Temperature and pressure effects upon hydrophobic interactions in natural waters

Abstract

Hydrophobic interactions are responsible for the formation of dimers and higher-order aggregates, and the formation of micelles and conformational changes of natural polymers in various transformation processes of organic substances in aqueous medium. There are indications that the strength of hydrophobic interactions increases with temperature and pressure. The effect of increased temperature and pressure upon hydrophobic interactions in natural waters has been studied by means of capacity current measurements using phase-sensitive a.c. voltammetry. Laboratory studies with model organic substances (oleic acid, humic acid and a mixture of these acids) and with natural seawater samples from various sampling areas in the Adriatic Sea have been performed. Samples were treated under different conditions of temperature and pressure (100–134°C and 1–3 atm). Surfactant activity was expressed in terms of the equivalent of the nonionic surfactant Triton X-100 (in mgl −1). The S values (surfactant activity after treatment/surfactant activity before treatment) were determined. Increased surfactant activity upon treatment of samples was observed both in laboratory studies and during the field experiment in an aquatic system exposed to continuous thermal pollution, indicating the transformation of the organic matter present. Natural seawater samples with initially low surfactant activity values (equivalent to 0.02–0.05 mg of Triton X-1001 −1) had S values between 2 and 17. For samples which initially had higher surfactant activity values (approximately 0.15 mg Triton X-1001 −1), neither qualitative nor quantitative changes of adsorbable organic matter could be detected with the method used. Oleic acid (0.010 mgl −1) and humic acid (0.75 mgl −1) had S values of 4.3 and 3.0 respectively, and their mixture had an S value of unity, i.e. the mixture was stable during the treatment at increased temperature and pressure.