STUDY OF MONOSTEARIN FILMS IN THE PRESENCE OF ELECTROLYTES

Changes in the structural characteristics and stability of monostearin monolayers in the presence of electrolytes in the aqueous subphase we re studied. Compression isotherms for monolayers on aqueous NaCl (0.1, 0.5, and 1 mol l(-1)) and CaCl2 (0.1 and 0.5 mol l(-1)) solutions wer e obtained using an automated Langmuir-type film balance, within a tem perature range 5-45 degrees C. Electrolytes promoted a liquid-condense d (LC) structure in the monolayer, the solid structure disappeared, an d a liquid-expanded (LE) structure was observed only at the highest te mperatures. Monolayers were more expanded in the presence of either el ectrolyte (NaCl or CaCl2). More-concentrated NaCl solutions produced a n increase in the expansion of the monolayer while the opposite effect was observed with CaCl2 aqueous solutions. From a thermodynamics poin t of view, it was deduced that the transition from the LC to the LE st ructure always involved absorption of heat and was accompanied by an i ncrease in entropy. Relationships between elasticities and monolayer s tructural characteristics are discussed. The results obtained are expl ained in terms of changes in monolayer-monolayer and monolayer-subphas e 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. Chan ges observed in the values of Delta H-t, and Delta S-t, for the LC to LE transition are consistent with the existence of these interactions.