Np. Hankins et al., MODELING EFFECTS OF PH AND COUNTERIONS ON SURFACTANT ADSORPTION AT THE OXIDE WATER INTERFACE/, Industrial & engineering chemistry research, 35(9), 1996, pp. 2844-2855
A model is presented that describes the effect of pH on the adsorption
of an isomerically pure anionic surfactant species at a mineral oxide
/water interface. A site-binding model, to account for effects of pH,
surface heterogeneities, and counterions, is incorporated into a patch
wise, phase-separation modeling approach, making it possible to predic
t both the surface charge and the counterion association beneath an ad
sorbed surfactant aggregate. Parameters for the site binding model on
or-alumina are obtained from experimental surface charge measurements.
The formation of both local monolayers (hemimicelles) and bilayers (a
dmicelles) is allowed, although the isotherms studied in this paper ar
e fit by parameter values that predict admicelle formation only, The m
odel is able to predict experimental measurements of the adsorption of
an isomerically pure, anionic surfactant species on alpha-alumina as
a function of pH. It reproduces several previously unexplained experim
ental observations; in particular, it offers an explanation for the ob
servation of significant adsorption of anionic surfactant above the po
int of zero charge (pzc) of a mineral oxide surface.