Abstract
The adsorption of mixed surfactants on solid substrates is important in a wide range of technological and industrial applications. Surfactant flooding in oil reservoirs is one of the most successful processes employed in enhanced oil recovery (EOR), where surfactant aqueous solutions are injected in the reservoir in order to decrease oil/water interfacial tension, leading to an increase in oil production. However, it suffers from a severe economic drawback due to the loss of large amounts of surfactant, which end up adsorbed on the rocks surface. This problem can be overcome with the use of systems of surfactant-modified silica particles that would be able to permeate through the rocks pores and either deliver the surfactants at the oil/water interface or act themselves as amphiphilic nanoagents, reducing the interfacial tension. For this purpose, we investigated the adsorption behavior of a nonylphenylethoxylate (NP10) on silica nanoparticles, mediated by the co-adsorption of a cationic surfactant (cetyltrimethylammonium bromide, CTAB). While NP10 alone shows meager adsorption, because of the lack of electrostatic interactions, CTAB adsorbs significantly on the oppositely charged silica surface. The adsorption isotherms of the binary surfactant mixtures at different molar ratios showed a sharp increase of NP10 adsorption on the silica/water interface in the presence of CTAB. Saturation values are reached at concentrations of NP10 higher than the critical micelle concentration, indicating significant adsorption on the nanoparticles. The adsorbed CTAB molecules would be probably acting as nucleating sites to form mixed aggregates with NP10 on the silica surface, through hydrophobic interactions with the CTAB hydrocarbon chain and would be responsible for the marked adsorption synergy between the two surfactants. The oil/water interfacial tension results suggest that the studied systems present a good potential as surfactant nanocarriers for EOR.
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