Sekh Mahiuddin

Ion specificity of the zeta potential of α-alumina, and of the adsorption of p-hydroxybenzoate at the α-alumina-water interface

The influence of inorganic anions (NO(3)(-), I(-), Br(-), Cl(-), SO(4)(2-), and S(2)O(3)(2-)) and of divalent cations (Ca(2+) and Mg(2+)) on the zeta potential and on the isoelectric point of alpha-alumina in aqueous medium has been studied. The effect of the anions is highly ion specific even at salt concentrations as low as 5x10(-4) M. This unexpected finding is in line with a recent report [Böstrom et al., J. Chem. Phys. 128 (2008) 135104]. It is also in agreement with an earlier theoretical prediction [B.W. Ninham, V.V. Yaminsky, Langmuir 13 (1997) 2097]. The results are consistent with the classical Hofmeister series, except for the case of NO(3)(-). Divalent anions (SO(4)(2-) and S(2)O(3)(2-)) decrease the magnitude of the zeta potential of alpha-alumina in aqueous medium, more precisely; S(2)O(3)(2-) produced large negative zeta potential (approximately -12 to -47 mV) within the pH range of the study without the isoelectric point (IEP) of alpha-alumina. However, the SO(4)(2-) decreased the zeta potential of alpha-alumina of different magnitudes (maximum approximately 25 mV at both ends of the experimental acidic and basic pH scale) with a minor shift of the IEP (approximately 0.5 unit) toward lower pH. Ca(2+) and Mg(2+) produce zeta potentials of alpha-alumina roughly equal to that of neat alpha-alumina but slightly higher than that of Na(+) at both sides of the IEP. We have shown further that the same ion specificity or equivalently competitive ion effects occur with the adsorption density of p-hydroxybenzoate onto alpha-alumina surfaces. The sequence of anions (with common cation) for the adsorption density of p-hydroxybenzoate on the alpha-alumina surfaces follows the Hofmeister series sequence: S(2)O(3)(2-) < SO(4)(2-) < Cl(-) > Br(-) > I(-) > NO(3)(-). The divalent cations (Ca(2+) and Mg(2+)) exhibit a roughly equivalent effect on the adsorption of p-hydroxybenzoate onto alpha-alumina surfaces. Using the frequency shifts of nu(as)(-COO(-)) and nu(s)(-COO(-)) in the DRIFT spectra of p-hydroxybenzoate after adsorption and other characteristic peaks, we have demonstrated that p-hydroxybenzoate forms outer-sphere complexes onto alpha-alumina surfaces at pH 5 and 6 and inner-sphere complexes at pH 7, 8, and 9 in the presence of 5x10(-4) M NaCl(aq).

Spectroscopic Studies of Catanionic Reverse Microemulsion: Correlation with the Superactivity of Horseradish Peroxidase Enzyme in a Restricted Environment

Catanionic microemulsions formed by dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfate (SDS), n-hexanol, dodecane, and citrate buffer have been characterized by using dynamic light scattering (DLS) and spectroscopic studies. While the DLS measurements provide information about the hydrodynamic diameters of the microemulsion droplets formed upon variation of the constituents, steady-state and time-resolved fluorescence emission experiments probe the polarity and the dynamics of the trapped solvent pool inside of the microemulsion droplets of nanometer dimension. In addition, time-resolved fluorescence anisotropy shows the rigidity of the confined solvent pool as well as the coupling between the motion of a solute and those of the solvent molecules. The results obtained from the DLS and those from the steady-state and time-resolved fluorescence emission studies have been found to correlate well with the superactivity of horseradish peroxidase enzyme in the catanionic microemulsions. Subsequently, the time-zero estimate for the dynamic Stokes shift in these microemulsions reveals that approximately 50% of the total solvent dynamical response is missed due to the limited time resolution employed in our experiments. The amplitude of the missing portion is similar to what has been observed recently for nanoscopic water by Fayer and co-workers (Piletic, I. R.; Tan, H.-S.; Fayer, M. D. J. Phys. Chem. B 2005, 109, 21273).

Specific ion effects on adsorption at the solid/electrolyte interface: A probe into the concentration limit

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.

Influence of Chain Length of Alcohols on Stokes’ Shift Dynamics in Catanionic Vesicles

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).

Concentration and temperature dependence of ultrasonicvelocity and isentropic compressibility in aqueous sodium nitrate andsodium thiosulfate solutions

Ultrasonic velocities (u/m s -1 ) of aqueous sodium nitrate and sodium thiosulfate solutions have been measured as functions of temperature (278.15⩽T/K⩽323.15) and concentration (0.006⩽m/mol kg -1 ⩽10.522). Isentropic compressibilities (κ S ), apparent molal isentropic compressibilities (K S,φ ) and hydration numbers (n h ) for both systems have been calculated. Sodium thiosulfate is more highly hydrated than sodium nitrate; the primary hydration numbers of NO 3 - and S 2 O 3 2- ions have been estimated to be 6 and 13, respectively. The values of κ S , K S,φ and n h indicate that a strong structural effect exists in the sodium thiosulfate system arising from strong ion–solvent and ion–ion interactions in different concentration regions.

Isentropic compressibility, effective pressure, classical sound absorption and shear relaxation time of aqueous lithium bromide, sodium bromide and potassium bromide solutions

Abstract Densities, speeds of sound and viscosities of aqueous lithium bromide, sodium bromide and potassium bromide solutions were measured as functions of concentration (0.0085≤ m (mol kg −1 )≤14.06) and temperature (273.15≤ T (K)≤323.15). Allied properties like isentropic compressibilities, effective pressure, classical sound absorption and shear relaxation time were calculated by using the measured data. The interaction in these three bromide solutions vary in the order of NaBr>KBr>LiBr. The primary hydration shells are saturated at 10.8, 5.1 and 5.8 mol kg −1 with 5.1, 10.9 and 9.6 number of water molecules in the primary hydration shell of lithium bromide, sodium bromide and potassium bromide solutions respectively. The cationic environment is found to influence the hydration phenomena of the anion. The non-Arrhenius temperature dependence of shear relaxation time were analysed by using the Vogel-Tammann-Fulcher (VTF) equation. The concentration dependence of shear relaxation time is different in these three bromide solutions. Such an effect is attributed to the competitive effects of hydrogen bonding, structure forming/breaking effect of ions and the formation of ion pairs.

Adsorption of 3,4-dihydroxybenzoic acid onto hematite surface in aqueous medium: Importance of position of phenolic –OH groups and understanding of the same using catechol as an auxiliary model

A comparative adsorption kinetics, isotherms, dissolution and surface complexation of 3,4-dihydroxybenzoic acid (3,4-DHBA) and 1,2-dihydroxybenzene (catechol) at the hematite/electrolyte interface were investigated. The kinetics at pH 10 and 298.15K suggested that the adsorption behaviour of 3,4-DHBA and catechol onto hematite surface is similar and attain same equilibration time of 60 min. The adsorption kinetics data of 3,4-DHBA and catechol fit the pseudo-second-order kinetic equation of nonlinear form best. The adsorption density of 3,4-DHBA at pH≥9 increases and thereby mimics the behaviour of catechol. The solubility of hematite depends on both pH of the suspension and concentration of adsorbate. The inner-sphere complex is formed by 3,4-DHBA and catechol onto hematite surface but the mode orientation is likely to be different in the pH range 5-8 and 9-10. The advance microscopic scanning in conjunction with the vibration spectroscopy would provide better pictorial presentation of the mode of orientation of 3,4-DHBA and catechol onto hematite surface at different pH.

Adsorption and surface complexation of trimesic acid at the α-alumina–electrolyte interface

Adsorption kinetics, adsorption isotherms and surface complexation of trimesic acid onto alpha-alumina surfaces were investigated. Adsorption kinetics of trimesic acid with an initial concentration of 0.5 mM onto alpha-alumina surfaces were carried out in batch method in presence of 0.05 mM NaCl (aq) at pH 6 and 298.15, 303.15 and 313.15 K. Adsorption isotherms were carried out at 298.15 K, pH 5-9, and 0.05 mM NaCl (aq) by varying trimesic acid concentration from 0.01 to 0.6 mM. Three kinetics equations such as pseudo-first-order, pseudo-second-order and Ho equations were used to estimate the kinetics parameters of the adsorption of trimesic acid on the alpha-alumina surfaces. Ho equation fits the experimental kinetics data significantly better and the estimated equilibrium concentration is in excellent agreement with the experimental value. The adsorption data were fitted to Freundlich and Langmuir adsorption model and the later best fits the adsorption isotherms. Comparison of adsorption density of trimesic acid with that of benzoic and phthalic acids follows the sequence: benzoic acid < trimesic acid < phthalic acid. The negative activation energy and the Gibbs free energy for adsorption indicate that the adsorption of trimesic acid onto alpha-alumina is spontaneous and facile. DRIFT spectroscopic studies reveal that trimesate forms outer-sphere complexes with the surface hydroxyl groups that are generated onto alpha-alumina surfaces in the pH range of the study.

Speeds of Sound and Viscosities of Potassium Thiocyanate in Water, Methanol, and Propylene Carbonate

Measured speeds of sound, structural relaxation times, and the viscosities of aqueous, methanolic, and propylene carbonate solutions of potassium thiocyanate are reported as functions of concentration (0.0049 ≤ m/mol kg–1 ≤ 22.924) and temperature (273.15 ≤ T/K ≤ 343.15). The primary solvation number of the SCN– ion has been estimated to be 11.6 in a water medium. The total solvation numbers corresponding to the concentration at which the isentropic compressibility isotherms converge for potassium thiocyanate in methanol and propylene carbonate media were estimated to be 5.2 and 1.9, respectively. Structural transition in aqueous potassium thiocyanate solution has been observed in the concentration range of c. 4.0–5.5 mol kg–1 which was assigned due to the counterbalance between the structural-breaking effect of the K+ ion and the ion-association of K+ and SCN– ions.

An Assessment of the Aggregation and Adsorption Behavior of the Sodium Dodecylsulfate–Cetyltrimethylammonium Bromide Mixed Surfactant System in Aqueous Medium

Sodium dodecylsulfate and cetyltrimethylammonium bromide mixtures are important catanionic systems, as they have an inherent tendency to form vesicle structures. Despite extensive studies on the phase behavior and microstructures, there is dearth of basic information on the aggregation and adsorption behavior of this mixed system. In this work the critical micelle concentration, surface tension reduction effectiveness, surface excess, mixed micelle and monolayer compositions, activity coefficients, interaction parameters, counterion binding and Gibbs energy terms of this mixed system are determined by measuring its surface tension and conductance as a function of composition. The dependence of mixed micelle composition on surfactant concentration has been successfully demonstrated.