Amphiphile self-aggregation: An attempt to reconcile the agreement-disagreement between the enthalpies of micellization determined by the van't Hoff and Calorimetry methods

In this article, discrepancies between the enthalpies of micellization of amphiphiles in aqueous solution determined by the methods of van't Hoff (VH) and calorimetry have been addressed. The contributions of the hydrophobic interaction, electrostatic interaction and the micellar size effect have been considered to assess the total picture of the amphiphile self-association process and related energetic parameters, especially the enthalpy and the specific heat capacity. Literature results on 23 amphiphile systems (six non-ionics, five anionics, and twelve cationics) have been analyzed, and the assessed enthalpies by VH method and direct calorimetry have been presented and compared. VH results considering participation of 5% of total amphiphile monomer to form micelle at cmc have been also compared. In addition to this, the changes in the standard specific heat of micellization for all the amphiphile aggregation processes evaluated by the VH and calorimetry procedures have been presented. The differences between the standard enthalpy of micellization (Delta H-m(o),) by the methods of VH and calorimetry are minor for nonionic surfactants but major for ionics, whereas the standard specific heat capacities of micellization (Delta C-pm(o)) by both the procedures fairly agree for all types of surfactants. Like Delta H-m(o) - Delta S-m(o) compensation observed in kinetic and equilibrium processes, a linear Correlation between Lt(T -> 0)Delta H-m(o) and Delta C-pm(o) has been observed with no distinction between the VH and calorimetry derived results for all the surfactant systems herein dealt with. (C) 2009 Elsevier Inc. All rights reserved.