Nanoporous surface wetting behavior: the line tension influence

The aim of this work is to develop a physical model to describe the evolution of the apparent contact angle for four different liquids on nanotextured alumina surfaces with different pore radius. The nanoporous alumina templates were fabricated by anodization of Al foil in a 0.3 M oxalic acid solution. Scanning electron microscopy was used to characterize the morphology of the surfaces. The templates are approximately 400 nm in thickness and consist of a well-ordered hexagonal array of uniform radius pores spaced 105 nm apart with pore radii from 12 to 42 nm. The wettability of nanoporous alumina templates was investigated using contact-angle measurements. We measured the contact angles using four liquids: water, ethylene glycol, aniline, and a mixture of ethylene glycol and aniline. We developed a new theoretical model for the contact angle on nanoporous surfaces as a function of the pore radius. This model is based on energy considerations and involves liquid penetration into the nanopores driven by the capillarity (Laplace's law). Because the air is compressed inside the pores, this model also includes the effect of the line tension. This is important because the three-phase line length is greatly enhanced in our nanoporous structures. For example: for a millimeter-sized droplet, the three-phase line around the perimeter of the droplet is a few millimeters long, whereas the total three-phase line within the pores can reach several tens of meters. Using our model, the line-tension value for our nanopore samples is positive and ranges from 4 to 13 × 10(-9) N, which falls within the wide interval from 10(-11) to 10(-5) N quoted in the literature. Nanoporous surfaces may allow the effect of line tension to be visible for micro- to macrodroplets.     

A study on the relationship between critical surface tension of wetting and oil agglomeration recovery of calcite

In this study, it was investigated that the relationship between the critical surface tension of wetting of calcite and agglomeration recovery depending on pH and amount of collector (Na oleate). For this purpose, effects of pH and the amount of collector on the agglomeration recovery were investigated and, also, zeta potential measurements and Fourier transform infrared spectrophotometer (FTIR) analyses have been carried out to determine the adsorption type of Na oleate on calcite surface. In this paper, the decisive rules could not put down as evidence of agglomeration success with the critical surface tension of wetting value as in the flotation, because there are different liquids as water and oil (bridging liquid) in the agglomeration system.     

Determination of line tension for systems near wetting

Contact angle measurements for three n-alkanes, heptane, octane, and nonane, on two different self-assembled surfaces (SAM) are reported as a function of drop size. These liquids all formed low contact angles (below 20 degrees ); the measurements were performed using an accurate method for systems with low contact angle, ADSA-D. The observed drop size dependence of the contact angles was interpreted using the modified Young equation. It was concluded that the observed drop size dependence of contact angles was due to line tension. The choice of systems also provided the opportunity to examine the behavior of the line tension for systems near wetting (i.e., low contact angles). It was determined that the line tension is positive and ranges from below 10(-7) to just below 10(-6) J/m for the systems studied; the observations suggested that the line tension decreases as the contact angle decreases and likely vanishes at complete wetting.     

Determination of critical micelle concentration of surfactants by capillary electrophoresis

Capillary electrophoresis (CE) has been proven to be a convenient and useful technique for the determination of the critical micelle concentration (CMC) of a surfactant in an electrophoretic system under operating conditions. In this review, methodological approaches to the determination of the CMC of surfactants by CE technique are described. The practical requirements for making such measurements and the CMC values of surfactants determined by CE methods are presented. In addition, difficulties and uncertainty, as well as misconceptions that may arise in the CMC determination are discussed.     

Interfacial tension and critical wetting phenomena of the binary liquid mixture cyclohexane-acetonitrile near the critical solution temperature

The determination of the capillary parameter for the partially miscible binary liquid system acetonitrile + cyclohexane has been reported. The system has closely matched densities and the data points are affected by gravity only when the fractional change of the temperature from the critical temperature is about 10^–6. The system also exhibits very interesting wetting properties. The interfacial tension has been fitted to a power law =0.061 (T_c-T)^1.26. The effect of doping with water and the instability of the wetting layer, which crucially depends on the interfacial tension, are studied.     

Determination of critical micelle concentration of non-ionic surfactants by electrocapillary curves

A method is described for the determination of the critical micelle concentration of non-ionic surfactants by measurement of their effect on the electrocapillary curves for a dropping mercury electrode. The method was compared with the iodine-solubilization, surface tension, and polarographic maximum suppression methods.     

Selective wetting of heterogeneous surfaces by solutions of surfactants

Selective wetting of dimethyldichlorosilane-modified glass plates by solutions of tetradecyltrimethylammonium bromide (TDTAB), a cationic surfactant, in p-xylene has been studied. When surfactant concentrations are lower than the critical micelle concentration (CMC), the contact angles under selective wetting conditions increase with increasing hydrophobic surface fraction. When surfactant concentrations are higher than CMC, contact angles are the same on all substrates studied. The adsorption of the surfactant on hydrophilic and hydrophobic regions of heterogeneous surfaces and the stability of wetting films are taken into account in interpreting the results.     

Determination of surface area of copolymer latex particles by surface tension measurements

The surface tension γ of four copolymer latices, of their respective serums, and of aqueous solutions of dispersant alone were measured at various dilutions. By extrapolating the surface excess of dispersant (calculated by the Gibbs adsorption equation) both at the aqueous solution surface and at the serum surfaces to 1/c = 0 (c being the bulk concentration of dispersant) the same limiting site area A_lim per adsorbed molecule was determined. Amounts of dispersant adsorbed by copolymer particles at various dilutions were determined from differences between the known total concentrations of the dispersant in latex and in serum at the same γ. These values were then extrapolated to the maximum adsorption at 1/c = 0 in latex. The surface area of copolymer particles was determined therefrom by using A_lim. The average particle radius calculated this way agrees reasonably well with electron microscope measurements. Thus it appears that the method for determining latex particle surface area by surfactant titration may be calibrated by means of the Gibbs adsorption equation, provided one uses A_lim and not the site area at the critical micelle concentration of dispersant.     

Dynamic surface tension and adsorption mechanisms of surfactants at the air-water interface

Recent advances in understanding dynamic surface tensions (DSTs) of surfactant solutions are discussed. For pre-CMC solutions of non-ionic surfactants, theoretical models and experimental evidence for a mixed diffusion-kinetic adsorption mechanism are covered. For micellar solutions of non-ionics, up to approximately 100 x CMC, the DST behaviour can also be accounted for using a mixed mechanism model. Finally, the first reported measurements of the dynamic surface excess Gamma(t), using the overflowing cylinder in conjunction with neutron reflection, are described.     

Determination of interfacial tension between two immiscible polymers with and without surfactants at the interface

A second-order drop deformation method for inferring interfacial tension between two immiscible polymers is proposed and shown to improve the accuracy of tension estimate appreciably. A small step-strain method, which uses a strong flow (capillary number >1) and short flow time approximately O(0.1s), is successfully developed to avoid complications caused by the surfactants for surfactant-laden drops. This method is demonstrated to give good tension estimates for a range of viscosity ratios and surface coverage.     

Adsorption of oleic acid and triolein onto various minerals and surface treated minerals

A systematic investigation of the adsorption of oleic acid was under-taken with various minerals and surface treated minerals, viz., kaolinite, treated kaolinites, montmorillonites, talcs, gibbsites, calcites and a treated calcite. Adsorption onto kaolinite, two of the treated kaolinites (amine and MgSiO₃ treated), talcs and gibbsites was well correlated by the Langmuir model, while adsorption on the treated calcite was well correlated by the Freundlich model. Adsorption on a cationic polymer-treated kaolinite was explained in terms of a cooperative mechanism. Adsorption onto montmorillonites was explained in terms of a penetrative mechanism involving exchangeable cations.Oleic acid adsorption was compared with triolein adsorption on one of the montmorillonites, two adsorbents produced by the surface treatment of this montmorillonite, and one of the talcs. The triolein adsorption of the montmorillonite was considerably less than its oleic acid adsorption, and was explained in terms of a cooperative mechanism. Triolein adsorption of the treated montmorillonites, and the talc was well correlated by the Langmuir model. Larger amounts of triolein were taken up by the treated montmorillonites than by the untreated montmorillonite. The triolein adsorption of the talc was greater than its oleic acid adsorption.     

Obtaining surface tension from contact angle data by the individual representation approach

Young equation is the fundamental equation of wetting theory in which the connection among the surface tensions, \(\gamma _{{\varphi \psi }} \) and the contact angle, θ _L, are given. The surface tension of solid surfaces, however, cannot be obtained directly from the Young equation. In this paper, the application of the individual representation theory is demonstrated for the determination of surface tensions of solids (or any phase pair) using experimentally obtained contact angle data. According to this approach, the state of the interfacial layers depends upon, by definition, the properties of the bulk phases in every heterogeneous system, and thus, it complements the traditional capillary theory.     

Thermodynamic approach to calculating the surface tension of single-component liquids by computer simulations

A thermodynamic method for computing the surface tension at a flat liquid-vapor interface by the Monte Carlo or molecular dynamics methods over a wide temperature range was proposed. The approach is based on the Gibbs separating surface method; it does not require information on the mechanical state of the surface layer.     

Kinetics of wetting of a glass surface by molten polyethylene: An empirical approach

The spreading kinetics of molten polyethylene was studied on a flat type A glass surface. Empirical equations are known in the literature which fit the experimental data only at short times. We present a relationship between the surface free energy and the shape of the melt drop interpreted by the θ contact angle. Relating the experimental θ values obtained at different times with the surface free energy (F), we obtain curves F,t. Using isothermal data at 150°C, we could fit them with a linear relationship between In (F-F_∞)/(F₀-F_∞) and In t/t₀, where o and ∞ are related to initial and equilibrium conditions.     

Determination of the surface characteristics of particulate fillers by inverse gas chromatography at infinite dilution: a critical approach

CaCO3 fillers were investigated by inverse gas chromatography (IGC) to determine the dispersion component of their surface tension as well as their acid-base character. Because of the high energy of the filler surface, it readily adsorbs water, thus the parameters measured by IGC depend on the conditioning temperature, as well as on the measurement conditions. As a consequence, the determined surface characteristics are not material constants; different fillers or the effect of coating can be compared only under standard conditions. The use of the same conditioning and measurement temperature eliminates the effect of measurement time. Under appropriate standard conditions the acid-base characteristics of the filler can be determined reliably. However, the accuracy of the determination and the value of the derived parameters depend very much on the selected approach and on the acid-base constants used for the probe molecules. A critical analysis of the approaches used in the current literature pointed out those that yield the most reasonable and accurate values. The results prove that the surface of CaCO3 is strongly basic in character. Coating significantly reduces basicity. Surprisingly, the filler coated with an amount of stearic acid resulting in minimum surface tension showed relatively strong acidity, which indicates a coating exceeding monolayer coverage and/or the uneven distribution of the surfactant on the surface.     

Binding ability of sodium valproate with cationic surfactants and effect on micellization: calorimetric,surface tension,light scattering and spectroscopic approach

Journal of Thermal Analysis and Calorimetry - The interactions of surfactant molecules with biologically active pharmaceutical ingredients remain an important area of research due to the necessity...     

Dynamic surface tension of aqueous solutions of ionic surfactants: role of electrostatics

The adsorption kinetics of the cationic surfactant dodecyltrimethylammonium bromide at the air-water interface has been studied by the maximum bubble pressure method at concentrations below the critical micellar concentration. At short times, the adsorption is diffusion-limited. At longer times, the surface tension shows an intermediate plateau and can no longer be accounted for by a diffusion-limited process. Instead, adsorption appears kinetically controlled and slowed down by an adsorption barrier. A Poisson-Boltzmann theory for the electrostatic repulsion from the surface does not fully account for the observed potential barrier. The possibility of a surface phase transition is expected from the fitted isotherms but has not been observed by Brewster angle microscopy.