surfactant-induced crystallisation of DI-aspartic acid

Abstract

Crystallisation of dl-aspartic acid beneath monolayers at the air-water interface was studied using seven different monolayer films namely stearic acid, methyl stearate, o-octadceyl carbonyl-l-tyrosine methyl ester hydrochloride, polycaprolactone, poly-1-isoleucine, poly-y-benzyl-l-glutamate, and nylon 6 6. These films were chosen to study the effect of small molecule and polymeric surfactants on the nucleation of dl-aspartic acid. The crystallisation was studied using optical microscopy and external reflection FTIR. The dl-aspartic acid crystal morphology was analysed using molecular modelling. It was found that the polymeric monolayers were more likely to nucleate dl-aspartic acid than the small molecule ones. In particular, nylon 6 6 and polycaprolactone were the best monolayer films at nucleating dl-aspartic acid. The crystals grown beneath these two films were typically grown in groups or lines, the nucleation density was larger and the crystals were also larger than for the other films. The and crystal faces were the main crystal faces that grew beneath all the films. External reflection FTIR was shown to be a good method of studying crystallisation beneath films, because peaks due to the film and dl-aspartic acid crystals can be seen. FTIR evidence of film reorganization during dl-aspartic acid crystallisation was found for the stearic acid, methyl stearate and nylon 6 6 films (spread from phenol-toluene). The FTIR data suggested that the nylon 6 6 and l-tyrosine monolayer films nucleated dl-aspartic acid crystals at earlier times compared with the other films.