J. Phys. Chem. 1981, 85,3311-3312
Acknowledgment. This paper is dedicated to Professor Joel Hildebrand in gratitude for his inspiration and friendship. We hope that in view of his long-standing interest in fluorocarbons he will find this work an appro-
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priate contribution to his centennial celebration. We thank Professor David M. Lemal for valuable advice and facilities for the synthesis of several compounds. This research was supported by NSF Grant No. GP 16232.
Criteria of Micellar Dissolution Kbro Shinoda Department of Applied Chemistty, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogayaku, Yokohama 240, Japan (Received: June I, 1981;In Final Form: July 9, 1981)
Conditions of micellar dissolution to soluble surfactantsare discussed. Simple and pictorial conditions for micellar dissolution are as follows: (1)the surfactant in the presence of a solvent has to be in the liquid state so that the solvent dissolves in the surfactant phase; (2) the solubility of solvent in the surfactant phase has to be infinite so that the surfactant disperses forming micelles, otherwise a two-phase solution is obtained; (3) the saturation concentration of surfactant in the solvent has to be small so that the micelles are thermodynamically stable above the saturation concentration of molecular dispersion. These criteria are equally valid for water-soluble polymers such as proteins.
Introduction I am very happy to participate in celebrating the 100th birthday of Professor Joel H. Hildebrand who is still very active as a distinguished scientist’ and never grows old. He wrote in Chem. Eng. News (March 12,1979), “I am so fortunate in being delightfully alive and experiencing the satisfaction of still discovering things that are taking their place in the beautiful structure of natural science.” He further wrote “43% of all my scientific publications have been published since my pseudo-retirement in 1952 (70 years old)”. We worked together in many occasions since 1956 and were coauthors of nine papers on the aspects of regular solution theorye2 A warm friendship has been maintained between us by frequent exchanges of letters, reprints, and visits. He has been a most influential teacher and educator to me in science and in life. Since regular solutions were well studied by Professor Hildebrand, I have directed most of my effort to studying less well explored systems, such as, surfactant solutions, solubilized solutions, microemulsions, emulsions and so on. the most striking feature of surfactants is their ability to dissolve due to the orientation, arrangement, and structure formation of molecules, molecules which otherwise are practically insoluble when randomly mixed. Hence, clarification of the conditions for micellar dissolution of a surfactant is important in the design of soluble surfactants. Why Is the Aggregation Number of Micelles Finite? In order to answer this question, we must known the shape of the liquid-liquid solubility curve of water-nonionic surfactant as a function of temperature, A nonionic surfactant dissolves in water forming micelles below the cloud point, but predominantly surfactant and water phases separate above the cloud p ~ i n t ~(solubility -~ curve). The cloud point curve is so flat and the critical composition (-1 wt %) is very close to the water axis that (1) J. H. Hildebrand, Annu. Reu. Phys. Chem., in press. (2) Foreword by J. H. Hildebrand in “Principles of Solution and Solubility”, Marcel Dekker, New York, 1978. (3) R. R. Balmbra, J. S. Clunie, J. M. Corkill, and J. F. Goodman, Trans. Faraday SOC.,58, 1661 (1962). (4) K. Shinoda, J. Colloid Interface Sci., 34, 278 (1970). (5) J. C. Lang and R. D. Morgan, J. Chem. Phys., 73, 5849 (1980).
we may state the following: If the solubility of water in the surfactant phase is infinite, the surfactant disperses forming micelles and the aggregation number is finite. If the solubility of water in the surfactant is finite, the aggregation number is effectively infinite (cf. Figure 1 in ref 4). This is a very simple and pictorial, but direct and important, principle to design soluble surfactants. Liquid State of the Surfactant Phase. Analyzing the solution behavior of ionic surfactant in water close to the Krafft point, I have given the physical meaning to the Krafft point that (1)the hydrated solid surfactant melts at the Krafft point and (2) the Krafft point is similar to a triple point having a surfactant coexist in three states: unassociated, micellar, and hydrated ~ o l i d .The ~ ~ satu~ ration concentration of a singly dispersed surfactant in water is so small due to the hydrocarbon tail that it is practically insoluble if the solid surfactant separates. Solubility at the Krafft point is about 0.2 wt % in the case of C12H25S0,Naas shown in Figure 1. If the temperature of the system is higher than the melting point of the hydrated (solid) surfactant, water dissolves into the surfactant phase. Since water is a small molecule it dissolves well in the hydrophilic portion of the surfactant phase. If an hydrophilic property of surfactant is strong, which is true in most ionic Surfactants, water will dissolve infinitely in the surfactant phase and finally pseudo-phase inversion occurs in the surfactant phase and the surfactant disperses in water as micelles. In order to dissolve with the formation of micelles, the surfactant has to be in a liquid state in the presence of solvent. Nonionic surfactants also may be insoluble below their Kraftt points. Actually, the nonionic surfactants C16H,30(CH2CH20)l~ and C18H370(CH2CH20)1J-I,whose Krafft points are 32.5 and 45.5 “C, respectively,8 dissolve forming micelles only in a temperature range between the Krafft point and the cloud point, about 61-62 oC.8 Infinite Aggregation of Ionic Surfactant, An ionic surfactant whose hydrophilic property is not strong enough (6) K. Shinoda, T. Nakagawa, B. Tamamushi, and T. Isemura, “Colloidal Surfactants”, Academic Press, New York, 1963, pp 6-8. (7) K. Shinoda and P. Becher, “Principles of Solution and Solubility”, Marcel Dekker, New York, 1978, pp 159-162. (8) H. Schott and S. K. Han, J. Pharm. Sci., 65, 979 (1976).
0022-3654/81/2085-3311$01.25/00 1981 American Chemical Society
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Shinoda
The Journal of Physical Chemlstty, Vol. 85, No. 22, 1981 I
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Concentration ef CaCh w t % in I o w t % aq c ~ ~ H ~ ~ o ( c H ~
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Figure 2. The effect of temperature and CaCI, concentration on the phase separation of micellar solution of 1 w i % aqueous CI2Hz5O(CH2CH2O),SO3.0.5Ca to two phases, liquid surfactant water phase. Due to the decrease of CaCI, in the system more water dissolves in the surfactant phase and finally a one-phase solution is obtalned.
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Temperature
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Figure 1. Phase diagram of the C,2H,5S0,Na-H20 system close to the Krafft point. The hydrated solid surfactant melts above the Krafft point and disperse in solution forming micelles. The micellar solution region (one phase) corresponds to a two-phase solution (solution excess solute phase) in an ordlnary system.
+
will not disperse as micelles, but a surfactant or liquid crystalline phase will separate above the saturation concentration, because the solubility of water in the surfactant is finiteeg Ionic surfactants in which the hydrophile-lipophile property is reasonably balanced, such as C12H25O(CH2CH20)2S03Cao.5, disperse forming micelles in water.1° However, an infinitely aggregated surfactant phase and water separate above a certain amount of added CaClz or temperature as shown in Figure 2. We may say that, if the solubility of water in a surfactant phase is finite, the surfactant aggregates infinitely. Only when the hydrophilic property of surfactant is strong enough, so that water dissolves infinitely, will the surfactant be soluble. This criterion is true regardless for both ionic and nonionic surfactants. The criteria for the dissolution of a hydrophilic polymer is the same.ll
Saturation Concentration of Singly Dispersed Species The other important and indispensable factor for micellar dispersion is the solubility of a surfactant in a sol(9)H.Kunieda and K. Shinoda, J. Phys. Chem., 82, 1710 (1978). (10)K. Shinoda and T. Hirai, J.Phys. Chem., 81,1842(1977). (11) K. Shinoda, Prog. Colloid Polym. Sci., 61,80 (1976).
vent. The solvent has to dissolve in the surfactant phase infinitely. However, the saturation concentration of a single dispersion of surfactant in the solvent has to be small, so that the surfactant aggregate is in equilibrium with the singly dispersed surfactant above the cmc, Le., the chemical potential of the micellar surfactant is equal to that of the singly dispersed surfactant. Otherwise a randomly mixed solution is formed. Micelle Formation in Nonpolar solvent^.'^.^^ In this respect the free energy energy decrease due to the aggregation of hydrocarbon chains (-640 cal/(mol/CH2)) is large enough to balance the free energy decrease due to dilution (or entropy of mixing) in aqueous s01ution.l~ In hydrocarbon media, however, the free energy decrease due to aggregation of a lyophobic group such as the oxyethylene chain is not large enough unless the size of the group is large. At the same time (1)the solvent has to dissolve in the surfactant infinitely, Le., the lyophilic fraction of the molecule has to be large, and (2) the melting point of the surfactant has to be low. These conditions make it difficult to prepare surfactants which form micelles in hydrocarbons. Water as a third substance will hydrate the hydrophilic group of the surfactants and enhance the aggregation tendency. From the liquid-liquid solubility curves of a hydrocarbon and nonionic surfactants without water, we estimate that the aggregation number is small, if aggregation occurs a t all15. In the presence of water, however, micelles of nonionic surfactant are formed and solubilization of water is observed.16 (12) H. F. Eicke and H. Christen, J. Colloid Interface Sci., 46, 417 (1974). (13) H. F. Eicke, Top. Curr. Chem., 87, 99 (1980). (14)K.Shinoda, J. Phys. Chem., 60, 1440 (1956). (15)K. Shinoda and H. Arai, J. Colloid Sci., 20, 93 (1965). (16)H.Saito and K . Shinoda, J. Colloid Interface Sci., 35,359(1971).
