Structural and thermodynamic characteristics of natural marine films derived from force–area studies

Natural marine film experiments in shallow offshore waters of the Baltic and Mediterranean Seas were carried out over the period 1990–1996 under calm sea conditions using a novel device for sampling and force–area studies. The sampler-Langmuir trough-Wilhelmy filter paper plate system ‘cuts out’ an undisturbed film-covered sea area (8 cm thick) to perform π–A studies without any initial physico-chemical sample processing. The limiting specific area Alim (268–3157 Å2 per molecule) and mean molecular weight (0.65–9.7 kDa) of microlayer surfactants were determined from the 2D virial equation of state applied to the isotherms. The π –A isotherms recorded in the temperature range 5–35°C exhibited several plateaus and ‘kinks’ corresponding to 2D phase transitions that allowed for the first time completing the phase diagram (π–T) of natural films. Enthalpy ΔH and entropy ΔSt of the first-order phase transitions (G → LE → LC) were evaluated using the Clausius–Clapeyron equation applied to the isotherms. Additional information on the film structure were derived from the entropy change ΔSc attributed to the π –A hysteresis, and the polymer film scaling theory adopted to marine films. The results obtained suggest that natural films are a complex mixture of biomolecules covering a wide range of solubilities, surface activity and molecular weight with an apparent structural organisation.