Author

Haiyan Li

Date of Award

12-2004

Level of Access

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor

Carl P. Tripp

Second Committee Member

Douglas W. Bousfield

Third Committee Member

David J. Neivandt

Abstract

Polymers/surfactants are used to control and modify the interfacial properties of particulate suspensions. The resulting properties are dependent on the amount of adsorbed surfactants/polymers on the surface as well as the nature of the aggregated structures. Often these interactions are complex and not well understood. In this thesis we have used Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) to identify surfactant, mixed surfactant, polyelectrolyte and mixed polyelectrolyte surfactant structures formed on charged Ti02 particles. In addition, the change occurring to the structure of an adsorbed polymer layer on Ti02 to flowing suspension of silica particles was studied. A particular focus has been on the headgroup bands of the surfactants and the hnctional groups of the polymer as this provides unique information on the nature of the aggregated structure.

For the measurement of cetyltrimethylammoniurn bromide (CTAB) onto Ti02 at pH 10.3, it is found that there are abrupt changes in the intensity of the symmetric bending mode of the CTAB headgroup and these abrupt changes have been correlated to different aggregated structures along the adsorption isotherm. Furthermore, by measuring spectra as a function of time it is possible to obtain information on the dynamics of the formation of aggregated CTAB structures on the surface. It is shown that aggregated hemimicellar, admicellar and micellar structures initially adsorb through intermediate structures that have a higher percentage of CTAB molecules bound directly to charged sites on the surface.

Mixed CTAB and SDS structures are produced when SDS is used to probe CTAB adsorbed hemimicelle, admicelle or micelle structures formed when CTAB was first added to a bare TiO2 surface at three different solution concentrations. Each CTAB structure was studied as a function of contact time with a solution containing the deutero form of the anionic surfactant, sodium dodecyl sulfate (SDS). By measuring the changes in the headgroup bands for both SDS and CTAB along with the change in the adsorbed amount of each surfactant as a function of time, a clearer picture emerges of the mixed surfactant structures formed on the surface. Specifically, it was shown that the SDS intercalates into the CTAB structure leading to a variety of mixed surfactant structures that depend on the surfactant concentrations in solution.

The adsorption of sodium polyacrylate (NaPA) on charged TiO2 particles and the subsequent interaction of the adsorbed polymer structure with cationic and anionic surfactants were also determined by ATR-FTIR. The nature of the polymer structure was deduced from the adsorbed amount in tandem with the information obtained from monitoring the change in the relative intensity of the COO- and COOH infrared bands. It is shown that the initial NaPA approaching the bare surface adopts a flat conformation with high bound fraction. Once the bare sites on the surface are covered, the accommodation of additional polymer on the surface requires the existing adsorbed layer to adopt a conformation with a lower bound fraction. When the adsorbed NaPA is probed with a solution containing the anionic surfactant, SDS, the SDS competes for surface sites and displaces some of the bound NaPA segments from the surface giving rise to an polymer layer adsorbed with an even lower bound fraction. In contrast, addition of a solution containing the cationic surfactant, CTAB results in the binding of the surfactant directly to the free COO- sites on the adsorbed polymer backbone. Confirmation of a direct interaction of the CTAB headgroup with the free COO- groups of the polymer is provided by intensity changes in the headgroup IR bands of the CTAB.

This vibrational approach was then used to study the adsorption of charged silica particles onto TiO2 particles coated with anionic or cationic polyelectrolytes. It is shown that the deposition of positively charged silica particles on a sodium polyacrylate coated TiO2 does not lead to any desorption of the polymer from the surface but rather to a change in the relative intensities of the bands due to COOH and COO- groups. From this change in band intensity, it is calculated that only about 6% of the COO- groups located in the loops and tails bind to the silica particle. This shows that the polymer bridges the two particles through an electrostatic interaction with the outer COO- groups. Similarly, in the case of the TiO2 particles coated with the cationic poly(diallyldimethylammonium) chloride, the deposition of negatively charged silica does not reduce the amount of polymer on the TiO2 surface but rather leads to an increase in intensity of the symmetric bending mode of the +N(CH3)3 group. This change in band intensity arises from the binding of these cationic sites of the polymer to the negative surface sites on the silica. The results show that once adsorbed on the TiO2 particle, the PDADMAC or the NaPA does not migrate to the silica particles.

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