ASSESSING MEDIUM EFFECTS IN TALSPEAK-LIKE SYSTEMS VIA CALORIMETRIC ENTROPY TITRATIONS
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This study investigated the application of calorimetric entropy titrations to the light lanthanides in TALSPEAK-like systems. Since TALSPEAK operates in an unusual ionic medium, this work was expanded to probe medium effects of the buffer by investigating three separate buffer systems. The work includes the determination of complexation of lactate with the light lanthanides at 2.0 M ionic strength, to provide internally consistent thermodynamic data for accurately modeling titration experiments. Luminescence titrations were conducted to validate the models for the titration experiments, and determine the stability constant for the metal-DTPA formation in these systems. Based on modeling using literature values, the reaction of DTPA with the lanthanides reaches an equilibrium with 80% of the metal complexed to DTPA in the 2.0 M total lactate and glycolate media under the specific conditions investigated. To corroborate these results, and predict changes in the hydration of the metal center in each of the buffers, luminescence lifetime measurements were conducted. These titrations suggest possible coordination modes for europium in these different media. Calorimetric entropy titrations were conducted to determine whether there are medium effects associated with the high concentrations of the buffer. A model defining a 1:1 metal-DTPA complex was developed to calculate the enthalpy, stability constant, Gibbs energy and entropy in the different buffers under the same conditions of pH, ionic strength, and metal concentration. An alternative model accounting for competing complexation equilibria was developed to determine the stability constant for metal-DTPA complex formation comparable to literature in the buffer medium. Comparison of this value to literature suggests the light lanthanides exhibit an increase in the stability constant for the metal-DTPA complex in the buffer systems relative to the constants determined in 2.0 M sodium perchlorate. The work presented here suggests the potential benefit of calorimetric entropy titrations to study these complex systems. Calorimetry offers a new way to directly probe the influence of the medium in systems that could not be measured previously, due to limitations in other experimental techniques.