Study of solvent and substituent effects on organic acids. Part III. Consistency of a thermodynamic and an electrostatic model with experimental results of ortho-benzoic acid derivatives

The experimental models of the transfer proton process of ortho-benzoic acid derivatives in DMSO-water solutions were used to check an electrostatic and a thermodynamic model.As in the case of the meta and para derivatives, the electrostatic model was found to be consistent with experimental results.     

Electrochemical nucleation with diffusion-limited growth. Properties and analysis of transients

A general equation for the potentiostatic transient for the case of electrochemical nucleation with diffusion-controlled growth has been published recently [L. Heerman, A. Tarallo, J. Electroanal. Chem. 470 (1999) 70]. This equation essentially is a correction to the model of Scharifker and Mostany [J. Electroanal. Chem. 177 (1984) 13] but leads to quite different predictions. This paper discusses the differences between these two models with special emphasis on the analysis of experimental transients. Numerical data are presented which allow the determination of the nucleation parameters (nucleation rate constant A and site density N₀). Finally, it is shown that plots of I/I_max versus t/t_max, which are widely used in the literature for the graphical representation of experimental results, are not very sensitive and can easily lead to misinterpretation of experimental results.     

Solubility of a spiroindolinonaphthoxazine photochromic dye in supercritical carbon dioxide: Experimental determination and correlation

The measurement and correlation of the experimental solubility of a spiroindolinonaphthoxazine photochromic dye (1,3-dihydro-3,3-dimethyl-1-isopropyl-6′-(2,3)-(dihydroindole-1-yl)spiro[2H-indole-2,3′-3H-naphtho[2,1-b] [1,4] oxazine]) in supercritical carbon dioxide (scCO₂) is reported. Results were obtained using a static analytical method, at 308.0, 318.0 and 328.0 K, and in a pressure range from 10.0 to 26.0 MPa. Solubility experimental data were correlated with three density-based models (Chrastil, Bartle and Méndez-Santiago–Teja models), with the Ziger–Eckert semi-empirical correlation and with two cubic equation-of-state (EOS) models, namely the Peng–Robinson EOS (PR-EOS) and the Soave–Redlich–Kwong EOS (SRK-EOS), together with the conventional van der Waals mixing and combining rules. Good correlation results were obtained between the calculated and experimental solubility, to all fitted models. Solubility results clearly indicate the feasibility of processing this dye, and possibly this class of photochromic dyes, using supercritical fluid technologies and processes, for example, supercritical fluid dye impregnation of polymer host materials.     

Batch and continuous fixed-bed column biosorption of thorium(IV) from aqueous solutions: equilibrium and dynamic modeling

Biosorption of thorium(IV) from aqueous solution by Cystoseira indica alga was investigated in batch and fixed-bed column experiments. In the batch study the effects of pH and initial concentration were investigated. The optimum pH for Th(IV) biosorption was found to be 3.5. The experimental isotherms obtained at different pH conditions were analyzed using three two-parameter models and three three-parameter models. Among the two-parameter models the Langmuir model and among the three-parameter models the Redlich–Peterson model vividly described the equilibrium data. The results showed that C. indica alga is a homogeneous biosorbent and Th(IV) biosorption is a favorable and physical process. The maximum biosorption capacity from the Langmuir model was 151.3, 195.7 and 120.6 mg/g at pH 2.5, 3.5 and 4.5, respectively. The continuous isotherm obtained from the column data was modeled by the Langmuir model and the maximum biosorption capacity was 283.8 mg/g. The experimental data were fitted by the use of an analytical and a numerical model, namely Clark and mass transfer models. The results showed that the mass transfer model adequately described the experimental data. Sensitivity analysis revealed that the value of k _in has more effect than the axial dispersion coefficient (D _z) on the shape of breakthrough curve.     

Finite element modelling of mode I delamination specimens by means of implicit and explicit solvers

The simulation of delamination using the Finite Element Method (FEM) is a useful tool to analyse fracture mechanics. In this paper, simulations are performed by means of two different fracture mechanics models: Two Step Extension (TSEM) and Cohesive Zone (CZM) methods, using implicit and explicit solvers, respectively.TSEM is an efficient method to determine the energy release rate components G_Ic, G_IIc and G_IIIc using the experimental critical load (P_c) as input, while CZM is the most widely used method to predict crack propagation (P_c) using the critical energy release rate as input.The two methods were compared in terms of convergence performance and accuracy to represent the material behaviour and in order to investigate their validity to predict mode I interlaminar fracture failure in unidirectional AS4/8552 carbon fibre composite laminates.The influence of increasing the loading speed and using mass scaling was studied in order to decrease computing time in CZ models.Finally, numerical simulations were compared with experimental results performed by means of Double Cantilever Beam specimens (DCB).Results showed a good agreement between both FEM models and experimental results.     

Chemometric approach in optimization of micellar liquid chromatographic separation of some halogenated phenols

Simultaneous optimization of separation quality and analysis time of the micellar liquid chromatography of nine chlorophenol isomers was investigated. The effect on retention of three experimental parameters was studied using multivariate analysis. The factors studied were the concentration of sodium dodecyl sulfate, propanol content and pH of the mobile phase. The experiments were performed according to the face-centred cube central composite design and the inverse form of the experimental retention times for analytes were fitted to the polynomial models. The results of the analysis of variance showed that the models obtained explain greater than 99% of the variance observed in the chromatograms. Good predictive ability of the models was verified as high values of the statistics R² and F were obtained for the linear relationship between predicted cross-validated and experimental values of the dependent variable. The study showed that the use of Pareto-optimality method, an approach from multi-criteria decision-making, allows the selection of the best possible combinations of separation and analysis time in micellar liquid chromatography of chlorophenols.     

Influence of excitation and solvation on the shift of tautomeric equilibrium. A quantum-mechanical study

Two quantum-mechanical models are proposed to described a shift of tautomeric equilibrium as a result of electronic excitation and change of environment. According to the first n PD MEP model which is used to estimate the relative solvation effect on the stability of tautomers in an excited state, the calculation of the interaction energy between a solvent (simulated by a set of n point dipoles, n PD) and an excited solute molecule is based on the molecular electrostatic potential (MEP) of the corresponding excited state. In the second n PDQ model, a solvent represented by a set of n point dipoles and quadrupoles (n PDQ) modifies the solute's hamiltonian via an electrostatic interaction contribution. Comparing the results of the calculation for isolated and solvated tautomers, the n PDQ model is used to estimate the influence of electronic excitation on the change of relative stability of tautomers existing in a solution. An application of both models to 2- and 4-oxopyridine predicts a shift of the tautomeric equilibria in their excited states in accordance with experimental evidence.     

Determination and correlation of the solubility of tricin in water and ethanol mixtures

The solubility of tricin in water and ethanol mixtures was measured over the temperature range of (288.15 to 328.15) K. The concentrations of tricin in the aqueous mixtures were assayed by the ultraviolet spectrophotometric method. The experimental solubility data indicated that the solubility of tricin increases with an increase in temperature and an enrichment in ethanol content. The two models, including the modified Apelblat equation and λh equation were used to correlate the experimental solubility data. The calculated solubility of tricin shows good agreement with the experimental results. Additionally, the estimation of thermodynamic properties including the activity coefficients, dissolution enthalpy, and entropy were obtained from the experimental data. Within the studied temperature range the dissolution process of tricin is endothermic, and the driving force is the entropy.     

Semiempirical studies of inner-core energy levels: IX. Esca shifts of silicon atoms in various chemical environments by the scc-mo method

The semiempirical self-consistent charge molecular orbital (SCC-MO) method has been applied to a number of representative molecules involving Si atoms in various chemical environments. The calculated point charges were correlated with Si(2p) inner-core energy level shifts by using ground state potential, relaxation potential and transition potential models (GPM, RPM and TPM). The results are in good agreement with experimental data. Various contributions to the relaxation energy are briefly discussed.     

Application of the MSA-based models in correlating the surface tension for single and mixed electrolyte solutions

Experimental values for surface tension of single and mixed electrolyte solutions were correlated using the models based on the perturbation theory. The Mean Spherical Approximation (MSA) model, coupled with the Ghotbi–Vera (GV) and the Mansoori et al. (BMCSL) equations of state, were used to correlate the experimental values of the surface tension. The results showed that the models can favourably correlate the experimental values for single electrolyte solutions. However, it was observed that the GV–MSA model can more accurately predict the surface tension for single electrolytes, especially at higher concentrations. Two different expressions for concentration dependency of cation hydrated diameters were used. Therefore, in terms of such dependency different forms of the models, i.e., GV–MSA1, GV–MSA2, BMCSL–MSA1 and BMCSL–MSA2 were introduced. It should be stated that the prediction of the surface tension for the mixed electrolyte solutions were made without introducing any new adjustable parameters. The results showed that GV–MSA2 model can predict more accurately the surface tension of electrolyte mixtures particularly at higher concentrations. Finally, the GV–MSA model was directly used to correlate the experimental results for the surface tension for both single and mixed electrolyte solutions with 2 and 4 adjustable parameters. The results showed that both of the models can accurately predict the experimental data of surface tension. These models can favourably fit and also, predict the surface tension of single electrolyte solutions with less than 1% average absolute relative deviation (AARD). The prediction capability of the proposed models is also acceptable for mixtures of electrolytes.     

Separation of propylbenzene and n-alkanes from their mixtures using 4-methyl-N-butylpyridinium tetrafluoroborate as an ionic solvent at several temperatures

Liquid-liquid extraction is the most common method for separation of aromatics from their mixtures with n-alkanes hydrocarbons. An ionic liquid (IL), 4-methyl-N-butylpyridinium tetrafluoroborate [(mebupy)(BF₄)], was evaluated as solvent for this separation. Liquid equilibria (LLE) for 2 ternary systems comprising tetradecane, or hexadecane + propylbenzene + [(mebupy)(BF₄)] were measured over a temperature range of 313-333 K and atmospheric pressure. The reliability of the experimental data was evaluated using the Othmer-Tobias correlation. The effect of temperature, n-alkane chain length and solvent to feed ratio upon solubility, selectivity, and distribution coefficient were investigated experimentally. In addition, the experimental results were regressed to estimate the interaction parameters between each of the 3 pairs of components for the UNIQUAC and the NRTL models as a function of temperature. Both models satisfactorily correlate the experimental data, however the UNIQUAC fit was slightly better than that obtained with the NRTL model.     

Densities,refractive indices,and derived properties of (cyclohexane,orn-heptane + an aromatic hydrocarbon) atT = 298.15 K

This paper reports experimental densities and refractive indices of (cyclohexane, or n -heptane  + o -xylene, or m -xylene, or p -xylene, or ethylbenzene) over the whole composition range at T =  298.15 K and at atmospheric pressure. Excess molar volumes and changes of refractive indices were calculated from the experimental data obtained. Partial excess molar volumes were also computed for all the mixtures studied. The results were fitted by means of the Redlich–Kister equation with the aid of F -test to optimize the number of parameters. Measurements were compared with other literature values. Different empirical and semiempirical models were applied in order to estimate physical property values and good agreement was obtained with experimental results.     

The variation of viscosity, refractive indices, compressibility, intermolecular free length, and excess molar volume of the acetophenone—ethyl acetate solutions at 303.15–323.15 K

Densities, viscosities, refractive indices and ultrasonic velocities of the binary mixtures of acetophenone with ethyl acetate were measured over the entire mole fractions at 303.15, 313.15, and 323.15 K. From the experimental results, excess molar volumes V ^E, viscosity deviation ??_??, refractive index deviation ??n _D , deviations in isentropic compressibility ???? _s and excess intermolecular free length ??L _f are calculated. The viscosity values were fitted to the models of Krishnan-Laddha and McAllister. The thermophysical properties under study were fit to the Jouyban-Acree model. The excess values were correlated using Redlich-Kister polynomial equation to obtain their coefficients and standard deviations. The data obtained fitted with the values correlated by the corresponding models very well. The results are interpreted in terms of molecular interactions occurring in the solution.     

Study of singlet—triplet coupling in glyoxal by level anticrossing spectroscopy. V. Nature of singlet—triplet interaction

We observe in glyoxal cooled in a supersonic free jet the fluorescence of individual rotational levels of the S₁ state excited by a cw laser. We use the technique of singlet—triplet magnetic resonance near an anticrossing to measure matrix elements V₃₁ as a function of rotational quantum numbers N_s, N_t, K_s, K_t. The experimental results are compared with theoretical models of singlet—triplet couplings and we show that the spin-vibronic interaction is the dominant singlet—triplet interaction in glyoxal.     

Flow birefringence and flexibility of the molecules of copolymers of para- and meta-aromatic polyamides

Flow birefringence (FB) of solutions of poly-p-phenyleneterephthalamide (PpPhTPhA), poly-m-phenyleneisophthalamide (PmPhIPhA) and 13 of their copolymers, with various compositions, has been investigated. The number S of monomer units in a Kuhn segment was determined for all samples from the experimental values of the shear optical coefficient Δn/Δτ. Molecular models for homo- and copolymers were considered. On the basis of these models, an equation expressing the dependence of the chain flexibility parameter 1/S on copolymer composition Z was derived. Good agreement between the experimental dependence of 1/S on Z and the theoretical value confirms the additivity of the structural and deformation mechanisms of flexibility in polymer chains.     

Perfluoroalkanes and perfluoroalkylalkane surfactants in solution: Partial molar volumes in n-octane and hetero-SAFT-VR modelling

Partial molar volumes at infinite dilution have been obtained for 3 perfluoroalkylalkanes (PFAA) in n-octane at 298.15 K from experimental apparent molal volumes. The results were interpreted estimating the contributions of the hydrogenated and perfluorinated segments to the partial molar volume. For this reason, partial molar volumes at infinite dilution for 3 perfluoroalkanes (PFA) in n-octane at 298.15 K have also been obtained. The results were further interpreted using the hetero-SAFT-VR equation of state, which models the perfluoroalkylalkanes as heterosegmented di-block chains. The cross interactions, both intra and intermolecular, were characterised using parameters developed in earlier studies for (alkane + perfluoroalkane) mixtures: the calculations are thus fully predictive. The theory is able to accurately predict the volumetric behaviour of the solutions of perfluoroalkylalkanes, without fitting to any experimental data.     

The study of the relationship between the new topological index A(m) and the gas chromatographic retention indices of hydrocarbons by artificial neural networks

The newly developed topological indices A_m1-A_m3 and the molecular connectivity indices ^mX were applied to multivariate analysis in structure-property correlation studies. The topological indices calculated from the chemical structures of some hydrocarbons were used to represent the molecular structures. The prediction of the retention indices of the hydrocarbons on three different kinds of stationary phase in gas chromatography can be achieved applying artificial neural networks and multiple linear regression models. The results from the artificial neural networks approach were compared with those of multiple linear regression models. It is shown that the predictive ability of artificial neural networks is superior to that of multiple linear regression method under the experimental conditions in this paper. Both the topological indices ²X and A_m1 can improve the predicted results of the retention indices of the hydrocarbons on the stationary phase studied.     

Stability of relaxed Lennard-Jones models made of 500 to 6000 atoms

We present a study of the stability of clusters models made of a numberN of atoms in the range 500 to 6000 atoms, freely interacting through the Lennard-Jones potential. The potential energy per atom, calculated for relaxed models, shows that stable models belong to an icosahedral sequence whenN<1600 and to a decahedral sequence beyond. A coexistence size range of both structures is discussed in connection with experimental results on argon clusters in free jet expansions.     

Theoretical,Equilibrium, Kinetics and Thermodynamic Investigations of Methylene Blue Adsorption onto Lignite Coal

The interaction of methylene blue (MB) dye with natural coal (collected from coal landfills of the Kosovo Energy Corporation) in aqueous solutions was studied using adsorption, kinetics, and thermodynamic data, and Monte Carlo (MC) calculations. In a batch procedure, the effects of contact duration, initial MB concentration, pH, and solution temperature on the adsorption process were examined. The Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) isotherms were used to examine the equilibrium adsorption data. The equilibrium data fit well to the Freundlich and Langmuir adsorption isotherm models; however, the Freundlich model suited the adsorption data to a slightly better extent than the Langmuir model. The kinetics experimental data was fitted using pseudo-first-order, first-order, pseudo-second-order, second-order, Elvoich equation, and diffusion models. The pseudo-second-order rate model manifested a superlative fit to the experimental data, while the adsorption of MB onto coal is regulated by both liquid film and intraparticle diffusions at the same time. Thermodynamic parameters, such as Gibbs free energy (ΔG⁰), enthalpy (ΔH⁰), and entropy (ΔS⁰) were calculated. The adsorption of MB was confirmed to be spontaneous and endothermic. The theoretical results were in agreement with the experimental ones.     

Solubility of (+)-Usnic Acid in Water, Ethanol, Acetone, Ethyl Acetate and n-Hexane

Using ultraviolet spectrophotometry, the solubility of (+)-usnic acid in water, ethanol, acetone, ethyl acetate and n-hexane were measured at temperatures from (278.15 to 338.15) K at atmospheric pressure. The results obtained led to the solubility order: ethyl acetate > acetone > n-hexane > ethanol > water. The solubility values were correlated with the Apelblat equation, the λh equation and the ideal model. The results showed that these three models agreed well with the experimental data.