Polysaccharide-surfactant interaction .1. Adsorption of cationic surfactants at the cellulose-water interface

The extent of adsorption (Gamma(2)(1)) of cetyltrimethylammonium bromide (CTAB), myristyltrimethylammonium bromide (MTAB), and dodecyltrimethylammonium bromide (DTAB) from aqueous solution onto a cellulose-water interface has been measured analytically in a wide range of surfactant concentrations below and above the critical micelle concentration (cmc) at different physicochemical conditions and in the presence of different electrolytes and urea. Gamma(2)(1) is found to increase with increase of bulk surfactant concentration C-2 until it reaches a maximum value Gamma(2)(m) when C-2 reaches a critical value, C-2(m). With further increase of C-2 beyond C-2(m), Gamma(2)(1) decreases from Gamma(2)(m) and becomes zero with attainment of surface azeotropic state at a surfactant concentration C-2(azeo). For C-2 > C-2(azeo), values of Gamma(2)(1) are negative due to the excess hydration of cellulose fibril and desorption of surfactant micelles from the surface to the bulk phase. The value of Gamma(2)(m) depends upon the different physicochemical conditions and presence of different electrolytes and urea. Values of C-2(m) lie considerably below the cmc in most cases. Gamma(2)(m) decreases with decrease of hydrocarbon chain length of surfactant molecules, and in the case of DTAB all values of Ttl are negative. The results also predict involvement of hydrophobic interaction in the adsorption process. The standard free energy change Delta G degrees for the transfer of surfactant molecules to 1 kg of cellulose at the state of surface saturation has been calculated using an integrated form of the Gibbs adsorption equation. The values of Delta G degrees follow the same order as those of Gamma(2)(m) The average slope of the linear plot of Delta G degrees vs Gamma(2)(m) is equal to -34.3 +/- 0.1 kJ/mol. This corresponds to the standard free energy change (Delta G(B) degrees) for the transfer of 1 mol of surfactant from the bulk solution to the cellulose surface when bulk. mole fraction of surfactant is altered from zero to unity. The values of Delta G(hi)degrees for different systems at high surfactant concentration (> C-2(azeo)) have been also calculated using a linear extrapolation method, and they are found to be positive in all cases due to excess positive hydration of cellulose.