Thermodynamics of naphthalene sorption to organoclays: Role of surfactant packing density

Temperature dependence of naphthalene sorption to four organoclays with different surfactant (CTMA(+)) packing densities was examined. The results showed that both Delta H degrees and Delta S degrees increase generally with CTMA(+) packing density. For organoclays with a low CTMA(+) packing density, the sorption process is driven by both the enthalpy term (Delta H degrees) and the entropy term (-T Delta S degrees), with values ranging from -4.7 to -7.5 kJ mol(-1) and -15.9 to -20.8 kJ mol(-1), respectively. As the CTMA+ packing density increases, the sorption process is driven by the entropy term (from -29.2 to -65.0 kJ mol(-1)) while it is opposed by the enthalpy term (from 7.9 to 40.5 kJ mol(-1)). These results indicate that the enthalpy demand for cavity formation within the surfactant aggregates and the mixing entropy of solute with surfactant aggregates both increase with the surfactant packing density. This means that the surfactant aggregates will form various organic phases as their packing density varies. Controlling the surfactant aggregates within an intermediate packing density range can improve the sorption capacities of the organoclays. (C) 2008 Elsevier Inc. All rights reserved.