Solubility of disperse yellow 54 in supercritical carbon dioxide with or without cosolvent

Extraction of disperse yellow 54 with supercritical carbon dioxide was conducted at 393.2 K and 30 MPa over a wide range of contact times. Saturated solubilities of the disperse dyestuff in supercritical carbon dioxide with or without cosolvent were also measured over the temperature and pressure ranges of 353.2 K to 393.2 K and 15 MPa to 30 MPa. Either ethanol or dimethyl sulfoxide, up to 5 mol%, was used as a cosolvent. As evidenced from the experimental results, the magnitudes of equilibrium solubility can be effectively enhanced in the presence of both two cosolvents. Dimethyl sulfoxide was found to yield higher solubility enhancement. Cosolvent effects were discussed on the basis of the Kamlet-Taft solvatochromic solvent parameters of cosolvents. The saturated solubility data were correlated with the Chrastil and the Mendez-Santiago and Teja equations. The Chrastil model correlated the solubility data to about within the experimental uncertainty. The correlated results of the Mendez-Santiago–Teja model supported the consistency of the solubility data over the entire experimental conditions.     

Supercritical phase behavior: The entrainer effect

One of the keys to lowering the cost of supercritical extraction processes is to find appropriate cosolvents (entrainers) that maintain or improve selectivity while increasing the solubility. This solubility enhancement is of great commercial interest; however until recently there was no way to predict either for which systems or under what conditions it would occur. We show that the entrainer effect is caused by a chemical association between the cosolvent and the solute. Further, we present a model that explicitly accounts for these chemical associations. Finally, we show that data and analysis techniques already established for associations in liquid solvents (especially spectroscopic and solvatochromic techniques) can be used to make qualitative predictions of the entrainer effect in a supercritical fluid.     

Diffusional release of a solute from a polymeric matrix — approximate analytical solutions

A refined integral method has been successfully applied to moving boundary problems encountered in the diffusional release of a solute from a polymeric matrix. The release kinetics has been analyzed for both erodible and non-erodible matrices with perfect sink and constant, finite external volume conditions. The range of applicability of these approximate analytical solutions has been established by comparison with available exact solutions. p]The approximate analytical solutions presented here are much more accurate than the pseudosteady-state results and much easier to use routinely than the exact solutions. For a dispersed solute, the results presented here are particularly useful for cases where the solute loading is not in great excess of the solute solubility in the matrix.     

Solubility of solids in near-critical conditions: effect of a third component

The effect of cosolvents on the solubility equilibria near the critical end-point of binary mixtures is analyzed. The problem has received recent attention for the particular case of ionic cosolvents (Mazo, R. M. J. Phys. Chem. B 2007, 111, 7288-7290), where a pronounced salting-out effect was predicted. Here we make a thermodynamic analysis to view the problem from a different perspective. Our conclusions are that, at the critical end-point, the Setchenov constant, which reflects the effect of cosolvents on solubility equilibria, diverges following the behavior of the osmotic susceptibility and that the sign of this divergence is encoded in the critical end-line of the ternary system.     

Measurement and correlation of solubility of benzamide in supercritical carbon dioxide with and without cosolvent

The experimental equilibrium solubility of benzamide in supercritical carbon dioxide was measured at temperatures between 308 K and 328 K and for pressures from 11.0 MPa to 21.0 MPa using a dynamic flow method. The effects of three cosolvents - ethanol, acetone and ethylene glycol, were investigated at a cosolvent molar concentration of 3.5%. The results showed that the solubility was enhanced by the presence of the three cosolvents, and ethanol exhibited the highest cosolvent effect. The solubility data in the absence and presence of cosolvents were correlated by two density-based models. The calculated results showed satisfactory agreement with the experimental data.     

Antihydrophobic cosolvent effects for alkylation reactions in water solution, particularly oxygen versus carbon alkylations of phenoxide ions

Antihydrophobic cosolvents such as ethanol increase the solubility of hydrophobic molecules in water, and they also affect the rates of reactions involving hydrophobic surfaces. In simple reactions of hydrocarbons, such as the Diels-Alder dimerization of 1,3-cyclopentadiene, the rate and solubility data directly reflect the geometry of the transition state, in which some hydrophobic surface becomes hidden. In reactions involving polar groups, such as alkylations of phenoxide ions or S(N)1 ionizations of alkyl halides, cosolvents in water can have other effects as well. However, solvation of hydrophobic surfaces is still important. By the use of structure-reactivity relationships, and comparing the effects of ethanol and DMSO as solvents, it has been possible to sort out these effects. The conclusions are reinforced by an ab initio computer model for hydrophobic solvation. The result is a sensible transition state for phenoxide ion as a nucleophile, using its oxygen n electrons to avoid loss of conjugation. The geometry of alkylation of aniline is very different, involving packing (stacking) of the aniline ring onto the phenyl ring of a benzyl group in the benzylation reaction. The alkylation of phenoxide ions by benzylic chlorides can occur both at the phenoxide oxygen and on ortho and para positions of the ring. Carbon alkylation occurs in water, but not in nonpolar organic solvents, and it is observed only when the phenoxide has at least one methyl substituent ortho, meta, or para. The effects of phenol substituents and of antihydrophobic cosolvents on the rates of the competing alkylation processes indicate that in water the carbon alkylation involves a transition state with hydrophobic packing of the benzyl group onto the phenol ring. The results also support our conclusion that oxygen alkylation uses the n electrons of the phenoxide oxygen as the nucleophile and does not have hydrophobic overlap in the transition state. The mechanisms and explanations for competing oxygen and carbon alkylations differ from previous proposals by others.     

Solubilities of poly(ethylene glycol)s in the mixtures of supercritical carbon dioxide and cosolvent

The solubilities of poly(ethylene glycol) (PEG6000) (M.W.=7500) in the mixtures consisting of supercritical carbon dioxide (CO₂) and cosolvent have been measured by observing the cloud points at 313.15 K and 16 MPa. Ethanol and toluene were used as cosolvents. The solubility of PEG6000 is extremely low in either CO₂ or ethanol, but becomes about 20 wt.% in a mixture of the two. The maximum solubility is achieved at about 50 wt.% (polymer-free) ethanol. The solubilities of PEG6000 in the mixtures of supercritical CO₂ and the cosolvent have been correlated by a regular solution model using the local compositions of solvents around a solute molecule, and an expanded liquid equation of state model.     

A cellular automata model of the soluble state

Cellular automata models of solubilities in a solvent (water) have been dynamically synthesized. Rules relatingwater‐water, water‐solute, and solute‐solute relationships have been systematically varied in order to assess their influence on the emergent property of solubility. The results reveal the prominent influence of rules governing the probabilities of solute‐water joining and breaking. This influence manifests itself in significant changes in the emergent properties of relative solubility and solubility changes with water“temperature”. The study demonstrates the validity and potential value of cellular automata to model solution phenomena. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solution thermodynamics and preferential solvation of hesperidin in aqueous cosolvent mixtures of ethanol,isopropanol, propylene glycol,and n-propanol

The solubility of hesperidin in some {cosolvent (1) + water (2)} mixtures expressed in mole fraction at temperatures from 293.15 K to 333.15 K reported by Xu et al. has been used to calculate the apparent thermodynamic functions, Gibbs energy, enthalpy, and entropy, of the dissolution processes by means of the van’t Hoff and Gibbs equations. Non-linear enthalpy–entropy relationships were observed for this drug in the plots of enthalpy vs. Gibbs energy of dissolution with positive or negative slopes regarding mixtures composition and/or cosolvent. Moreover, the preferential solvation of hesperidin by the cosolvents was analysed by using the inverse Kirkwood–Buff integrals observing that this drug is preferentially solvated by water in water-rich but preferentially solvated by cosolvents in mixtures 0.20 (or 0.24) ≤ x₁° ≤ 1.00. Furthermore, a new mathematical model was proposed for correlating/predicting the solubility of hesperidin in binary solvent mixtures at various temperatures.     

FTIR study of hydrogen bonding of stearic acid with ethanol, dimethyl sulfoxide,and acetonitrile in supercritical CO_2

FTIR spectroscopy was used to study the hydrogen bonding of stearic acid with ethanol, dimethyl sulfoxide (DMSO),and acetonitrile in supercritical CO_2 at 318.15 K, and 12.5 and 16.5 MPa. The concentrations of the cosolvents range from 0—0.6 mol·L~(-1). The area percentage of absorption bands for hydrogen-bonded and nonhydrogen-bonded species was obtained from the IR spectra. The acid and the cosolvents can form hydrogen bond even when their concentrations are very low. At fixed solute concentration, the extent of hydrogenbonding increases with cosolvent concentration. At higher ethanol concentrations, it seems that one stearic acid molecule can hydrogen bond with more than one ethanol molecules simultaneously. It is seen that the strength of the hydrogen bond formed by the acid and the cosolvents is in the order: DMSO>ethanol>acetonitrile.     

Correlation for the variations with temperature of solute solubilities in high temperature water

Methods for estimating solute solubilities in high temperature water both below and above its boiling point (under pressure) are needed for applications of this medium in processing applications such as sub-critical water extraction, reaction chemistry in heated water, and in the material sciences. There is a paucity of data and correlative methods for estimating solute solubilities under these conditions; the limited existing methods are based on a limited solubility data base, and in some cases predicted solubility values are in quite serious disagreement with experimentally derived data. Here available solute solubility data both above and below the boiling point of water has been correlated for diverse solute types consisting of hydrocarbons, essential oil components, pesticides, polyphenolic compounds, as well as solutes exhibiting high solubility in water under the stated conditions. Utilizing solubility data from diverse sources, appropriate conversions and equations have been derived for converting all solubility data to a mole fraction basis, while the other required physicochemical parameters, such as melting point, boiling point, critical properties, have been estimated, when necessary, largely by group contribution-based methods. A solubility model based on such physicochemical parameters and critical properties of the solutes was derived. An excellent correlation is obtained for x_{c (estimated)}versus x_c using this approach and the prediction of solute solubility in water as a function of temperature was found to be excellent for 431 data points representing the solubility of 34 solutes in the temperature range between 298 and 573 K.     

Thermochemical Investigations of Tautomeric Equilibrium. Variation of the Calculated Equilibrium Constant with Binary Solvent Composition

Abstract

A conventional nonelectrolyte solution model which has led to successful predictive equations for solute solubility and infinite dilution chromatographic partition coefficients is extended to systems containing a tautomeric solute dissolved in a binary solvent mixture. The derived expression predicts that the tautomeric solute concentration in a binary solvent is a geometric average of the pure solvent ratios and permits calculation of solute-solvent association constants from variation of the stoichiometric tautomeric solute concentration ratios as a function of binary solvent composition. Experimental data for phenylazonaphthol dissolved in aqueous-ethanol and aqueous-acetone solvent mixtures is discussed in relation to the theoretical model.     

A unified cosolvency model for calculating solute solubility in mixed solvents

Organic solvents are amongst the most powerful solubilization agents for a large number of water-insoluble drugs. A number of equations has been reported for mathematical representation of solute solubility in mixed solvents. The question is then posed--is there a mathematical difference between these models? To address this point, it has been demonstrated that all cosolvency models could be made equivalent by using algebraic manipulations. In order to familiarize the readers with the available cosolvency models, they are briefly reviewed. The models can be divided into two mathematical categories, i.e. linear and non-linear models. The linear models include: the log-linear, extended Hildebrand solubility approach, excess free energy equations, combined nearly ideal binary solvent/Redlich-Kister equation and Margule equations which can be converted to a general single model which expresses the logarithm of mole fraction solubility of a solute as a power series of volume fraction of the cosolvent. The non-linear models include the mixture response surface methods, two step solvation model and modified Wilson model which can be converted to a non-linear general form. Also, it has been shown that both the general single model and a non-linear general model are mathematically identical. To show the applicability of the models on real experimental data, 35 data sets have been collected from the literature. Both linear and nonlinear models produced comparable accuracies when an equal number of constant terms was employed in numerical analyses.     

One-dimensional model for water and aqueous solutions. III. Solvation of hard rods in aqueous mixtures

A simple one-dimensional model for aqueous solution is applied to study the solvation thermodynamics of a simple solute (here, a hard-rod particle) in mixtures of waterlike particles and a cosolvent. Two kinds of cosolvents are considered, one that stabilizes and one that destabilizes the "structure of water." The results obtained for the Gibbs energy, entropy, enthalpy, and heat capacity of solvation are in qualitative agreement with experimental data on the solvation of argon and methane in mixtures of water and ethanol and of water and p-dioxane.     

Mechanistic Implications of the Tetrameric Cubic Structure of Lithium Enolates

The tetrameric cubic structure established for lithium enolates in the solid state is used as a model to discuss several facets of enolate chemistry: solubility, effects of cosolvents, addition to aldehyde and acid chloride, influence of mixed aggregates on diastereo- and enantioselectivities.     

Thermodynamic quantities relative to solution processes of Naproxen in aqueous media at pH 1.2 and 7.4

Based on van’t Hoff and Gibbs equations, the thermodynamic functions Gibbs free energy, enthalpy, and entropy of solution, mixing and solvation of naproxen (NAP) in water at pH 1.2 and 7.4, were evaluated from solubility values determined at several temperatures. The solubility at pH 7.4 and 25.0°C was almost 150 times higher with respect to pH 1.2. The enthalpies of solution were positive and greater for pH 1.2, while the entropies of solution were both negative, thereby implying a greater molecular organization at pH 7.4. The results were discussed in terms of solute–solvent interactions.