(631b) 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.