5126
J . Phys. Chem. 1984,88, 5 126-5 129
Principles of Attainlng Very Large Solubilization (Microemulsion): Inclusive Understanding of the Solubilization of Oil and Water in Aqueous and Hydrocarbon Media K6z6 Shinoda,* Hironobu Kunieda, Tsutomu Arai, and Hiroyuki Saijo Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Yokohama 240, Japan (Received: February 13, 1984: In Final Form: May 1 , 1984)
It is intrinsically important to change the hydrophile-lipophile balance (HLB) of a surfactant mixture continuously by various devices in order to attain a large solubilization or ultimately complete mixing of hydrocarbon and water with less surfactant. The maximum solubilization of hydrocarbon (or water) was observed close to the surfactant phase separation at which the hydrophile-lipophile property of a surfactant mixture balances for a given system. The relative solubilities of water and oil in the surfactant phase change with the HLB of a surfactant mixture. When the hydrophile-lipophileproperty of a surfactant is balanced, three phases, Le., water, surfactant, and oil, coexist at low surfactant concentration. When it is hydrophilic the solubility of water in the surfactant phase is infinite and an aqueous solubilized micellar solution + oil system is obtained. When it is lipophilic the solubility of oil in the surfactant phase is infinite and a reversed micellar solution of hydrocarbon + water system is obtained. These conceptual interpretations on the solubilization of oil and water in surfactant aggregates were embodied on the phase diagrams presented. The devices of cosurfactants, surfactants, and their combinations yielded very large solubilization. The hydrophile-lipophile balance (HLB) of a surfactant is certainly a function of variables such as a surfactant composition, temperature, valence of the counterions, salt concentration, etc. It is argued that the HLB of a surfactant in the system and the HLB number of the surfactant should not be confused.
Introduction In order to attain a large solubilization (microemulsion), the selection of an optimum hydrophilic chain length of or an optimum t e m p e r a t ~ r e is ~ , the ~ most important factor in nonionic surfactant solutions, because the HLBS of a nonionic surfactant changes with the hydrophilic chain length and temperature. The solubilization of hydrocarbon as well as water was maximum when the hydrophile-lipophile property of the surfactant balances. It is evident that solubilization should be studied as a function of the HLB of the surfactant or the surfactant mixture in ionic surfactants as well. In order to change the HLB of ionic surfactants, the change of the valency of counterion~,69~ the addition of salt^,^^^ the introduction of side chains, e.g., dialkyl type surfactant~,’“’~ and the mixing with lipophilic c o ~ u r f a c t a n t ~are *’~ effective. If the HLB of an ionic surfactant is nearly balanced, such as in Aerosol OT, the solution property of the ionic surfactant undergoes changes from water soluble to oil soluble with the change of salt con~entration,~ temperature, and the type of oil,l5 etc. However, ionic surfactants obtainable in clean form are usually too hydrophilic that the HLB of such surfactants do not change continuously from water-soluble to oil-soluble by mixing with ordinary cosurfactant, such as R,OH, or by the addition of salts. In the present investigation, oil-soluble surfactants which are also reasonably lipophobic and water-soluble surfactants which are reasonably less hydrophilic were combined to change the HLB (1) Shinoda, K.; Ogawa, T. J. Colloid Interface Sci. 1967, 24, 56. (2) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 24, 10. (3) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1968, 26, 70. (4) Friberg, S.; Lapczynska, I. Prog. Colloid Polym.Sci. 1975, 56, 16. ( 5 ) Becher, P. “Emulsions: Theory and Practice”, 2nd ed.;Reinhold: New York; 1966; p 232. (6) Shinoda, K.; Hirai, T. J. Phys. Chem.1977, 81, 1842. (7) Shinoda, K.; Hanrin, M.; Kunieda, H.; Saito, H. Colloids Surf.1981, 2. 301. (8) Shinoda, K. Pure Appl. Chem. 1980,52, 1195. (9) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (IO) Sagitani, H.; Suzuki, T.; Nagai, M.; Shinoda, K. J. Colloid Interface Sci.1982, 87, 1 1 . (11) Shinoda, K.; Sagitani, H. J. Phys. Chem. 1983, 87, 2018. (12) Graciaa, A,; Barakat, Y.; El-Emary, M.; Fortney, L.; Schechter, R.; Yiv, S.; Wade, W. H. J. Colloid Interface Sci.1982, 89, 209. (13) Barakat, Y.; Fortney, L. N.; Schechter, R. S.; Wade, W. H.; Yiv, S. H.; Graciaa, A. J. Colloid Interface Sci. 1983, 92, 561. (14) Shinoda, K.; Kunieda, H.; Obi, N.; Friberg, S. E. J. Colloid Interface Sci. 1981, 80, 304. (15) Eicke, H. F. J. Colloid Interface Sci. 1979, 68, 440.
0022-3654/84/2088-5126$01.50/0
of a surfactant mixture continuously from hydrophilic to lipophilic, Le., from water-soluble to oil-soluble, to attain a large solubilization. Experimental Secton Materials. R120CH2CH2S04Calj2 is the same material that was used in previous s t u d i e ~ .R12S04Na ~ and R16S04Nawere synthesized at Kao Soap Co. from the corresponding extrapure alkanols. R12S04Nawas purified by extraction of unreacted dcdecanol with ethyl ether and recrystallized from H20-C2HSOH (1:l by weight). Pentaethylene glycol monododecyl ether, diethylene glycol monooctyl ether, and glycerol mono(2-ethylhexyl) ether were synthesized and purified at Nikko Chemicals Co. Decane and tetradecane were Tokyo Kasei Co. extrapure materials. Liquid paraffin was Exxon’s Crystol 70. NaCl was extrapure grade. All water used was twice distilled. Procedure. Varying amounts of surfactant, cosurfactant, water, and oil were sealed in ampules. A series of ampules were well shaken for 24-72 h in a thermostat. Longer chain hydrocarbons such as liquid paraffin needed longer shaking time (72 h) to reach equilibrium. Then, the ampules were stored for 12 h and the solubilization limit was determined by observing the presence or absence of excess oil droplets concentrated at the meniscus of the solution. Since the coalescence rate of the emulsion droplets is usually very fast in a three-phase region in which water, surfactant, and oil phases coexist, the boundary between three- and two-phase regions was relatively easily determined after agitation. Results and Discussion Continuous Change in the HLB of the Surfactant with Variables. Before the discussion of the results, it seems necessary to clarify the definition of HLB and HLB number to avoid confusion. Becher said in his authoritative book on the s ~ b j e c t“An , ~ HLB number is the number assigned to each surfactant. The HLB stand for hydrophile-lipophile balance.” Due to the definition of Griffin,16 it is apparent that an HLB number is not a function of variables such as temperature, pressure, salt concentration in solution, etc. On the other hand, as Clayton” has drawn attention to the concept of a balanced emulsifying agent embodied in a series of patents dating back to 1933,5the hydrophile-lipophile balance (HLB) of a surfactant is a balance of emulsifier (surfactant) (16) Griffin, W. C. J. SOC.Cosmet. Chem. 1949, 1, 31 1 ; 1954, 5, 249. (17) Clayton, W. “Theory of Emulsions”, 4th ed.; Blakiston Co.: New York, 1943; p 127.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 5127
Principles of Attaining Very Large Solubilization
/I
e 2
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-
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.
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Figure 1. Phase diagram of H20-C14H30containing a 5 wt % system of C12HzsO(CH2CH20)5H as a function of temperature. The abscissa is the weight fraction of C14H30 in solvents. D means surfactant phase, and L.C. means liquid crystalline phase.
H20
CIOHZZ
CIOHZZ conceived by many scientists. The HLB of a surfactant in a system weight froction of H20 t CIOHZZ certainly changes with the composition of the s ~ r f a c t a n t , ~ - ' ~ Figure 2. Phase diagram of H20/C12H2sOCH2CH2S04Cal~2/glycerol t e m p e r a t ~ r e , the ~ . ~type of oils,18 the types and amounts of admono(2-ethylhexyl) ether/CloHz2a t 25 O C . The total surfactant conditions in water,19 etc. HLB and HLB number should be used centration is 5 wt %. The abscissa and ordinate are the weight fractions carefully. of solvents and surfactants, respectively, in Figures 2-5. Efficient surfactant molecules including cosurfactants have to be composed of strongly hydrophobic (lipophilic) and lipophobic consisting of water, surfactant, and oil phases, is observed at (hydrophilic) groups. Because the insolubility of a surfactant in intermediate temperatures (HLB temperature) between two both water and oil is important to depress the saturation concritical solution temperatures at which the hydrophile-lipophile centrations of singly dispersed species in water and hydrocarbon, property of surfactant just balances. Solubilized micellar solutions most of the surfactant in the solution is in an aggregated state W, and 0, are one phase, because oil and water dissolve comregardless of the fact that the aggregation number is finite pletely in the water and oil phases in these regions, respectively. (micelle) or infinite (surfactant phase or liquid crystal). Although The solubilization is at a maximum just before the separation of the solubility of the monodispersed surfactant in solvents should the water (or oil) phase from the surfactant phase occurs. The be. small, the solubility of solvent (oil and water) in the surfactant solubility of oil (or water) in the solution, Le., in the surfactant phase has to be large. In the extreme case, the solubility of water phase, increases further and finally it becomes infinite, Le., critical (or hydrocarbon) in the surfactant phase is infinite, Le., water mixing occurs with the temperature elevation (or depression). It (or hydrocarbon) and surfactant are mutually soluble, this coris apparent that micellar dispersion, surfactant phase separation, responds to the formation of a micellar solution of the surfactant and reversed micellar dispersion occur due to the competitive in water (or hydrocarbon).20 Critical solution of surfactant and dissolution of water and oil in the surfactant phase with change solvent, in other words, infinite solubility of the solvent in a of temperature, i.e., a change of the HLB in the surfactant. Hence, surfactant, is a necessry condition for micellar dispersion, Le., solubilization in aqueous and hydrocarbon media should be studied soluble surfactants.2@22 When the hydrophile-lipophile property inclusively.22 A similar phase diagram is observed not only as of a surfactant is just balanced, however, the solubilities of water a function of temperature3 but also by varying the size of hyand oil in the surfactant are both finite.22 Hence, a three-phase drophilic the concentration of added salt,Ig the hydrosystem consisting of water, surfactant, and oil phases is obtained carbon chain length of the oils,'I2 etc. Thus, the HLB of a nonionic in dilute solution, but the water and/or oil phases disappear and surfactant changes continuously from hydrophilic (water soluble) a one-phase (surfactant) or two-phase system is obtained in more to lipophilic (hydrocarbon soluble) via surfactant phase separation concentrated solution. with these variables. These conditions are clearly shown in the phase diagram of a A temperature change usually does not affect the HLB of an water-nonionic surfactant-hydrocarbon system as a function of ionic surfactant appreciably. The HLB of an ionic surfactant is temperature in Figure 1.23 Water dissolves in a nonionic suralso a function of (1) the composition of mixture with other factant phase infinitely at lower temperature via liquid crystalline surfactants, (2) the number of hydrocarbon chains of the surphases of different water content and an aqueous micellar solution, factant, (3) the concentration of added salts, and (4) the types W,, is obtained. On the other hand, the hydrocarbon will dissolve of counterions, but not much with the chain length of hydrocarbons infinitely at higher temperature and a reversed micellar solution, (solvents) or temperature.8 Om,is obtained.23 Such behavior is understandable because the Phase diagrams of (1) H20/R120CHzCH2S04Cal,2/inonionic surfactant is hydrophilic at lower temperatures and R80CH2CH(OH)CH20H/C,oH22, (2) 3 wt. % NaCl aquelipophilic at higher temperatures. Below the surfactant-oil solOU~/R,~S~~N~/R~O(CH~C (3)H3~wtO % ) NaCl ~H/C~~H~~, ubility curve in figure 1, the surfactant and oil phases separate aqueous/RlzS04Na/i-R80CHzCH(OH)CH20H/C10H22, and (4) and above the water-surfactant solubility curve water and the 1.0 wt % NaCl aqueous/Rl6SO4Na/i-R80CH2CH(OH)surfactant phases separate. Therefore the three-phase region, 111, CH20H/liquid paraffin (Exxon's Crystol70) at 25 OC are shown in Figures 2-5. (18) Shinoda, K.; Arai, H. J . Phys. Chem. 1964, 68, 3485. In these diagrams, the HLB of the ionic surfactants is changed (19) Shinoda, K.; Takeda, H. J. Colloid Interface Sci. 1970, 32, 642. by varying the composition of the hydrophilic and lipophilic (20) Shinoda, K. J . Colloid Interface Sci. 1970, 34, 278.
(21) Shinoda, K. J . Phys. Chem. 1981,85, 3311. (22) Shinoda, K. Prog. Colloid Polym. Sci. 1983, 68, 1.
(23) Kunieda, H.; Shinoda, K. J . Dispersion Sci. Technol. 1982, 233, 3.
(24) Shinoda, K.; Kunieda, H. J . Colloid Interface Sci. 1973, 42, 381.
5128 The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 concentrotion o f surfactants 3
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surfactants. A successive change from water soluble to oil soluble via surfactant phase separation occurs with a change in composition of the surfactants. The solubilization of water or oil is very large at the compositions for which the hydrophile-lipophile property of a surfactant mixture balances. Hence, it is evident that the conclusions described above in nonionics are equally applicable to mixtures of ionic surfactants and cosurfactants as well, provided the HLBs of two surfactants are reasonably close. A system containing the calcium salt of an ionic surfactant is shown in Figure 2.6 In this system the solubilization is much larger compared to that of sodium dodecyl sulfate and it is possible to obtain one-phase regions at any solvent composition at 5 wt % of total surfactant per system. Between the one-phase regions
a q u e ~ u s / C ~ ~ H ~ ~ S O ~ N a / g lmono(2-ethylhexyl) ycerol ether/CloHz2 (solubilized solutions A and B in Figure 4). The total surfactant concentration is 5 wt %.
W, (aqueous micellar solution) and 0, (nonaqueous micellar solution), a lamellar liquid crystalline phase was observed. Addition of salt was unnecessary in this system in order to change the HLB of the ionic surfactant from hydrophilic to lipophilic. When the HLB of an ionic surfactant is strongly hydrophilic, such as RI2SO4Na,addition of salt was necessary to suppress the hydrophilic property of the ionic surfactant to change the HLB of the surfactant mixture continuously. If salt is not added, the solubilization of oil in aqueous micellar solution was very small as indicated by the 0% NaCl curve on the left-hand side of Figures 3 and 4. Namely, if a too lipophilic cosurfactant or a too hydrophilic ionic surfactant without salt was used, the solution behavior of the ionic surfactant would have behaved differently from a nonionic surfactant and the two cases could not be treated uniformly. Due to the use of a sophisticated cosurfactant in the presence of salt, the solubilization of oil (and water) increased remarkably near the three-phase region compared with that of ionic surfactant alone. The solution behavior of the surfactant mixtures continuously changed from water soluble to oil soluble. In Figure 5, sodium hexadecyl sulfate alone does not dissolve forming micelles at 25 OC because the Krafft point of the surfactaqt fs higher. Due to the mixing with the cosurfactant, glycerol
J . Phys. Chem. 1984, 88, 5129-5132 I
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total surfactant swtx/system Figure 7. The effect of temperature on the composition of the cosur-
factnat in the one-phase region, W, (solution C in Figure 5), in the system 1 wt % NaCl aqueous/Ci6H33S04Na/glycerolmono(2-ethylhexyl) etherkquid paraffin. The total surfactant concentration is 5 wt %.
mono(2-ethylhexyl) ether, the Krafft point of C16Hj3S04Nais depressed and the surfactant dissolves well at 25 “C. The solubilizing power of the system is very large, because 16 wt 75 of liquid ’ per system of the surfactant. paraffin was solubilized in 5 wt % According to our measurements 27 wt % of hexadecane is solubilized in the same surfactant solution. The value is nearly equal
5129
to that of decane in RI2SO4Na(Figure 4). Since the solubilization decreases with the hydrocarbon chain length of the solubilizate, we confirm that the use of a longer chain surfactant is effective in enhancing the solubilizing power. Effect of Temperature on the HLB of an Ionic Surfactant and Cosurfactant Mixture, i.e.,on the Composition of a One-phase Region. The phase diagram was very sensitive to a temperature change in a nonionic surfactant solution as shown in Figure 1, which means that the HLB of a nonionic surfactant changes sensitively with temperature. It is highly desirable, however, to get solubilized solutions which are independent of temperature change. In order to test the effect of temperature on the HLB of ionic surfactant cosurfactant, the temperature dependence of the surfactant composition in solubilized solutions indicated by A, B, and C in Figures 4 and 5 were studied. The results are plotted in Figures 6 and 7. It is evident from Figures 6 and 7 that the compositions of one-phase regions, Le., aqueous micellar region, W,, and nonaqueous micellar region, Om,in Figures 4 and 5, are both scarcely dependent on the temperature change. This is a remarkable fact for many practical purposes, because the solubilizing power is so much enhanced in these systems and stable to temperature changes.
+
Registry No. R,2iOCHiCH2S041/2Ca, 41343-91-3; RL2S04Na,15121-3; R$04Na, 1120-01-0;C12H25(OCH2CH2)50H, 3055-95-6; RsO(CH2CH,0)H, 19327-37-8; i-R,OCH,CH(OH)CH,OH, 70445-33-9; C10H22, 124-18-5;Ci4H30, 629-59-4.
Thermodynamic Analysis of the Potentiometric Titration with Separation of Phases Hiroshi Maeda Department of Chemistry, Faculty of Science, Nagoya University, Nagoya 464, Japan (Received: December 19, 1983; In Final Form: April 18, 1984)
The effect of phase separation on the potentiometric titration is theoretically examined. The fundamental equation is derived from the Gibbs-Duhem relation. The same equation has been derived on the basis of multiple equilibria [A. Shatkay and I. Michaeli, J . Phys. Chem., 70, 3777 (1966)]. Two procedures are presented to evaluate the free energy of dissociation of uncharged polymers: a procedure in terms of the degrees of ionization of the two phases in equilibrium and another in terms of the degree of ionization averaged over the whole system.
Introduction A procedure to evaluate the free energy difference of uncharged polymers between two discrete conformations from potentiometric titrations has been de~elopedl-~ and successfully applied to the conformational change of poly(methacry1ic and the helix-coil transition of polypeptide^.'-^,^-'^ When the procedure (1) Wada, A. Mol. Phys. 1960, 3, 409. (2) Zimm, B. H.; Rice, S. A. Mol. Phys. 1960, 3, 391. (3) Hermans, Jr., J.; Scheraga, H. A. J. Am. Chem. SOC.1961,83, 3283. (4) Nagasawa, M.; Holtzer, A. J . Am. Chem. SOC.1964, 86, 538. (5) Leyte, J. C.; Mandel, M. J . Polym. Sci., Part A-2 1964, 1879. (6) Mandel, M.; Leyte, J. C.; Stadhouder, M. G. J . Phys. Chem. 1967,71,
603.
(7) Michaeli, I. J. Polym. Sci., Part C 1968, 4169. (8) Crescenzi, V.; Quadrifoglio, F.; Delben, F. J. Polym. Sci. Part A-2 1972, 10, 357. (9) Hermans, Jr., J. J . Phys. Chem. 1966, 70, 510. (10) Hermans, Jr., J. J . Am. Chem. SOC.1966, 88, 2418. (11) McDiarmid, R.; Doty, P. J . Phys. Chem. 1966, 79, 2620.
0022-3654/84/2088-5 129$01.50/0
is applied to the evaluation of the stability of the p structure, another important secondary structure frequently found in proteins, the resulting free energy difference cannot be assigned to a well-defined reaction, because aggregation and/or precipitation of polypeptides usually occur as the p structure is f ~ r m e d . l ~ - ’ ~ The effect of aggregation on the procedure was examined thermodynamically.I6 Thermodynamic characterizations of potentiometric titrations with separation of phases have been d e ~ e l o p e d . ’ ~ However, -~~ neither the above-mentioned procedure nor the free energy change (12) Olander, D. S.; Holtzer, A. J. Am. Chem. SOC.1968, 90, 4549. (13) Senior, M.; Gorrell, S.; Hamori, E. Biopolymers 1971, 10, 2387. (14) Pederson, D.; Gabriel, D.; Hermans, Jr., J. Biopolymers 1971, 10,
2133.
(15) Maeda, H.; Ikeda, S . Biopolymers 1975, 14, 1623. (16) Maeda, H. J . Phys. Chem. 1975, 79, 1680. (17) Linderstrerm-Lang, K. Arch. Biochem. 1946, 11, 191. (18) Snell, F. M.; Neilsen, E. B. J . Phys. Chem. 1961, 65, 2015. (19) Shatkay, A.; Michaeli, I. J . Phys. Chem. 1966, 70, 3777. (20) Michaeli, I. J . Phys. Chem. 1967, 71, 3384.
0 1984 American Chemical Society
