Hydrophobicity " plays crucially important roles in a variety of phenomena in aqueous environments . It is widely believed that the hydrophobicity originates from the inability of nonpolar solutes to participate in hydrogen bonds of water . If this is true , the hydrophobicity should be strengthened when the hydrogen bonding is enhanced , for example , by lowering the temperature . However , there are experimental observations manifes 七 ing that the hydrophobicity is weakened at low

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... . Here we clarify the true physical origin of the hydrophobicity by investigating cold denaturation of a protein as an important example . The major conclusions are as follows : The hydrophobicity is caused primarily by the entropic excluded − volume effect ; and its characteristics are ascribed to the interplay of the exceptionally small molecular size and the strongly attractive interaction of water , and not necessarily to its hydrogen − bonding properties .