2 research outputs found
Combined Molecular Dynamics Simulation–Molecular-Thermodynamic Theory Framework for Predicting Surface Tensions
A molecular
modeling approach is presented with a focus on quantitative
predictions of the surface tension of aqueous surfactant solutions.
The approach combines classical Molecular Dynamics (MD) simulations
with a molecular-thermodynamic theory (MTT) [Y. J. Nikas, S. Puvvada, D. Blankschtein, Langmuir 1992, 8, 2680]. The MD component is used to calculate thermodynamic
and molecular parameters that are needed in the MTT model to determine
the surface tension isotherm. The MD/MTT approach provides the important
link between the surfactant bulk concentration, the experimental control
parameter, and the surfactant surface concentration, the MD control
parameter. We demonstrate the capability of the MD/MTT modeling approach
on nonionic alkyl polyethylene glycol surfactants at the air–water
interface and observe reasonable agreement of the predicted surface
tensions and the experimental surface tension data over a wide range
of surfactant concentrations below the critical micelle concentration.
Our modeling approach can be extended to ionic surfactants and their
mixtures with both ionic and nonionic surfactants at liquid–liquid
interfaces
Discrete Fractional Component Monte Carlo Simulation Study of Dilute Nonionic Surfactants at the Air–Water Interface
We
present a newly developed Monte Carlo scheme to predict bulk
surfactant concentrations and surface tensions at the air–water
interface for various surfactant interfacial coverages. Since the
concentration regimes of these systems of interest are typically very
dilute (≪10<sup>–5</sup> mol. frac.), Monte Carlo simulations
with the use of insertion/deletion moves can provide the ability to
overcome finite system size limitations that often prohibit the use
of modern molecular simulation techniques. In performing these simulations,
we use the discrete fractional component Monte Carlo (DFCMC) method
in the Gibbs ensemble framework, which allows us to separate the bulk
and air–water interface into two separate boxes and efficiently
swap tetraethylene glycol surfactants C<sub>10</sub>E<sub>4</sub> between
boxes. Combining this move with preferential translations, volume
biased insertions, and Wang–Landau biasing vastly enhances
sampling and helps overcome the classical “insertion problem”,
often encountered in non-lattice Monte Carlo simulations. We demonstrate
that this methodology is both consistent with the original molecular
thermodynamic theory (MTT) of Blankschtein and co-workers, as well
as their recently modified theory (MD/MTT), which incorporates the
results of surfactant infinite dilution transfer free energies and
surface tension calculations obtained from molecular dynamics simulations