a Holographic Interferometric Study of the Temperature Dependence of Diffusion of Aqueous Sodium-Chloride in Water, from 32 Degrees C to 50 Degrees C.
Diffusion in liquids is a macroscopic phenomenon that can give us insight into the local environment of molecules in a liquid. The diffusion rate is a measure of the kinetic energy of the molecules as well as the intermolecular distances between molecules. The diffusion constant for a variety of liquid systems has been measured many times using a wide range of methods (1). However, the majority of techniques used are either destructive or disruptive, and therefore inaccurate. With the development of holographic interferometry a method of studying liquids without disturbing them has become available. This dissertation will investigate the development of holographic interferometry as a tool for studying diffusion and look closely at one system of special interest. Dr. George Kreishman, of the University of Cincinnati Department of Chemistry, reported the detection of an apparent second order phase transition in bulk water structure in tetramethylammonium chloride solutions by proton nuclear magnetic resonance spectroscopy (2). His measurements indicated that the entropic increase with temperature is continuous but a discontinuity appears in the first derivative at about 42(DEGREES)C. This discontinuous behavior of the chemical shift of bulk water protons suggests the existence of a second order phase change in the water structure (which he terms a biphasic change). Other experimental work has also indicated a possibility of a chloride ion induced phase transition in water. Biphasic behavior as a function of temperature was seen in the mean ionic activity coefficients of NaCl in H(,2)O (3), the conductance of Cl('-) in H(,2)O (4), the reduction potentials of the saturated calomel and silver-silver chloride reference electrodes (5), the aggregation properties of purine, and the absorbance of purine (6). In all of these systems the transition temperature for biphasic behavior is 42(DEGREES)C (+OR-) 1(DEGREES)C. Such a phase change must be characterized by changes in the structure of the liquid. The thermodynamic variables that show a discontinuity during such a phase change are , , , and , (7). We monitor the thermodynamic variables directly. A change in volume, for example, will cause a change in density which will alter the "vaporization energy" of a liquid. The diffusion constant is closely related to the vaporization energy, so as we monitor the diffusion constant, we can detect changes in the density. Any biphasic behavior should be seen as a change in the temperature dependence of the diffusion constant for the system. A further discussion of the relationship between the vaporization energy and the diffusion constant is given in Chapter III. In order to investigate this apparent phase transition we chose to look at the diffusion of aqueous NaCl into H(,2)O. We measured the diffusion constant for this system over the temperature range 32(DEGREES)C to 50(DEGREES)C using holographic interferometry. We analyzed the interferograms using a new method that we recently developed (8). A complete discussion of this method is given in Chapter IV followed by a description of the experimental procedure and results in Chapters V and VI, respectively. We conclude with a discussion of the results in chapter VII.
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- Physics: Optics