Thermodynamics of Cationic Surfactant Sorption onto Clinoptilolite
E.J. Sullivan, J.W. Carey, and R.S. Bowman
ABSTRACT
Sorption enthalpies of hexadecyltrimethylammonium bromide (HDTMA) as monomers and micelles, and tetraethylammonium bromide (TEA) were used with surfactant, counterion, and co-ion sorption isotherms to infer the conformation, sorption mechanism, and relative stability of the sorbed surfactants on clinoptilolite. The average value of the sorption enthalpy was -10.375 kJ/mol for monomers, -11.98 kJ/mol for micelles, and +3.029 kJ/mol for TEA. Sorption of monomers produced a lower sorption plateau than equivalent micelle sorption (maxima 145 mmol/kg, 225 mmol/kg). Analysis of the sorption data demonstrated a change in sorption mechanism at the external cation exchange capacity (ECEC) of clinoptilolite. Sorption data from below and above the ECEC were fit to a simple polynomial model and the Gibbs free energy of sorption (Go) and sorption entropies were calculated. Resultant values of Go were -9.27 and -14.38 kJ/mol for HDTMA monomers and micelles, respectively, for sorption below the ECEC, and -16.11 and -23.10 kJ/mol, respectively, for sorption above the ECEC. The value for TEA was -1.041 kJ/mol, indicating weaker sorption than for HDTMA. Monomer sorption to clinoptilolite exceeded the ECEC even when the solution concentration was below the critical micelle concentration. Hydrophobic (tail-tail) components of Go were the driving force for sorption of HDTMA both below and above the ECEC. A significant kinetic effect was observed in the sorption isotherms with a period of rapid sorption followed by slow equilibration requiring 7 days to achieve steady state for HDTMA; TEA equilibration occurred within 24 hours.
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