  We study some aspects of hydrophobic interaction between molecular rough and
flexible model surfaces. The model we use in this work is based on a model we
used previously (Eun, C.; Berkowitz, M. L. J. Phys. Chem. B 2009, 113,
13222-13228), when we studied the interaction between model patches of lipid
membranes. Our original model consisted of two graphene plates with attached
polar headgroups; the plates were immersed in a water bath. The interaction
between such plates can be considered as an example of a hydrophilic
interaction. In the present work we modify our previous model by removing the
charge from the zwitterionic headgroups. As a result of this procedure, the
plate character changes; it becomes hydrophobic. By separating the total
interaction (or potential of mean force, PMF) between plates into the direct
and the water-mediated interactions we observe that the latter changes from
repulsive to attractive, clearly emphasizing the important role of water as a
medium. We also investigate the effect of roughness and flexibility of the
headgroups on the interaction between plates and observe that roughness
enhances the character of the hydrophobic interaction. The presence of a
dewetting transition in a confined space between charge-removed plates confirms
that the interaction between plates is strongly hydrophobic. In addition, we
notice that there is a shallow local minimum in the PMF in case of
charge-removed plates. We find that this minimum is associated with the
configurational changes that flexible headgroups undergo, as the two plates are
brought together.
