Adsorption of Cationic Surfactants on Silica. Surface Charge Effects Tatiana P. Goloub, † Luuk K. Koopal,* and Bert H. Bijsterbosch Department of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreyenplein 6, 6703 HB Wageningen, The Netherlands Marianna P. Sidorova Department of Colloid Chemistry, St. Petersburg University, Universitetskij pr. 2, 198904 St. Petersburg, Russia Received October 31, 1995. In Final Form: February 5, 1996 X The surface charge of Aerosil OX50 in an aqueous dispersion has been studied in the presence of cationic surfactants (alkylpyridinium chlorides and trimethylammonium bromide) as a function of pH and salt concentration. The measurements were complemented with surfactant isotherms. To investigate the effect of the aliphatic surfactant tail, the surface charge-pH curves have also been measured in the presence of tetramethylammonium bromide (TMAB). At low salt concentration the surfactant isotherms show a two-step behavior. In the first step the surface charge follows the surfactant adsorption, indicating that head-on adsorption occurs. In the second step the surface charge increases only slightly. Comparison of the results for the surfactants and TMAB shows that the surface charge adaptation is strongly affected by the hydrocarbon tail and that the kind of head group is of secondary importance. Up to the first pseudoplateau, the molecules adsorb with their head groups and tails on the surface. For the second part of the isotherm, surfactant self assembly occurs. At high salt concentration the initial surface charge is relatively large and the surface charge adjustment is small. Due to the relatively large surface charge the surface is less hydrophobic and hydrophobic interactions with the surface become less probable. Instead hydrophobic tail-tail interactions become important also in the lower part of the isotherm. Surfactant self assembly occurs both before and after the surface charge is compensated by the surfactant charge. Introduction Silica is a classical sorbent to study adsorption of cationic surfactants on negative surfaces. The large impact of surfactant adsorption phenomena on silica for many technical applications has led to a large number of experimental and theoretical studies. The main experi- mental features of adsorption of cationic surfactant on silica have been demonstrated in refs 1-10. The adsorp- tion appeared to be dependent on surface charge density, pH, surfactant structure, and electrolyte concentration. On the basis of these results it was established that the driving forces for adsorption of cationic surfactants on silica are the Coulombic attractions between surfactant ions and charged surface groups and the hydrophobic interactions between hydrocarbon moieties. But specific interactions between the surfactant ions and the surface, leading to a better screening of the surface charge density (or briefly the surface charge), can also have a pronounced effect on adsorption processes especially at surfactant concentrations below the critical micelle concentration (cmc). In most of the preceding references little attention has been given to this aspect. Recent work on silica 11,12 and rutile 13 surfaces has shown not only that adsorption of surfactants varies strongly with pH but also that the ionization of the surface groups increases upon adsorption of the ionic surfactant. This change in surface charge leads to a change of pH in solution. The fact that often only the initial pH is specified is the reason why it is so difficult to make a quantitative comparison between adsorption isotherms obtained in different studies. Actu- ally, the most widely used method to determine adsorption isotherms is to measure the surfactant depletion from a solution brought in contact with adsorbent. The pH change that occurs depends on the surface area to solution volume ratio and on the surfactant concentration. If no pH control is made, the pH values may vary along the isotherm and from isotherm to isotherm. In general, the surface charge of silica is well studied. Silica has its isoelectric point (iep) at about pH 2 and a salt concentration dependent surface charge. 14-18 The charge-pH curves have a characteristic shape that is different from that of other well studied metal oxide surfaces. 19-21 Direct potential measurements using the ISFET technique 22 and modeling studies 21 indicate that the surface potential of silica as a function of pH is highly † Present address: Department of Colloid Chemistry, St. Pe- tersburg University, Universitetskij pr. 2, 198904 St. Petersburg, Russia * Corresponding author. 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