The effect of molecular weight and degree of substitution on the interactions between carboxymethyl cellulose and cetyltrimethylammonium bromide

The behavior of mixtures of carboxymethyl cellulose (CMC) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated in the dilute range at the liquid-air interface by means of surface tension and ellipsometry, and in the bulk solution by conductivity and turbidity measurements. CMC behaves like a polyanion under the experimental conditions (pH ∼ 7.5). The effect of the CMC molecular weight and degree of substitution on the complexation with CTAB was studied. Keeping the CMC concentration constant at 1.0 g/L, we observed three critical concentrations as the CTAB concentration was increased. Only the CMC degree of substitution (DS) exerted influence on the critical concentration values, evidencing the cooperative binding and the electrostatic interaction as the main driving forces for the complex formation. The critical concentration values found at the liquid-air interface are in accordance with those determined in the bulk solution. The co-adsorption of CMC and CTAB was observed at the liquid-air interface, where the molecular packing became denser with increasing CTAB concentration.