The influence of the alkyl chain length and the concentration of
exchangeable sodium cations at the
solid/water interface on the energetics of cationic surfactant
adsorption on a hydrophilic silica surface at
free pH has been studied. Adsorption isotherms, electrophoretic
mobilities of the silica particles, and
differential molar enthalpies of displacement for
benzyldimethyldodecylammonium bromide (BDDAB) on
the original silica sample (SilNa) at different ionic strengths (pure
deionized water and 0.1 M NaBr solution)
were compared, as well as those on a washed silica sample (SilH).
The stability of silica suspensions, as
characterized by the changes in turbidity, was analyzed with the
intention of shedding light on the possible
orientation of the adsorbed surfactant ions. Adsorption of BDDAB
was also compared with that of
benzyldimethyloctylammonium bromide (BDOAB) on SilNa. Individual
surfactant adsorption occurs by
ion pairing or cation exchange mechanisms, depending on the purity of
silica sample and composition of
the bulk phase. A significant endothermic contribution to the
total enthalpy of displacement upon individual
surfactant adsorption derives from desorption of the very structured
interfacial water due to the specific
adsorption of a hydrophobic surfactant moiety. In the presence of
sodium cations at the silica surface, this
endothermic contribution dominates over others because the enthalpy of
adsorption for surfactant ionic
heads and the enthalpy of desorption for sodium cations cancel each
other out to a great extent. For
moderate interfacial concentrations of sodium, the overall enthalpy
change is endothermic in a certain
adsorption interval, whereas for high concentrations it is always
exothermic. Individual surfactant
adsorption at free pH causes the pH of the equilibrium bulk solution to
decrease, indicating an increase
in the surface charge of silica.
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