Influence of glycerol on the formation of lyotropic mesophases —microscopic texture observations for determining preliminary phase diagrams of binary K-soap/glycerol systems

Abstract

Analogously to aqueous K-soap/water systems already examined, the glycerol-containing systems KC _n /G (KC _n ;n=12, 14, 16, 18, 22; G=glycerol) are also able to build up hexagonal, lamellar, optically isotropic, gel-like and crystalline phases. These preliminary phases have been identified by texture observations of contact samples and singular concentrations with a polarizing microscope. The appertaining phase regions have been plotted in the binary phase diagrams.

Correspondences and differences between these systems have been elucidared by drawing a comparison. Mosaic texture and “oily streaks” are typical of the lamellar phase. Spherulites are mainly found in the heterogeneous two-phase region lamellar/isotropic. The textures of the hexagonal phase are of fan-like morphology. The appearance of the gel phase texture resembles globular or curd-like structures.

The influences exerted by the increasing chain lengths of the K-soaps (KC _n ,n=12–22) on the phase regions in the binary systems (KC _n /G) can be described as follows. The concentrations required for forming the hexagonal and the lamellar phase respectively are shifted toward lower K-soap concentrations. The concentration range in which the hexagonal phase is stable is diminished. The temperature range in which the hexagonal phase is stable becomes larger. The upper temperature limit of the lamellar phase region is lowered.

Binary aqueous and glycerol-containing K-soap systems have the following common features: The hexagonal phase is built up at low soap concentrations. The lamellar phase is formed at high soap concentrations. The lamellar phase is formed at high soap concentrations. An optically isotropic region is inserved between the lamellar and the hexagonal phase in aqueous and glycerol-containing systems of the types KC₁₄, KC₁₆ and KC₁₈. The temperature of the transition hexagonal↔isotropic phase (H↔S) runs through a maximum value. On increasing the chain length the formation of the hexagonal phase is shifted in the direction of lower soap concentrations.

Aqueous and glycerol-containing K-soap mixtures differ in the following essential points: The lyotropic mesophases (H, L, I) of aqueous systems are formed at considerably lower soap concentrations than the corresponding phases of glycerol-containing systems. The lamellar phases of aqueous systems reach the regions of very low soap concentrations. The lyotropic mesophases of aqueous systems are built up at temperatures lower than the corresponding ones of glycerol-containing mixtures. In aqueous systems the concentration range of the lamellar phase increases with increasing chain length, in contrast to glycerol-containing systems where it is diminished.