engleski

Characterization of Sea-Surface Microlayers by Monolayer Techniques, Brewster Angle Microscopy and Fractal Analysis

The sea surface microlayer (SSM) is a thin layer between the ocean and the atmosphere with special physical, chemical and biological properties that either affects or is affected by global climate change. Actually, it hosts the main exchange processes of organic and inorganic matter, gases, water vapour, heat and energy between those two departments. The environmental significance of surface microlayer is widely accepted, although there are still many gaps in our understanding of the morphology and chemical composition affecting the mechanisms of exchange processes. It is of considerable interest to get insight into morphology, structural changes and build-up mechanisms of the natural SSM. In this study the microlayers were investigated without any pre-treatment and as ex-situ reconstructed films after previous microlayer extraction by organic solvents of different polarity. Monolayer studies, particularly measurements of surface pressure-area (π-A) isotherms, have been applied for investigations of physical states and elastic properties of different microlayers throughout determination of the surface compression modulus. New insight into formation, temporal variations and morphology of the sea-surface microlayers, as well as into processes occurring in this film can be gained by using Brewster angle microscopy (BAM), which was used for optical characterization and visualization of original microlayers and reconstructed films. BAM images have been recorded under surface pressure control to correlate the morphology and the monolayer phase state of the natural microlayers and ex-situ reconstructed films. The images of some films show liquid condensed domains as small bright areas in the liquid expanded phase which is visualised as dark background and are dependent on the surface pressures. Homogeneous films without domains have also been observed. Adsorbed films and surface microlayers often exhibit fractal characteristics and valuable information can be gained through fractal analysis. Box counting method provides efficient means for estimation of fractal dimension of two dimensional structures from its images. Fractal analysis of BAM images of a considered microlayer was used to infer the fractal dimension D of the microlayer and its change during the compression. The observed microlayer structures are fractal, and changes of the fractal dimension are correlated with processes and phase transitions in the layer. Generally the increase of fractal dimension proportional to compression was observed. The compression process ends in rather compact and homogeneous non-fractal film with D approaching 2. However, the observed variations in the fractal dimension enabled identification of second order phase transition – percolation that resulted in abrupt change of compressibility modulus. Thus, providing evidence that the macroscopic changes of the order parameter are associated with a short range order on molecular level in the layer. The different patterns in the change of fractal dimension observed for hydrophobic and hydrophilic material extracts are correlated with different – characteristic lateral interactions in the monolayer. Application of fractal analysis to BAM images provides additional complementary means for characterization of materials, structures and dynamical processes at the interface giving information on (second order) phase transitions, interactions and aggregation mechanisms at the interface and structural changes under applied pressure.

monolayer; compressibility; lipid; fractal dimension; BAM; percolation

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