Abstracts 1991 - 1993

Tomasko, D.L., Knutson, B.L., Pouillot, F., Liotta, C.L., and Eckert , C.A. , Spectroscopic Study of Structure and Interactions in Cosolvent-Modified Supercritical Fluids. Journal of Physical Chemistry, 1993. 97 (45): p. 11823-11834.

Cosolvents can be used to tailor supercritical fluid solvents for increased selectivity by increasing the solvent polarity or polarizability and participating in specific interactions with solutes. Specific interactions in these fluids are often inferred from thermodynamic measurements although independent confirmation of the structure of cosolvent/solute interactions is lacking. The solutes 2-naphthol, 5-cyano-2-naphthol, and 7-azaindole exhibit well-characterized spectral responses to an organized, hydrogen-bonded solvent environment in liquids. We describe fluorescence and UV absorption experiments to probe the structure and interaction between polar cosolvents and these solutes in a variety of supercritical fluids. In addition, new solubility data are reported for 7-azaindole in supercritical CO2 with and without methanol and in supercritical ethane. The results suggest that the interaction between cosolvent and solute can be strong yet exhibit no evidence for actual complexes in solution.

Hait, M.J., Liotta, C.L., Eckert, C.A., Bergmann, D.L., Karachewski, A.M., Dallas, A.J., Eikens, D.I., Li, J.J.J., Carr, P.W., Poe, R.B., and Rutan, S.C., Space Predictor for Infinite Dilution Activity-Coefficients. Industrial & Engineering Chemistry Research, 1993. 32 (11): p. 2905-2914.

A new formulation (SPACE) is presented for the prediction of infinite dilution activity coefficients in nonionic solutions. Statistical methods have been used to develop a critically evaluated gamma(infinity) database at 25-degrees-C and to determine optimum mathematical expressions. The predictive method stems from earlier work using additive contributions to the cohesive energy density, but all compound-specific adjustable parameters have been avoided by using known molecular properties, especially the solvatochromic parameters for dipolarity/polarizability and for hydrogen bonding. This method has the advantages of including explicitly ''chemical'' interactions and of being uncoupled from any specific g(E)(x) expression. For the present, it is limited to monofunctional molecules at 25-degrees-C, and it has not yet been applied to water. Although it appears somewhat more accurate for this database than the UNIFAC method, it should be viewed as a complementary technique-an additional tool for solvent selection by the design engineer.

Li, J.J., Dallas, A.J., Eikens, D.I., Carr, P.W., Bergmann, D.L., Hait, M.J., and Eckert, C.A., Measurement of Large Infinite Dilution Activity-Coefficients of Nonelectrolytes in Water by Inert-Gas Stripping and Gas-Chromatography. Analytical Chemistry, 1993. 65 (22): p. 3212-3218.

An inert gas stripping gas chromatography method was described for the determination of infinite dilution activity coefficients (gamma2infinity) of organic compounds in water. The infinite dilution activity coefficients for 31 compounds including many halogenated and aromatic compounds were measured. In general, the gamma2infinity values measured by gas stripping are in very good agreement with gamma2infinity values estimated from the inverse solubilities and other experimental measurements. This suggests that when there is no directly measured data available, gamma2infinity values estimated from the inverse of solubility can be used for the development and test of the models predicting limiting activity coefficients. A major advantage of this method relative to headspace gas chromatography that improves its precision and accuracy is the fact that the absolute detector response is unimportant. The detector only needs to be linear.

Vanalsten, J.G. and Eckert , C.A. , Effect of Entrainers and of Solute Size and Polarity in Supercritical-Fluid Solutions. Journal of Chemical and Engineering Data, 1993. 38 (4): p. 605-610.

New experimental data are presented for the solubility of several solid solutes and solute mixtures in entrainer-doped supercritical fluid carbon dioxide. These results, along with others from the literature, are used to demonstrate the effects of solute size (appreciable) and solute polarity (small) in pure supercritical fluids. Solvent effects appear to be far more important than solute structure, and polar and hydrogen-bonding entrainers exhibit large effects on both solute loading and selectivity. Thus, it appears that entrainers can be used to tailor supercritical fluid solvents for specific applications.

Howell, W.J., Alger, M.M., and Eckert , C.A. , Application of the Lcpt Model to Solid-Liquid Equilibria for Binary Compound-Forming Alloys. Aiche Journal, 1993. 39 (9): p. 1519-1526.

The linear chemical-physical theory (LCPT) model for liquid metal solution thermodynamics has been extended to the determination of the liquidus curves for binary intermetallic compound-forming systems. The equations developed include corrections to the observed melting point temperature and heat of fusion for compounds that dissociate partially on melting. The primary advantages of the LCPT model for solid-liquid equilibria are the small number of physically realistic parameters required, ease of implementation, and wide applicability. In addition, the model also permits the incorporation of compounds in modeling the liquidus curves that are not necessary for representing the liquid-phase thermodynamic properties. For the seven systems studied, the agreement between calculated and experimentally measured liquidus curves is quite good.

Tomasko, D.L., Hay, K.J., Leman, G.W., and Eckert, C.A., Pilot-Scale Study and Design of a Granular Activated Carbon Regeneration Process Using Supercritical Fluids. Environmental Progress, 1993. 12 (3): p. 208-217.

A technology which has great potential for environmental control and waste remediation is contaminant removal and separation with supercritical fluids (SCF's) or supercritical fluid extraction (SFE). Pressure tuning of solvent power allows SCF processes to adapt to a wide variety of small batch oriented separations typified by environmental cleanup operations. The ability of supercritical CO2 to extract model contaminant compounds from GAC and subsequently drop out most of the contaminant in a liquid phase has been investigated in a pilot scale apparatus. Typical desorption profiles indicate an 85% removal of the compound from the carbon which allows for reuse. The desorption results have been interpreted with a generalized desorption-mass transfer model. The results of the pilot plant studies have been applied to the design of a fixed-site GAC regeneration unit consisting of a three-element desorber with two-stage flash separation. Optimization of the process centers around minimizing the cost of recycling the SCF through an efficient recompression scheme and cycle configuration in the desorber unit. An economic evaluation shows a processing cost of 10.6cent/lb (23cent/kg) GAC which compares favorably with thermal regeneration and incineration. This non-destructive process allows re-use of the GAC while maintaining a high adsorbate capacity, which reduces carbon replacement costs and significantly decreases the need for carbon disposal by landfill or incineration.

Macnaughton, S.J., Tomasko, D.L., Foster, N.R., and Eckert , C.A. , Design Considerations for Soil Remediation Using Supercritical-Fluid Extraction. Process Safety and Environmental Protection, 1993. 71 (B2): p. 124-128.

The technical feasibility of soil remediation using supercritical fluid extraction has been proven at both the laboratory and demonstration plant scale. The design of a flexible soil remediation plant that can be optimised for each particular application requires a detailed understanding of the behaviour of supercritical fluid systems. The influence of the phase behaviour of cosolvent systems, co-solute interactions, solid matrix effects, transport property behaviour and solute solubility phenomena on the scale-up and subsequent design of a soil remediation plant is discussed.

Trampe, D.B. and Eckert , C.A. , A Dew-Point Technique for Limiting Activity-Coefficients in Nonionic Solutions. Aiche Journal, 1993. 39 (6): p. 1045-1050.

A new measurement technique for activity coefficients at infinite dilution has been developed involving the construction of a very dilute vapor phase and the accurate determination of its dew point. This method is especially applicable to systems of low solute relative volatility, precisely where other methods such as differential ebulliometry and headspace gas chromatography become less precise. Data are reported for gamma(infinity) for N, N-dimethylformamide, dimethylsulfoxide, and N-methyl pyrrolidone in water over a range of temperatures.

Poe, R.B., Rutan, S.C., Hait, M.J., Eckert, C.A., and Carr, P.W., Developing Models for Infinite Dilution Activity-Coefficients Using Factor-Analysis Methods. Analytica Chimica Acta, 1993. 277 (2): p. 223-238.

This paper illustrates a novel use of factor analysis to characterize models for infinite dilution activity coefficients. Different forms of a modified cohesive energy density (MOSCED) equation were used to create synthetic data matrices to be analyzed by factor analysis. The abstract factors from these matrices were used to target test the different model parameters. By using the synthetic data, the factors contributing to the different forms of the MOSCED equation that were most susceptible to noise were identified. Factor analysis and target testing were then used to characterize a database of experimental infinite dilution activity coefficients for 29 solvents and 31 solutes. The factor analysis results for one form of the MOSCED equation were used to evaluate different physicochemical scales for dispersion, dipolarity and hydrogen bonding interactions.

Eckert , C.A. and Knutson, B.L., Molecular Charisma in Supercritical Fluids. Fluid Phase Equilibria, 1993. 83 : p. 93-100.

The unique properties of supercritical fluids (SCF) solutions near the critical region are often a manifestation of local properties which are significantly different than bulk properties. A variety of spectroscopic measurements, as well as thermodynamic and theoretical treatments, support the existence of local density and local composition augmentations in the vicinity of large solute molecules in SCF solutions. Opportunities to exploit this concept of molecular charisma exist for a variety of engineering application including separations using cosolvent/SCF systems and chemical reactions in SCF solvents. The component of cosolvent effect on solubilities in SCF systems resulting from molecular charisma is discussed and related to specific solute/cosolvent interactions and cosolvent concentration. As a consequence of two effects of molecular charisma, the potential to adjust reaction rates and reaction selectivities in SCF solvents is also enhanced. Reaction rates may be altered through the concentration of reactants at the reaction site. Additionally, modification of local solvent strength in SCF solutions, a property of the local reactant environment, can alter reaction rates and selectivities.

Ekart, M.P., Bennett, K.L., Ekart, S.M., Gurdial, G.S., Liotta, C.L., and Eckert , C.A. , Cosolvent Interactions in Supercritical Fluid Solutions. Aiche Journal, 1993. 39 (2): p. 235-248.

The addition of cosolvents to supercritical fluid (SCF) solvents can have large effects on solubilities, giving engineers the ability to tailor loadings and selectivities of solutes for difficult separations. It is necessary to have a better understanding of the special intermolecular interactions that occur in SCF solutions to predict the effects of cosolvents. We use a SCF chromatographic technique to acquire a database of cosolvent effects for a variety of cosolvents and solutes, examination of the cosolvent effects shows evidence of hydrogen bonding, charge transfer complex formation, and dipole-dipole coupling between solute and cosolvent molecules. SCF solvents, carbon dioxide, ethane, and fluoroform, are compared, and then the use of the chromatograph to measure solubilities is discussed.

Tomasko, D.L., Knutson, B.L., Coppom, J.M., Windsor, W., West, B., and Eckert, C.A., Fluorescence Spectroscopy Study of Alcohol Solute Interactions in Supercritical Carbon-Dioxide. Acs Symposium Series, 1993. 514 : p. 220-227.

The addition of cosolvent to a supercritical fluid (SCF) can enhance both solubilities and selectivities through specific interactions with the solute of interest. Characterizing the phase behavior in these solutions calls for a molecular level understanding of the local solvent structure and interaction strength around the solute. We describe fluorescence spectroscopy experiments to probe the structure and interaction between methyl alcohol cosolvent and the solute 7-azaindole. The results indicate that the interaction between cosolvent and solute in these systems is quite different than the usual hydrogen bonding observed in liquids. Specifically, there is a clear interaction between solutes and alcohol even at 0.5% alcohol but there is no evidence for stoichiometric hydrogen bonded complexes in supercritical fluid solutions.

Ekart, M.P., Bennett, K.L., and Eckert , C.A. , Effects of Specific Interactions in Supercritical Fluid Solutions - a Chromatographic Study. Acs Symposium Series, 1993. 514 : p. 228-235.

The addition of a small amount of a second component, a cosolvent, to a supercritical fluid can alter phase equilibria dramatically, giving engineers the capability to fine tune the properties of the solvent over a wide range. However, this ability remains largely unexploited in engineering applications due in part to the scarcity of experimental data and lack of accurate models. We have developed a new chromatographic technique to measure these cosolvent effects rapidly. This approach was verified by comparison to literature data obtained by other methods. With this technique, cosolvent effects can be measured in a variety of systems leading to a better understanding of the special intermolecular interactions that occur in supercritical fluid solutions.

Bergmann, D.L. and Eckert , C.A. , Limiting Activity-Coefficients of Nonelectrolytes in Aqueous-Solutions. Acs Symposium Series, 1992. 509 : p. 218-228.

Infinite dilution activity coefficients of non-electrolytes in water are very useful for the economical design of both separation systems for specialty chemicals and for environmental control. However, the unique properties of water result in very large nonidealities and present special challenges in measurement, correlation, and prediction. A number of measurement techniques that exist specifically for determining limiting activity coefficients are reviewed. They provide a modest base of reasonably reliable data. Current estimation techniques are limited. Certainly more good data are needed for both design and correlation development.

Knutson, B.L., Tomasko, D.L., Eckert , C.A. , Debenedetti, P.G., and Chialvo, A.A., Local Density Augmentation in Supercritical Solutions - a Comparison between Fluorescence Spectroscopy and Molecular-Dynamics Results. Acs Symposium Series, 1992. 488 : p. 60-72.

Substantial evidence suggests that in highly asymmetric supercritical mixtures the local and bulk environment of a solute molecule differ appreciably. The concept of a local density enhancement around a solute molecule is supported by spectroscopic, theoretical, and computational investigations of intermolecular interactions in supercritical solutions. Here we make for the first time direct comparison between local density enhancements determined for the system pyrene in CO2 by two very different methods--fluorescence spectroscopy and molecular dynamics simulation. The qualitative agreement is quite satisfactory, and the results show great promise for an improved understanding at a molecular level of supercritical fluid solutions.

Tomasko, D.L., Knutson, B.L., Eckert , C.A. , Haubrich, J.E., and Tolbert, L.M., Fluorescence Investigation of Cosolvent Solute Interactions in Supercritical Fluid Solutions. Acs Symposium Series, 1992. 488 : p. 84-91.

One of the challenges to the development of supercritical fluid (SCF) processes is understanding the solution thermodynamics on a molecular scale. Fluorescence spectroscopy has been shown to probe the local environment around chromophores. We now extend its use to SCF systems containing a cosolvent. Often a cosolvent is chosen to interact specifically with a given solute through hydrogen bonding or chemical complexation. In this work we investigate such interactions by fluorescence spectroscopy. Probe molecules such as 2-naphthol and its 5-cyano-derivative are effective chromophores for studying acid/base interactions since both are relatively strong photo-acids. In addition, 2-naphthol is a common solute for which SCF solubility and physical property data exist. Ultimately, spectroscopic information will be used to develop a clearer picture of the specific interactions which induce large cosolvent effects on solubility in SCF solutions.

Eckert, C.A., Ekart, M.P., Knutson, B.L., Payne, K.P., Tomasko, D.L., Liotta, C.L., and Foster, N.R., Supercritical Fluid Fractionation of a Nonionic Surfactant. Industrial & Engineering Chemistry Research, 1992. 31 (4): p. 1105-1110.

Supercritical fluid extraction (SFE) is demonstrated for the removal of unreacted dodecanol (lauryl alcohol) from nonionic surfactants (poly(oxyethylene) dodecyl ethers, C12En) and fractionation based on polymer chain length. Carbon dioxide, pure and with 1.0 and 3.5 mol % methanol cosolvent, propane, and ammonia were used as supercritical solvents. Experimental solvent loadings and distribution coefficients were used to develop a batch extraction model and continuous stagewise extraction model. Propane and ammonia exhibited higher loadings while the CO2/methanol mixtures showed greater selectivity for removing the dodecanol and C12E1. Conclusions regarding the choice of solvent for specific separations are discussed.

Howell, W.J. and Eckert , C.A. , Chemical Physical Model for the Interfacial Thermodynamics of Liquid-Metal Solutions. Industrial & Engineering Chemistry Research, 1991. 30 (7): p. 1500-1506.

The surface thermodynamics using chemical-physical theory (STCPT) model is developed to represent the surface tensions of compound-forming liquid metal alloys. The STCPT model uses an approach to modeling the interfacial thermodynamics wherein the surface is treated not as a separate phase, but rather the system is viewed as consisting of a bulk phase that is acted upon by a surface force. In this treatment, this surface force is analogous to other forces that may act upon the system, e.g., electrical and magnetic forces. The STCPT model provides an excellent model for the surface tension isotherms for a wide range of binary systems that exhibit various types of behavior. Compared with previous models, the STCPT model provides more accurate representations of the surface tension isotherms for compound-forming systems with fewer model parameters. Possible extensions are also discussed.

Hansen, A.R. and Eckert , C.A. , Volumetric Measurements for Some Liquid-Metals at High-Pressures and Temperatures. Journal of Chemical and Engineering Data, 1991. 36 (2): p. 252-255.

Molar volume data for liquid mercury, tin, lead, and bismuth were obtained for temperatures up to 650-degrees-C and pressures up to 3 kbar. The dilatometric technique used to determine this information is somewhat novel, since the higher melting points of most of these metals require special crucible and furnace configurations.

Bergmann, D.L. and Eckert , C.A. , Measurement of Limiting Activity-Coefficients for Aqueous Systems by Differential Ebulliometry. Fluid Phase Equilibria, 1991. 63 (1-2): p. 141-150.

New data have been determined by differential ebulliometry for limiting activity coefficients of binary systems of water. Results are reported with acetone, N,N-dimethylformamide, ethyl acetate, isopropanol, methanol, nitromethane and tetrahydrofuran over a modest range of temperatures. The results are used to demonstrate the limitations of several current excess Gibbs energy expressions in representing both vapour-liquid equilibrium and liquid-liquid equilibrium data from limiting activity coefficient measurements.

Trampe, D.M. and Eckert , C.A. , Calorimetric Measurement of Partial Molar Excess-Enthalpies at Infinite Dilution. Journal of Chemical and Engineering Data, 1991. 36 (1): p. 112-118.

An asymmetric isothermal flow calorimeter was used to obtain excess enthalpies of binary liquid mixtures of nonelectrolytes in the dilute region, and these data were used to calculate partial molar excess enthalpies at infinite dilution. A variety of solutes and solvents were examined to characterize the wide range of intermolecular interactions that may occur in solution. Comparisons were made to the values of the partial molar excess enthalpy at infinite dilution obtained from the temperature dependence of limiting activity coefficient data and to heats of solution at infinite dilution.

Hansen, A.R. and Eckert , C.A. , Interrelating Excess Thermodynamic Functions Using Fusion Properties. Aiche Journal, 1991. 37 (1): p. 48-58.

A new model that interrelates the liquid-phase excess Gibbs energy, enthalpy and volume, called the GHV model, is developed and discussed. The model is based on the fact that most mixtures are immiscible in solid phases and, therefore, have steep solid-phase boundaries. This single assumption allows the model to be applied potentially to all types of mixtures, including organic, metallic electrolytic and polymeric solutions. The model, which predicts that excess properties are related to one another through various fusion properties, pure component volumes, and enthalpies of fusion in particular, has been applied to a wide variety of organic and liquid metal mixtures. The results show at least qualitative validity without adjustable parameters and more quantitative ability with "effective" fusion properties. The model is also shown to rationalize some sign, magnitude and shape phenomena.

Karachewski, A.M., Howell, W.J., and Eckert , C.A. , Development of the Avec Model for Associating Mixtures Using Nmr-Spectroscopy. Aiche Journal, 1991. 37 (1): p. 65-73.

Common models for excess Gibbs energy account primarily for physical forces and do not represent strong specific interactions. However, purely chemical models neglect strong nonspecific forces and combined chemical-physical treatments involve an unreasonable number of adjustable parameters. One way to ameliorate this limitation is to make independent measurements of the parameters by a nonthermodynamic technique. We report an investigation of hydrogen bonding, a localized and very directional specific interaction, conducted in various alcohol mixtures using Fourier transform nuclear magnetic resonance to measure the number of complexed species in solution. A chemical-physical association model with variable equilibrium constants has been developed. The distinguishing feature of this model is that the species in solution do not have equal probability of forming yet require only a single chemical interaction parameter determined from fitting NMR data and a physical interaction parameter determined from infinite dilution activity coefficients. The AVEC model successfully predicts vapor-liquid equilibrium and excess enthalpy as well as liquid-liquid equilibrium, an especially stringent test.

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