Dilatometric investigation of phase transitions and structures of anhydrous soaps

Abstract

The molar volume of a number of anhydrous soaps consisting of straight and branched carbon chains has been measured as a function of the temperature between 25 °C and the melting point of the soap concerned. In the homologous series of the straight-chain soaps investigated (sodium and lithium salts of fatty acids with chain lengths from 11 to 19 and 14 to 18 carbon atoms respectively) a significant “even — odd” alternation of the molar volume in the crystalline state and of the volume changes at all phase transitions up to about 250 °C was found. These results indicate the existence of different structures in the series of the even and odd numbered anhydrous sodium and lithium soaps, both in the crystalline and in the mesomorphic phases. It is assumed that the structural differences are due to different modes of hydrocarbon chain packing. This assumption is supported by an analysis of the magnitude of the transition dilatations, which indicates the presence of ordered chain structures in the mesomorphic soap phases (“ribbon”-structure). It is suggested that the hydrocarbon chains in these structures possess a lowdegree of order in lateral directions, but a high degree of order in the packing of the methyl end groups. Including results reported in the literature, it has been found that the magnitude of the molar melting dilatation in the series of alkali metal stearates strongly decreases with increasing size of the cation, and is significantly lower than that of other paraffinic compounds. It is concluded, that due to theCoulomb forces between the polar groups, the molecules in the melt of anhydrous soaps are more closely packed than in the melt of other long-chain compounds. This applies also to the branched-chain soaps investigated (sodium salts of 2-methyltetradecanoic and 2-methylhexadecanoic acid) in which the number and temperature of the phase transitions are considerably lower and the molar melting dilatation is slightly lower than in straightchain soaps having the same chain length.