Monolayer and Brewster Angle Microscopy Studies of Poly(methyl methacrylate)−Monopalmitin Mixed Systems at the Air−Water Interface

Abstract

Mixed monolayers of poly(methyl metacrylate) (PMMA) and monopalmitin (Mp) were used for the study of their interactions. A thorough analysis of surface pressure (pi)-area (A) isotherms with the Langmuir monolayer technique, complemented with Brewster angle microscopy (BAM) images was performed. Mixed films show two phase transitions at a surface pressure of 14.5 mN/m and at 20-21 mN/m, respectively. Moreover, mixed monolayers show two well-defined collapses: one, corresponding to the lipid (at surface pressures of 50-51 mN/m) and the another one, ascribed to the PMMA, at surface pressure values of 57-58 mN/m. When the mean molecular areas of the mixed films (A(1,2)) were plotted versus film composition (X(1) or X(2)), positive deviations from the ideal behavior were observed at surface pressures below 15 mN/m, which were mainly attributed to a change in the conformation of the PMMA molecules at the surface. However, at higher surface pressures, the areas per monomer unit of the mixed monolayers obey the additivity rule, attributed to the fact that the film components form an immiscible system in these conditions.

Approach to Knowledge of the Interaction between the Constituents of Contact Lenses and Ocular Tears: Mixed Monolayers of Poly(methyl methacrylate) and Dipalmitoyl Phosphatidyl Choline

“…(7) The existence of the ideal behavior of the PMMA-DPPC mixed system allows us to argue that contact lenses in the eye do not alter the structural characteristics of the phospholipid (DPPC) in tears. This contrasts with previous results that we obtained in the study of other lipids (cholesterol and monopalmitin) and proteins (lysozyme) with PMMA, where it was observed 36 that cholesterol does not interact with the polymer because both are immiscible at the interface air/water; on the contrary, monopalmitin interacts with PMMA, 37 causing a change in its coiled arrangement in the pure PMMA monolayer to a flatter configuration in the mixed film. Also, the PMMA-lysozyme mixed system shows negative deviations from ideality with a maximum deviation in the mixed monolayer with composition X PMMA ≈ 0.25, which is attributed to the formation of a complex between both components with 1:3 (PMMA/lysozyme) stoichiometry.…”

“…Therefore, cholesterol molecules PMMA-monopalmitin mixed films exhibit two phase transitions (like the PMMA-DPPC system): one corresponds to pure glyceride, and the other corresponds to PMMA. 37 In addition, all mixed films show two well-differentiated collapses at the same surface pressure as that of the pure components (similar to the PMMA-cholesterol system). Besides, the A m -X PMMA plots are linear at high surface pressures (as in PMMAcholesterol and PMMA-DPPC mixed systems), but at low surface pressures, that is, below the phase transition corresponding to glyceride, the plots show the existence of positive deviations from linearity that is attributed to the fact that the monopalmitin molecules, horizontally oriented on the water at low surface pressures, interact with the polymer molecules and thus cause their unfolding from the coiled arrangement in the pure PMMA monolayer (where, because of steric hindrance, it can be argued that some monomer units must be out the surface) to a flatter configuration with most monomer units at the surface.…”

“…The lysozyme, a main protein component in tears, shows two different macroscopic behaviors in mixed films with PMMA. 38 At low PMMA composition, a miscible system with negative deviations from ideality is formed, showing a maximum deviation in the mixed monolayer with composition X PMMA ≈ 0.25. This behavior is attributed to the formation of a complex with 1:3 (PMMA/lysozyme) stoichiometry.…”