Electrochemical wettability control on conductive TiO2 nanotube surfaces modified with a ferrocene redox system

This work reports on an electrochemical system which allows the control of surface wettability properties by voltage induced changes in contact angle (Θ) of ΔΘ  50°. For this we used conductive TiO₂ nanotubular layers that were modified with ferrocene coupled to the TiO₂ surface via triethoxysilane. To enhance the hydrophobic character of the nanotubular TiO₂ surface, also mixed organic monolayers namely perfluorotriethoxysilane, were explored. Formation of the ferrocene and mixed organic monolayer was confirmed by X-ray-photoelectron-spectroscopy (XPS). Contact angle combined with electrochemical measurements show that ferrocene in these monolayers can successfully be switched from Fe²⁺ to Fe³⁺ and that this change in the redox state considerably alters the wetting properties. Using a conductive nanotube substrate allows us to amplify this change by a factor of more than 10, and thus this surface can be used to trigger significant wetting alterations.     

Influence of geometry on steady dewetting kinetics

The experimental velocity dependences of the receding contact angles θ_r and the critical velocities of liquid film entrainment U^R_cr are independent of the geometry of the solid surface but significantly influenced by its material properties. These results are interpreted on the basis of published equations and the analysis shows that these theoretical relationships do not describe the experimental results adequately over the entire steady velocity range (0 ⩽U ⩽U^R_cr).The negligible effect of geometry on the steady dewetting kinetics can be qualitatively explained in the framework of the Cox—Voinov hydrodynamic model. This effect is weaker for liophobic systems (large θ_o), where the solid surface properties are more important.     

A novel poly(p‐phenylene) containing alternating poly(perfluorooctylethyl acrylate‐co‐methyl methacrylate) and polystyrene grafts by combination of atom transfer radical polymerization and Suzuki coupling processes

A poly(p‐phenylene) (PP), carrying perfectly alternating, well‐defined poly(perfluorooctylethyl acrylate‐co‐methyl methacrylate) [P(FEA‐co‐MMA)] and polystyrene (PS) side chain grafts, was synthesized by the combination of atom transfer radical polymerization (ATRP) and Suzuki cross‐coupling processes. First, dibromobenzene and diboronic ester functional macromonomers of P(FEA‐co‐MMA) and PS, respectively, were prepared by ATRP. In the second step, PP with lateral alternating P(FEA‐co‐MMA) and PS chains was synthesized by a Suzuki coupling reaction in the presence of Pd(PPh₃)₄ catalyst. The wetting behavior of the polymers was studied by measurements of the static contact angle θ of thin films (200?400 nm thickness) using water and n‐hexadecane as wetting liquids. The obtained fluorinated PP showed high static contact angles with both interrogating liquids, exhibiting simultaneously hydrophobic (θ_w = 111°) and lipophobic (θ_h = 67°) properties. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

The wettability of polytetrafluoroethylene by aqueous solution of cetylpyridinium bromide and propanol mixtures

Measurements of advancing contact angles (θ) were carried out for aqueous solutions of cetylpyridinium bromide (CPBr) and propanol mixtures at constant CPBr concentration equal to 1 × 10⁻⁵, 1 × 10⁻⁴, 6 × 10⁻⁴, 1 × 10⁻³ M, respectively, on polytetrafluoroethylene (PTFE). The obtained results indicate that the wettability of PTFE by aqueous solutions of these mixtures depends on their composition and concentration. In contrast to Zisman, there is no linear dependence between the cos θ and surface tension of aqueous solutions of CPBr and propanol mixtures (γ_LV), but a linear relationship exists between the adhesion tension and the surface tension of aqueous solutions of CPBr and propanol mixtures which have a slope equal to −1, and between cos θ and the reciprocal of the surface tension of solution. The slope equal to −1 and the intercept on the cos θ axis close to −1 suggest that adsorption of CPBr and propanol mixtures and the orientation of their molecules at aqueous solution–air and PTFE–aqueous solution interfaces are the same. This also suggests that the work of solution adhesion to the PTFE surface does not depend on the concentration of propanol and CPBr. Extrapolation of the straight line to the point corresponding to the surface tension of solution, which completely spreads over the PTFE surface, gives the value of the critical surface tension of PTFE wetting equal to 24.84 mN/m. This value is higher than PTFE surface tension (20.24 mN/m) and the values of the critical surface tension of PTFE wetting determined by other investigators from the contact angle of nonpolar liquids (e.g. n-alkanes). The differences between the value of the critical surface tension obtained here and those which can be found in the literature were discussed on the basis of the simple thermodynamic rules. Using the measured values of the contact angles and Young equation the PTFE–aqueous solution interfacial tension was determined. The values of PTFE–aqueous solution interfacial tension were also calculated from Miller and co-workers equation in which the correction coefficient of nonideality of the surface monolayer was introduced. From comparison of the obtained values it appears that good agreement exists between the values of PTFE–solution interfacial tension calculated on the basis of Young and Miller and co-workers equations in the whole range of propanol concentration.     

A review of factors that affect contact angle and implications for flotation practice

Contact angle and the wetting behaviour of solid particles are influenced by many physical and chemical factors such as surface roughness and heterogeneity as well as particle shape and size. A significant amount of effort has been invested in order to probe the correlation between these factors and surface wettability. Some of the key investigations reported in the literature are reviewed here.It is clear from the papers reviewed that, depending on many experimental conditions such as the size of the surface heterogeneities and asperities, surface cleanliness, and the resolution of measuring equipment and data interpretation, obtaining meaningful contact angle values is extremely difficult and such values are reliant on careful experimental control. Surface wetting behaviour depends on not only surface texture (roughness and particle shape), and surface chemistry (heterogeneity) but also on hydrodynamic conditions in the preparation route. The inability to distinguish the effects of each factor may be due to the interplay and/or overlap of two or more factors in each system. From this review, it was concluded that:

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Surface geometry (and surface roughness of different scales) can be used to tune the contact angle; with increasing surface roughness the apparent contact angle decreases for hydrophilic materials and increases for hydrophobic materials.

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For non-ideal surfaces, such as mineral surfaces in the flotation process, kinetics plays a more important role than thermodynamics in dictating wettability.

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Particle size encountered in flotation (10-200 μm) showed no significant effect on contact angle but has a strong effect on flotation rate constant.

There is a lack of a rigid quantitative correlation between factors affecting wetting, wetting behaviour and contact angle on minerals; and hence their implication for flotation process. Specifically, universal correlation of contact angle to flotation recovery is still difficult to predict from first principles. Other advanced techniques and measures complementary to contact angle will be essential to establish the link between research and practice in flotation.     

Simultaneous spreading and evaporation: Recent developments

The recent progress in theoretical and experimental studies of simultaneous spreading and evaporation of liquid droplets on solid substrates is discussed for pure liquids including nanodroplets, nanosuspensions of inorganic particles (nanofluids) and surfactant solutions. Evaporation of both complete wetting and partial wetting liquids into a nonsaturated vapour atmosphere are considered. However, the main attention is paid to the case of partial wetting when the hysteresis of static contact angle takes place. In the case of complete wetting the spreading/evaporation process proceeds in two stages. A theory was suggested for this case and a good agreement with available experimental data was achieved. In the case of partial wetting the spreading/evaporation of a sessile droplet of pure liquid goes through four subsequent stages: (i) the initial stage, spreading, is relatively short (1–2 min) and therefore evaporation can be neglected during this stage; during the initial stage the contact angle reaches the value of advancing contact angle and the radius of the droplet base reaches its maximum value, (ii) the first stage of evaporation is characterised by the constant value of the radius of the droplet base; the value of the contact angle during the first stage decreases from static advancing to static receding contact angle; (iii) during the second stage of evaporation the contact angle remains constant and equal to its receding value, while the radius of the droplet base decreases; and (iv) at the third stage of evaporation both the contact angle and the radius of the droplet base decrease until the drop completely disappears. It has been shown theoretically and confirmed experimentally that during the first and second stages of evaporation the volume of droplet to power 2/3 decreases linearly with time. The universal dependence of the contact angle during the first stage and of the radius of the droplet base during the second stage on the reduced time has been derived theoretically and confirmed experimentally. The theory developed for pure liquids is applicable also to nanofluids, where a good agreement with the available experimental data has been found. However, in the case of evaporation of surfactant solutions the process deviates from the theoretical predictions for pure liquids at concentration below critical wetting concentration and is in agreement with the theoretical predictions at concentrations above it.     

Thermochemistry of the solid complex Gd(Et<Subscript>2</Subscript>dtc)<Subscript>3</Subscript>(phen)

Summary A ternary solid complex Gd(Et₂dtc)₃(phen) has been obtained from reactions of sodium diethyldithiocarbamate (NaEt₂dtc), 1,10-phenanthroline (phen) and hydrated gadolinium chloride in absolute ethanol. The title complex was described by chemical and elemental analyses, TG-DTG and IR spectrum. The enthalpy change of liquid-phase reaction of formation of the complex, Δ_rH^Θ_m(l), was determined as (-11.628±0.0204) kJ mol^-1 at 298.15 K by a RD-496 III heat conduction microcalorimeter. The enthalpy change of the solid-phase reaction of formation of the complex, Δ_rH^Θ_m(s), was calculated as (145.306±0.519) kJ mol^-1 on the basis of a designed thermochemical cycle. The thermodynamics of reaction of formation of the complex was investigated by changing the temperature of liquid-phase reaction. Fundamental parameters, the apparent reaction rate constant (k), the apparent activation energy (E), the pre-exponential constant (A), the reaction order (n), the activation enthalpy (Δ_rH^Θ_≠), the activation entropy (Δ_rS^Θ_≠), the activation free energy (Δ_rG^Θ_≠) and the enthalpy (Δ_rH^Θ_≠), were obtained by combination of the thermodynamic and kinetic equations for the reaction with the data of thermokinetic experiments. The constant-volume combustion energy of the complex, Δ_cU, was determined as (-18673.71±8.15) kJ mol^-1 by a RBC-II rotating-bomb calorimeter at 298.15 K. Its standard enthalpy of combustion, Δ_cH^Θ_m, and standard enthalpy of formation, Δ_fH^Θ_m, were calculated to be (-18692.92±8.15) kJ mol^-1 and (-51.28±9.17) kJ mol^-1, respectively.     

The Neumann and Young equations for nematic contact lines

The Neumann and Young equations for three-phase nematic contact lines have been derived using the momentum balance equation and classical liquid crystal physics theories. The novel finding is the presence of bending forces, originating from the anchoring energy of nematic interfaces, and acting on the contact line. The classical Neumann triangle or tensile force balance becomes in the presence of a nematic phase the Neumann pentagon, involving the usual three tensile forces and two additional bending forces. The Young equation that describes the static contact angle of a fluid in contact with a rigid solid is again a tensile force balance along the solid, but for nematics it also involves an additional bending force. The effects of the bending forces on contact angles and wetting properties of nematic liquid crystals are thoroughly characterized. It is found that in terms of the spreading coefficient, bending forces enlarge the partial wetting window that exists between dewetting and spontaneous spreading. Bending forces also affect the behaviour of the contact angle, such that spreading occurs at contact angles greater than zero and dewetting at values greater than pi. Finally, the contact angle range in the partial wetting regime is always less than pi.     

The Neumann and Young equations for nematic contact lines

The Neumann and Young equations for three-phase nematic contact lines have been derived using the momentum balance equation and classical liquid crystal physics theories. The novel finding is the presence of bending forces, originating from the anchoring energy of nematic interfaces, and acting on the contact line. The classical Neumann triangle or tensile force balance becomes in the presence of a nematic phase the Neumann pentagon, involving the usual three tensile forces and two additional bending forces. The Young equation that describes the static contact angle of a fluid in contact with a rigid solid is again a tensile force balance along the solid, but for nematics it also involves an additional bending force. The effects of the bending forces on contact angles and wetting properties of nematic liquid crystals are thoroughly characterized. It is found that in terms of the spreading coefficient, bending forces enlarge the partial wetting window that exists between dewetting and spontaneous spreading. Bending forces also affect the behaviour of the contact angle, such that spreading occurs at contact angles greater than zero and dewetting at values greater than pi. Finally, the contact angle range in the partial wetting regime is always less than pi.     

Wetting and dewetting behaviour of hygroscopic liquids: Recent advancements

This review covers the most recent researches on wetting and dewetting phenomena related to the application of hygroscopic liquids in industry and technologies. Hygroscopic liquids actively absorb moisture from the surrounding air; therefore, they are used in the processes required for the moisturizing of surfaces or preventing the icing as well as control of evaporation rate and the humidity. The air humidity and wettability of substrates were shown to be crucial parameters affecting the wetting/dewetting kinetics of hygroscopic liquids. It is the adsorption of moisture on the hydrophilic surface that promotes the formation and spread of the precursor layer. The latter, in turn, is the droplet-spreading driver. The absorption of moisture by the liquid itself gives only a slight effect on wetting. The work devoted to the dewetting of hygroscopic liquids during evaporation, and the influence of thermal effects arising from contact with moist air on the wetting kinetics is also considered.     

Wetting of Solids by Aqueous Solutions of Surfactant Binary Mixtures: 2. Wetting of High-Energy Surface

Wetting of glass by aqueous solutions of binary mixtures of cationic and nonionic surfactants was studied in the range of overall concentrations c ₀ = 10^–8–10^–2 M at the molar fraction of cationic surfactant = 0.2, 0.5, and 0.8. It was established that the character of glass wetting is determined by the presence of cationic surfactant in the mixture: contact angle isotherms (c ₀) exhibit maximum, as in the case of individual cationic component solutions. Maximal values are virtually independent of the nature of cationic surfactant and its molar fraction in the mixture. It was shown that the synergistic effect in glass wetting is controlled by the chemical structure of surfactant cation.     

Dynamic Contact Angles of Water in Ultrathin Capillaries

Velocities of motion V of advancing meniscus of water in quartz capillaries with radii from 45 to 270 nm was directly measured using an optical microscope. In the case, when the meniscus advanced over the wetting film that is remained after the previous meniscus receding, hysteresis was not observed, and the wetting was complete. When the meniscus advanced over the yet unwetted surface, the dynamic contact angle _d greatly depended on V, this dependence was the more pronounced, the smaller the r value. As the velocity V increases to 10^–3 cm s^–1, the value of _d rises to 60°–70° reaching the plateau. Preliminary adsorption of water vapors on the capillary surface markedly decreases the values of _d. The results obtained cannot be explained in terms of hydrodynamic and barrier theories of the contact angle. It was assumed that the controlling factor is the kinetics of vapor adsorption on the capillary surface in front of advancing meniscus.     

聚合物熔体膜在基体表面的润湿和铺展行为

聚合物熔体膜在基体表面上的润湿和铺展行为受铺展系数和Hamaker常数影响。对于不能在基体表面上铺展的聚合物膜,当处于其玻璃化温度以上时,聚合物熔体膜将破裂,出现非连续区域。随着体系处于聚合物玻璃化温度以上时间的延长,非连续部分尺寸不断增长,增长速率与表面张力、聚合物粘度、聚合物液滴在基体表面的平衡接触角等因素有关,平衡后聚合物以液滴的形式在基体表面稳定存在。将带功能端基聚合物加入不能在基体表面上铺展的聚合物中,通过修饰聚合物与基体界面或改变聚合物熔体膜的表面张力,可以使原来不能在基体表面铺展的聚合物保持稳定。本文综述了聚合物熔体膜的铺展和润湿动力学研究进展,并归纳了使聚合物熔体膜稳定的方法。     

Criterion of the existence of compensation relationship for non-isothermal kinetics

If, for a series of similar-type chemical transformations in non-isothermal kinetics, identical or closely similar values of T _cr are observed in the equation 1/T _cr =1/T _si + + (R/E _i)·ln (E _i q/RT _si ² the existence of the compensation relationship lnA _i= =E _i/RT _cr may regularly be assumed.     

Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate

Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate were studied under monomer‐starved conditions. To investigate the importance of the kinetics of the water phase in the nucleation process, the monomer feed rate was used as a variable to alter the monomer concentration in the aqueous phase. The emulsifier concentration in the feed was employed to alter the particle stability. Particle formation and coagulation were discussed in terms of critical surface coverage ratios. Particle coagulation occurred if the particle surface coverage dropped below θ_cr1 = 0.25 ± 0.05. The secondary nucleation occurred above a critical surface coverage of θ_cr2 = 0.55 ± 0.05. The number of particles remained approximately constant if the particle surface coverage was within θ_cr1 = 0.25 < θ < θ_cr2 = 0.55. This surface coverage band is equivalent to the surface tension band of 42.50 ± 5.0 dyne/cm that is required to avoid particle formation and coagulation in the course of polymerization. The kinetics of the water phase was shown to play an important role during homogeneous and micellar nucleations. For any fixed emulsifier concentration in the feed and above θ_cr2, the number of secondary particles increased with monomer concentration in the aqueous phase. Moreover, the presence of micelles in the reaction vessel is not the only perquisite for micellar nucleation to occur, a sufficient amount of monomer should be present in the aqueous phase to enhance the radical capture by partially monomer‐swollen micelles. The rate of polymerization increased with the surfactant concentration in the aqueous phase. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3612–3630, 2000     

Fast dynamic wetting of polymer surfaces by miscible and immiscible liquids

The initial stages of spontaneous spreading of a solvent drop (toluene) on the surface of a soluble polymer (polystyrene) have been studied with a high-speed camera. For drops of 1–4 μL volume, the increase in contact radius r can be described by a power law r μ t^a r \propto {t^{\alpha }} , with the spreading exponent α = 0.50 and for the first ≈8 ms. Thereafter, the three-phase contact line was pinned leading to a macroscopic static contact angle of Θ₀ = 12–15°. The insoluble liquids ethanol (α = 0.47, Θ₀ = 0) and water (α = 0.35, Θ₀ = 90°) showed a slower spreading. We attribute the fast spreading of toluene to the strong interaction with the polymer, like in reactive wetting. The finite macroscopic contact angle indicates the formation of a ridge by softening of polystyrene due to permeated toluene and the subsequent plastic deformation by the surface tension of the liquid. This interpretation is supported by experiments on polymers grafted from a silicon wafer. Toluene completely wets polymer brush surfaces. Transport of toluene through the vapor phase plays a significant role.     

Approaches to determine the surface tension of small particles: Equation-of-state considerations

The primary purpose of this paper is a clarification of the question how sensitive five approaches to the determination of solid surface tensions are to the form of the equation of state for interfacial tensions, which is used in the interpretation of the experimental results. The five approaches are (1) adhesion, (2) phagocytosis, (3) sedimentation volumes, (4) solidification front, and (5) contact angles. Three equation-of-state-type relations, i.e., that due to Neumann et al., the unmodified Good equation, and Antonow's rule, are considered. The first three techniques depend on thermodynamic models in a way that requires of the equation of state only symmetry, γ₁₂ = f(γ_1v, γ_2v) = f(γ_2v, γ_1v), and zero as the minimum interfacial tension, γ₁₂ = f(γ_1v, γ_2v) = 0 when γ_1v = γ_2v. All three equations (and many similar ones, which one might consider) satisfy these requirements and hence produce identical results. In other words, the validity of these three techniques and the results which they produce are not sensitive to details of the equation of state used. The last two techniques, the solidification front technique and the contact angle method, present more stringent requirements for the equation-of-state relation used. The solidification front technique eliminates Antonow's rule from further consideration because of this equation's intrinsic inability to predict particle rejection by advancing solidification fronts, a frequent experimental observation. This technique also eliminates those equation-of-state relations which violate the minimum interfacial tension condition. Finally, the contact angle technique is the most discriminating tool with which to study the merit of equation-of-state relations. Of the relations considered here, only that due to Neumann et al. yields, in conjunction with contact angle data, values for the solid surface tension which are in agreement with those obtained from the other techniques.     

Wetting–dewetting films: The role of structural forces

The liquid wetting and dewetting of solids are ubiquitous phenomena that occur in everyday life. Understanding the nature of these phenomena is beneficial for research and technological applications. However, despite their importance, the phenomena are still not well understood because of the nature of the substrate's surface energy non-ideality and dynamics. This paper illustrates the mechanisms and applications of liquid wetting and dewetting on hydrophilic and hydrophobic substrates. We discuss the classical understanding and application of wetting and film stability criteria based on the Frumkin–Derjaguin disjoining pressure model. The roles of the film critical thickness and capillary pressure on the film instability based on the disjoining pressure isotherm are elucidated, as are the criteria for stable and unstable wet films. We consider the film area in the model for the film stability and the applicable experiments. This paper also addresses the two classic film instability mechanisms for suspended liquid films based on the conditions of the free energy criteria originally proposed by de Vries (nucleation hole formation) and Vrij–Scheludko (capillary waves vs. van der Waals forces) that were later adapted to explain dewetting. We include a discussion of the mechanisms of nanofilm wetting and dewetting on a solid substrate based on nanoparticles' tendency to form a 2D layer and 2D inlayer in the film under the wetting film's surface confinement. We also present our view on the future of wetting–dewetting modeling and its applications in developing emerging technologies. We believe the review and analysis presented here will benefit the current and future understanding of the wetting–dewetting phenomena, as well as aid in the development of novel products and technologies.     

Thermodynamics and phase relationships of the ternary lanthanum-uranium-oxygen system

In the selected regions La:(La + U) = 0.05 and O:(La + U) = 2.00 of the ternary system lanthanum-uranium-oxygen emf measurements on solid state galvanic cells, coulometric titrations, and X-ray diffraction techniques were used to obtain phase boundaries and thermodynamic data in the temperature range from 600 to 1000°C. For the first time order disorder transformations of La_1−yU_yO_2+x up to 15 mole% lanthanum are reported. The transformation temperature is 1415°K for UO_2.23; 1397°K for La_0.05U_0.95O_2.23, and 1449°K for La_0.15U_0.85O_2.23. The vibrational entropy component of excess oxygen in M_1−yU_yO_2+x is estimated.     

Contact angle hysteresis

In thermodynamic equilibrium, the contact angle is related by Young's equation to the interfacial energies. Unfortunately, it is practically impossible to measure the equilibrium contact angle. When for example placing a drop on a surface its contact angle can assume any value between the advancing Θ_a and receding Θ_r contact angles, depending on how the drop is placed. Θ_a − Θ_r is called contact angle hysteresis. Contact angle hysteresis is essential for our daily life because it provides friction to drops. Many applications, such as coating, painting, flotation, would not be possible without contact angle hysteresis. Contact angle hysteresis is caused by the nanoscopic structure of the surfaces. Here, we review our current understanding of contact angle hysteresis with a focus on water as the liquid. We describe appropriate methods to measure it, discuss the causes of contact angle hysteresis, and describe the preparation of surfaces with low contact angle hysteresis.