Adsorption of benzoic and 4-heptylbenzoic acid on different silica substrates from organic and aqueous solution

In studies of the adsorption of surface-active molecules from organic phase on mineral surfaces in relation to wettability alterations, the system has normally been made water-free. However, since water is an inherent part of all oil-bearing formations, it is of interest to study adsorption in solid/water/oil systems. In this paper we therefore study the adsorption of benzoic (BzCOOH) and 4-heptylbenzoic (4-HBA) acid from organic solution on different silica substrates in the presence of water. The results show that the adsorption is enhanced in the presence of water and that 4-HBA has a higher affinity for the silica surface than does BzCOOH. Variations in the surface properties of the silica substrates had no significant effect on the adsorption. In studies of the adsorption of organic acids from aqueous phase, the acid has been added either as the corresponding acid salt or as the acid itself. However the adsorption behavior of the acid salt is not necessarily representative of the adsorption behavior of the acid. To investigate this more closely, we studied the adsorption of benzoic acid added to water both as sodium benzoate (BzCOO(-)Na(+)) and as BzCOOH. The results revealed a significant increase in the maximum adsorption when BzCOO(-)Na(+) was used as adsorbate instead of BzCOOH. (C) 2000 Academic Press.