The effect of polyethylene glycol 400 (PEG) dissolved at various concentrations (0-40% v/v) in water, on the interfacial transport of methyl nicotinate across an aqueous-isopropyl myristate interface was investigated with a rotating diffusion cell. At four temperatures studied (20-37 degrees C), the presence of PEG decreased the rate of solute transfer both into and out of the organic phase in a concentration-dependent fashion. The bulk partition coefficient of the solute (organic/aqueous) increased with increasing PEG in the aqueous phase. Analysis of the temperature dependence of the interfacial transfer kinetics allowed thermodynamic activation energy parameters for the phase transport process to be determined. Although the free energy of activation (Delta G(not equal)) for transfer was not affected by PEG, the relative enthalpic and entropic contributions were dramatically altered. At PEG concentrations of 10-40% v/v the enthalpic portion of Delta G(not equal) was decreased by about a factor of two, while the entropic contribution (which is large and positively favorable in the absence of PEG) was reduced considerable such that it was totally eliminated at higher PEG levels. These observations suggest novel and direct experimental evidence for the concept that high PEG concentrations substantially alter water structure at an aqueous solution-organic liquid biomembrane model interface. The results support the hypothesis that the critically important function of PEG in inducing cell-cell and liposome-liposome fusion is to remove the hydration layer that impedes the close apposition of converging phospholipid bilayers.