MOLECULAR STRUCTURE AND ORGANIZATION IN ORGANIC MONOLAYERS AT AQUEOUS/VAPOR INTERFACES

Abstract

The goal of this thesis is to understand how the asymmetry of interfaces affects the structure of adsorbed surfactants and organization within the monolayer. These studies employ a variety of experimental techniques including surface tensiometry and vibrational sum frequency spectroscopy, a nonlinear optical method having surface specificity. The first studies in this thesis examine the ability of different neutral organic surfactants to form films at the aqueous/vapor interface. Specifically, structure and organization within monolayers formed by insoluble and soluble alcohols at aqueous/vapor interfaces were investigated. Relatively simple organic molecules were used to isolate both intermolecular interactions within adsorbed films and the competition between attractive and repulsive forces experienced between monolayer monomers and the aqueous subphase. Results of the experiments allowed us predict that linear alcohols form tightly packed monolayers at the aqueous/vapor interface. This organization allows the alcohol OH group to make strong H-bonds with the water subphase while the hydrocarbon chains interact with each other through attractive van der Waals forces. Our studies showed that the interplay between the van der Waals attraction and the hydrophobic repulsion is the primary factor in determining the equilibrium interfacial structures of 2- and 3-position alcohols. The primary conformer structures predicted for 2-position alcohols include all-trans conformations for insoluble monolayers and a model containing two gauche defects for soluble monolayers. In an effort to model these results we initiated a series of classical molecular dynamics simulations designed to develop molecular insights into the equilibrium structures inferred from experiments. Computer simulations were also used to separate and compare the individual forces contributing to film organization.

Our studies in the last part of the thesis focus on the effect of charged soluble surfactants on the structure and organization of phospholipid monolayers adsorbed to aqueous/air interfaces. The self driven spreading of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) on aqueous surfaces to form monolayers was a matter interest in these experiments. The effect of surfactants as a potentially competing surface active species was explored with a function of surfactant bulk phase concentration. The results showed significantly different effects depending on whether the surfactant was anionic or cationic.