| Abstract: | Individual surface-active particles (SAP) in aqueous environmental samples can be characterized through their coalescence at a model hydrophobic interface. Each coalescence event is recorded as an electrical signal. As the electrical signal is a current transient (time on a millisecond scale), the sequence of fast processes leading to the formation of a condensed film can be studied from the moment of first attachment at the interface.In the stratified Krka Estuary, SAP are present in maximum concentrations (107−5 × 108 particles l−1 min the size range > 1 μm) at the halocline (freshwater-seawater interface). This highly surface-active fraction of particulate organic matter can be viewed as oily droplets. SAP are fluid, and are readily transformed at interfaces to films of molecular thickness (surface area per particle is in the range 10−5–10−2 cm2).Comparison of response of individual SAP with model particles, and the frequency of their appearance in estuarine samples, shows that a majority of estuarine SAP has, in terms of viscosity and film permeability, the properties of oleic acid. A significant fraction of SAP have more rigid structures, comparable to moleic acid + cholesterol model particles. A small fraction ( > 1%) behaves as phytoplankton cells with fluid outer membranes.We have also presented evidence for a strong interaction between elemental mercury and SAP, which is not sensitive to the molecular structure of the organic phase. The puzzling accumulation of mercury at the halocline of the Krka Estuary may be regarded as the result of interaction of elemental mercury with SAP. |
