Thermal diffusion with stepwise association equilibria in aqueous solutions of the ionic dyes methylene blue and methyl orange

Soret coefficients are measured conductimetrically for dilute aqueous solutions of the stepwise-associating ionic dyes methylene blue (chloride salt) and methyl orange (sodium salt) at 25°C. In contrast to the behavior of other dilute aqueous electrolytes, the Soret coefficients of the dyes increase with concentration. An approximate treatment of thermal diffusion of stepwise-associating solutes is developed to interpret the results. The analysis is used to estimate the intrinsic enthalpies of transport of the monomeric and the associated forms of the dyes. The enthalpy of association makes a large reactive contribution to the enthalpy of transport of methyl orange.     

Quinary Mutual Diffusion Coefficients of Aqueous Mannitol + Glycine + Urea + KCl and Aqueous Tetrabutylammonium Chloride + LiCl + KCl + HCl Solutions Measured by Taylor Dispersion

Taylor dispersion is widely used to measure binary mutual diffusion. Studies of three- and four-component solutions show that the dispersion method is also well suited for multicomponent diffusion measurements, including cross-coefficients for coupled diffusion. Numerical procedures are reported here to calculate mutual diffusion coefficients from dispersion profiles measured for solutions of any number of components. The proposed analysis is used to measure the sixteen quinary mutual diffusion coefficients of five-component aqueous mannitol + glycine + urea + KCl solutions and aqueous NBu₄Cl + LiCl + KCl + HCl solutions. Mannitol, glycine, urea and KCl interact weakly at the low solute concentrations used (0.010 mol·dm^?3). The diffusion coefficients of this system are compared with pseudo-binary predictions. Strong coupling of the NBu₄Cl, LiCl, KCl and HCl fluxes is interpreted by using ionic conductivities and Nernst equations to calculate limiting quinary diffusion coefficients for mixed electrolytes that interact by the electric field generated by ion concentration gradients.     

Quaternary diffusion coefficients of NaCl−MgCl2−Na2SO4 −H2O synthetic seawaters by least-squares analysis of taylor dispersion profiles

Equations are developed to calculate the nine inter diffusion coefficients of four-component solutions by least-squares analysis of the refractive index profiles across Taylor dispersion peaks. The pre-exponential weighting factors and the eigenvalues of the diffusion coefficient matrix are used as the fitting parameters instead of the diffusion coefficients. This simplifies the fitting equations and speeds up the convergence. The analysis is tested on dilute aqueous solutions of sucrose + glycine + urea, and then used to determine the interdiffusion coefficients of aqueous NaCl+MgCl₂+Na₂SO₄ solutions at compositions corresponding to synthetic seawaters of salinities 5, 15, 25 and 35. The results are used to predict the concentration profiles that develop when seawater interdiffuses with fresh water.     

The Thermal Diffusion of Hydrogen Chloride in Water–Monoatomic Alcohol Mixtures at 298 K

Standard entropies of the thermal diffusion transference of HCl, entropies of the moving H⁺ and Cl⁻ ions, and Soret coefficients of this electrolyte at 298 K were obtained from a thermoelectrochemical investigation of HCl in water + alcohol solvent systems. The thermoelectric powers of the silver/silver chloride and para-benzoquinone/hydroquinone thermocells have been measured. Effects of the mixed solvent composition and the nature of an alcohol on the thermal diffusion characteristics were discussed using the theories of De Bethune and Agar.     

Experimental measurement and thermodynamic modelling of clathrate hydrate equilibria and salt solubility in aqueous ethylene glycol and electrolyte solutions

We present the extension of a recently developed method for modelling saline water to the thermodynamic prediction of phase behaviour for mixed salt–organic clathrate hydrate inhibitor aqueous solutions. Novel freezing point, boiling point and salt solubility data have been generated for NaCl–ethylene glycol (EG) and KCl–EG aqueous solutions. These data have been used in the optimisation of binary interaction parameters between salts and ethylene glycol. The extended thermodynamic model is capable of predicting complex vapour–liquid–solid (VLSE) equilibria for aqueous electrolytes and/or organic inhibitor solutions over a wide range of pressures, temperatures and inhibitor concentrations. Reliable hydrate dissociation data for two mixed salt–organic inhibitor quaternary systems (CH₄–H₂O–NaCl–EG and CH₄–H₂O–KCl–EG) have been measured at pressures up to 50 MPa. These data are used to validate the predictive capabilities of the model for hydrate equilibria. Good agreement between experimental data and predictions is observed, demonstrating the reliability of the developed model.     

Prediction of solubilities of salts,osmotic coefficients and vapor–liquid equilibria for single and mixed solvent electrolyte systems using the LIQUAC model

The electrolyte model LIQUAC has been used up till now to predict osmotic coefficients, mean ion activity coefficients, the vapor–liquid equilibrium (VLE) behavior, the solubility of gases in single and mixed solvent electrolyte systems, and solubilities of salts in aqueous solutions. In this paper, the required expressions for the calculation of salt solubilities not only in aqueous systems, but also in organic solvents and water–solvent electrolyte systems were deduced in detail based on the LIQUAC model with a fixed reference state and thermodynamic relations. Four salts (NaCl, KCl, NH₄Cl and NaF) and two solvent (water and methanol) were selected to test the derived expressions. The results show that the LIQUAC model with a fixed reference state can be used to predict osmotic coefficients, solubilities of salts in aqueous solutions, vapor–liquid equilibria, and the solubilities of salts in water–organic solvent systems with strong electrolytes.     

The thermal diffusion characteristics of hydrogen and alkali metal chlorides in aqueous-methanolic solutions at 298.15 K

The paper presents the results of experimental studies of thermoelectrochemical systems comprising silver chloride and quinhydrone electrodes in aqueous-methanolic solutions of hydrogen, lithium, potassium, and cesium chlorides. These results and literature data are used to calculate the standard entropies of transfer of electrolytes, Soret thermal diffusion coefficients, and entropies of moving ions at 298.15 K. The influence of the nature of electrolytes and mixed solvent composition on the characteristics of thermal diffusion transfer in the systems studied is discussed.     

Thermal diffusion behavior of nonionic surfactants in water

We studied the thermal diffusion behavior of hexaethylene glycol monododecyl ether (C12E6) in water by means of thermal diffusion forced Rayleigh scattering (TDFRS) and determined Soret coefficients, thermal diffusion coefficients, and diffusion constants at different temperatures and concentrations. At low surfactant concentrations, the measured Soret coefficient is positive, which implies that surfactant micelles move toward the cold region in a temperature gradient. For C12E6/water at a high surfactant concentration of w1 = 90 wt % and a temperature of T = 25 degrees C, however, a negative Soret coefficient S(T) was observed. Because the concentration part of the TDFRS diffraction signal for binary systems is expected to consist of a single mode, we were surprised to find a second, slow mode for C12E6/water system in a certain temperature and concentration range. To clarify the origin of this second mode, we investigated also, tetraethylene glycol monohexyl ether (C6E4), tetraethylene glycol monooctyl ether (C8E4), pentaethylene glycol monododecyl ether (C12E5), and octaethylene glycol monohexadecyl ether (C16E8) and compared the results with the previous results for octaethylene glycol monodecyl ether (C10E8). Except for C6E4 and C10E8, a second slow mode was observed in all systems usually for state points close to the phase boundary. The diffusion coefficient and Soret coefficient derived from the fast mode can be identified as the typical mutual diffusion and Soret coefficients of the micellar solutions and compare well with the independently determined diffusion coefficients in a dynamic light scattering experiment. Experiments with added salt show that the slow mode is suppressed by the addition of w(NaCl) = 0.02 mol/L sodium chloride. This suggests that the slow mode is related to the small amount of absorbing ionic dye, less than 10(-5) by weight, which is added in TDFRS experiments to create a temperature grating. The origin of the slow mode of the TDFRS signal will be tentatively interpreted in terms of a ternary mixture of neutral micelles, dye-charged micelles, and water.     

Development of a Modified Bromley's Methodology for the estimation of ionic media effects on solution equilibria: Part 3. Application to the construction of thermodynamic models

The development of the Modified Bromley's Methodology (MBM) is extended for the estimation of the activity coefficients of individual species in aqueous solution at 25°C in single and mixed ionic media. The estimation is compared with literature data of activity coefficients of mixtures of electrolytes in water and applied to (a) the prediction of the ionic product of water in aqueous solutions containing different salts which are commonly used as background electrolytes (NaCl, KNO₃ and NaClO₄) and (b) the equilibrium constants of the Cr(VI)–H₂O system.     

Activity coefficients for the system NaCl+Na2SO4+H2O at various temperatures. Application of Pitzer's equations

The mean activity coefficients of NaCl in the system NaCl+Na₂SO₄+H₂O at various compositions were determined in the temperature range 5–45°C from the emf of potentiometric cells. By processing the results using Pitzer's equations the mixing parameters describing the non-ideal behavior of electrolytes were calculated. The temperature coefficients of the mixing parameters were determined and found not to be significant. The mixing parameters and temperature coefficients calculated for the binary mixture can be used to describe the behavior of multicomponent systems containing NaCl and Na₂SO₄, and eventually sea water.     

Explosivity Conditions of Aqueous Solutions

This paper focuses on the conditions for explosive boiling and gas exsolution of aqueous solutions from a thermodynamic point of view. Indeed, the kinetic nature of these processes, hence their explosivity, can be assessed by considering their relation with the spinodal curve of these liquids. First, the concepts of mechanical and diffusion spinodals are briefly described, which allows us to introduce the notions of superspinodal (explosive) transformations and subspinodal (non-explosive) ones. Then, a quantitative study of spinodal curves is attempted for the binary systems H₂O–CO₂ and H₂O–NaCl using equations of state having a strong physical basis. It is shown that dissolved gaseous components and electrolytes have an antagonist effect: dissolved volatiles (like CO₂) tend to shift the superspinodal region towards lower temperatures, whereas electrolytes (like NaCl) tend to extend the metastable field towards higher temperatures.     

Entropy characteristic of solvation and thermal diffusion of hydrogen chloride in water-1-propanol solutions: A thermoelectrochemical determination

Thermogalvanic cells with silver chloride and quinhydrone electrodes in the HCl-H₂O-1-PrOH system are studied experimentally. The results are used to determine standard entropies of thermal diffusion transport of hydrogen chloride, entropies of mobile H⁺ and Cl^? ions, and Soret coefficients of the electrolyte at 298 K. Thermal diffusion entropies and partial molar entropies of said ions in water-1-propanol solutions are calculated within the Agar model. The results are interpreted with the application of basic concepts of the De Bethune theory concerning thermal diffusion transport in electrolytes.     

Thermodynamic Properties of Aqueous Glucose–Urea–Salt Systems

In this work, the thermodynamic behavior of aqueous solutions containing the solutes NaCl, glucose, and/or urea is investigated. These substances are vital components for living bodies and further they are main components of blood serum. Osmotic coefficients were determined by cryoscopic measurements in single-solute and multi-solute aqueous solutions containing salts (NaCl, KCl, CaCl₂), glucose, and/or urea. The results show that NaCl determines the osmotic coefficients in the urea/glucose/NaCl/water system. Investigation of the effect of different salts on osmotic coefficients revealed ion-specific effects. At physiologically important solute concentrations in typical blood serum solutions, the osmotic coefficients were found to be in the range of 0.90–0.93. In a second step, the state of water in different glucose/salt/water and urea/salt/water systems was investigated. Depending on the kind of salt, the chemical potential of water in urea/salt/water is either higher or lower than in glucose/salt/water systems at equal nonelectrolyte concentrations. This result was found to be independent of the salt molality. Finally, the investigated systems were modeled with the Pitzer model and the ePC-SAFT equation of state, which allowed predicting of the properties of these multi-solute aqueous solutions.     

Ternary mutual diffusion of isoniazid in aqueous sodium chloride,sodium hydroxide,and hydrochloric acid at T = 298.15 K

Ternary mutual diffusion coefficients measured by Taylor dispersion method (D₁₁, D₂₂, D₁₂, and D₂₁) are reported for aqueous solutions containing isoniazid and different electrolytes (NaCl, NaOH, or HCl) at T = 298.15 K at different carrier concentrations. These diffusion coefficients have been measured having in mind a better understanding of the structure of these systems and the thermodynamic behaviour of isoniazid in different media. For example, it is possible to make conclusions about the influence of these electrolytes in diffusion of isoniazid, and to obtain information concerning the number of moles of each component transported per mole of the other component driven by its own concentration gradient.     

Hartley–Crank Equations for Coupled Diffusion in Concentrated Mixed Electrolyte Solutions. The CaCl2 + HCl + H2O System

Nernst—Planck equations and ionic conductivities are used to calculate accurate limiting interdiffusion coefficients D _ik ^o for mixed electrolyte solutions. The electrostatic mechanism for coupled electrolyte diffusion is investigated by calculating the electrostatic contribution to each D _ik ^o coefficient to give the flux of each electrolyte driven by the electric field, which is generated by the migration of ions of different mobilities. Ternary diffusion coefficients are measured for dilute aqueous K₂SO₄ + KOH and Li₂SO₄ + LiOH solutions. Because of the different mobilities of K⁺ and Li⁺ ions relative to SO ₄ ^2– ions, diffusing K₂SO₄ drives cocurrent flows of KOH, but diffusing Li₂SO₄ drives counterflows of LiOH. To describe coupled diffusion in concentrated mixed electrolyte solutions, the Hartley–Crank theory is used to correct the limiting D _ik ^o coefficients for nonideal solution behavior, viscosity changes, ionic hydration, and the zero-volume flow constraint. Diffusion coefficients predicted for concentrated aqueous CaCl₂ + HCl solutions are compared with recently reported data. The large amount of HCl cotransported by the diffusing CaCl₂ is attributed to the salting out of HCl by CaCl₂ and to the migration of H⁺ ions in the diffusion-induced electric field, which slows down the Cl^– ions and speeds up the less-mobile Ca²⁺ ions to maintain electroneutrality along the CaCl₂ gradient.     

Standard characteristics of thermodiffusion of hydrogen chloride in the water-2-propanol system at 298.15 K

The initial and stationary temperature voltage coefficients for symmetric thermogalvanic cells with silver/silver chloride and quinhydrone electrodes in the HCl-H₂O-2-PrOH system are measured at 298.15 K. From the experimental data obtained, the standard entropies of thermodiffusional transfer of the electrolyte, the standard entropies of moving hydrogen and chlorine ions, and the Soret coefficients of HCl in aqueous solutions of 2-propanol are calculated. The influence of the composition of the mixed solvent and the nature of ions on the entropy characteristics is discussed.     

Soret Coefficients for Aqueous Polyethylene Glycol Solutions and Some Tests of the Segmental Model of Polymer Thermal Diffusion

A thermogravitational cell is used to measure Soret coefficients (s) for dilute binary aqueous solutions of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycol (PEG) fractions with average molecular weights from 200 to 20,000 g-mol^–1. The cell design allows the top and bottom halves of the solution column to be withdrawn and injected into a high-precision HPLC differential refractometer detector for analysis. Previously reported mutual diffusion coefficients D and the measured Soret coefficients are used to calculate thermal diffusion coefficients D _T. s and D vary with the PEG molecular weight M as M ^+0.53 and M ^–0.52, respectively; hence, D _T = sD is essentially independent of M. The segmental model of polymer thermal diffusion predicts D _T = D_seg U _S/RT ², where D _seg is the segment diffusion coefficient, U _S the solvent activation energy for viscous flow, R the gas constant, and T the temperature. The predicted D _T values, although independent of M, are too large by a factor of five. Additional tests of the segmental model are provided using literature data for polystyrene + toluene, n-alkane + CCl₄, and n-alkane + CHCl₃ solutions. Agreement with experiment is not obtained. In particular, the measured D _T values for the alkane solutions are negative.     

The effect of counterions on coacervation and solubilization in oil-external and middle-phase microemulsions

The effect of hydrated radii, valency, and concentration of counterions on the coacervation of aqueous petroleum sulfonate solutions and on the solubilization capacity of oil-external and middle-phase microemulsions was investigated. The critical electrolyte concentration (CEC) for coacervation increased with Stokes' hydrated radii of monovalent counterions. The CEC for CaCl₂ was much lower than that predicted by either the Stokes' hydrated radii or the ionic strength. For mixed electrolytes containing NaCl and CaCl₂, it was concluded from the shift in CEC that 1 mole of CaCl₂ is equivalent to 16 to 19 moles of NaCl. The changes in relative concentrations of NaCl and CaCl₂ for coacervation exhibited additive behavior. The maximum solubilization of brine in oil-external microemulsions occurred at a specific salt concentration. For mixed electrolytes containing NaCl and CaCl₂, the shift in electrolyte concentration for maximum solubilization showed that 1 mole of CaCl₂ is equivalent to about 4 moles of NaCl. These results suggest that the equivalence ratio of CaCl₂ to NaCl is strikingly different in aqueous solutions and oil-external microemulsions. For solubilization in middle-phase microemulsion containing mixed NaCl and CaCl₂, it was concluded from the shift in optimal salinity that 1 mole of CaCl₂ is equivalent to about 16 moles of NaCl. Here also the changes in NaCI and CaC1₂ concentrations showed additive behavior. The equivalence ratio of CaCl₂ and NaCl appears to be independent of oil chain length in the present study. As shown by the equivalence ratio of CaCl₂ to NaCl, the formation of middlephase microemulsions appears to be similar to coacervation of aqueous surfactant solutions and quite different from the solubilization of water in oil-external microemulsions.     

The Soret effect in dilute polymer solutions: influence of chain length, chain stiffness, and solvent quality

Thermal diffusion in dilute polymer solutions is studied by reverse nonequilibrium molecular dynamics. The polymers are represented by a generic bead-spring model. The influence of the solvent quality on the Soret coefficient is investigated. At constant temperature and monomer fraction, a better solvent quality causes a higher affinity for the polymer to the cold region. This may even go to thermal-diffusion-induced phase separation. The sign of the Soret coefficient changes in a symmetric nonideal binary Lennard-Jones solution when the solvent quality switches from good to poor. The known independence of the thermal diffusion coefficients of the molecular weight is reproduced for three groups of polymers with different chain stiffnesses. The thermal diffusion coefficients reach constant values at chain lengths of around two to three times the persistence length. Moreover, rigid polymers have higher Soret coefficients and thermal diffusion coefficients than more flexible polymers.     

Effect of electrolytes on the sensitivity of thermal-lens measurements in aqueous media

The thermooptical properties of aqueous solutions of strong electrolytes (H₂SO₄, HCl, NaCl, Na₂SO₄, and NaOH) are considered. The thermal lens signal depends on the nature of the electrolyte. The effect of an electrolyte is not the sum of the effects of the constituent ions. The largest gain in the sensitivity of thermal-lens measurements is achieved with sulfuric acid (sensitivity enhancement factor of 2 for 0.6 M H₂SO₄ versus water) and sodium chloride (sensitivity enhancement factor of 1.5 for 3 M NaCl), while the effect of hydrochloric acid is negligible.