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During the time period of the PRF grant I have employed two undergraduate students with PRF funds to study the adsorption behavior of the polyelectrolyte poly(sodium 4-styrene sulfonate) (PSS) to TiO₂ films coated onto a ZnSe internal reflection element in the presence of the surfactant centyl pyridinium bromide monohydrate (CPBM).  Fourier transform infrared spectroscopy is coupled with attenuated total internal reflection spectroscopy (FTIR ATR) and multivariate data analysis is employed to understand the adsorption behavior of PSS and PSS/CPBM aggregates at the TiO₂/ water interface.  Lab supplies need for the students research has also been purchased with PRF funds.  In addition, the work of a master’s student who is supported on separate NSF and internal funding is being leveraged towards research outlined in the PRF grant. 

Significant progress has been made towards quantitating the amount of polyelectrolyte that adsorbs to the TiO₂ films and how addition of CPBM affects the adsorption.  One of the significant findings of the research is that the amount of PSS that adsorbs to the TiO₂ surface can be affected by the addition of CPBM but at high enough concentrations of solution phase PSS the amount of PSS at the TiO₂/water interface is essentially independent of CPBM concentration for the range of CPBM concentrations studied.  The concentration of CPBM does affect the amount of PSS that accumulates at the TiO₂/water interface for a given change in solution phase PSS concentration.  The amount of PSS adsorbed at the TiO₂/water interface increases by up to 2.5 times for a given change in solution phase PSS concentration at high CPBM concentrations relative to the PSS adsorption for the same change in solution phase PSS concentrations with no CPBM present.  The reason for this is attributed to formation of CPBM/PSS aggregates that are not as easily solvated by water and adsorption to the TiO₂ surface becomes more energetically favorable.  However, it is surprising that the overall amount of PSS that accumulates at the TiO₂ water interface is relatively consistent within the uncertainties of the experiments for studies with and without CPBM present.  Another interesting finding is that for systems near the saturation limit of CPBM, the CPBM accumulates at the TiO₂ surface and is not displaced until enough PSS is added to force the CPBM into solution likely as an aggregate with PSS.

In addition to quantitating the amount of PSS and CPBM at the TiO₂/water interface, exchange of CPBM between the solution phase and PSS at the TiO₂/water interface has also be observed and quantitated.  For instance, the ratio of CPBM to PSS was found to vary from 190/1 to 60/1 for a CPBM concentration of 1.08 x 10^-4 M and PSS concentration ranging from 1.25 x 10^-7 to 1.85 x 10^-6 M.  The multivariate data analysis my group has employed allows separation of the infrared bands due to the PSS from infrared bands due to CPBM and insures that the behavior of the two species can be separated.

Results from the PRF funded research were presented at a poster session of the colloids and surfaces division at the ACS national meeting in Salt Lake City in March of 2009.  The poster was entitled “In Situ Infrared and UV-Vis Spectroscopy Studies of Polyelectrolyte Interactions at the TiO₂/Water Interface”.  Two co-authors on the poster, Sarah Hayden and Ryan Scheffelmaier, are the undergraduates who have been funded on the PRF grant.  Some of the travel expenses to this meeting were funded by the PRF grant.  A manuscript is currently in preparation for submission to the journal Langmuir with both Sarah Hayden and Ryan Scheffelmaier as co-authors.  During the time Sarah and Ryan have been working on this research they have been exposed to advanced spectroscopic and data analysis techniques not normally encountered in the undergraduate curriculum.  Sarah plans to apply to Ph.D. programs in physical chemistry this year and Ryan plans to apply to pharmacy school this year.

Future research will involve carrying out studies similar to what has been described above but at different pH and ionic strengths as outlined in the PRF grant.  However, while we did have some success with CPBM as a surfactant we have moved on to another surfactant, oxphenonium bromide.  This is mainly because this surfactant has a distinctive carbonyl band that is easily seen in the infrared spectra and should make the separation of the surfactant signal from the PSS signal much more straight forward.   In addition, we also hope to employ some temperature dependent studies of PSS and PSS/surfactant adsorption to yield a direct measurement of the enthalpy of adsorption.  Finally, we well continue some UV-Vis spectroscopy studies that were started in the last year on the PSS/surfactant/TiO₂ system.  While this was not part of the original PRF proposal we have found that these studied can provide valuable information about the solution phase behavior of the PSS and surfactant which can be correlated in our infrared surface studies.  Such information helps us to understand the behavior we see at the TiO₂/water interface.