The influence of urea on the kinetics of interfacial transfer

The phase transfer kinetics of methyl nicotinate (2-pyridine carboxylic acid, methyl ester) across a water-isopropyl myristate interface have been determined in the presence and absence of a large concentration of urea dissolved in the aqueous phase. Using a rotating diffusion cell, rate constants have been measured at four temperatures covering the range 20-37°C; the temperature dependence of the kinetics has enabled the thermodynamic parameters for interfacial transfer to be derived. Although urea appeared to have little effect on the rate of solute transfer organic → aqueous at the concentration used (6M), it was found to reduce the rate constant in the opposite direction by a factor of two. The free energy barrier (ΔG^{{Plimsoll sign}}_#,±) to transfer at the interface was relatively insensitive to the presence of urea; however, the favorable entropic contribution to (ΔG^{{Plimsoll sign}}_#,±) was significantly reduced when urea was introduced into the system. Interpretation of the results is proposed in which both the solute solvation behavior and the relative ordering of the interface with and without urea present are taken into consideration.