Characterization of aquatic natural microlayers by monolayer techniques and Brewster angle microscopy

Natural samples of sea- and lake-surface microlayer and samples which have been previously extracted by organic solvent and then reconstructed on the water surface were studied by monolayer techniques and Brewster angle microscopy (BAM). Surface pressure-area (π-A) and surface potential-area (ΔV−A) isotherms have been measured. Simultaneously BAM video images have been recorded. From π-A isotherms, thermodynamic parameters describing the states of monolayers, namely compressional modulus C−1s and compressibility have been calculated. The π-A isotherms of all three investigated microlayers (natural sample of sea-surface microlayer and ex-situ reconstructed film) were of expanded type without phase transition and the ΔV values are between 250–300 mV for the compressed films at 15 mN/m. However, a significant difference in morphology of the investigated microlayers was noticed. The BAM images depended on surface pressure. At lower surface pressures liquid condensed domains surrounded by liquid expanded phase were observed, and by compressing the domains become smaller and densely packed. This clearly indicates the presence of two phases showing that π-A isotherms are insufficient for film characterization. BAM pictures of one of the studied samples showed conformational changes of domains during compression and expansion of surface layer. One of the samples showed homogeneous and continuous film without any defined structure in the whole area region.