How a range of metal ions influence the interfacial tension of n-decane/carboxylic acid/water systems: The impact of concentration, molecular- and electronic structure

Abstract

The data from more than 700 pendant droplet interfacial tension (IFT) measurements has been scrutinized to reveal the relation between salinity/surfactant structure and IFT in a brine – decane/carboxylic acid system. The large number of measurements enabled an experimental uncertainty of around 1% in the IFT range around 30 mN/m. Thus, small changes and trends in IFT as a function of salinity and surfactant structure become apparent. Five carboxylic acids are chosen to resemble natural naphthenic acids; individually, they only differ by a CH2-group starting from cyclohexane carboxylic acid and ranging to 5-cyclohexyl pentanoic acid. The salt concentration was varied from 0.01 M to 1.0 M for a range of chlorides of monovalent and divalent metal cations: LiCl, NaCl, KCl, MgCl2, CaCl2, SrCl2, BaCl2, and ZnCl2. For Na+, NaBr, NaI, were also investigated. The variation in IFT with salinity and carboxylic acid is subtle with divalent ions, not surprisingly, inducing larger IFT changes than monovalent ones when the concentration is varied. Zn2+ gives rise to a surprisingly large IFT reduction (up to 7.5 IFT units in the range from 0.01 M to 1.0 M) and clearly stands out as a special case. For the three smallest chain acids the IFT increases as a function of salinity whereas, for the system involving the two largest ones, 4-cyclohexyl butanoic acid and 5-cyclohexyl pentanoic acid, IFT decreases as a function of salinity. The distinct behavior of Zn2+ and the sudden crossover in the variation of IFT as a function of salinity when increasing the chain length of the carboxylic acid is taken to indicate that structure of both the (solvated) electrolyte and the carboxylic acid– particularly with regard to the orientation at the interface - are important factors in determining the magnitude and variation in IFT as a function of physical parameters.