Langmuir 1996,21, 2979-2984
2979
Binding Constants of Symmetric or Antisymmetric Electrolytes and Aggregation Numbers of Oil-in-Water Type Microemulsions with a Nonionic Surfactant Keiichi Yoshihara,? Hiroyuki Ohshima,*J Nobuyuki Momozawa,ll Hideki Sakai,? and Masahiko Abe*rtl§ Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278,Japan, Faculty of Pharmaceutical Sciences, Science University of Tokyo, 12, Ichigaya-Funakawaracho, Shinjuku, Tokyo 162,Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3,Kagurazaka, Shinjuku-ku, Tokyo 162,Japan, and Faculty of Industrial Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278,Japan Received October 19, 1994. I n Final Form: May 15, 1995@ The effects of electrolytes (symmetric and antisymmetric) on the phase behavior, the apparent cloud point, the g-potential, the particle size, and the surface charge density of an oil-in-water type (Omtype) microemulsion forming in solutions of a nonionic surfactant (tetraethylene glycol monodecyl ether, CloPOE4)ln-decanelbrinehave been examined. Additions of NaSCN andor Ca(SCN)2result in increasing the temperature where the Om-type microemulsions formed, while additions of NaC1, NaN03, CaC12, andor NH2CONH2 decrease the temperature. When the concentrations of NaSCN andlor Ca(SCN)2 increase, the apparent cloud points of the ON-type microemulsion increase, while there is a decreasein the particle sizes of the microemulsion droplets. On the other hand, as the concentrations of NaCl, NaN03, CaC12, or NHzCONHz increase, the apparent cloud point of the microemulsion decreases slightly, but their particle sizes increase. The electrophoretic mobilities of the microemulsion droplets can be measured only in the systems containing NaSCN or Ca(SCN12. Moreover, the 5-potential and the surface charge density show negative values at a given salinity, though CloPOE4 does not possess a charged component in a molecule. Applying formulas described in this article, the adsorption of anion (SCN-) to the hydrophilic moieties of CloPOE4microemulsions is shown to be greater than those of cations (Na+or Ca2+). "he binding constants ( K ) of anion and cation to the hydrophilic groups of CloPOE4 microemulsions are found to be K N =~0 M-l, Kca = 0.2 M-l, 10.0M-l < KSCN< 12.0 M-' and the adsorption density of CloPOE4 molecules on the oillwater interface (N) is found to be N = 0.3 nm-2; the surfactant (CloPOE4)numbers per ON-type microemulsion droplet decrease (a few thousands to a few hundreds) with the increasing concentration of NaSCN andor Ca(SCN12.
Introduction Microemulsions are generally defined as clear thermodynamically stable disper~ionsl-~ of two immiscible liquids containing an appropriate amount of surfactant (andlor surfactant with cosurfactants).* They differ markedly from both macro- and miniemulsions in this respect, since these two types depend upon intense agitation for their f ~ r m a t i o n . The ~ dispersed phase consists of small droplets with diameters in the range of 10-100 nme4 The applications of microemulsion systems are of industrial and practical importance for enhanced oil recoveryg and for preparation of ultramicroparticles. In this regard, many scientists have studied the phase
* Author to whom correspondence may be addressed at Faculty of Science a n d Technology, Science University of Tokyo. + Faculty of Science a n d Technology. Faculty of Pharmaceutical Sciences. Institute of Colloid a n d Interface Science. Faculty of Industrial Science and Technology. Abstract published in Advance A C S Abstracts, J u l y 15, 1995. (1)In Solvent Properties of Surfactant Solutions; Shinoda, K, Ed.; Marcel Dekker: New York, 1967;p 4. (2)Danielsson, I.; Lindman, B. Colloids Surf. 1981,3, 230. (3)Friberg, S. E. Colloid Surf. 1982,4,20. (4)Rosen, M. J.Surfactant andlnterfaciul Phenomena, 2nd ed.; Wiley Interscience: New York, 1989. (5)Gillberg, G.; Lehtinen, H.; Freberg, S.E. J.Colloid Interface Sci. 1970,33, 40. (6)Ekwall, P.; Mandell, L.; Fontell, K. J.Colloid Interface Sci. 1970, 33,215. (7)Shinoda, K.;Kunieda, H. J. Colloid Interface Sci. 1973,42,382. (8)Kunieda, H.; Sato, T. Yukagaku 1979,28,627.
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behavior12and preparation of microemulsionsystems.13-16 However, few studies have been made on the surface state and aggregation numbers of microemulsions. In our previous paper,17 we have reported on the influence of pH on the [-potential of the microemulsion formed by the amphoteric surfactant (Na,Na-dimethylW-lauroyllysine)in the presence of NaC1. The amphoteric surfactant shows nonionic-surfactant-likebehavior in the isoelectric region (pH 5-7) where the net charge derived from the head groups of the surfactant becomes almost zero. However, the surface charge densities of the microemulsion become negative in the isoelectric region because of the preferential adsorption of anions compared to cations. Furthermore, the binding constants of inorganic electrolytes and the aggregation numbers of the oilin-water ( O N ) type microemulsions are able to be (9)Wilson, L.A., Jr.Improved Oil Recovery by Surfactant and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; Academic Press: New York, 1977. (10)Kandori, K.; Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1987,115,579. (11)Keyima, P.M.; Qutubuddin, S.J.Mater. Sci. Lett. 1989,8,171. (12)Shinoda, K.; Kunieda, H. J.Colloid Interface Sci. 1973,42,382. (13)Microemulsions; Prince, L. M., Ed.; Academic Press: New York, 1977. (14)Microemulsions; Robb, I. D., Ed.; Prenum Press: New York, 1982. (15)Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Marcel Dekker: New York, Basel, 1988. (16)Gillberg, G. In Emulsions and Emulsion Technology; Lissant, K. J., Ed.; Surfactant Science Series 6;Marcel Dekker: New York, 1984; Part 3,pp 1-43. (17)Abe, M.; Kuwabara, A.; Yoshihara, K.; Ogino, K.; Kim, M. J.; Kondo, T.;Ohshima, H. J . Phys. Chem. 1994,98,2991.
0743-746319512411-2979$09.00/00 1995 American Chemical Society
Yoshihara et' al.
2980 Langmuir, Vol. 11, No. 8, 1995 estimated from the (-potential values and particle sizes of the microemulsion droplets. In this paper, we report on the effect of symmetric and antisymmetric electrolytes on the Om-type microemulsions of the nonionic surfactantln-decanehaltsystem and the preferential adsorption of anions to hydrophilic moieties in the nonionic surfactant microemulsion. We also discuss the binding constants of electrolyte ions and the aggregation numbeFs of the ON-type microemulsions by (-potential measurement.
presence ofsymmetric(1-1) or antisymmetric (2- 1)electrolytes, respectively, NA is Avogadro's number, e is the elementary electric charge, COis the concentration of electrolytes, k is the Boltzmann constant, and T is the absolute temperature. According to the numericalvalues of KU,RKU)can be expressed by the equations.1s-20 Symbol x stands for K a .
Experimental Section Preparation of Microemulsion. Tetraethylene glycol monodecyl ether (CloPOE4)was supplied by Nikko Chemicals Co., Ltd., Tokyo, Japan. The gas chromatographic analysis revealed no peaks of other homologs. Pure n-decanepurchased from Tokyo Chemical Industries was reagent grade and was used without further purification. Sodium thiocyanate (NaSCN), calcium thiocyanate (Ca(SCN)Z),potassium iodide (KI), sodium chloride (NaCI),calcium chloride (CaClZ),sodium nitrate (NaN03), and urea (NHzCONH2) purchased from Wako Pure Chemical Industries were reagent grade and were used without further purification. Degassed doubly distilled water was used in making all solutions. The ON-type microemulsions were produced from 4 g of aqueous solutionsof 1.5 x M nonionicsurfactant and various amounts of oil and electrolytes in Teflon-cappedglassed tubes. Aseries ofthe glass tubes were well shaken and allowed to attain equilibrium in the thermostat for at least several hours.
where
Measurement of Apparent Cloud Points and Solubilization Limits. The apparent cloud points and solubilization
limits were determined by the naked eye. The concentration of M. After the addition ofknown amounts CloPOE4was 1.5 x of electrolytes, the solutions were slowly heated in thermostats. Near the cloud point and solubilization limits, the heating and cooling rates were 1 Wmin. The temperatures of the sudden onset of turbidity on heating and sudden clearing on cooling were measured in duplicate. Thesefourvalues werein agreement within 0.5 "C. Measurement of Electrophoretic Mobilities and Calculation of 5-Potentials. Electrophoretic mobilities (U)of the ON-type microemulsions were measured in aqueous media of
different ionic strengths with the System 3000 particle electrophoresis apparatus (PenKem, Inc., New York). This is equipped with a 5-mW He-Ne laser (A = 632.8 nm). The electrophoretic mobilities were measured at different temperatures, because the ON-type microemulsions with the added various electrolytes have different equilibrium states. The temperature was kept constant at 23 "C in the case of added NaSCN, Ca(SCN)2,and/or KI, and at 20 "C in the case of NaC1, CaC12, NaN03, and/or NH2CONH2. The calculation of
