Abstract
Mixed fatty acids or mixed phospholipids systems with saturated-unsaturated hydrocarbon chains are of biological interest. In this work, the monolayers of oleic acid-stearic acid (OA-SA) and palmitoyloleoylphosphatidylcholine-dipalmitoylphosphatidylcholine (POPC-DPPC) have been studied. From the surface pressure-area isotherms, elastic modulus values and virial equation coefficients can be obtained. Thermodynamic treatment also yields excess (GE) and mixing (ΔGmix) free energies. Results indicate positive GE values, that is, molecular interactions in the mixed films are less favourable, due to the presence of unsaturation; however, the mixture is slightly favourable due to the entropic factor that affords positive ΔGmix values. For the OA-SA system, a high SA content and surface pressure facilitate the phase separation, even though a certain miscibility between both components still remains. For the POPC-DPPC system, the most favourable mixing conditions occur for XPOPC ≈ 0.4. For these mixed systems, the values of the elastic modulus are more similar to those of more fluid components (OA or POPC); analysis of the virial coefficients shows that the b1 virial coefficient values lie between those of the individual components and are higher than values suitable for an ideal mixing.
Highlights
Fatty acids and phospholipids have been widely studied with the Langmuir technique, because they are amphiphilic compounds, ideal for forming ordered and compact monolayers, and for their biological interest in systems as biomembranes and tear films
Ocko et al [33] found that stearic acid and elaidic acid were poorly miscible in monolayers at all surface pressures, and phase separation was always observed
They suggest that in the DPPG-DOPE system, and due to the presence of the double bonds in the acyl chains of DOPE molecule, the distance between DPPG and DOPE molecules is greater; this weakens the strength of molecular interactions
Summary
Fatty acids and phospholipids have been widely studied with the Langmuir technique, because they are amphiphilic compounds, ideal for forming ordered and compact monolayers, and for their biological interest in systems as biomembranes and tear films. Wydro et al [44] found that DPPG mix non-ideally with DPPE, DSPE and DOPE They suggest that in the DPPG-DOPE system, and due to the presence of the double bonds in the acyl chains of DOPE molecule, the distance between DPPG and DOPE molecules is greater; this weakens the strength of molecular interactions. Their results indicate that DPPG mix more favourably with PE possessing saturated acyl chains as compared to DOPE, and suggests the existence of a phase separation for DPPG-DOPE monolayers. In other works [43], the influence of cholesterol in mixed films has been studied; the results indicate an important effect of cholesterol in the monolayer structure
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