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Advances in Colloid and Interface Science (v.132, #1)
Recent progress in the determination of solid surface tensions from contact angles by H. Tavana; A.W. Neumann (pp. 1-32).
Advancing contact angles of different liquids measured on the same solid surface fall very close to a smooth curve when plotted as a function of liquid surface tension, i.e., γlvcos θ versus γlv. Changing the solid surface, and hence γsv, shifts the curve in a regular manner. These patterns suggest that γlvcos θ depends only on γlv and γsv. Thus, an “equation of state for the interfacial tensions” was developed to facilitate the determination of solid surface tensions from contact angles in conjunction with Young's equation. However, a close examination of the smooth curves showed that contact angles typically show a scatter of 1–3° around the curves. The existence of the deviations introduces an element of uncertainty in the determination of solid surface tensions.Establishing that (i) contact angles are exclusively a material property of the coating polymer and do not depend on experimental procedures and that (ii) contact angle measurements with a sophisticated methodology, axisymmetric drop shape analysis (ADSA), are highly reproducible guarantees that the deviations are not experimental errors and must have physical causes. The contact angles of a large number of liquids on the films of four different fluoropolymers were studied to identify the causes of the deviations.Specific molecular interactions at solid–vapor and/or solid–liquid interfaces account for the minor contact angle deviations. Such interactions take place in different ways. Adsorption of vapor of the test liquid onto the solid surface is apparently the only process that influences the solid–vapor interfacial tension ( γsv). The molecular interactions taking place at the solid–liquid interface are more diverse and complicated. Parallel alignment of liquid molecules at the solid surface, reorganization of liquid molecules at the solid–liquid interface, change in the configuration of polymer chains due to contact with certain probe liquids, and intermolecular interactions between solid and liquid molecules cause the solid–liquid interfacial ( γsl) tension to be different from that predicted by the equation of state, i.e., γsl is not a precise function of γlv and γsv. In other words, the experimental contact angles deviate from the “ideal” contact angle pattern.Specific criteria are proposed to identify probe liquids which eliminate specific molecular interactions. Octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS) are shown to meet those criteria, and therefore are the most suitable liquids to characterize surface tensions of low energy fluoropolymer films with an accuracy of ±0.2 mJ/m2.
Keywords: Solid surface tensions; Contact angle; Molecular interactions; Fluoropolymers; Liquids with bulky molecules
Thin wetting films from aqueous solutions of surfactants and phospholipid dispersions by L. Alexandrova (pp. 33-44).
The microscopic thin wetting film method was used to study the stability of wetting films from aqueous solution of surfactants and phospholipid dispersions on a solid surface. In the case of tetradecyltrimethylammonium bromide (C14TAB) films the experimental data for the receding contact angle, film lifetime, surface potential at the vapor/solution and solution/silica interface were used to analyze the stability of the studied films. It is shown that with increasing C14TAB concentration charge reversal occurs at both (vapor/solution and solution/silica) interfaces, which affects the thin-film stability. The spontaneous rupture of the thin aqueous film was interpreted in terms of the earlier proposed heterocoagulation mechanism. The presence of the mixed cationic/anionic surfactants was found to lower contact angles and suppresses the thin aqueous film rupture, thus inducing longer film lifetime, as compared to the pure amine system. In the case of mixed surfactants hetero-coagulation could arise through the formation of ionic surfactant complexes. The influence of the melting phase-transition temperature Tc of the dimyristoylphosphatiddylcholine (DMPC) on the stability of thin films from dispersions of DMPC small unilamellar vesicles on a silica surface was studied by measuring the film lifetime and the TPC expansion rate. The stability of thin wetting films formed from dispersions of DMPC small unilamellar vesicles was investigated by the microinterferometric method. The formation of wetting films from diluted dispersions of DMPC multilamellar vesicles was studied in the temperature range 25–32 °C. The stability of thin film of lipid vesicles was explained on the basis of hydrophobic interactions. The results obtained show that the stability of wetting films from aqueous solutions of single cationic and mixed cationic–anionic surfactants has electrostatic origin, whereas the stability of the phospholipid film is due to hydrophobic interaction.
Keywords: Surfactant; Phospholipid; Three-phase contact; Thin wetting film