Effect of fatty acids on the mixing behaviour of pyridine and diisopropylamine in aqueous mixtures

Pyridine and diisopropylamine (DIPA) are groundwater pollutants. We examine the mixing behaviour of these amines in water in the presence of long-chain fatty acids (in the context of water purification) and in the presence of short-chain fatty acids produced by pollutant-degrading bacteria (in the context of pollutant migration in groundwater). This knowledge is currently lacking. Pentanoic acid is water soluble and has a short tail: it does not induce pyridine-water separation, but increases the fuzziness of pyridine clusters, as seen by small-angle neutron scattering (SANS). Decanoic and dodecanoic acid have longer tails. They decrease the miscibility of pyridine in water, because they compete against pyridine for hydrogen (H) bonding with water, as seen by computer simulations. Dodecanoic acid forms crystalline lamellar films at air-water interfaces, as seen by synchrotron X-ray reflectometry. These films trap separated pyridine droplets, enabling low-energy water purification, as shown by attenuated total-reflectance Fourier-transform infrared spectroscopy. While pyridine is an H bond acceptor, diisopropylamine donates H bonds to fatty acids. In aqueous mixtures, DIPA is intercalated in decanoic acid or pentanoic acid micelles dispersed in water, as seen by SANS. Pentanoic acid is fully miscible in water and thus enhances the solubility of DIPA in water, potentially enhancing its migration in groundwater. This fundamental study can aid management of polluted sites, by offering insights regarding amine migration in groundwater and by demonstrating the potential of energy-efficient water purification approaches.