SOLVENT EXCLUSION EFFECT PREDICTED BY THE SCALED PARTICLE THEORY AS AN IMPORTANT FACTOR OF THE HYDROPHOBIC EFFECT

With the aim at extracting an important factor of the hydrophobic effect, we have analyzed the physical meaning of a basic formula for the free energy of cavity formation given by the scaled particle theory (SPT). It is shown that, when a solute is introduced into a solvent, the region around the solute with a thickness of solvent radius becomes inaccessible to solvent particles and, as a result of it, their configuration entropy decreases. As a prediction from SPT, it is concluded that the solvent exclusion effect is an important factor of the hydrophobic effect because this decrease in entropy is greatest for aqueous solvent composed of water molecules having the smallest size among the ordinary liquids in nature. By combining the above with the fact that changes in enthalpy and entropy of hydrating water around a cavity exactly compensate with each other, it is suggested that hydrating water is energetically similar to bulk water, and the term ''highly-structured'' is not appropriate to describe it.