Factors influencing the contact angle value. The contact angle,as a characteristic of the properties of solid surfaces

A survey of publications concerning the properties of solids in relation to wetting phenomena is presented. Factors influencing the contact angle value as well as problems of objective approach to research into wetting phenomena are discussed. Peculiarities of the direct and reverse processes during the formation of the solid—liquid—vapor three-phase contact and the inevitability of contact angle hysteresis for polar solids and liquids are analyzed. It is suggested that contact angle hysteresis is due to high energy of the interaction between the liquid and the solid and hence a long relaxation time of the three-phase contact system. Specific features of the response of a solid surface to all surface processes (“chemomechanics”) is discussed. Cleaning of solid surfaces as well as surface preparation for repeated measurements is considered. It is shown that good reproducibility of results is possible if conditions for sample preparation are met. The results of determination of the activation energy for wetting of glass surface with water are presented. The influence of the structure of solids (their hardness) on the contact angle values is demonstrated. Inevitability of the presence of different-type active sites characterized by different dissociation constants (pK_a) on the surface of solids is discussed. The pK_a values and content of these surface sites obtained from potentiometric titration and wetting data are estimated. The estimates thus obtained are in reasonable agreement with each other and can thus be used in practical applications. However, potentiometric titration is currently inappropriate for evaluating the content of individual surface sites as well as the surface charge.     

Kinetics of free-surface decomposition of dolomite single crystals and powders analyzed thermogravimetrically by the third-law method

The results of our thermogravimetric experiments on the decompositions of dolomite crystals and powders and some data reported in the literature were used for the determination of the E parameter of the Arrhenius equation by the third-law method and estimation of the self-cooling effect on the results of these determinations. The experimental values of the E parameters, obtained under different conditions by the third-law method, are in excellent agreement with the theoretically predicted values based on the mechanism of congruent decomposition of CaMg(CO₃)₂ into gaseous CO₂, CaO and MgO with the simultaneous condensation of low-volatility CaO and MgO molecules. The second important result of this study is the first quantitative comparison of absolute rates of decomposition of powder samples and single crystals. Based on these results, a simple procedure was proposed for the determination of the E parameter by the third-law method from the data obtained for powder samples. It consists in the evaluation of the absolute decomposition rate of a powder sample (reduced to the unit of the outer surface area of a pellet formed by the powder sample in a cylindrical crucible). The value received is lowered by the empirical factor and then used for the calculation of the E parameter by the third-law method. The value of this factor (2.8±0.4) does not depend on the temperature, residual pressure of air in the reactor, grain size and mass of a powder sample. This procedure permits to expand the application of the third-law method to the determination of decomposition kinetics for many solids available only in the powder form.     

Factors affecting interaction of radiocesium with freshwater solids

The paper aims at the analysis of principal factors affecting the interaction of radiocesium with freshwater solids, important for the migration of radiocesium in rivers. Uptake and release of radiocesium by bottom sediment and suspended solids from a small stream were studied as a function of contact time during the uptake and releases, of concentration of the solid phases and of temperature, using laboratory model experiments. Kinetics of the uptake were found to be singificantly affected by temperature and concentration of the solid phase. The kinetics and the concentration effect can be quantitatively described using kinetic model of two parallel or consecutive reactions. Kinetic parameters for the model were determined. Distribution coefficient K_d is independent of sediment concentration in the range of 20–70 mg·dm^?3 but passes through a maximum at higher concentration values. Release of radiocesium adsorbed on the freshwater solids was found to be quite rapid and a simple kinetics of the release from freshwater solids was observed. The amount released decreased with increasing contact time of radiocesium with solid phase. Quantitative evaluation of the release revealed partial irreversibility of radiocesium uptake on the solids studied. Results obtained are compared with literature data and conclusions are drawn on the importance of the factors studied for modeling of radiocesium migration in rivers.     

Measurement and thermodynamic model study on equilibrium solubility in the ternary system KCl-KBr-H2O at 323.15 K

The solubilities and the refractive indices of the KCl-KBr-H₂O system at 323.15 K were studied with the isothermal dissolution method. The phase diagram and refractive index diagram were plotted for this system at 323.15 K. There is only one crystallization field of the solid solution K(Cl, Br). The system belongs to the solid solution type. The refractive indices of the equilibrium solution varies regularly with w(KCl) increasing. The calculated refractive index data are in good agreement with the experimental data. The parameter Ψ_{K, Cl, Br} at 323.15 K was fitted using the measured solubility data in this study. The equilibrium constant equation for the equilibrium solids at 323.15 K were obtained using the different Pitzer parameters from the literature or this work with a method using the activity product constant. The calculated solubilities agree well with experimental values.     

Comparison of adsorption affinity of polyacrylic acid for surfaces of mixed silica–alumina

The influence of solution pH (in the range 3–9) on the adsorption of polyacrylic acid (PAA) on the mixed silica–alumina surface (SA-3: SiO₂ 97 %–Al₂O₃ 3 % and SA-96: SiO₂ 4 %–Al₂O₃ 96 %) was investigated. The following methods were applied in experiments: spectrophotometry, viscosimetry, potentiometric titration, and microelectrophoresis, which enable determination of adsorbed amount of the polymer, thickness of its adsorption layers, surface charge density, and zeta potential of solid particles in the presence and absence of PAA, respectively. The obtained results indicate that rise of solution pH causes the decrease of PAA adsorption and the increase of its adsorption layer thickness on surfaces of both solids. Moreover, significantly higher adsorption of polyacrylic acid was obtained on the SA-96 surface. This is a result of more favorable electrostatic interactions occurring between the adsorbing polymer chains and the SA-96 surface and formation of a greater number of adsorbate-adsorbent connections through hydrogen bridges.     

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.     

On the dynamics of H2 adsorption on the Pt(111) surface

The dynamics and kinetics of the dissociation of hydrogen over the hexagonal close packed platinum (Pt(111)) surface are investigated using Car–Parrinello molecular dynamics and static density functional theory calculations of the potential energy surfaces. The calculations model the reference energy‐resolved molecular beam experiments, considering the degrees of freedom of the catalytic surface. Two‐dimensional potential energy surfaces above the main sites on Pt(111) are determined. Combined with Car–Parrinello trajectories, they confirm the dissociative adsorption of H₂ as the only adsorption pathway on this surface at H₂ incindence energies above 5 kJ/mol. A direct determination of energy‐resolved sticking coefficients from molecular dynamics is also performed, showing an excellent agreement with the experimental data at incidence energies in the 5–30 kJ/mol range. Application of dispersion corrections does not lead to an improvement in the prediction of the H₂ sticking coefficient. The adsorption reaction rate obtained from the calculated sticking coefficients is consistent with experimentally derived literature values.     

Influence of the chemical and textural properties of k2cr2o7 supported on solids,in the oxidation of cholesterol

Several K₂Cr₂O₇ supported reagents on inert solids have been obtained by deposition of K₂Cr₂O₇ on different solids.A1PO ,BPO and solid mixtures BPO -A1PO are used as supports. The K₂Cr₂O₇ is deposited on solid as large crystals detected by an X-ray diagram in the Cr50 and Cr25 series giving solids with very low surface areas.The number of acid and oxidizing sites is dtermined.Solids supports with small surface areas and large pore diameter give K₂Cr₂O₇ supported reagents with higher activity than those obtained from solids with high surface areas and large pores.     

Amides as a model system of low molar mass algal organic matter. Influence on the adsorption of p-nitrophenol on activated carbon

In this study, the adsorption equilibrium and diffusivity parameters of p-nitrophenol were estimated for water containing different concentrations of secondary amides. Commercial powdered activated carbon was used as an adsorbent. The external mass transfer coefficient (k_f), the surface diffusion coefficient (D_s) and the standard free Gibbs energy were calculated for p-nitrophenol in the presence of different secondary amide concentrations. The analysis established that there are correlations between structural parameters of amides, on the one hand, and diffusion and thermodynamic parameters for p-nitrophenol adsorption process, on the other. It was noticed that voluminous hydrophobic amides decreased the adsorption capacity of p-nitrophenol on activated carbon. On the basis of the results obtained for external mass transfer coefficients, it is assumed that amides cause the reduction of adsorption capacity of p-nitrophenol onto activated carbon by concentrating at the solid/liquid interface.     

Dynamics of gaseous I2 on an iron surface: a study of adsorption and exchange processes at room tempeature

The quasi-classical trajectory method has been applied to investigate the interaction of I₂ molecules with an Fe(100) surface. Rate constants in the temperature range 273 ≤T ≤ 373 K were calculated under different conditions. The emphasis was on a sensitivity study with respect to (a) parameters of the potential governing the interaction process and (b) characteristics of the solid surface encountered by the I₂ molecule. The unknown potential parameters were chosen so that for a rigid homogeneous surface the calculated rate constant for iodine adsorption at T = 298 K fits the experimental value k = 2.0 × 10³ cm/s. When a corrugated or dynamic surface was assumed, this value decreased by 20%. However, making the surface dynamic and dissipative caused the calculated rate constants to be almost identical to the rigid homogeneous surface.     

Comparison of methods concerning the selection of a function describing the thermal decomposition of solids

Determination of the kinetic parameters of the thermal decomposition of solids usually requires a knowledge of the function describing the mechanism of decomposition. Research was made to obtain a method that enables the selection of one of the 16 functions used in the literature in such a way that it best describes experimental data in the isothermal measurements. The results obtained indicate that the best fit of experimental and calculated data is gained by using the weighted least-squares method with the equation g(α) = kt + b and using criteria based on the minimization of the difference between α_exp and α_calc     

The potential dependence and kinetics of HCOOH adsorption on rhodium electrodes

The formic acid adsorption on an electrochemically prepared rhodium electrode has been studied by the radiochemical method. Electrochemical properties of the rhodium electrode surface in 0.5 M H₂SO₄ have been investigated by cyclic voltammetry. It has been shown that starting from E=0.20 V the rate of HCOOH adsorption is markedly potential dependent being practically independent of the electrode potential up to E=0.20 V. It seems that the HCOOH adsorption process may be explained on the basis of the two-sites kinetics model. The data obtained for HCOOH adsorption on a rhodium electrode have been compared with those for a platinum electrode reported previously.     

Intrinsic adsorption properties of CO2 on 5A and 13X zeolite

Data for the adsorption of CO₂ on 5A (CaA) and 13X (NaX) zeolite are critically evaluated. In addition, fresh data for the adsorption of CO₂ on 13X zeolite is reported. Three intrinsic properties are examined: q _max , the saturation loading, K _H , the Henry constant, and (?ΔH) _q , the isosteric heat of sorption. Below a reduced temperature T _r , of 0.9, the q _max values for both 5A and 13X zeolites are similar to theoretical values that may be derived using zeolitic crystallographic properties and the sorbate density calculated using the Rackett equation. For the region 0.9 ≤ Tr ≤ 1.0, the calculated q _max values exceed the theoretical values similarly calculated, indicating that the molecules have a smaller molar volume than in a similar liquid phase. This is a similar result to that observed in ionic liquids. Linear regressed equations are derived for q _max for the region 0.9 ≤ Tr ≤ 1.25. The Henry constant values for 5A are remarkably consistent for the five studies examined, with a correlation coefficient, R, of 0.999 for the van’t Hoff equation, but for the seven studies examined in 13X the data is more disperse as indicated by a correlation coefficient R of 0.899 for the van’t Hoff equation. The values of (?ΔH) _q , the isosteric heat of sorption are in agreement with the literature. An explanation is advanced for the discrepancy between the higher heats of sorption values obtained calorimetrically from those obtained from isosteric adsorption studies. Finally, the fresh data is observed to fit the Toth model with regression coefficients of 0.999. However, the parameters obtained for the Toth equation by regression are significantly different from the intrinsic properties derived earlier, indicating the difficulty of deriving intrinsic parameters from isotherm fits.     

Optimization of Microwave-Assisted Extraction for the Determination of Glycyrrhizin in Menthazin Herbal Drug by Experimental Design Methodology

In the present study, a microwave-assisted extraction (MAE) method has been investigated for the extraction of glycyrrhizin from Menthazin herbal drug. The extracted samples have been analyzed by a developed reversed-phase liquid chromatography with ultraviolet detection. The separation was performed by a Eurospher-100 C₈ reversed-phase column (250 × 4.6 mm i.d., 5 μm) and the mobile phase consisted of methanol:acetonitrile:water:glacial acetic acid (30:30:40:1 v/v/v/v) with a flow rate of 0.8 mL min^?1. The extraction procedure has been screened by a two level full factorial design for determination of statistically significant parameters. Thereafter, the identified parameters, extraction temperature, time and solvent volume were optimized by a Box–Behnken design. The proposed mathematical model was based on analysis of variance results and correctly explained the behavior of the response in the experimental domain. R ² value adjusted for numbers of degrees of freedom was 0.9915 and P-value for lack of fit, 0.8499 at the 95% confidence level, P > 0.05. The optimal condition identified were extraction temperature, 70 °C, time, 13.8 min and solvent volume 2.0 mL. To evaluate the applicability of the proposed MAE method, results were compared with those obtained with the liquid extraction method. Extraction efficiency and precision were higher when MAE has been used. The proposed method allows extracting the glycyrrhizin in a small quantity of solvent and faster than the liquid extraction method.     

An improved methodology for adsorption characterization of unmodified and modified silica gels

An improvement in the adsorption characterization of the surface and structural properties of unmodified and modified mesoporous silica gels is presented. This improvement was achieved by selection of proper macroporous silica as the reference solid for adsorption characterization of porous silica gels. Experimental illustration is provided for unmodified and n-octyl-modified silica gels of different bonding density. The surface and structural properties of these silica gels were characterized by utilizing the standard adsorption data for both unmodified and octyl-modified LiChrospher Si-1000 macroporous silica gels. It was shown that the standard nitrogen adsorption data have an appreciable influence on the analysis of the pore size and surface properties of silica gels. This analysis can be improved by selecting the reference solid of the surface properties close to those of the silica gel studied.     

Estimation of solid vapor pressures of pure compounds at different temperatures using a multilayer network with particle swarm algorithm

Solid vapor pressures (P^S) of pure compounds have been estimated at several temperatures using a hybrid model that includes an artificial neural network with particle swarm optimization and a group contribution method. A total of 700 data points of solid vapor pressure versus temperature, corresponding to 70 substances, have been used to train the neural network developed using Matlab. The following properties were considered as input parameters: 36 structural groups, molecular mass, dipole moment, temperature and pressure in the triple point (upper limit of the sublimation curve), and the limiting value P^S → 0 as T → 0 (lower limit of the sublimation curve). Then, the solid vapor pressures of 28 other solids (280 data points) have been predicted and results compared to experimental data from the literature. The study shows that the proposed method represents an excellent alternative for the prediction of solid vapor pressures from the knowledge of some other available properties and from the structure of the molecule.     

Theoretical model for moisture adsorption on ionic liquids: A modified Brunauer-Emmet-Teller isotherm approach

The adsorption of atmospheric water on the air-liquid interface of ionic liquids is analyzed by means of a modified version of the Brunauer-Emmet-Teller (BET) multilayer adsorption isotherm including lateral interactions between adsorbed molecules, treated in a mean-field fashion. Recently reported experimental results of water adsorption on hydrophobic ionic liquids of the 1-alkyl-3-methyl imidazolium tetrafluoroborate (C_nMIM-BF₄) family are analyzed in the present theoretical framework. The calculated values of the lateral interaction are seen to be compatible with the Keesom dipole-dipole interaction in water, confirming the validity of the multilayer assumption hypothesis. A somehow surprisingly ordered hydrophilic-like adsorption of atmospheric water is suggested to take place in the free surface of hydrophobic ILs of the imidazolium family.