STRUCTURE AND THERMOTROPIC PHASE-BEHAVIOR OF SODIUM AND POTASSIUM CARBOXYLATE IONOMERS

A molecular complex is formed between long-chain carboxylic acids and their alkali salts in a 1:1 mixture. These so-called ''acid soaps'' or carboxylate ionomers have multilamellar bilayer-type structures in so lid state, which are retained in the presence of excess water, resembl ing the dispersions (gels) formed by typical two-chain amphiphiles, e. g. lipids. The special arrangement of hydrogen-bonded pairs of carboxy lic acid and carboxylate groups into a unique ''head-group'' is suppor ted by frequency shifts and partial or total disappearance of the char acteristic vibrations of carboxylic acid dimers and of carboxylate gro ups. The existence of well-ordered hydrocarbon chains is demonstrated by the existence and polarization properties of the methylene rocking and wagging propagation modes. The gel to liquid-crystal phase transit ion of the hydrated acid soaps shows practically no cation dependence, unlike the corresponding phase transition in neutral soaps which vari es considerably with the nature of the counterion. There is spectrosco pic evidence to suggest a cooperative process that involves ''melting' ' of the alkyl chains and disintegration of the hydrogen-bonded carbox ylate-carboxylic acid complex, followed by a cation-dependent equilibr ium that favors the formation of acid dimers at elevated temperatures and some form of hydrogen-bonded ion pair aggregates at intermediate t emperatures.