Thermodynamic Behaviour of Mixed Films of an Unsaturated and a Saturated Polar Lipid. (Oleic Acid-Stearic Acid and POPC-DPPC)

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 (G E ) and mixing (∆G mix ) free energies. Results indicate positive G E 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 ∆G mix 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 X POPC ≈ 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 b 1 virial coefficient values lie between those of the individual components and are higher than values suitable for an ideal mixing.

A Deeper Insight into the Interfacial Behavior and Structural Properties of Mixed DPPC/POPC Monolayers: Implications for Respiratory Health

“…The value of b 0 is lower than 1, indicating that two-dimensional phospholipid aggregates may be formed under low surface pressure. The b 0 value of the POPC monolayer is higher than that of DPPC, and b 1 is positive, indicating that the main interaction between phospholipid molecules is repulsive [ 37 ], which is consistent with the result of A exc . The value of ( b 1 ) 12 ( X POPC = 0.4) is 0.2274 and is the lowest, significantly, while the corresponding value of A exc is close to 0 at a low surface pressure.…”

Langmuir monolayer studies of non-ionic surfactants and DOTMA for the design of ophthalmic niosomes

“…3 ), all while retaining the shape of the isotherm of pure Tween 60, the majority component. From a molecular point of view, DOTMA molecules feature two hydrophobic C18:1 oleic chains that make it surface active, in spite of having a net charge (a trimethylammonium group) in its polar headgroup [ 18 , 31 , 32 ], so it was expected to remain at the interface along with Tween 60 molecules, probably intercalating and interacting with them, as shown later.

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“…This is consistent with the intercalation of unsaturated hydrocarbon chains, in this case in an amount proportional to DOTMA concentration, among saturated, single-chain stearic residues of Tween 60 molecules. The expanding effect of oleic chains has been reported in the literature for oleic acid/stearic acid [ 32 ] and saturated/unsaturated chain phospholipid interactions [ 33 ].…”

Lysozyme Influence on Monolayers of Individual and Mixed Lipids

“…References to previous studies can be found in [1][2][3]. Some articles on single fatty acids and single phospholipids published in the last years are reported in reference [4]. Recent articles related to mixtures of fatty acid and a phospholipid are those of [5][6][7][8][9][10][11][12][13][14][15], and articles related to mixtures of cholesterol and a phospholipid are those of [16][17][18][19][20][21][22].…”