Journal of Colloid and Interface Science 297 (2006) 797–804 www.elsevier.com/locate/jcis Interaction of cationic surfactants with carboxymethylcellulose in aqueous media Jitendra Mata a,∗ , Jaykumar Patel a , Nirmesh Jain b , Gautam Ghosh c , P. Bahadur a a Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, India b School of Chemistry, University of Sydney, Sydney, Australia c UGC-DAE CSR, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 10 August 2005; accepted 9 November 2005 Available online 27 December 2005 Abstract We have examined the polymer–surfactant interaction in mixed solutions of the cationic surfactants, i.e., dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetradecyltriphenylphos- phonium bromide, and tetradecylpyridinium bromide and a semiflexible anionic polyelectrolyte carboxymethylcellulose in water and aqueous salt solutions by various techniques: tensiometry, viscosimetry or ion-selective electrode method, and dynamic light scattering. We have investigated the effect of varying surfactant chain length, head group size, counterion, and ionic strength on the critical aggregation concentration (CAC) of mixed polymer surfactant systems and the collapse of the polymer molecule under different solution conditions. The CAC decreases with in- creasing alkyl chain length. Above a certain surfactant concentration, mixed aggregates start growing until their macroscopic phase separation. The growth is more rapid with greater surfactant tail length and with increasing head group size. This is attributed in both cases to the increasing hydrophobic interaction between polymer and surfactant. Among surfactants with monovalent halide counterions, iodide induces the strongest binding, reflected by the onset of growth of the mixed aggregates at low surfactant concentration. This is perhaps related to the decreasing hy- dration of the counterion from chloride to iodide. The surfactant concentration at which the viscosity of the solution starts to decrease sharply is smaller than the CAC, and probably reflects polymer chain shrinkage due to noncooperative binding.  2005 Elsevier Inc. All rights reserved. Keywords: Carboxymethylcellulose; Tensiometry; Noncooperative binding 1. Introduction The study of interactions between polymers and surfactants in aqueous solutions has attracted significant interest in recent years because of their widespread applications and relatively complex behavior. In particular, the association between poly- electrolytes and oppositely charged surfactants, both in bulk and at interfaces, has been extensively studied, and many re- cent reviews and books covering different aspects are available [1–5]. For these oppositely charged systems, the strong electrosta- tic attraction between the charged groups leads to bulk com- plexation above a surfactant concentration called the critical * Corresponding author. Fax: +91 261 2256012. E-mail address: jitendramata@yahoo.com (J. Mata). aggregation concentration (CAC), which can be several orders of magnitude below the critical micelle concentration (CMC) of the surfactant. This CAC is usually determined either by surface tension measurements (the tension remains almost con- stant with increasing surfactant concentration over a range of concentrations above CAC) or by surfactant-specific electrodes that are sensitive to the nonassociated surfactant molecules. It is generally admitted that surfactant micelles start to form much above CAC, decorating the polymer chains, at least when these chains are sufficiently flexible. Recent calorimetry [6] and elec- trical birefringence [7,8] experiments with cationic surfactants and anionic polymers have revealed that polymer/surfactant as- sociation already occurs below CAC. It has been proposed that this binding below CAC is noncooperative, and becomes more cooperative above CAC only. Chatterjee et al. [9] investigated the interaction of a cationic surfactant, CTAB, and an anionic 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.11.022