Add time:09/27/2019         Source:sciencedirect.com

Changes in the structural characteristics and stability of Monostearin (cas 123-94-4) monolayers in the presence of electrolytes in the aqueous subphase were studied. Compression isotherms for monolayers on aqueous NaCl (0.1, 0.5, and 1 mol 1−1) and CaCl2 (0.1 and 0.5 moll−1) solutions were obtained using an automated Langmuir-type film balance, within a temperature range 5–45°C. Electrolytes promoted a liquid-condensed (LC) structure in the monolayer, the solid structure disappeared, and a liquid-expanded (LE) structure was observed only at the highest temperatures. Monolayers were more expanded in the presence of either electrolyte (NaCl or CaCl2). More-concentrated NaCl solutions produced an increase in the expansion of the monolayer while the opposite effect was observed with CaCl2 aqueous solutions. From a thermodynamics point of view, it was deduced that the transition from the LC to the LE structure always involved absorption of heat and was accompanied by an increase in entropy. Relationships between elasticities and monolayer structural characteristics are discussed. The results obtained are explained in terms of changes in monolayer-monolayer and monolayer-subphase interactions. The ions have to be hydrated in the bulk phase, so the hydrogen-bonding between water molecules and the monoglyceride polar head groups are replaced by van der Waals interactions with ions. Changes observed in the values of ΔHt and ΔSt for the LC to LE transition are consistent with the existence of these interactions.

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