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

The extent of adsorption (Γ 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. Γ 2 1 is found to increase with increase of bulk surfactant concentration C 2 until it reaches a maximum value Γ 2 m when C 2 reaches a critical value, C 2 m . With further increase of C 2 beyond C 2 m, Γ 2 1 decreases from Γ 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 Γ 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 Γ 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. Γ 2 m decreases with decrease of hydrocarbon chain length of surfactant molecules, and in the case of DTAB all values of Γ 2 1 are negative. The results also predict involvement of hydrophobic interaction in the adsorption process. The standard free energy change Δ G ° 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 Δ G ° follow the same order as those of Γ 2 m . The average slope of the linear plot of Δ G ° vs Γ 2 m is equal to −34.3 ± 0.1 kJ/mol. This corresponds to the standard free energy change (Δ G B °) 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 Δ G hi ° 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.