Structure, Stability and Crystal Growth of Polymorphs and Polytypes of Long-Chain Aliphatic Compounds

Abstract

This article reviews recent studies on crystal structure, thermodynamic stability and crystal growth of polymorphs and polytypes of long-chain aliphatic compounds. Particular attention is paid to the n-fatty acids, since numerous studies have recently been made on these compounds. Details of their molecular structures are described, based on X-ray crystallographic and vibrational spectroscopic data. Using stearic acid, an outstanding model substance, the thermodynamic stability of the different polymorphic as well as polytypic modifications is studied, by precisely measuring the solubility data of each modification (using single crystals). Calculations of molecular interactions combined with vibrational spectroscopic data are also used to evaluate the thermodynamic stability. A few examples of solid-state transformations, where polymorphic and polytypic conversions occur at the same time, are examined by electron microscopic and vibrational spectroscopic methods. The characteristics of nucleation probability and crystal growth rate of different polymorphs and polytypes from various organic solutions are subsequently described. It is found that, depending on the polymorph, the effects of temperature, supersaturation and solvents appear in a different manner. Finally thin film growth by vacuum deposition is studied by electron microscopic observation and analyzed with respect to the importance of the molecular orientation and the underlying vapor growth mechanisms.