A New Mechanism for Nucleation beneath Monolayer Films?

Abstract

The nucleation of the amino acids, aspartic acid and asparagine monohydrate, beneath monolayer films has been investigated as a function of film material and surface pressure. For the first time, nucleation has been shown to occur preferentially beneath films at low to medium surface pressures and not at high surface pressures as previously found. At high surface pressures, where the film was close-packed, the nucleation rate was low. In contrast, the nucleation rate was optimum under films at low to medium surface pressures. Since the close-packed films were unable to induce significant nucleation promotion, it is highly unlikely that this optimum nucleation at lower surface pressures occurs beneath close-packed film islands, but rather it is induced by the inherent nature of the films at lower surface pressures. We believe these novel results arise from both the substantial adsorption of the amino acid zwitterions between the film molecules and the exploitation of the greater compressional freedom of films at lower surface pressures, which enables greater lattice mismatches between the film and nucleating crystal face to be accommodated. Based on these findings, a new mechanism for nucleation beneath monolayer films is proposed. By using molecular modeling it was possible to demonstrate the existence of an electrostatic and geometric correlation between the film and nucleating crystal face in all cases. Therefore nucleation beneath the monolayers was governed by both strong adsorption upon the film and a correspondence between the structure and geometry of the film and nucleating crystal face, the latter correspondence being facilitated by the greater compressibility of the lower surface pressure films.