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The tear film has an important role on the quality of the vision process and is of importance in the fields of ophthalmology and optometry. The lipid layer, the outermost part of the tear film, can be investigated with the Langmuir technique, as well as the interaction of components of the aqueous part of the tear film with those of the lipid layer. Dysfunctions in the stability and composition of the tear film are the causes of eye diseases, as the dry eye syndrome. For the treatment of this disease, the artificial tears are used, and one type of them are the lipid tears, which can be investigated with the Langmuir technique. This technique can also be used to investigate the influence on the lipid layer of other artificial tear components or medicines instilled in the eye. This review presents investigations where the Langmuir technique has been used in relation with the tear film. In particular, the surface properties of natural tears and of four commercial lipid-containing artificial tears have been reported and discussed in connection with their composition. [ABSTRACT FROM AUTHOR]

Abstract.: Artificial lipid bilayers have been extensively studied as models that mimic natural membranes (biomimetic membranes). Several attempts of biomimetic membranes inserting ubiquinone (UQ) have been performed to enlighten which the position of UQ in the lipid layer is, although obtaining contradictory results. In this work, pure components (DPPC and UQ) and DPPC:UQ mixtures have been studied using surface pressure-area isotherms and Langmuir-Blodgett (LB) films of the same compounds have been transferred onto solid substrates being topographically characterized on mica using atomic force microscopy and electrochemically on indium tin oxide slides. DPPC:UQ mixtures present less solid-like physical state than pure DPPC indicating a higher-order degree for the latter. UQ influences considerably DPPC during the fluid state, but it is mainly expelled after the phase transition at $ \approx$ 26 mN·m^-1 for the 5:1 ratio and at $ \approx$ 21 mN·m^-1 for lower UQ content. The thermodynamic studies confirm the stability of the DPPC:UQ mixtures before that event, although presenting a non-ideal behaviour. The results indicate that UQ position can be tuned by means of the surface pressure applied to obtain LB films and the UQ initial content. The UQ positions in the biomimetic membrane are distinguished by their formal potential: UQ located in 'diving' position with the UQ placed in the DPPC matrix in direct contact with the electrode surface ( -0.04±0.02 V), inserted between lipid chains without contact to the substrate ( 0.00±0.01 V) and parallel to the substrate, above the lipid chains ( 0.09±0.02 V). Graphical abstract: [ABSTRACT FROM AUTHOR]

Abstract.: The relevance of biomimetic membranes using galactolipids has not been expressed in any extensive experimental study of these lipids. Thus, on the one hand, we present an in-depth article about the presence and role of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) in thylakoid membranes, their physical states and their applications. On the other hand, we use the Langmuir and Langmuir-Blodgett (LB) techniques to prepare biomimetic monolayers of saturated galactolipids MGDG, DGDG and MGDG:DGDG 2:1 mixture (MD)--biological ratio--. These monolayers are studied using surface pressure-area isotherms and their data are processed to enlighten their physical states and mixing behaviour. These monolayers, once transferred to a solid substrate at several surface pressures are topographically studied on mica using atomic force microscopy (AFM) and using cyclic voltammetry for studying the electrochemical behaviour of the monolayers once transferred to indium-tin oxide (ITO), which has good optical and electrical properties. Moreover, MD presents other differences in comparison with its pure components that are explained by the presence of different kinds of galactosyl headgroups that restrict the optimal orientation of the MGDG headgroups. Graphical abstract: [ABSTRACT FROM AUTHOR]

Classical and atomistic nucleation models have been tested in several aqueous systems dealing with electrocrystallisation. A lot of reported experimental nucleation data have been used and in a wide range of overpotentials. The critical nucleus size has been calculated in those cases not reported in the original work, and the results obtained with the classical and atomistic models have been tabulated, compared and discussed. Small values for the critical nucleus size occur in most of the systems. [ABSTRACT FROM AUTHOR]