Molecular weight distribution of nonionic surfactants: II. Partition coefficients of normal distribution and homogeneous p, t-octylphenoxyethoxy-ethanols (OPE's)

Partition coefficients (K_D = C_H2O/C_isooctane) below the critical micelle concentration have been determined for a homogeneous and normal (Poisson) distribution series of p, t-octylphenoxyethoxyethanols (OPE_1–10) in the system, isooctane-water, at 25°C. As the ethylene oxide (EO) chain length increases from 1 to 10, K_D increases by four orders of magnitude. The calculated standard free energy change (ΔG_EO⁰) of transport of 1 mole of ethylene oxide from the isooctane to the aqueous phase is −0.60 kcal. mole⁻¹ for single species OPE's. Assuming a Poisson distribution of molecular weights and weak interactions between OPE molecules of varying EO chain length, one can calculate to a good approximation the partition coefficients of normal distribution OPE's from the partition coefficients of single species OPE's. Similarly, one can calculate the molecular weight distribution and average molecular weight of a given OPE in both the isooctane and aqueous phases as a function of the phase volume ratio of isooctane and water. An order of magnitude estimate of the partition coefficients of normal distribution and single species OPE's can be made from appropriate c.m.c. data. A 35°C. temperature increase decreases the partition coefficient of single species OPE₉ by an order of magnitude. The corresponding standard enthalpy (ΔH°) and standard entropy (ΔS°) changes for the transfer of 1 mole of OPE₉ from the isooctane phase to the aqueous phase are −12.9 kcal. mole⁻¹ and −43 e.u., respectively. The partition coefficient of single species OPE₉ increases with increasing concentration at concentrations in excess of the onset of micellization.