
X8
0.96 0.90 0.88 0.90 0.90
0.11 0.12 0.10 0.09 0.36
X~
X~
k Moles pentanol/ Moles oil
I Moles pentanol/ Moles soap
AG. kcal/ mole
0.86 0.90 0.86 0.80 0.83 0.80 0.85 0.71
0.08 0.10 0.08 0.06 0.06 0.08 0.09 0.04
0.09 0.11 0.09 0.07 0.07 0.08 0.10 0.04
6.0 9.0 6.0 4.0 5.0 4.0 5.5 2.5
-- 1.39 --1.32 --1.39 -- 1.53 -- 1.55 1.41 --1.34 --1.75
Chain length
X~
x8
C12 C14 C16 CIs Cs0
0.91 0.90 0.85
0.12 0.14 10.0 --1.22 0.11 0.12 9.0 --1.29 0.02 0.02 5.5 --2.26 No microemulsification No microemulsification
0.95 0.94 0.92 0.90
0.12 0.11 0.10 0.t0
0.13 0.12 0.11 0.11
18.5 15.5 12.0 9.0
0.94 0.91 0.89 0.88 0.88
0.11 0.13 0.11 0.10 0.09
0,13 0.14 0.13 0.12 0.10
15.0 10.5 8.0 7.0 7.0
C~ Cs C10 C12 C14 Counterion
k
I
&G,
Moles pentanol/ Moles oil
Moles pentanol/ Moles soap
kcal/ mole
0.12 0.13 0.11 0.10 0.04
23.5 9.0 7.5 9.5 9.5
--1.29 --1.22 --1.32 --1.38 --1.94
Dodecyl sulfates
Li + Na + K+ Rb + Cs + NH~ + (CH3)4N + AMP Na carboxylate
-
-
Long chain carbo×ylates k
Moles pentanol/ Moles oil
I
Moles pentanol/ Moles soap
AG~ keal/ mole
K carboxylate C12 C~4 C1~ C18
--1.27 --1.30 --1.33 --1.34
Rb carboxylate C12 C14 C16 C18 C20
--1.28 --1.19 --1.24 --1.28 --1.36
a40.0 ml benzene; 1.44 ml water; 0.0032 moles suffactant. Ammonium carboxylates and AMP carboxylates did not form microemulsions with benzene.
Journal of Colloid and Interface Science, Vol. 44, No. 2, August 1973
246
GERBACIA AND ROSANO V Polossium Polminole Q Potossium Myris~ote Potossium Loumte • Potossium Steorote
50
-a~o ~N 30 ~
2O I0 r
;__
T
Moles oil Mo]es soap
Fzo. 1. Variation of the chain length of potassium carboxylates for n-hexadecane microemulsions.
into the water phase to give a concentration of 1.37 X 10-3 M. The volume of pentanol was increased until a transient ~'i = 0 was obtained (Fig. 2). I t was observed that it took less alcohol to reduce ~,i to zero when injecting into the oil phase than when injecting into the water phase. After injection of the pentanol, the interfacial tension returned to approximately the same value as prior to injection. I t is to be noted that the rate of diffusion (as determined by the maximum 7q) is greatly
decreased in the presence of the adsorbed monolayer of surfactant. Microemulsions are usually prepared either by titrating with one of the components (as was done in these experiments) or by mixing pure components together. Either method allows transport of the amphiphatic component through the interface to occur. To avoid this and determine if diffusion is a necessary condition for microemulsification an aqueous solution of surfactant and pentanol was pre-
30
25~
/
_
o
o,,we,e, ,JJ~ ~ 5 ~ I\
I\
i 0
/ ~
om
lilt I00
J .,,""%~-" " * / " * / * I 200
~ o o . i 300
(injection into oil) oil/SOS (injection into oqueous phose) oil/SOS (injection into oil} i I i i • 400 500 600 700
Time ( s e c )
F i e . 2. V a r i a t i o n of t h e n - h e x a d e c a n e / a q u e o u s i n t e r f a c i a l tension vs time. IournaI of Colloid and Interface Selenee, Vol. 44, No. 2, August 1973
MICROEMULSIONS
pared. The solution contained the amount of pentanol that would be present in the final microemulsion as determined by the distribution data in Tables 1 and 2. For example, 1.44 ml of water, 0.0032 moles of SDS and 0.024 moles of pentanol were used in one of the solutions. Another solution of oil and its ratio of pentanol was prepared. For example, 0.042 moles of pentanol and 40.0 ml of n-hexadecane was used in conjunction with the aqueous solution described above. The two solutions were then mixed together for approximately twenty minutes at 30°C but no microemulsion resulted. It is to be emphasized that these components when combined in the usual way (i.e., by titrating the alcohol) with the amounts described would have resulted in the formation of a microemulsion. Microemulsions generally form almost immediately when the correct conditions are provided. These systems prepared in this way were not even stable with regard to phase separation upon standing for a few minutes. The actual solutions used were: A.
0.92 g SDS 1.44 ml water 2.61 ml pentanol mixed with 40.0 ml n-hexadecane 4.57 ml pentanol B.
0.92 g SDS 1.44 ml water 3.16 ml pentanol mixed with 60.0 ml benzene 7.95 ml pentanol DISCUSSION
It is apparent from the above experiments that it is possible for the interracial tension of a system to drop to zero for a certain period of time due to redistribution of amphiphatic molecules while the equilibrium 3'~ remains positive. The requirement of diffusion across the interface in these systems was demonstrated in the
247
experiments last described. The diffusion proc, ess has been mentioned before as a necessary condition for spontaneous emulsification (15, 16) but it is not a sufficient condition for microemulsification in these systems since pentanol does not produce microemulsions in all of the dodecyl sulfate systems. It is reasonable that the surfactant must be able to stabilize the system against coagulation and coalescence after the cosurfactant has lowered 3'i sufficiently to cause dispersion. The ability of the surfactant to accomplish this would depend upon the type of interfacial film it forms with the alcohol and oil present. The larger requirement for alcohol in the systems with the bulkier counterions may be explained by the consideration that surfactants of this type would produce a more expanded interracial film permitting faster diffusion of the alcohol through the interface and, therefore, the maximum film pressure would not be maintained for as long a period of time. The same reasoning would explain the effect of increasing the chain length of the surfactant. The shorter chain lengths would give a more expanded film and permit faster diffusion of the alcohol. If the chain length is too long, however, diffusion through the interface would be too slow and the effect of the alcohol in lowering 3'i would be diminished. Therefore, one would expect that at a certain chain length of the surfactant, microemulsions would not form. This was seen in several of the carboxylate systems. It must be assumed that this would also occur with the sulfate surfactants at a chain length greater than C14. It has previously been pointed out that strong association between the surfactant and cosurfactant is not necessary for microemulsification and that nonionic and ionic amphiphatie molecules, in general, adsorb independently (16, 17). The calculated free energies for adsorption of the pentanol are all small, indicating little association between the surfactant and cosurfactant [in agreement with Rosano et al. (5)-]. The absence of strong interracial complexing between the surfactant and cosurfactant was also indicated in N M R
Journal of Colloid and Interface Science, Vol. 44, No. 2, August 1973
248
GERBACIA AND ROSANO
studies of w / o m i c r o e m u l s i o n in CC14 stabilized b y long chain sulfates (18). ACKNOWLEDGMENT We would like to thank Mr. Eric Gluck for his technical assistance in this investigation. REFERENCES 1. HOAR, T. A N D SCh'ULMAN,J. H., Nature (London) 152, 102 (1943). 2. STOECKENIUS,W., SCIIULMAN,J. H., AND PRINCE~ L. M., Kolloid-Z. 169, 170 (1960). 3. PRINCE, L. M., J. Colloid Interface Sci. 23, 165 (1967). 4. PRINCE, L, M., J. Colloid Interface Sci. 29, 2, 216 (1969). 5. ROSANO, H. L., PEISER, R. C., AND EYDT, A., Rev. Fran. Corps Gras 16, 4, 249 (1969). 6. ADAMSON,A. W., J. Colloid Interface Sci. 29, 2, 261 (1969). 7. WINSOR,P. A., Trans. Faraday Soc. 22, 376 (1948). 8. ibid. 46, 762 (1950). 9. SCtIULMAN,J. H., STOECHENIUS,W., AND PRINCE, L. M., ]. Phys. Chem. 63, 1677 (1959).
10. SCI~ULX*-AN,J. H., AND McRoBERTS, T. S., Trans. Faraday Soc. 42B, 165 (1946). 11. BoweoTT, J. E., AND SCmYLM~AN,J. H., Z-Electrochem. 59, 4, 283 (1955). 12. COOKE, C. E., AND SCHULMAN,J. H., "Proc. 2nd Scandinavian Syrup. Surface Activity," Stockholm, p. 231 (1965). 13. ROSANO, H. L., GER.BACIA, W., FEINSTEIN, M., AND SWAINE, J. Colloid Interface Sci. 36, 298 (1971). 14. JASPER.,J. J., ANDHOUSEMAN,B. L., J. Phys. Chem. 67, 1548 (1963). 15. MCBAIN, J. W., AND WOO, T., Proc. Roy. Soc. A163, 182 (1937). 16. KAMINSKI, A., AND McBAIN, J. W., Roy. Soc. A198, 447 (1949). 17. DAVIES, J. T., AND RIDEAL, E. K., "Interfacial Phenomena," p. 364. Academic Press, New York (1961). 18. GEI~BAClA,W., ANDROSANO,H. L., to be published. 19. SCHULMAN,J. H., AND MONTAGNE,J. B., Ann. N.Y. Acad. Sci. 92, 366, 2 (1961). 20. PRINCE, L. M., J. Soc. Cosmetic Chem. 21, 193 (1970). 21. GILLBERG, G., LEHTINEN, H., AND--FRIBEI~C,S., d. Colloid Interface Sd. 33, 40 (1970).
Journal of Colloid and Interface Science, Vol. 44. No. 2. August 1973
Received June 5, 1972; accepted November 1, 1972 The influence on the the formation of water-in-oil microemulsions of the chain length and cation of the surfactant and the nature of the solvent were studied. From NMR and free energy of adsorption of the alcohol, it was concluded that the alcohol-surfactant interaction is weak. Measurement of the change in the water-oil interracial tension (-~) while alcohol was injected into one of the phases was recorded. It was found that ~ may be temporarily lowered to zero while the alcohol diffused through the interface. It would therefore, be possible for a dispersion to occur spontaneously (while ~,~ = 0). The role of the surfactant would be to lower "rs and stabilize the system against coalescence. INTRODUCTION Microemulsions are transparent emulsions of high stability. Like macroemulsions additional a m p h i p h a t i c components are needed to form these emulsions. Usually, two additives, a surfactant and a cosm'factant are required. Unlike macroemulsions they are optically clear, the spherical droplet diameters of the dispersed phase being less than 1400 A. The dispersed component can be an oil and the continuous phase water or an aqueous solution, in which case the system is described as an oil-in-water (o/w) microemulsion, or the oil and water can exchange roles, in which case one speaks of a water-in-oil (w/o) microemulsion. Microemulsions of other immiscible liquids are also possible, b u t only the w / o t y p e will be discussed here. Since the term microemulsion was first introduced b y Schulman (1), t h e y have been studied b y him and others using a wide v a r i e t y of techniques. Numerous explanations of their stability have been offered (1-9, 21) b u t as y e t no theory explaining their formation and s t a b i l i t y has proved completely satisfactory. This paper will consider one aspect of micro-
emulsion formation which has not been given much attention. I t was originally suggested b y Schulman t h a t microemnlsions would form when the suri a c t a n t and cosurfactant in the right ratio produced a mixed adsorbed film t h a t would reduce the interfacial tension (~'i) between the oil and water below zero. There would then be free energy in the a m o u n t - ' y i d A for dispersion, where A is the interfacial area. The interfacial tension in the presence of the mixed film is given b y "Yi = "Yo/w-- wi, where ~'o/, is the o / w interfacial tension without the film present and ~-i is the spreading pressure of the film. A t equilibrium 7i becomes zero. If the concept of zero interfacial tension is accepted, stabilization of the microemulsion is implied. However, this model does not seem conceptually valid since a ~,~ = 0 would not require the dispersed phase to be distributed in spherical droplets as is found in the systems under discussion (2). Another objection raised b y Prince (3), was the necessity of attaining
242 Journal of Colloidand Interface Science. Vol.44. No. 2, August 1973
Copyright ~ 1973 by AcademicPress. Inc. All rights of reproductionin any form reserved.
MICROEMULSIONS very high equilibrium, interfacial film pressures ( > 55 dyne/cm for alkanes; > 35 dyne/cm for benzene) in order to obtain a 71 = 0. To alleviate this difficulty Prince (3) maintained that it was not ~'ozw but (~'o/,~)a (i.e., the oil/water interfacial tension of the oil and water with the cosurfactant, e.g., pentanol present which should be taken into account. This would eliminate the necessity for such high film pressures. Prince (3, 4) therefore, attributed to the cosurfactant the role of lowering the initial interfacial tension to permit the formation of microemulsions. (%/w)~ is, however, an equilibrium expression but it does, nevertheless, express the role of the cosurfactant as more than just an association moiety of the suffactant. The dynamic role of the cosurfactant in lowering the interracial tension has not as yet been considered in trying to explain microemulsion formation, although the effect is mentioned in the literature (14). The transient lowering of ~'i due to transport of the cosuffactant through the interface and the lack of strong interfacial complexing between the cosurfactant and the surfactant in the process of microemulsion formation is the subject of this paper. MATERIAL The n-hexadecane and benzene were reagent grade (Eastman Organic Chemicals, Rochester, NY) and were used without further purification. Lauryl sulfates were prepared from sodium lauryl sulfate (A&S Corporation, NY) by an ion exchange process described by Rosano, et al. (5). The remaining long chain sulfates and carboxylic acids were practical grade (Eastman Organic Chemicals). The LiOH, NaOH, KOH, NH4OH and (CHa)4NOH were reagent grade (J. T. Baker Co., Phillipsburg, NJ). The CsOIt and RbOH were 99.5% pure (Amend Drug and Chemical Co. Inc., NY). The tetraethylammonium hydroxide was reagent grade (Matheson Coleman and Bell, Cincinnati, OH). The 2-amino-2-methyl1-propanol (AMP) was a commercial sample
243
(Commercial Solvents Corp., NY). Distilled water was used in all experiments. PREPARATION OF THE MICROEMULSIONS The microemulsions for Tables 1 and 2 were prepared by adding to 40.0 ml of oil and 0.0032 moles of the sulfate or carboxylic acid, 1.44 ml of water or 2.2 N base solution (in the case of the carboxylic acids) and titrating the mixture to clarity with alcohol at 30°C. Ten or twenty milliliters of oil was then added causing a transition from the clear microemulsion to a macroemulsion with a concomitant increase in turbidity. Clarity was reproduced in the sample by again titrating with alcohol. This procedure was repeated several times to get a sufficient number of points to plot. INTERFACIAL TENSION The interfacial tension of water against oil as a function of time was monitored by using an automated Wilhelmy slide method (11). A sandblasted Pt blade prewet with water was attached by a thread to a microforce transducer. The signal from the transducer was amplified by a transducer amplifier (transducer and amplifier were manufactured by HewlettPackard Corporation). The signal was then plotted as a function of time on a calibrated strip-chart recorder (Sargent-Welch Corporation). If the blade was drawn through an aqueous-oil interface, the signal was proportional to ~'i. No hysterysis was observed on drawing the blade through the interface from either side, therefore it was assumed that the contact angle was zero (14). Pentanol was injected into one of the phases by using an agla micrometer syringe accurate to -4-0.00005 ml (Burroughs-Welcome and Company, Durham, NC). Gentle stirring was maintained in the phase in which the alcohol was being injected by using a magnetic stirrer (aqueous phase) or a motorized glass stirrer (oil phase). Several injections were made into each phase so that the systems in Fig. 2 contain varying amounts of alcohol. Fifty ml each of the oil and aqueous phase were used. Sodium
Journal of Colloid and Interface Science, Vol. 44, No. 2, A u g u s t 1973
244
GERBACIA AND ROSANO TABLE I
TABLE I--Contlnued
WATER-n-HEXADECANE MICIZOEMULSIONS AT Chain length X~
X,
0.97 0.96 0.93 0.88 0.86
0.36 0.35 0.29 0.20 0.21
Chain length
k I Moles Moles penpentanol/ tano]/ Moles Moles oil soap 0.57 0.5 0.41 0.25 0.27
28.5 24.5 14.0 7.5 6.5
Xi
X,
0.78 0.88 0.88 0.83 0.78 0.78 0.92 0.93 0.90
0.22 0.22 0,22 0.24 0.22 0.22 0.28 0.31 0.28
Xi
X~
k Moles pentanol/ Moles oil
0.29 0.28 0.28 0.31 0.29 0.29 0.38 0.45 0.40
2" Moles pentanol/ Moles soap
3.5 7.5 7.5 5,0 3.5 3.5 11.5 13.5 8.5
Chain length
k Moles pentanol/ Moles oil
I Moles pentanol/ Moles soap
AG8 kcal/ mole
~G~ kcaI/ mole
C12 C14 Cls Cls C20
0.33 0.33
No microemulsification No microemulsification 0.14 0.16 0.5 --0.53 --0.64 0.12 0.13 0.5 No microemulsification
40.0 ml n-hexadecane; 1.44 mI water; 0.0032 moles surfactant. Ammonium carboxylates did not form microemulsions in hexadecane.
AG~ kcal/ mole
RESULTS
0.88 0.83 0.75 0.71
0.26 0.36 7.5 0.20 0.25 5.0 0.17 0.20 3.0 0.14 0.16 2.5 No microemulsification
--0.73 --0.86 --0.90 --0.99
0.91 0.83 0.83 0.85
0.27 0.21 0.18 0.15
0.38 0.27 0.23 0.17
10.0 5.0 5.0 5.5
--0.72 --0.82 --0.90 --1.05
0.31 0.23 0.21 0.18
0.44 0.30 0.26 0.22
7.0 2.5 2.5 2.5
--0.63 --0.68 --0.74 --0.83
0.88 0.71 0.71 0.71
I Moles pentanol/ Moles soap
--0.75 --0.84 --0.84 --0.76 --0.75 --0.75 --0.72 --0.66 --0.69
Rb carboxylate C12 C14 C16 Cls
k Moles pentanol/ Moles oil
dodecyl sulfate (SDS) solution was introduced into the aqueous phase b y injecting an appropriate a m o u n t of an SDS solution. Sufficient time was allowed for the interface to reach equilibrium before alcohol was injected. This was judged b y the stability of 3'~ as a function of time. All systems were m a i n t a i n e d at 30°C in a thermostated breaker 4.90 cm in diameter. The interface was cleaned initially before a n y injections were done b y using suction through a narrow pipet.
K carboxylate CI2 C14 C16 C1s
X,
AMP carboxylate --0.59 --0.60 --0.70 --0.89 --0.85
Long-chain carboxylates
Na carboxylate
Long-chain carboxylates Xi
zxa, kcal/ mole
Dodecyl sulfates
Counterion
C12 C14 CI6 Cls C20
Na carboxylate
Long-chain sodium sulfates
CG Cs Clo C~2 C14
Li+ Na+ K Rb + Cs+ NH4+ (CFI~)4N+ (C~Hs)2N+ AMP
30°Ca
The graphs of moles of alcohol per mole of surfactant versus moles of oil per mole of surfactant gave straight lines for small additions of oil to the original microemulsions (Fig. 1). The intercept (/) was taken to be the n u m b e r of moles of alcohol at the interface per mole of surfactant. The a m o u n t of pentanol dissolved in the dispersed phase was assumed negligible. The slope (k) gave the solubility of the alcohol in the continuous phase (5). From the data the free energy per mole for the absorption of alcohol into the interphase from the continuous phase was calculated using the formula
AG. = - - R T In (Xa~/Xa ~) where Xa; and Xa ~ are the mole fraction of pentanol ill the interphase and the continuous phase, respectively.
Journal of Colloid and Interface Science, Vol. 44, No. 2, August 1973
245
MICROEMULSIONS
The intercepts, slopes and AG~ are tabulated in Tables 1 and 2 for several oil-surfactant combinations. I t was noted that all 2xG~ are small negative values for these systems. When n-hexadecane was used as the oil phase, the requirement for pentanol diminished and the free energy for adsorption decreased slightly as the chain length of the sulfate surfactant was increased. The same trend was shown with the carboxylates although it was less marked. In the case of the value of I a leveling was noted after C12. The same trends were noted when benzene was substituted for n-hexadecane and carboxylates were used as the surfactants. However, no leveling in the value of I was observed except when rubidium was the counter ion and then it occurred at a higher value, Cls. The values of I were significantly higher with benzene than in n-hexadecane in the case of the carboxylates. In all cases the zXG, was lower in benzene than in n-hexadecane. The behavior of the long-chain sulfates in benzene was slightly different in that a minimum value was reached and C10 and Cs, C1~ and C14 demonstrated the same requirements for cosurfactant. The effect of changing the counterion is not as clear cut. The stericly larger counterions such as tetramethyl and tetraethylammonium ion required the largest amount of pentanol in the interface in hexadecane but these cations do not show any significant difference from Cs+, K +, Li +, Rb +, and NH4 +, in benzene when the dodecyl sulfate was the anion. The value I does not show any ordering in terms of the size of the counterion for the alkali cations and it is felt that this type of experiment is not sensitive enough to detect fine differences in very similar counterions. The effect of transport of pentanol through the oil-aqueous interface was studied to determine its relevance in microemulsion formation. Fifty milliliters of n-hexadecane was placed gently on 50 ml of water. The effect of diffusion of pentanol was measured for different volumes of pentanol injected into the aqueous or oil phase. Measurements were also made with SD S in the aqueous phase. The SDS was injected
TABLE II WATER-BENZENE MICROE~ULSlONS AT 30°C ~ Chain length
Long chain sodium sulfates X~
X8
0.96 0.90 0.88 0.90 0.90
0.11 0.12 0.10 0.09 0.36
X~
X~
k Moles pentanol/ Moles oil
I Moles pentanol/ Moles soap
AG. kcal/ mole
0.86 0.90 0.86 0.80 0.83 0.80 0.85 0.71
0.08 0.10 0.08 0.06 0.06 0.08 0.09 0.04
0.09 0.11 0.09 0.07 0.07 0.08 0.10 0.04
6.0 9.0 6.0 4.0 5.0 4.0 5.5 2.5
-- 1.39 --1.32 --1.39 -- 1.53 -- 1.55 1.41 --1.34 --1.75
Chain length
X~
x8
C12 C14 C16 CIs Cs0
0.91 0.90 0.85
0.12 0.14 10.0 --1.22 0.11 0.12 9.0 --1.29 0.02 0.02 5.5 --2.26 No microemulsification No microemulsification
0.95 0.94 0.92 0.90
0.12 0.11 0.10 0.t0
0.13 0.12 0.11 0.11
18.5 15.5 12.0 9.0
0.94 0.91 0.89 0.88 0.88
0.11 0.13 0.11 0.10 0.09
0,13 0.14 0.13 0.12 0.10
15.0 10.5 8.0 7.0 7.0
C~ Cs C10 C12 C14 Counterion
k
I
&G,
Moles pentanol/ Moles oil
Moles pentanol/ Moles soap
kcal/ mole
0.12 0.13 0.11 0.10 0.04
23.5 9.0 7.5 9.5 9.5
--1.29 --1.22 --1.32 --1.38 --1.94
Dodecyl sulfates
Li + Na + K+ Rb + Cs + NH~ + (CH3)4N + AMP Na carboxylate
-
-
Long chain carbo×ylates k
Moles pentanol/ Moles oil
I
Moles pentanol/ Moles soap
AG~ keal/ mole
K carboxylate C12 C~4 C1~ C18
--1.27 --1.30 --1.33 --1.34
Rb carboxylate C12 C14 C16 C18 C20
--1.28 --1.19 --1.24 --1.28 --1.36
a40.0 ml benzene; 1.44 ml water; 0.0032 moles suffactant. Ammonium carboxylates and AMP carboxylates did not form microemulsions with benzene.
Journal of Colloid and Interface Science, Vol. 44, No. 2, August 1973
246
GERBACIA AND ROSANO V Polossium Polminole Q Potossium Myris~ote Potossium Loumte • Potossium Steorote
50
-a~o ~N 30 ~
2O I0 r
;__
T
Moles oil Mo]es soap
Fzo. 1. Variation of the chain length of potassium carboxylates for n-hexadecane microemulsions.
into the water phase to give a concentration of 1.37 X 10-3 M. The volume of pentanol was increased until a transient ~'i = 0 was obtained (Fig. 2). I t was observed that it took less alcohol to reduce ~,i to zero when injecting into the oil phase than when injecting into the water phase. After injection of the pentanol, the interfacial tension returned to approximately the same value as prior to injection. I t is to be noted that the rate of diffusion (as determined by the maximum 7q) is greatly
decreased in the presence of the adsorbed monolayer of surfactant. Microemulsions are usually prepared either by titrating with one of the components (as was done in these experiments) or by mixing pure components together. Either method allows transport of the amphiphatic component through the interface to occur. To avoid this and determine if diffusion is a necessary condition for microemulsification an aqueous solution of surfactant and pentanol was pre-
30
25~
/
_
o
o,,we,e, ,JJ~ ~ 5 ~ I\
I\
i 0
/ ~
om
lilt I00
J .,,""%~-" " * / " * / * I 200
~ o o . i 300
(injection into oil) oil/SOS (injection into oqueous phose) oil/SOS (injection into oil} i I i i • 400 500 600 700
Time ( s e c )
F i e . 2. V a r i a t i o n of t h e n - h e x a d e c a n e / a q u e o u s i n t e r f a c i a l tension vs time. IournaI of Colloid and Interface Selenee, Vol. 44, No. 2, August 1973
MICROEMULSIONS
pared. The solution contained the amount of pentanol that would be present in the final microemulsion as determined by the distribution data in Tables 1 and 2. For example, 1.44 ml of water, 0.0032 moles of SDS and 0.024 moles of pentanol were used in one of the solutions. Another solution of oil and its ratio of pentanol was prepared. For example, 0.042 moles of pentanol and 40.0 ml of n-hexadecane was used in conjunction with the aqueous solution described above. The two solutions were then mixed together for approximately twenty minutes at 30°C but no microemulsion resulted. It is to be emphasized that these components when combined in the usual way (i.e., by titrating the alcohol) with the amounts described would have resulted in the formation of a microemulsion. Microemulsions generally form almost immediately when the correct conditions are provided. These systems prepared in this way were not even stable with regard to phase separation upon standing for a few minutes. The actual solutions used were: A.
0.92 g SDS 1.44 ml water 2.61 ml pentanol mixed with 40.0 ml n-hexadecane 4.57 ml pentanol B.
0.92 g SDS 1.44 ml water 3.16 ml pentanol mixed with 60.0 ml benzene 7.95 ml pentanol DISCUSSION
It is apparent from the above experiments that it is possible for the interracial tension of a system to drop to zero for a certain period of time due to redistribution of amphiphatic molecules while the equilibrium 3'~ remains positive. The requirement of diffusion across the interface in these systems was demonstrated in the
247
experiments last described. The diffusion proc, ess has been mentioned before as a necessary condition for spontaneous emulsification (15, 16) but it is not a sufficient condition for microemulsification in these systems since pentanol does not produce microemulsions in all of the dodecyl sulfate systems. It is reasonable that the surfactant must be able to stabilize the system against coagulation and coalescence after the cosurfactant has lowered 3'i sufficiently to cause dispersion. The ability of the surfactant to accomplish this would depend upon the type of interfacial film it forms with the alcohol and oil present. The larger requirement for alcohol in the systems with the bulkier counterions may be explained by the consideration that surfactants of this type would produce a more expanded interracial film permitting faster diffusion of the alcohol through the interface and, therefore, the maximum film pressure would not be maintained for as long a period of time. The same reasoning would explain the effect of increasing the chain length of the surfactant. The shorter chain lengths would give a more expanded film and permit faster diffusion of the alcohol. If the chain length is too long, however, diffusion through the interface would be too slow and the effect of the alcohol in lowering 3'i would be diminished. Therefore, one would expect that at a certain chain length of the surfactant, microemulsions would not form. This was seen in several of the carboxylate systems. It must be assumed that this would also occur with the sulfate surfactants at a chain length greater than C14. It has previously been pointed out that strong association between the surfactant and cosurfactant is not necessary for microemulsification and that nonionic and ionic amphiphatie molecules, in general, adsorb independently (16, 17). The calculated free energies for adsorption of the pentanol are all small, indicating little association between the surfactant and cosurfactant [in agreement with Rosano et al. (5)-]. The absence of strong interracial complexing between the surfactant and cosurfactant was also indicated in N M R
Journal of Colloid and Interface Science, Vol. 44, No. 2, August 1973
248
GERBACIA AND ROSANO
studies of w / o m i c r o e m u l s i o n in CC14 stabilized b y long chain sulfates (18). ACKNOWLEDGMENT We would like to thank Mr. Eric Gluck for his technical assistance in this investigation. REFERENCES 1. HOAR, T. A N D SCh'ULMAN,J. H., Nature (London) 152, 102 (1943). 2. STOECKENIUS,W., SCIIULMAN,J. H., AND PRINCE~ L. M., Kolloid-Z. 169, 170 (1960). 3. PRINCE, L. M., J. Colloid Interface Sci. 23, 165 (1967). 4. PRINCE, L, M., J. Colloid Interface Sci. 29, 2, 216 (1969). 5. ROSANO, H. L., PEISER, R. C., AND EYDT, A., Rev. Fran. Corps Gras 16, 4, 249 (1969). 6. ADAMSON,A. W., J. Colloid Interface Sci. 29, 2, 261 (1969). 7. WINSOR,P. A., Trans. Faraday Soc. 22, 376 (1948). 8. ibid. 46, 762 (1950). 9. SCtIULMAN,J. H., STOECHENIUS,W., AND PRINCE, L. M., ]. Phys. Chem. 63, 1677 (1959).
10. SCI~ULX*-AN,J. H., AND McRoBERTS, T. S., Trans. Faraday Soc. 42B, 165 (1946). 11. BoweoTT, J. E., AND SCmYLM~AN,J. H., Z-Electrochem. 59, 4, 283 (1955). 12. COOKE, C. E., AND SCHULMAN,J. H., "Proc. 2nd Scandinavian Syrup. Surface Activity," Stockholm, p. 231 (1965). 13. ROSANO, H. L., GER.BACIA, W., FEINSTEIN, M., AND SWAINE, J. Colloid Interface Sci. 36, 298 (1971). 14. JASPER.,J. J., ANDHOUSEMAN,B. L., J. Phys. Chem. 67, 1548 (1963). 15. MCBAIN, J. W., AND WOO, T., Proc. Roy. Soc. A163, 182 (1937). 16. KAMINSKI, A., AND McBAIN, J. W., Roy. Soc. A198, 447 (1949). 17. DAVIES, J. T., AND RIDEAL, E. K., "Interfacial Phenomena," p. 364. Academic Press, New York (1961). 18. GEI~BAClA,W., ANDROSANO,H. L., to be published. 19. SCHULMAN,J. H., AND MONTAGNE,J. B., Ann. N.Y. Acad. Sci. 92, 366, 2 (1961). 20. PRINCE, L. M., J. Soc. Cosmetic Chem. 21, 193 (1970). 21. GILLBERG, G., LEHTINEN, H., AND--FRIBEI~C,S., d. Colloid Interface Sd. 33, 40 (1970).
Journal of Colloid and Interface Science, Vol. 44. No. 2. August 1973