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J. Phys. Chem. B 2007, 111, 8402-8410
Phase Behavior of Aqueous Polyion-Surfactant Ion Complex Salts: Effects of Polyion Charge Density† Jens Norrman,‡ Iseult Lynch,§ and Lennart Piculell*,‡ DiVision of Physical Chemistry 1 Centre for Chemistry and Chemical Engineering, Lund UniVersity, P.O. Box 124, SE-221 00 Lund, Sweden, and Irish Centre for Colloid Science and Biomaterials, School of Chemistry and Chemical Biology, UniVersity College Dublin, Belfield, Dublin 4, Ireland ReceiVed: NoVember 6, 2006; In Final Form: January 24, 2007
The effect of varying the fraction of charged monomer units of the polyion in aqueous polyion-oppositely charged surfactant complex salts has been investigated. The complex salts used were based on cetyltrimethylammonium (C16TA+) with three different polymeric counterions: poly(acrylate) (PA-) or poly(acrylate) copolymerized with either dimethylacrylamide (PA-/DAM) or N-isopropylamide (PA-/NIPAM). The charge density of the polyion was varied by either adding poly(acrylic) acid (PAA) to the C16TAPA complex salt (annealed charges) or by varying the fraction of uncharged units in the C16TAPA/DAM or C16TAPA/NIPAM complex salts (quenched charges). The formed phases were studied visually between crossed polarizers and by small angle X-ray scattering (SAXS). Both types of complex salts (annealed and quenched) formed hexagonal phases at high fractions of charged monomers and low water contents. Upon increasing the water content, a cubic phase of the Pm3n space group was found. Upon further addition of water, a miscibility gap with the cubic phase in equilibrium with pure water was found. Decreasing the fraction of charged monomers in the annealed complex salt resulted in an increase of the curvature of the surfactant aggregates. Only at very low (50%) content and low NIPAM ( 20%), there was a bimodal distribution of molecular weights. This bimodal distribution was most likely caused by the attractive interaction between AA and NIPAM/DAM.13,14 In all of these copolymers, there was a primary peak at around 50-100 kg/mol and a secondary peak at higher molecular weight. This peak at higher molecular weight most probably represents aggregated copolymers and not individual polymers with higher molecular weight. We therefore conclude that the molecular weight of the individual polymers is of the order of 50-100 kg/mol. This molecular weight is in line with those expected from using a mixed solvent system with ethanol as the chain transfer agent (as compared with molecular weights on the order of 106 in benzene alone). The Complex Salts. The complex salts were prepared by titrating the hydroxide forms of the surfactant with the polyacid (homopolymer) or by titrating the polyacid with the hydroxide form of the surfactant (copolymers). The first procedure is the same as that used in our previous work,3 and the second procedure is derived from this procedure. The complex salts based on the quenched copolymers will be named C16TAPA/Z, where Z is the DAM or NIPAM monomer. The first step of the synthesis was to convert C16TABr into C16TAOH by ion exchange.3 The ion-exchange resin (Dowex SBR, dry mesh 20-50, from Sigma) was charged by stirring in an excess amount of 1 M NaOH for 2 h and then rinsed with water until the rinsing water reached pH 7. C16TABr (10 g) was then dissolved in a plastic beaker containing a large excess (100 g) of the charged ion-exchange resin and 100-200 mL of water. The solution was stirred until all of the surfactant was dissolved. The slurry was filtered, and the filtrate and additional rinsing water were added to a fresh batch of 100 g resin and 100 mL water, which was stirred for another 2 h. The last step was repeated once. The alkaline solution now contained C16TAOH at a concentration of approximately 0.05 M. Titrimetric analysis of this solution gave a bromide content below the detection limit of the analysis used (
