Jerzy Zajac

Adsorption microcalorimetry used to study interfacial aggregation of quaternary ammonium surfactants (zwitterionic and cationic) on powdered silica supports in dilute aqueous solutions

Abstract Titration microcalorimetry has been used to compare interfacial aggregation of some zwitterionic and cationic quaternary ammonium surfactants on powdered silica substrates with micellisation in free aqueous solution. Enthalpy of dilution measurements allowed determining the molar enthalpies of micellisation for two zwitterionic amphiphiles, (dodecyldimethylammonio)butanoate and (dodecyl dimethylammonio)-1-propanesulfonate, and two cationics, benzyldimethyldodecylammonium bromide (BDDAB) and tetradecyltrimethylammonium bromide (TTAB). The effect of temperature and ionic strength on adsorption of zwitterionic surfactants on a macroporous, hydrophilic silica surface was studied so as to prove the micelle-like character of surface-bound aggregates. Interfacial aggregation of cationic surfactants on a non-porous, a macroporous, and a mesoporous solid sample in dilute aqueous solutions was inferred from adsorption and calorimetry measurements at room temperature. Special attention was given to the control of the solid surface purity. Interfacial aggregation in such systems results from the interplay of the same molecular interactions that drive the formation of micelles in free solution, with the influences and constraints of the solid–solution interface.

Competitive Solubilization of Phenol by Cationic Surfactant Micelles in the Range of Low Additive and Surfactant Concentrations

Competitive interactions of phenol (PhOH) with micellar aggregates of hexadecyltrimethylammonium bromide (HTAB) against 1-butanol (BuOH) in aqueous solutions at surfactant concentrations close to the critical micelle concentration (CMC), BuOH concentration of 0.5 mmol kg(-1), and phenol contents of 1, 5, or 10 mmol kg(-1) have been investigated at 303 K by means of (1)H NMR spectroscopy, titration calorimetry, and solution conductimetry. The solubilization loci for phenol were deduced from the composition-dependence of the (1)H chemical shifts assigned to various protons in the surfactant and additive units. Since in pure HTAB solutions phenol is already in competition with Br(-), addition of 1 mmol kg(-1) NaBr to the system weakens the phenol competitiveness. The presence of butanol in the HTAB micelles causes phenol to penetrate deeper toward the hydrophobic micelle core. For higher phenol contents, the butanol molecules are constrained to remain in the bulk solution and are progressively replaced within the HTAB micelles by the aromatic units. The competitive character of phenol solubilization against butanol is well supported by changes in the thermodynamic parameters of HTAB micellization in the presence of both of the additives.

Micellization Behavior of Long-Chain Substituted Alkylguanidinium Surfactants.

Surface activity and micelle formation of alkylguanidinium chlorides containing 10, 12, 14 and 16 carbon atoms in the hydrophobic tail were studied by combining conductivity and surface tension measurements with isothermal titration calorimetry. The purity of the resulting surfactants, their temperatures of Cr→LC and LC→I transitions, as well as their propensity of forming birefringent phases, were assessed based on the results of 1H and 13C NMR, differential scanning calorimetry (DSC), and polarizing microscopy studies. Whenever possible, the resulting values of Krafft temperature (TK), critical micelle concentration (CMC), minimum surface tension above the CMC, chloride counter-ion binding to the micelle, and the standard enthalpy of micelle formation per mole of surfactant (ΔmicH°) were compared to those characterizing alkyltrimethylammonium chlorides or bromides with the same tail lengths. The value of TK ranged between 292 and 314 K and increased strongly with the increase in the chain length of the hydrophobic tail. Micellization was described as both entropy and enthalpy-driven. Based on the direct calorimetry measurements, the general trends in the CMC with the temperature, hydrophobic tail length, and NaCl addition were found to be similar to those of other types of cationic surfactants. The particularly exothermic character of micellization was ascribed to the hydrogen-binding capacity of the guanidinium head-group.

What are the main contributions to the total enthalpy of displacement accompanying the adsorption of some multivalent metals at the silica–electrolyte interface?

The integral molar enthalpies of displacement, Δdplh, accompanying adsorption of Mg(II), Ca(II), Sr(II), Ba(II), Cd(II), Co(II), Zn(II), and Eu(III) cations from aqueous solutions of metal nitrate onto Spherosil XO75LS at 298K were determined based on the combination of liquid flow calorimetry and classical adsorption for two degrees of surface coverage: 0.035μmolm(-2) and 0.08μmolm(-2), in the presence of 0.1molL(-1) sodium nitrate in the aqueous phase at pH 5, 6, and 7. The displacement was shown to be endothermic and quite independent of the chemical specificity of the adsorbing metal. Two enthalpy effects were postulated to contribute mostly to the positive Δdplh values, depending on the experimental pH value: (i) cation dehydration upon adsorption and (ii) deprotonation of surface silanols to create negatively charged SiO(-) sites. Changing proportions among the various adsorbed species, including free Eu(3+) or Cd(2+) cations and hydrolyzed Eu(OH)(2+), Eu(OH)2(+) or Cd(OH)(+) cations, were accepted to explain the downward trends in Δdplh with increasing extent of adsorption for Eu(III) and Cd(II).

Bulk hydrolysis and solid-liquid sorption of heavy metals in multi-component aqueous suspensions containing porous inorganic solids: are these mechanisms competitive or cooperative?

Fundamental aspects of the removal of heavy metals from aqueous streams under conditions of competition among the various species have been studied between pH 3 and 9 on Spherosil XO75LS, ordered mesoporous MCM-41 and MCF silicas, as well as a MCF sample grafted with (3-aminopropyl)methoxydimethylsilane (AMPS-MCF). Cd(II), Co(II), Pb(II), or Sr(II) nitrate solutions were used to determine the percentage of metal uptake by each solid at 298 K as a function of the pH of the equilibrium solution, at an initial metal concentration of 0.0001 mol L(-1) and the ionic strength being fixed with 0.01 mol L(-1) NaNO(3). Almost complete retention of the heavy metals on the four solid samples was observed, with the process beginning at pH values smaller than those marking the onset of bulk precipitation of a given metal in free solution. The heavy metal-uptake mechanism was regarded as hydrolysis-like phenomenon in metal-containing solid suspensions. Weak adsorption of metal species from slightly acidic and neutral solutions was a kind of nucleation step. Adding cadmium to an equimolar solution containing cobalt, lead, or strontium showed no significant effect on the retention of the main metal component. This indicated the great independence of the retention mechanisms.

The effect of chelating anions on the retention of Co(II) by γ-alumina from aqueous solutions under the unadjusted pH condition of supported catalyst preparation

This study analyzes the effect of the addition of acetate, citrate, and nitrilotriacetate anions on the retention of Co(II) cations by the γ-alumina surface in view of the preparation of alumina supported cobalt catalysts. The emphasis was placed on the way the Co(II) species attach to the solid surface when adsorbed from aqueous solutions under the unadjusted pH condition. The individual adsorption isotherms onto γ-Al2O3 support for cobalt and a given ligand were determined by following the solution depletion method in single-solute and bi-solute systems. These adsorption data were supplemented by the results of potentiometric titrations. In the case of bi-solute systems, the adsorption procedures allowed either co-impregnation of γ-alumina with equimolar solutions of cobalt and ligand salts or pre-impregnation of γ-alumina with the ligand anions and the subsequent adsorption of cobalt. Changes in the pH of the equilibrium solid-liquid suspension were also monitored along the adsorption isotherms. The adsorption of Co(II) onto γ-Al2O3 in the presence of acetate and nitrilotriacetate led to the formation of the type A (i.e., solid-metal-ligand) ternary complexes. The use of citrate anions together with Co(II) cations was shown to improve the impregnation process through the formation of ternary complexes of type B (i.e., solid-ligand-metal). The comparison with a system containing tricarballylate anions allowed concluding that the presence of the hydroxyl group in the citrate anion enhanced its affinity for the alumina surface by contributing to the inner-sphere character of its surface-bound complexes.