Molecular Modeling of Select Organic Molecules at the Air-Water Interfaces

A large sector of industrial chemicals are amphiphilic organic compounds. When dissolved in water, amphiphlies are attracted to the air-water, i.e. nonpolar-polar, interface. The hydrophilic or polar head group of the compound is attracted by hydrogen bonding to water while the hydrophobic or alkyl chain group is attracted to the nonpolar air.  In the concrete industry, amphiphilic compounds are used to reduce shrinkage cracking in portland cement concrete by reducing the surface tension of the pore water within the cement microstructure. The cement pore water contains sodium, potassium, calcium, sulfate and to a lesser extent other ions. Assembly and concentration of the amphiphilic compound at the air-water interface in the presence of various ions is critical to the understanding of compound effectiveness.  To elucidate the behavior of select compounds, a combined experimental and computational approach was used.  This study summarizes ongoing molecular dynamics simulations and experiments using three compounds: butyl glycolate, 2-(2-butyoxyethoxy) ethyl acetate, and hexylene glycol.  Molecular dynamics simulations were performed to simulate the interactions of the test compounds pure water and simulated pore solution.  Interactions between various groups were modeled using the COMPASS force field. In addition, surface tension experiments were also used to elucidate the process and reconcile model predictions and experimental observations.