Abstract
Atmospheric aerosols contain organic molecules that serve as cloud condensation nucleation sites and affect the climate. Several experimental and simulation studies have been dedicated to investigate their surface propensity, but the mechanisms that drive them to the water surface are still not fully understood. In this molecular dynamics (MD) simulation study, primary alcohols are considered as a model system representing polar organic molecules. We find that the surface affinity of n-alcohols increases linearly with the length of the hydrophobic tail. By decomposing the adsorption free energy into enthalpy and entropy contributions, we find that the transition from bulk to surface is entropically driven, compatible with the fact that the hydrophobic effect of small solutes is of entropic origin. The enthalpy of surface adsorption is nearly invariant among different n-alcohols because the loss of solvent-alcohol interactions is balanced by a gain in solvent-solvent interactions. Structural analysis shows that, at the surface, the linear alcohols prefer an orientation with the hydrophobic tail pointing out from the surface, whereas the hydroxyl group remains buried in the water. This general behaviour is likely transferable to other small molecules with similar structures but other functional groups that are present in the atmosphere. Therefore, the present study is a step forward toward a general description of organic molecules in aerosols.
Highlights
IntroductionAs a complement to experiments, molecular dynamics (MD) simulations have been used to study organic molecules at water interphases,[13,14,16,17,18,19,20,21] providing an atomistic interpretation of the experimental signals
The contributions from enthalpy and entropy to the potential of mean force (PMF) are analyzed, and the enthalpy is further decomposed into contributions from solute–solvent and solvent–solvent interactions
The PMF along a reaction coordinate corresponding to the center of mass (COM) distance between a single alcohol molecule and the water slab was calculated from umbrella sampling simulations (Fig. 2)
Summary
As a complement to experiments, molecular dynamics (MD) simulations have been used to study organic molecules at water interphases,[13,14,16,17,18,19,20,21] providing an atomistic interpretation of the experimental signals. Walz et al.[13] used MD simulations to reveal structural arrangements of surfaceenriched pentanol that could give rise to experimental XPS spectra. Combining experimental observations with simulations is a fruitful approach to overcome limitations of each of the methods alone
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