Phase Diagrams and NMR Studies of Some Ternary Sodium Deoxycholate-Surfactant-Water Systems C. LA MESA, 1 A. KHAN, K. FONTELL, AND B. LINDMAN Chemical Center. University of Lund, Box 124, S-22100 Lund, Sweden Received May 2, 1984; accepted July 18, 1984 The isotropic solution phase of sodium deoxycholate and water may dissolve large amounts of another surfactant, anionic, cationic, or nonionic. If the other surfactant forms liquid crystalline phases, those may incorporate rather large amounts of the bile salt and, in addition, new liquid crystalline phases may be formed. In the present study the "guest" surfactant has been either sodium diethylhexylsulfosuceinate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, or Triton X-100. The phase diagrams for the ternary systems (pseudo- for the ionic surfactants) have been determined and the structures of the various phases have been studied by polarizing microscopy, NMR, and low-angle X-ray diffraction. © 1985 Academic Press, Inc. INTRODUCTION The bile acid salts belong to the association colloids but their association behavior is not as clear-cut as for ordinary surfactants, the association of which becomes pronounced above a critical concentration, a CMC. How- ever, aqueous solutions of the bile acid salts may solubilize lipids which normally are insoluble in water (1-3). Aqueous solutions of bile acid salts have been subject to extensive study due to the relations between their physico-chemical properties and in vivo pro- cesses (4). Systematic studies have shown that binary aqueous bile salt systems do not form liquid crystalline phases (3). On the other hand it has been observed that the liquid crystalline phases of many surfactant systems take up bile acid salts and, in some cases, the for- mation of new liquid crystalline phases has been observed (5-8). Experimental studies of some ternary sys- tems of bile acid salt/lipid/water have sug- gested a connection between the metabolic processes and the phase transitions occurring 1Permanent address: Ist. Chimica-Fisica,Fac. Scienze Mat. Fis. Nat., University of Rome, Rome, Italy. Journal of Colloid and Interface Science, Vol. 103, No. 2, February 1985 between isotropic solution phases, micellar or reversed micellar, and liquid crystalline phases (3). Due to the shape of the bile acid molecule there are geometrical restrictions for the aggregates of binary bile acid salt/ water systems and only a limited number of locations are possible when, for instance, a straight-chain surfactant is added, A particular location of a "guest molecule" in an aggregate or on its surface may lead to special conclu- sions as regards the solubilization mechanism (3) and the interpretation of the interaction between such aggregates and membranes may be affected (9). Several different basic molecular arrange- ments have been suggested for the bile salt aggregates in binary and ternary systems: (i) the bile salt molecules aggregate first into hydrogen-bonded dimers which in turn form somewhat larger aggregates (10). This implies that one has abandoned the distinct separa- tion in polar and nonpolar regions which is the driving force for micelle formation of ordinary surfactants. (ii) the bile salt mole- cules aggregate back-to-back forming small primary aggregates which, in turn, may form secondary agglomerates when the concentra- tion is increased (5, 6). When "guest mole- 373 0021-9797/85 $3.00 Copyright © 1985 by Academic Press, Inc. All fights of reproduction in any form reserved.