THE EXISTENCE OF SMALL SURFACE AGGREGATES - SURFACE MICELLES ON POLAR CHARGED SURFACES

The adsorption of tetradecylpyridinium (TDPy) and lauryl sulphate (LS) ions on polar, low charged adsorbents, i.e. fumed silica (Aerosil Ox 50) and fumed titanium dioxide (P25), was studied, as well as in the presence of chemisorbed octadecyldimethylsilane (ODDMS) residues on the adsorbent surfaces. The influence of different counterions on the adsorption of TDPy and of ODDMS groups density is demonstrated on silica at low pH values and, consequently, low surface charge density. The adsorption on ODDMS groups is confirmed for TDPy on surface-modified TiO2. The LS anions, not adsorbed to bare silica, are also adsorbed to the negatively charged silica surface in the presence of ODDMS groups. The results are discussed taking into consideration different proposals for the structure of the adsorbates, in particular the state of saturation adsorption. The behaviour of the surfactant ions can be best explained assuming small surface aggregates (micelles) if the surface density of primary adsorption sites, including ODDMS anchor groups, is lower than 0.3 mmol m-2.