Structure of organic ionic melt mesophases

Abstract

At their first melting points T f a number of organic salts pass into mesophases which show ‘liquid crystal' behaviour. Structural studies are described on the mesophases of sodium w-butyrate (mesophase range 252-324 °C) and sodium isovalerate (mesophase range 188- 280 °C). X-ray findings indicate that much of the detailed molecular ordering in the crystalline states of these salts is lost at T f except for the dominant ‘long spacings' between successive layers of cations. These are probably packed adjacent to charged carboxyl groups. ‘Long spacings' persist until the second melting point T cl is reached, when optically isotropic melts are formed. Although closely similar in some ways, this pair of salts exhibits interesting differences of thermal behaviour in the mesophases, with respect to the temperature dependence of their long spacings, and with respect to certain optical properties. These characteristics are tentatively interpreted in terms of differences of molecular packing of the alkane ‘tails’ of the anions. Evidence has also been obtained of preferred molecular orientation effects in layers of melt adjacent to walls of Lindemann glass, used to contain the mesophases for X-ray studies. In these layers ordering of the long-spacings persists for a few degrees above the clearing temperatures T cl attributed to the mesophase in bulk.