RESUMO
To study the influence of the chemical nature of headgroups and the type of counterion on the process of micellization in mixed surfactant systems, the cmc's of several binary mixtures of surfactants with the same length of hydrocarbon tail but with different headgroups have been determined as a function of the monomer composition using surface tension measurements. Based on these results, the interaction parameter between the surfactant species in mixed micelles has been determined using the pseudophase separation model. Experiments were carried out with (a) the nonionic/anionic C(12)E(6)/SDS ((hexa(ethyleneglycol) mono-n-dodecyl ether)/(sodium dodecyl sulfate)), (b) amphoteric/anionic DDAO/SDS ((dodecyldimethylamine oxide)/(sodium dodecyl sulfate)), and (c) amphoteric/nonionic C(12)E(6)/DDAO mixed surfactant systems. In the case of the mixed surfactant systems containing DDAO, experiments were carried out at pH 2 and pH 8 where the surfactant was in the cationic and nonionic form, respectively. It was shown that the mixtures of the nonionic surfactants with different kinds of headgroups exhibit almost ideal behavior, whereas for the nonionic/ionic surfactant mixtures, significant deviations from ideal behavior (attractive interactions) have been found, suggesting binding between the head groups. Molecular orbital calculations confirmed the existence of the strong specific interaction between (1) SDS and nonionic and cationic forms of DDAO and between (2) C(12)E(6) and the cationic form of DDAO. In the case for the C(12)E(6)/SDS system, an alternative mechanism for the stabilization of mixed micelles was suggested, which involved the lowering in the free energy of the hydration layer. Copyright 2000 Academic Press.
RESUMO
The colloidal stability of quartz suspension was determined over a wide range of pH in aqueous copper nitrate where the state of Cu(II) is changed from mainly aqua ions and monohydroxyl complexes in the acid and neutral pH to polynuclear hydroxo complexes and colloidal precipitated copper hydroxide at higher pH. Two regions of instability were observed and in both cases the particles were shown to have low electrophoretic mobility. In the neutral pH region, the uptake of Cu(II) was sufficient to reduce the mobility of the particles to zero, while in the high-pH region evidence suggested coagulation between precipitated Cu(OH)2 and the quartz particles. It was shown that in all cases the coagulation was reversible and that the uptake of Cu(II) was dependent on the uncharged surface hydroxyl density. Studies of the coagulation kinetics showed that extended time scales were involved (several minutes in the neutral pH region to tens of minutes at high pH). Copyright 1998 Academic Press.
RESUMO
The dynamic surface tension of aqueous solutions of ethyl (hydroxyethyl) cellulose (EHEC) and hydrophobic modified ethyl (hydroxyethyl) cellulose (HM-EHEC) were determined using the maximum bubble pressure method. Values were monitored over surface lifetimes ranging from 0.15 to 2.5 s (after dead time corrections). In the low concentration range (<100 ppm) HM-EHEC was shown to be more surface active, and the presence of salt (0.1 M) was shown to increase the surface activity of both polymer systems. The results were compared with surface tension measurements carried out over longer time scales, determined using the du Nouy ring technique. From these results, isotherms were constructed relating surface tension to surface aging time. Although the results could not be directly correlated to interfacial diffusion models, the isotherms were found to be comparable to previous data reported for lower-molecular-weight poly(oxyethylene ether) surfactants and a higher-molecular-weight EHEC polymer determined using the pendant drop technique. Essentially, the isotherms could be divided into distinct regions: induction period, fast fall region where surface coverage occurs fairly rapidly, and finally the meso-equilibrium region. From the Gibbs equation, the number of segments of polymer per unit surface area at saturated adsorption levels was calculated over regions of different aging times, and the results are discussed in terms of configuration changes of the polymer at the interface. Copyright 1997Academic Press
RESUMO
It was demonstrated that the presence of tetraalkylammonium counterions in aqueous foams and thin film lamella stabilized by sodium dodecyl sulfate (at concentrations below the CMC) can act either to promote or to prevent foam stability. The increase in stability was explained by the congenial incorporation of the smaller and more hydrophilic counterions (tetraethylammonium cations and, to some extent, tetrabutylammonium cations) into the interfacial anionic layers. It was suggested that this increased the interfacial cohesive interaction and resiliency within the thin film lamella. This causes regular thin film drainage rates (within reasonable agreement with Reynold's theory) and was shown to promote black films at low surfactant concentrations in model horizontal thin film lamella. Also, high bulk foam lifetimes were observed, as determined by handshaking. The decrease in stability was associated with rapid drainage rates and was explained by the penetration of larger more hydrophobic tetrapentylammonium cations in the layers. This could disrupt and weaken the molecular interactions (less cohesion). This result is quite different from previously reported studies describing the action of tetraalkylammonium counterions on sodium dodecyl sulfate (at concentrations above the CMC). At these higher surfactant concentrations, a defoaming action was generally observed and could be explained in terms of a reduction in the dynamic stability (Gibbs surface elasticity).
RESUMO
The surface tension of aqueous solutions of simple inorganic electrolytes (36 in total) have been measured by the maximum bubble pressure method as a function of electrolyte concentration up to 1 M. In most cases the surface tension increased, however in a minority of cases, certain combinations of cations and anions had a negligible effect or decreased surface tension. Results were analysed in terms of surface tension/electrolyte concentration gradients (d(Deltagamma)/dc) and this parameter was found to correlate with the entropies of ion hydration, Jones-Dole viscosity coefficients and dissolved oxygen gradients. Calculation of Gibbs surface deficiencies for selected electrolytes were carried out using the raw surface tension data. Discussion of the surface tension/electrolyte concentration gradients was extended to the mechanism of inhibition of bubble coalescence by electrolytes. The Gibbs-Marangoni effect did not provide a satisfactory explanation for the inhibition of coalescence for all electrolytes and from the present study we suggest that dissolved gas (microbubble) gradients between macroscopic bubbles plays an important role in the coalescence process.
