Partition coefficients for a pH-degradable 1 3-dioxolane alkyl ethoxylate surfactant 4 CH3O (CH2CH2O)(5) (6)-CH2 22 (CH2)(12)CH3 2 (CH2) CH3 1 3 dioxolane or cyclic ketal surfactant CK-2 13-E-5 (6) (ave) between isooctane- and water rich phases of 2- and 3-phase microemulsion systems (K-n) were determined as functions of the ethoxylate size n and temperature for the neat surfactant and its binary surfactant mixtures to understand the partitioning of alkyl ethoxylates possessing a broad distribution of ethoxylate size and to determine conditions required for formation of 3-phase microemulsion systems at an optimal temperature where phase separation occurs rapidly important for protein purification via proteins selective partitioning to the middle phase driven by affinity to the second surfactant of the binary mixture A semi-empirical thermodynamic mathematical model described the partitioning data well provided optimal temperature values consistent with phase diagrams and theory and demonstrated that the tail region of CK-2 13-E-5 (6) (ave) is more polar than the hydrophobes of fatty alcohol ethoxylates The addition of Aerosol-OT (AOT) removed the temperature sensitivity of CK-2 13 E-5 S-6 ave partitioning producing 3-phase microemulsion systems between 20 degrees C and 40 degrees C Analysis of the bottom phases of the 2- and 3 phase microemulsion systems formed by CK-2 13-E-5 6 ave via small-angle neutron scattering demonstrated the presence of spherical monodisperse oil-in-water microemulsions (C) 2010 Elsevier Inc All rights reserved