Liquid crystallinity in surfactant systems

Abstract

The phase equilibria of surfactants in twoand three component systems were studied by water deuteron NMR, polarizing microscope and differential scanning calorimetric methods. The ternary phase diagrams of ionic surfactants of types C8 SO4, C12 SO4 with Ca 2 +, Mg 2 ÷ and Ba 2 ÷ counterions and of hexadecyltrimethylammoniumsulphate with water and decanol; and the binary phase diagrams of calcium, magnesium and barium di-2-ethylhexylsulphosuccinate with water, reveal that the homogeneous liquid crystalline and isotropic solution phases form in these surfactant systems have many characteristics which are different from those formed in the corresponding systems with monovalent counterions. The most important difference, however is, that the lamellar liquid crystals form, with divalent counterions, have limited swelling capability and these systems often lead to the formation of reverse hexagonal liquid crystal at high decanol and low water contents. The mixed ionic surfactant system sodium and calcium di-2-ethylhexylsulphosuccinate with water forms two lamellar liquid crystals; one is stable at high water contents, and the other at low water contents, and these two liquid crystals are found to coexist. A similar observation can be made for the ternary system calcium dodecylmonooxyethylenesulphate decanolwater. The following important observations may be made from the study of phase equilibria of uncharged surfactants hexadecyltrimethylammonium n-alkyl carboxylates (C16A+RA -, where R = 1,3,7,11,13 and 15 carbons) with water: (I) all surfactant systems form micellar solutions above their respective Krafft point; (2) when R = 1, the system forms cubic and hexagonal liquid crystals and for R = 2, only a hexagonal liquid crystal is obtained; (3) when R = 7, there appears to be a hexagonal liquid crystal occupying a very narrow zone immediately after the micellar solution region and a lamellar liquid crystal also occupied a narrow area at high surfactant concentration, the two-phase region (two liquid crystals) occupies a large region and (4) when R ~ 11, only the lamellar liquid crystall is found to exist. Some of the experimental findings observed for the ionic and uncharged surfactant systems may be explained qualitatively by existing theories.