Surface tension of aqueous solutions of water-soluble organic and inorganic compounds

Surface tension of pure and mixed aqueous solutions of both inorganic and organic species that are relevant in atmospheric droplets has been studied in a laboratory investigation. Following the Gibbsian theory of surfaces experimental data have been fitted by the Szyszkowski equation. As a function of the carbon-chain length trends in the maximum surface excess Γ_max and the inverse surface activity β have been clearly identified for pure aqueous solutions of homologous series of n-alcohols and mono-carboxylic acids, while these parameters oscillate for the homologous series of di-carboxylic acids. For mixed aqueous solutions of water-soluble organic and inorganic compounds, e.g. cis-pinonic acid and sodium chloride, a further decrease in surface tension has been observed, which can be interpreted as an interaction between the inorganic and organic solutes and an enrichment of the surface-active organic compound in the surface layer. Taking this effect into account surface-tension data of real atmospheric samples become more plausible, especially when also slightly soluble or water-insoluble organic compounds having a high surface activity at low concentrations are considered.