Polarization Microscopy of Short Chain Sodium Carboxylate Mesophases

Abstract

Polarization microscopy has been used to study the ionic mesophases of melts of short chain sodium alkyl carboxylates. The smectic A or neat structure is confirmed for all the sodium carboxylates studied: isovalerate, n-butyrate, n-valerate, n-hexanoate and n-heptanoate. The domain sizes and transition temperatures are reported and an empirical correlation between birefringence and the order parameter is observed. The birefringence is predicted to be negative for all the compounds studied. It is interesting to note that sodium isovalerate exhibits quite different properties from those found for the other n-alkyl carboxylates. Both the birefringence temperature coefficient and the average domain size (500 μm) are unusually large for sodium isovalerate.