The ζ-potential of polyacrylonitrile fiber Vonnel P was measured in aqueous acid solution (pH3) of anionic surface active agents-sodium dodecyl sulfate (SDS) and sodium tetradecyl sulfate (STS). The amounts adsorbed of surface active agents per unit area of the fiber surface were calculated from these ζ-potential values. With increasing the concentration of surface active agent, the sign of the ζ-potential value of this fiber changed from positive to negative and the amount adsorbed of surface active agent at the fiber surface increased because of the electrostatic attractive force and van der Waals' force between the fiber and those agents.
The free energy of adsorption for surface active agent
ΔG was calculated from the slope of the ζ-log
CS (
CS : the concentration of surface active agent) curve, etc.
ΔG decreased with rise of temperature and with increasing the chain length of surface active agent. The heat of adsorption
ΔH and the entropy of adsorption
ΔS for SDS were-6.8 kcal·mol
-1 and 5.3 cal·mol
-1·deg
-1, respectively. The corresponding parameters for STS were-6.1 kcal·mol
-1 and 8.7 cal. mol
-1·deg
-1, respectively. These thermodynamic data indicate the importance of entropic interaction (the formation of hydrophobic bond) as a driving force to adsorption for SDS and STS at the fiber surface.
From the occupied areas of surface active agent molecules on the fiber at the maximum adsorption of those agents, it was suggested that SDS and STS were coarsely adsorbed on the fiber.
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