Specific Ion Effects on Counterion Distribution in Surfactant Adsorbed Films Studied through Surface Tensiometry and Total Reflection XAFS

In this thesis, “the specific ion effect on interfacial and colloidal phenomena” was investigated by examining the counterion distribution in the surfactant adsorbed films with the aid of X-ray absorption fine structure under total reflection condition (TR-XAFS) followed by solvation structure analysis as well as by evaluating thermodynamic physicochemical variables through surface tensiometry followed by the analysis based on our original thermodynamic formulation. In Chapter 1, the scientific background for this study was summarized. In Chapter 2, the materials and experimental apparatus used for this study were summarized. In Chapter 3, the relation between the counterion distribution and the surface charge density was investigated for the single surfactant systems of dodecyltrimethylammonium bromide (DTAB), hexadecyltrimethylammonium bromide (HTAB), DTAB with added NaBr and 1-decyl-3-methylimidazolium bromide (DeMIMBr). In the TR-XAFS technique, the spectra evaluated through the extended X-ray absorption fine structure (EXAFS) analysis were expressed as linear combinations of two specific spectra corresponding to fully hydrated bromide ions (free-Br) and partially dehydrated bromide ions bound to the hydrophilic groups of surfactant ions (bound-Br) in the adsorbed film, and the ratio of bound-Br was determined to evaluate the counterion distribution. The values of the bound ratio of bromide ions of HTAB and DTAB with added NaBr were rather close to that of DTAB at a fixed surface density of surfactant ions which indicated that the difference in chain length and adding electrolyte are not so much influential on counterion distribution, whereas the bound ratio of DeMIMBr was evidently smaller than that of DTAB due to the effective packing of imidazolium rings at surface which provided the conceivable situation that the adsorbed molecules were definitely not homogeneously dispersed but a part of them were molecularly dispersed around the domains (islands or clusters) with the bound Br ions. In Chapter 4, the distribution and miscibility of counterions in the electrical double layers of adsorbed films containing binary counterions was investigated by relating the bound ratio to the surface composition of counterions and the excess Gibbs energy. It was suggested that in the DTAB-DTAC system the mixing of Cl ions had practically no influences to the distribution of Br ions in the adsorbed film and ideal mixing was observed because the nature of counterions were similar to each other. In the HMIMBr-HMIMBF4 system the hydrogen bonding between BF4 ion and imidazolium cation was essential, which caused a kind of segregation of the counterion distribution. In the DTAB-DTABF4 systems, on the other hand, preferential binding of counterions due to lower adsorption energy of BF4 and a remarkable difference in size between them yielded the preferable counterion mixing in the electrical double layer. In Chapter 5, the effect of valence and size of counterions on the miscibility and distribution of binary counterions was clarified by employing the DTAB-DTA2SO4 system and adopting the same methodology as shown in Chapter 4 with newly derived thermodynamic equations. The evaluated composition of counterions in the adsorbed film revealed that the space among larger SO4 2 is effectively occupied by smaller Br ions. The determined ratio of bound-Br revealed that the vicinity of hydrophilic heads of surfactant ions is occupied by monovalent Br ions preferentially even to divalent SO4 2ions. The obtained negative excess Gibbs energy in the adsorbed film was reasonably explained from the enthalpy and entropy contributions respectively arising from the preferential binding of Br ions having smaller hydration free energy and adsorption energy and the sharing the confined space by SO4 2and Br ions having different size. From the results of Chapter 4 and 5, the three essential factors that cause the specific ion effect, the size and hydration of counterions and the hydrogen bonding with surfactant ions and counterions was suggested. In Chapter 6, the effect of the arrangement of hydrophilic head groups in the adsorbed films on counterion distribution was investigated with employing the HTAB-DTAB mixed system. From the obtained smaller value of the ratio of bound-Br than that for DTAB, it was suggested that the staggered arrangement of surfactant ions enhances the inhomogeneous distribution of ion species similarly to DeMIMBr system demonstrated in Chapter 3. From these results, it was suggested that both the density and arrangement of surface charge due to surfactant ions could influence counterion distribution in the electrical double layer. At the last of this chapter, the perspective of the study on the specific counterion effect using TR-XAFS technique was presented.