A Study of Hydrogen-Bonded Solutions With NMR Spectroscopy
McNiel, Marianne Marie
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https://hdl.handle.net/2142/69788
Description
Title
A Study of Hydrogen-Bonded Solutions With NMR Spectroscopy
Author(s)
McNiel, Marianne Marie
Issue Date
1987
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
Several thermodynamic models have been proposed for alcohol-hydrocarbon solutions using chemical-physical theories with the alcohol assumed to self-associate through hydrogen bonds. These models do an excellent job of correlating vapor-liquid equilibria, excess enthalpy and mutual solubility data. However, they have no predictive capabilities because of the large number of unknown parameters. Also, cross prediction of other thermodynamic properties from the fitted parameters is usually not possible.
One of the reasons for the failure of these models is that the hydrogen-bond association models are incorrect, and incorrect equilibria association parameters are obtained. Phase equilibria data are not very sensitive to the association models so cross prediction of other properties is not possible. Independent verification of association models and equilibrium parameters is necessary. Fitting a model to activity coefficient data is not a stringent test of the association models for hydrogen bonding as will be shown.
We have used NMR (nuclear magnetic resonance) spectroscopy measurements to test chemical-physical models for associated systems and to obtain equilibrium constants and heats of formation for hydrogen bonding. Our work indicates that a trimer rather than a dimer is the dominate species in dilute 2-propanol and 1-butanol in cyclohexane. From these results, a new association model that includes a cyclic trimer and linear higher order species has been developed. This model is able to represent NMR data, phase behavior and to cross predict the excess enthalpy. It is compared to several other models including the continuous linear association model of Flory (19) used by Renon and Prausnitz (17) to describe the thermodynamics of alcohol-hydrocarbon solutions. Our model has been shown to be superior for predicting phase behavior, for representing the activity coefficients at infinite dilution at both ends and for the cross prediction of the excess enthalpy. The model should be an excellent tool for representing the thermodynamic behavior of alcohols dilute in hydrocarbons and other inert solvents.
An improved technique for obtaining equilibrium constants from NMR spectroscopy in solvated systems is described and shown to be successful. The technique allows for variation in the inert solvent mole fractions of at least 0.02 in cases where the equilibrium constant is not expected to be large ($>$20).
For solvated systems, it is shown that cross prediction of the excess enthalpy is possible using the parameters determined from NMR spectroscopy.
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