Adsorption of organic acid at the mineral oxide-electrolyte interface has been explored. The adsorption of 2,4-dihydroxybenzoic acid onto α-alumina illustrates that specific ion effects show up at very low salt concentration (<0.05 mM). These surprising Hofmeister effects occur at salt concentrations an order of magnitude lower than in a previous study ( J. Colloid Interface Sci. 2010, 344, 482 ). Salts enhance adsorption and specifically at ≤0.05 mM. With increasing concentration of ion, the adsorption density decreases. The results are accounted for by incorporating the ion size and dispersion forces in the theoretical modeling based on ab initio calculations of polarizabilities. The order appears to be governed by ion size, determining the maximum concentration that ions can attain near the surface due to close packing.

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In this paper, we explore the effects of the chain length of simple monohydroxy alcohol (C(n)OH, 2 ≤ n ≤ 8) and benzyl alcohol (C(6)H(5)CH(2)OH) upon the fluorescence dynamics of a dipolar solute probe, coumarin 153 (C153), in vesicles formed in aqueous solutions of two oppositely charged (cationic and anionic) surfactants in the presence of 0.05 mol kg(-1) alcohol. The catanionic vesicles are composed of 70 mol % sodium dodecyl sulfate (SDS) and 30 mol % cetyltrimethylammonium bromide (CTAB). The presence of alcohols of different chain length improves the stability of the catanionic vesicles. Dynamic light scattering (DLS) experiments suggest a gentle increase in the hydrodynamic diameter of the catanionic vesicle with alcohol chain length up to n=4 and then an abrupt increase for the rest of the alcohols considered. The polarity and dynamics of the catanionic vesicles, probed by the steady-state and time-resolved fluorescence spectroscopy, indicate a signature of confined water. Quantities measured from fluorescence experiments of these vesicles also show a mild variation for alcohols of chain length n ≤ 4, followed by a sudden variation for alcohols with n > 4. Interestingly, pentanol and benzyl alcohol in catanionic vesicles showed roughly similar effects due to their equivalent chain length. All of these data are remarkably correlated with the recently observed depression of the solubility temperature of catanionics with alcohol chain length (Langmuir2009, 25, 12516-12521).

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