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We have continued to characterize the sensitivity of spectroscopic shifts of three organic dyes in response to their chemical environment.  Specifically, we have studies di-8-ANNEPS, RH-237, and Coumarin 30.  Our approach has been to dissolve each dye in dozens of pure solvents and record their absorbance and fluorescence spectra.  We then correlate the frequency of maximum absorbance or fluorescence (ν_max) with the solvent parameters π*, α, β, and E.  These parameters measure a solvent’s dipolarity/polarizability, hydrogen bond (HB) donating ability, HB accepting ability, and excess polarizability.  These correlations take the form

ν_max = C + sπ* + aα + bβ + eE

The coefficients obtained for absorbance measurements made on di-8-ANEPPS, RH-237, and Coumarin 30 are found in the table below.

The results for coumarin 30 indicate that the dye’s frequency of maximum absorbance decreases as the polarity and HB donating ability increases.  From this, it can be concluded that the excited state is more polar and has stronger HB accepting ability than the ground state.  More importantly, it shows that the spectroscopy of the dye is sensitive to its chemical environment and that observed spectral shifts can be related to specific intermolecular interactions.  With several such dyes of varying dependence, it should be possible to characterize the interaction ability of surfactant solutions used in enhanced oil recovery processes.  We are currently pursuing the characterization of additional dyes for this purpose.

The statistics for di-8-ANEPPS and RH-237 are below those normally obtained for LSER studies, particularly when one considers that five solvents were significant outliers and thus not included in the analysis.  Given the structural similarity of the dyes, it is also surprising to see such different coefficients, both in magnitude and sign.  The outliers for di-8-ANEPPS were benzene, toluene, chloroform, chlorobenzene, and chloromethane – all nonpolar compounds.  One possible explanation for this is that these dyes change their ground and excited state conformations depending on the polarity of the solvent, leading to different types of electronic transitions upon absorption.  The LSER model is not designed to account for such changes.  We are looking into this possibility using computational chemistry and have also started collaborating with a computational group at the University of Chicago .

Future dye studies

Given the success we had in characterizing coumarin 30, we are now pursuing other coumarin derivatives.  While many literature reports exist about a variety of derivatives (they are popular laser dyes), solubility issues and commercial availability have limited our efforts.  We have obtained coumarin 6, 7, and 153, however, and have started absorbance and fluorescence studies on them.

Expansion MEKC selectivity triangle analysis via Drake’s DU-SCI program

Our review of literature related to the characterization of surfactant systems led us to the work of Fu and Khaledi (J. Chromatogr. A 2009, 1216, 1891-1900).  They used a triangle scheme and a compilation of 74 LSERs to group surfactant systems according to their types and strengths of intermolecular interactions with solutes.  Based on their work, we have developed a new visualization tool for comparing two surfactant systems and determining how chemically similar they are in terms of their interactions with solutes.  In effect, we correlate LSER coefficients for any two surfactant systems.  The stronger the correlation, the more similar the two systems are.  We then plot the correlation coefficient, slope, and intercept for the correlation in three-dimensional space.  Applying this to the data set of 74 systems allows for 2701 comparisons between systems, as shown in the accompanying figure.  The 3-D plot makes visual identification of similar systems (high correlations) and different systems (low correlation coefficients) quite easy.  Additional chemical information is gained by differentiating unity slopes from non-unity slopes and non-zero intercepts from intercepts equal to zero.

Additionally, and perhaps more importantly, while Fu and Khaledi’s work focuses on LSERs in MEKC, our new methodology and visualization tool are applicable to reversed phase, normal phase, gas, and supercritical fluid chromatography, as well as other models of intermolecular interactions such as Snyder’s hydrophobic subtraction model.  Thus, while the work developed from our interest in micellar systems, it has broader implications for comparing a variety of separation systems.  We are in the process or developing two manuscripts related to various aspects of this work.

ACS-related activities and impact of the work on students:

ACS National Meeting, Washington, D.C. :  I presented our initial work on our visualization tool at the American Chemical Society Fall Meeting this past August.  I have also submitted an abstract to a special session about innovations in separation science at the upcoming Pittcon meeting. 

Midwest ACS Meeting:  Two of my students, Andy Johnson and Ryan Johnson, have recently submitted abstracts to present our work at the regional ACS meeting in Iowa City in October.  Both students are planning to pursue graduate degrees in chemistry.  The opportunity to present their work at a regional meeting will help prepare them for this goal.

Project SEED:  This year, just as last, two local high school students participated in the research.  They were funded through the ACS Project Seed program. 

Summary: 

Overall, our work on the spectroscopic characterization of surfactant systems continues, with an increased attention to and need for computational studies.  We have also branched out into developing a new method that allows for easy visual comparison of literally thousands of separation systems.  Some of the work has already been presented at a national meeting and we have submitted abstracts for additional regional and national venues.  We also anticipate the submission of manuscripts based on the research that has resulted from our interests in characterizing surfactant systems.