Impact of Multi-Interface Surfactant Adsorption on Wettability in Dense Nonaqueous Phase Liquid Systems

Abstract

The wettability, and hence the distribution and transport, of dense nonaqueous phase liquid (DNAPL) wastes in the subsurface is strongly affected by surfactants that are present in the wastes. To understand their impact, this work examined the dependence of the contact angle of quartz=tetrachloroethylene (PCE)=water systems containing the anionic surfactant Aerosol OT (AOT) or=and the nonionic surfactant hexaoxyethylene glycol mono-ndodecyl ether (C 12 E 6 ) on the surfactants' adsorption at both the PCE=water and silica=water interfaces. Results showed that C 12 E 6 enhanced oil wetness at low pH. However, the system reverted to water-wet upon the addition of AOT, which by itself, did not alter the wettability of the system. To mechanistically explain such behavior, it was proposed that, based on measurements of adsorption onto quartz and calculations of surface excess using an equation-of-state approach, the increased oil-wetness in systems containing C 12 E 6 is due to the entropy-driven attraction between the oxyethylene groups of the adsorbed C 12 E 6 at both the PCE=water and water=silica interfaces. Addition of AOT reduced the adsorbed C 12 E 6 at both interfaces, lowering the attraction between PCE and silica. Thus, a consideration of surfactant adsorption at both interfaces, and the subsequent interaction between the molecules sorbed at both interfaces, is critical to explaining the wettability behavior of waste DNAPLs.

Acid–Base Polymeric Foams for the Adsorption of Micro-oil Droplets from Industrial Effluents

“…This adsorption is driven by the electronic property of acid–base polymeric foams. Under acidic or basic conditions, these polymers can acquire charge by protonation or dissociation of functional groups, such as carboxyl, hydroxyl, and amine groups. − Although these polymers are hydrophilic in nature, they are able to switch from a hydrophilic surface to an oleophilic surface by the adsorption of oppositely charged oil droplets. − The oil field effluents contain crude oil microdroplets, which are highly stable due to the presence of anionic surfactants at the oil–water interface. − At a specific pH, if the adsorbent surface (i.e., foam surface) has a positive charge and the oil droplets have a negative charge, the droplets will be attracted to the adsorbent surface, switching the surface from hydrophilic to oleophilic (Figure ). ,,− To invoke switchable wetting, bulk pH can be varied to alter the surface charge polarity of the foam and oil droplets, enabling effective adsorption …”