Size and shape dependence of hydrophobic hydration at the level of primitive models

Abstract

The hydrophobic hydration of apolar solutes of different shape and size is studied at an elementary level using two types of extended primitive models of water and representing the solute by hard spheres, cylinders, and spherocylinders, respectively. The structure of the first hydration shell around cylindrical particles is determined and compared to that around spherical ones and at a hard structureless flat wall. It is found that while the two studied models of water give the same hydration structure for small non-polar particles, larger solutes are hydrated in a different way. Whereas one model does not show significant size and shape dependence, the other predicts significant changes in the orientation and hydrogen bonding of water molecules in the vicinity of the hydrophobic surface. These results are in agreement with those found for spherical solutes and confirm sensitivity of hydration phenomena to details of models of water, particularly to the strength and geometry of hydrogen bonding.