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 benz oic (BzCOOH) and 4-heptylbenzoic (4-HBA) acid from organic solution on diff erent 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 highe r affinity for the silica surface than does BzCOOH. Variations in the surfa ce properties of the silica substrates had no significant effect on the ads orption. 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 aci d itself. However the adsorption behavior of the acid salt is not necessari ly representative of the adsorption behavior of the acid. To investigate th is more closely, we studied the adsorption of benzoic acid added to water b oth as sodium benzoate (BzCOO(-)Na(+)) and as BzCOOH. The results revealed a significant increase in the maximum adsorption when BzCOO(-)Na(+) was use d as adsorbate instead of BzCOOH. (C) 2000 Academic Press.