Liquid Crystalline Structures Occurring in Aqueous Systems of a Totally Fluorinated Fatty Acid and Some of its Salts

Abstract

Liquid crystalline phases exist in binary fluoro-carbon surfactant/water systems above the T_c temperature of the surfactant. The internal structure is often lamellar but other structures may occur. The present report deals with X-ray diffraction studies at room temperature mainly of the lamellar phases existing in the binary system of heptadecafluorononanoic acid (C₈F₁₇COOH) and water, and in the corresponding systems of some of its salts, the counterion being sodium, lithium, cesium, ammonium, tetramethylammonium, dimethylammonium or diethylammonium. In the acid/water system and in the systems of the dimethyl and diethylammonium salts, the behaviour of the lamellar phase is “ideal”, that is, the thickness of the fluorocarbon (bi)layers and the areas per polar head group at the interfaces between polar and nonpolar regions are independent of concentration. The lamellar phases of systems where the counterion is ammonium or tetramethylammonium show variations in these parameters. A few individual measurements have been performed in the liquid crystalline phases of systems where the counterion was sodium, lithium or cesium.