Published: April 25, 2011 r2011 American Chemical Society 5868 dx.doi.org/10.1021/jp110247r | J. Phys. Chem. B 2011, 115, 5868–5876 ARTICLE pubs.acs.org/JPCB Physicochemical Properties of Methylcellulose and Dodecyltrimethylammonium Bromide in Aqueous Medium Marcos A. Villetti,* ,‡ Clara I. D. Bica, § Irene T. S. Garcia, § Fabiano V. Pereira, † Francieli I. Ziembowicz, ^ Carmen L. Kloster, ‡ and Cristiano Giacomelli ^ ‡ Departamento de Física, Centro de Ci ^ encias Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900 Santa Maria-RS, Brazil § Instituto de Química, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre-RS, Brazil † Departamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte-MG, Brazil ^ Departamento de Química, Centro de Ci ^ encias Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900 Santa Maria-RS, Brazil ’ INTRODUCTION Interactions involving nonionic polymers and ionic surfac- tants in aqueous solutions have generated considerable inter- est in the research community because their physicochemical properties are very important in many industrial applications (pharmaceutical formulations, cosmetics products, food ad- ditives, enhanced oil recovery, paints and coating products etc.). 1À11 The amphiphilic nature of surfactants grants them special properties to introduce interactions with water-soluble polymer, especially those with hydrophobic segments. 12 Methyl- cellulose is a hydrophobically modified nonionic water-soluble polysaccharide produced by the methylation of native cellulose. MC has been used, for example, in pharmaceutical industry to produce polymeric films that can act as protective layers for tablets and rate controlling barriers to drug release. 13 Cationic surfactants such as n-alkyltrimethylammonium bro- mide homologues (RTAB, with R equal to C 16 ,C 14 , and C 12 ) are important pharmaceutically because of their bactericidal activity against a wide range of gram-positive and some gram-negative organisms. 13 In addition, aqueous MC solutions exhibit an phe- nomenon of gelling upon heating in contrast to natural polymers such as gelatin, agarose or carrageenan that gel upon cooling. 14,15 Several factors, such as pH, presence of salts, type and concentra- tion of surfactants, and so forth, can alter the solÀgel transition of MC. It was observed that the surfactant molecules have a tendency to aggregate around such hydrophobic segments of the poly- mer chains in aqueous solution, inducing changes in the thermal behavior of cellulose derivatives during the gelation process. 12,16,17 It is established that polymerÀsurfactant interaction can be described by two critical concentrations in a system of fixed polymer concentration and increasing amounts of surfactant. The first represents the concentration at which surfactant molecules start to bind to the polymer chain and is known as critical aggregation concentration (CAC). Since CAC is, in general, lower than the CMC of the surfactant, this means that the polymerÀsurfactant complex is energetically more stable than surfactant molecules in regular micelles. The second critical concentration occurs when the sites on polymer are saturated with surfactant molecules and this point is called polymer saturation point (PSP). 18,19 These critical concentrations can be determined through different techniques such as electrical conductivity, 20À23 viscosity, 11 microcalorimetry, 24 tensiometry, 22,23,25,26 and steady-state fluorescence. 25,27À31 The interaction of cellulose ethers with cationic surfactants compared with anionic ones has been found to be considerably weak. 32,33 A few works describing the interaction of MC with hexadecyltrimethylammonium bromide (CTAB) surfactant with a longer alkyl chain, have been published. 18,34,35 Li et al. 35 recently reported the effect of surfactant type on solÀgel transition of MC as studied by micro-DSC and rheology. The authors found that both CTAB and sodium dodecyl sulfate (SDS) impart salt-out and salt-in Received: October 26, 2010 Revised: March 24, 2011 ABSTRACT: Interactions between uncharged polymers and cationic surfactants are considered weaker than interactions with the anionic analogues. This work describes the binding occurring between methylcellulose (MC) and the cationic surfactant DTAB in aqueous medium. In the absence of salt, MCÀDTAB exhibits a maximum in hydrodynamic radius, R h,slow , with the increase in the surfactant concentration. Otherwise, in presence of salt the MCÀDTAB system shows only a linear increase of R h,slow . CAC is lower than the CMC, which is taken as an evidence of binding between the cationic surfactant and neutral polymer that induces the aggregation process. Static light scattering, rheology and micro-DSC results highlight the hydrophobic MCÀDTAB association. Salt-out and the salt-in effects were observed in presence of DTAB, with a clear transition at concentration values close to the CMC, as judged from rheological and micro DSC measurements. Indeed, DTAB affects both the pattern of the solÀgel transition and the gel strength.