pubs.acs.org/Langmuir Competitive Solubilization of Phenol by Cationic Surfactant Micelles in the Range of Low Additive and Surfactant Concentrations Radhouane Chaghi, Louis-Charles de Menorval, Clarence Charnay, Ga :: elle Derrien, and Jerzy Zajac* Institut Charles Gerhardt, Equipe Agr egats, Interfaces et Mat eriaux pour l’Energie, CNRS UMR 5253, Universit e Montpellier 2, C.C. 1502, Place Eug ene Bataillon, 34095 Montpellier cedex 5, France Received October 16, 2008. Revised Manuscript Received December 27, 2008 Competitive interactions of phenol (PhOH) with micellar aggregates of hexadecyltrimethylammonium bromide (HTAB) against 1-butanol (BuOH) in aqueous solutions at surfactant concentrations close to the critical micelle concentration (CMC), BuOH concentration of 0.5 mmol kg -1 , and phenol contents of 1, 5, or 10 mmol kg -1 have been investigated at 303 K by means of 1 H NMR spectroscopy, titration calorimetry, and solution conductimetry. The solubilization loci for phenol were deduced from the composition-dependence of the 1 H chemical shifts assigned to various protons in the surfactant and additive units. Since in pure HTAB solutions phenol is already in competition with Br - , addition of 1 mmol kg -1 NaBr to the system weakens the phenol competitiveness. The presence of butanol in the HTAB micelles causes phenol to penetrate deeper toward the hydrophobic micelle core. For higher phenol contents, the butanol molecules are constrained to remain in the bulk solution and are progressively replaced within the HTAB micelles by the aromatic units. The competitive character of phenol solubilization against butanol is well supported by changes in the thermodynamic parameters of HTAB micellization in the presence of both of the additives. Introduction The dissolution of linear or cyclic alcohols into water by the action of surfactant micelles has attracted considerable atten- tion and interest over several decades (refs 1-15 and references therein). When added in low or moderate amounts, alcohol mole- cules can distribute between the micelles and the surrounding aqueous phase in a way that is dependent mainly on their hydro- phobic-hydrophilic character. They appear to inhabit mostly the outer portions of the surfactant micelles (i.e., the polar mantle, micelle-solution interface). Moderate and long-chain alcohols are commonly thought to pack within mixed micelles oriented in the same manner as the surfactant units, with the polar group being located in the hydrated headgroup domain and the hydro- phobic tail pointing toward the nonhydrated inner core. Never- theless, the depth of additive penetration into the micelle structure may be subject to change in function of the overall alcohol content in the system and the degree of packing of the surfactant units in the host aggregate. Based on modeling studies of the system octanol/sodium octanoate/water, Aamodt et al. claimed that the first incorporated octanol molecule per micelle was located in the inner core due to the large decrease in surface area per molecule in the micelle. 16 According to recent investigations made on the solubilization of phenol by cationic micelles of hexadecyltri- methylammonium bromide (HTAB) by means of titration calori- metry and 1 H NMR, 15 the aromatic units were located pre- ferentially in the headgroup region of cationic micelles by an enthalpy-driven solubilization mechanism, but some additional molecules were simultaneously forced to penetrate deeper toward the hydrophobic micelle core. It should be emphasized that this “solubilization” behavior, observed for low HTAB concentra- tions in the vicinity of the critical micelle concentration (CMC) and additive contents much below their critical phase-separation concentrations, was at variance with the solely “interfacial” phenol location within the cationic micelles of HTAB formed in a concentrated surfactant solution (50 mM HTAB and 0.01 M NaBr). 13 Even such a subtle factor as solvent isotope effect was shown to alter the solubilization mechanism for both cyclic and linear alcohol additives, 15,17 which called for much caution when interpreting the results coming from different measurements carried out on apparently the same systems. The list of potential uses of micellar solubilization, eagerly cited in numerous papers, includes advanced drug delivery systems, microreactors for a variety of chemical, biocatalytic and enzy- matic reactions, or templating units in the preparation of nanosize *Corresponding author. 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