Introduction
The adsorption of cationic and anionic surfactants on rutile is studied as a function of surfactant concentration and ionic
In a series of papers Koopal and co-workers (1-6) have strength in solution. The pH is kept constant for each series of discussed surfactant adsorption on mineral oxides. In these isotherms. Special attention is given to the extent of adaptation papers special attention was paid to the effect of the salt of the surface charge as a function of surfactant adsorption. Experconcentration on surfactant isotherms measured at a fixed imental results are complemented with model calculations based on the self-consistent (mean) field lattice theory for adsorption pH and to the pH effect on the surfactant adsorption at a and association (SCFA) to confirm the correctness of the given given salt concentration. It was shown that isotherms meainterpretation. The adsorption behavior of sodium nonyl benzene sured in electrolyte solutions of different concentration intersulfonate (SNBS) and dodecyl or tetradecyl pyridinium chloride sect at the so-called cip or common intersection point (1, 2, (DPC/TPC) on rutile is broadly similar. Isotherms can be divided 4, 5, 7). The presence of such a cip was first noted by De in two regions by the (approximately) common intersection point Keizer et al. (8) who also showed that in the absence of (cip) between isotherms measured at different salt concentrations. specific adsorption of other ions and/or activity effects the Before the cip adsorption occurs head-on (hemi-micelles); after cip should correspond to the isoelectric point of the particles the cip both head-on and head-out adsorption (ad-micelles) occur. covered with surfactant.
Intersection points similar to the cip occur in the proton excess and
Another issue is the comparison of the adsorption behavior the net surface charge/surfactant charge plots. Although broadly of anionic and cationic surfactants on similar surfaces. Acsimilar in their adsorption behavior, the SNBS and DPC/TPC systems differ markedly if the charge situation near the interface cording to theoretical calculations (2, 4, 5) their behavior is considered. SNBS follows simple rules; the strongly adsorbing should be very similar. In literature, however, the compari-SNBS head groups can compete easily with the salt ions for the son is mostly made between cation adsorption on silica and surface sites. The point where the surface charge and the surfacanion adsorption on a metal oxide surface such as aluminum tant charge balance, the equivalence point, and the cip in the oxide or titanium oxide. As indicated by Bo Β¨hmer and Koopal isotherms coincide. Moreover, the cip corresponds with the isoelec- (2,4), there are reasons to believe that such a comparison tric point (iep). The DPC and TPC systems behave in a more is biased by the fact that the charging characteristics of silica complicated way, due to their relatively weak interaction with the are quite different from that of metal oxides (6, 9, 10). Most surface. The cip in the isotherms and the cip in the net surface/ recently, the adsorption of the negatively charged surfactant surfactant charge plots occur at much lower adsorption values sodium nonyl benzene sulfonate, SNBS, on positively than the equivalence point. Co-ions thus contribute at the cip to charged rutile (TiO 2 ) was compared with that of the posithe charge balance in the surface region. In this situation the iep will occur at lower adsorption values than the cip. α§ 1996 Academic tively charged surfactant dodecyl pyridinium chloride, DPC, Press, Inc. on negatively charged rutile (1). In Fig. 1 the isotherm of Key Words: ad-micelles; adsorption in hemi-micelles; adsorption (a) SNBS and (b) DPC at several salt concentrations are in nonyl benzene sulphonate; adsorption dodecyl pyridinium chloreplotted to illustrate that the adsorption is, broadly speaking, ride; adsorption tetradecyl pyridinium chloride; adsorption rutile; similar as expected on theoretical grounds. The isotherms adsorption on adaptation; surface charge surface charge; adjustof Fig. 1 also show the characteristic four region shape and ment surfactant; ionic strength effect on adsorption theory; selfthe presence of the cip.
consistent field (SCF) for adsorption.
Although an isotherm gives information about the surface coverage of surfactant as a function of the bulk surfactant concentration it gives no direct information concerning the