Polysaccharide−Surfactant Interaction. 1. Adsorption of Cationic Surfactants at the Cellulose−Water Interface

Abstract

The extent of adsorption (Γ21) 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. Γ21 is found to increase with increase of bulk surfactant concentration C2 until it reaches a maximum value Γ2m when C2 reaches a critical value, C2m. With further increase of C2 beyond C2m, Γ21 decreases from Γ2m and becomes zero with attainment of surface azeotropic state at a surfactant concentration C2azeo. For C2 > C2azeo, values of Γ21 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 Γ2m depends upon the different physicochemical conditions and presence of different electrolytes and urea. Values of C2m lie considerably below the cmc in most cases. Γ2m decreases with decrease of hydrocarbon chain length of surfactant molecules, and in the case of DTAB all values of Γ21 are negative. The results also predict involvement of hydrophobic interaction in the adsorption process. The standard free energy change ΔG° 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 ΔG° follow the same order as those of Γ2m. The average slope of the linear plot of ΔG° vs Γ2m is equal to −34.3 ± 0.1 kJ/mol. This corresponds to the standard free energy change (ΔGB°) 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 ΔGhi° for different systems at high surfactant concentration (>C2azeo) 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.