The behaviour of water molecules in the presence of nonpolar solutes is one of the basic issues to be considered when trying to understand the mechanism of protein folding. It is commonly believed that hydrophobicity of nonpolar residues is related to the partial ordering of water molecules around them. Yet this behaviour is not completely understood even at the microscopic level, and rarely accounted for in simple models of protein folding, where the introduction of effective hydrophobic potentials is usually preferred to more accurate descriptions. In this work we introduce a model where the role of water can be considered explicitly. Our analytical results for the free energy, energy and entropy changes upon wetting a nonpolar monomer are close to those obtained in experiments and simulations. Resorting to a suitable approximation, we also solve the model in the case of a hydrophobic homopolymer, and find out that both a "cold" and "warm" swelling of the polymer take place, reminding  the behaviour of real proteins. The model can be extended to describe heteropolymers and proteins, and can be efficiently used in computer simulations.

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