Extremely Strong Interaction of Sodium Decyl Sulfate and Decyltrimethylammonium Bromide in Molecular Aggregates

Sodium dodecyl benzene sulfonate (SDBS) and N,N-dimethyldodecan-1-amine oxide (DDAO) in single and mixed systems as corrosion inhibitors of zinc in hydrochloric acid

The influence of surfactant synergism between sodium dodecyl benzene sulfonate (SDBS) and N,N-dimethyldodecan-1-amine Oxide (DDAO) on zinc corrosion in 0.05 M hydrochloric acid (HCl) solutions at 25 °C was investigated. Firstly, solutions of SDBS and DDAO with mole fractions of 0.00, 0.25, 0.50, 0.75 and 1.00 were prepared and their surface tension and critical micelle concentration (CMC) values in water and in 0.05 M HCl were measured as a function of total surfactant concentration. The SDBS/DDAO mixed system exhibited a strong synergism in 0.05 M HCl with a highly negative interaction parameter β (average β = −23.46), according to regular solution theory. Secondly, the adsorption of single surfactants SDBS and DDAO and SDBS/DDAO surfactant mixture on 2.0% zinc powder was investigated by the depletion method to find out the role of synergism in the adsorption tendency of these surfactants on the zinc surface and thus their corrosion inhibiting effect. The adsorption tendency of single surfactant and the mixed surfactant systems onto 2.0% zinc powder followed the order: SDBS > 0.75 SDBS/0.25 DDAO ≈ 0.25 SDBS/0.75 DDAO > DDAO > 0.50 SDBS/0.50 DDAO. Finally, the corrosion of zinc was investigated using the potentiodynamic polarization technique. It was found that SDBS and DDAO act as efficient corrosion inhibitors for zinc in 0.05 M HCl solution with increasing corrosion inhibition efficiency when they are mixed. Additionally, images of scanning electron microscopy were obtained for zinc sheets in solutions containing single and mixed SDBS/DDAO surfactants in the presence and absence of 0.05 M HCl. The microscopic images show an improvement in the protection of the zinc surface against acid attack in the presence of single and mixed SDBS/DDAO surfactants.

Study on the Complex System of Sodium Lauryl Diphenyl Ether Disulfonate and Dodecyl Dimethyl Hydroxyethyl Ammonium Chloride

Surface chemical properties of the mixed system of sodium lauryl diphenyl ether disulfonate (C12-MADS, an anionic surfactant with double hydrophilic groups) and dodecyl dimethyl hydroxyethyl ammonium chloride (K3, a cationic quaternary ammonium surfactant) along with different proportion have been studied at 298 K and 318 K. Synergistic parameters of the mixture in the mixed micelles and the mixed adsorption layer were also calculated by using the regular solution theory. The composition of the mixed micelles and of the mixed adsorption layer was obtained. The results show that the surface chemical properties of C12-MADS/K3 mixture are better than either of single system, and when the solution ratio (α1 = [n (K3)/n (C12-MADS + K3)]) is 0.7 and 0.9, the surface tension curve has a double inflection point which indicates a synergistic effect between the surfactant molecules. The complex system showed a strong synergistic effect in the mixed adsorption layer and the mixed micelles. The mixed system of C12-MADS/K3 has synergism in surface tension reduction effectiveness, surface tension reduction efficiency and mixed micelle formation. When α1 is 0.67, the synergistic effect is the strongest.

Miscibility of sodium chloride and sodium dodecyl sulfate in the adsorbed film and aggregate

The adsorption, micelle formation, and salting out of sodium dodecyl sulfate in the presence of sodium chloride were studied from the viewpoint of their mixed adsorption and aggregate formation. The surface tension of aqueous solutions of a sodium chloride–sodium dodecyl sulfate mixture was measured as a function of the total molality and composition of the mixture. Phase diagrams of adsorption and aggregate formation were obtained by applying thermodynamic equations to the surface tension. Judging from the phase diagrams, sodium chloride and sodium dodecyl sulfate are miscible in the adsorbed film at very large composition of sodium chloride and in the salted-out crystalline particle, while they are immiscible in the micelle. The miscibilities in the adsorbed film, micelle, and crystalline particle increase in the following order: particle > adsorbed film > micelle. The difference in miscibility among the oriented states was ascribed to the difference in geometry between the adsorbed film and micelle and to the interaction between bilayer surfaces in the particle.

Alkaline phosphatase refolding assisted by sequential use of oppositely charged detergents: a new artificial chaperone system

A novel artificial chaperone system, based on combination of oppositely charged detergents, was elaborated to refold soluble alkaline phosphatase. Upon dilution of urea-denatured alkaline phosphatase to a nondenaturing urea concentration in the presence of the capturing agent, complexes of the detergent and non-native protein molecules are formed and thereby the formation of protein aggregates is prevented. The so-called captured protein is unable to refold from the detergent-protein complex states unless a stripping agent is used to gradually remove the detergent molecules. In that respect, we used detergents with variable charges and tail lengths to initiate and complete the refolding process. The results obtained from various analyses (fluorescence, UV, circular dichroism, surface tension, turbidity measurements and activity assays) indicated that the extent of refolding assistance was different due to detergents structure and also the length of hydrophobic portion of each detergent. These observed differences were attributed to the strong electrostatic interactions among the capturing and stripping detergents used in this investigation. Collectively it is expected that protein refolding process can be achieved easier, cheaper and more efficient, using the new technique reported here.

Influence of organic acid and organic base interactions on interfacial properties in NAPL--water systems

Interfacial properties play an important role in determining the transport and distribution of waste nonaqueous phase liquids (NAPLs) in groundwater. To develop a better understanding of the solute interactions governing the interfacial properties of waste NAPLs, this study examined the interfacial tension and contact angle of a tetrachloroethylene/water/quartz system containing octanoic acid and dodecylamine as a function of pH. The results showed that interactions between these solutes affected the system's interfacial properties significantly, producing a positive synergism. For example, octanoic acid, which by itself does not affect wettability, could reverse the wettability of quartz in the presence of dodecylamine. The significant reduction in interfacial tension and increase in contact angle around neutral pH was, based on the results of speciation modeling, attributed to the formation of a complex composed of the protonated organic base and deprotonated organic acid, whose formation also peaks around neutral pH. Thus, measures of the content of only one class of compounds, such as the base number, are inadequate descriptors of a NAPL's ability to alter wettability.

Thermotropic Phase Behavior of Cationic Gemini Surfactants and Their Equicharge Mixtures with Sodium Dodecyl Sulfate

The lyotropic phase behavior for the neat cationic gemini surfactants alkanediyl-alpha,omega-bis(alkyldimethylammonium bromide), designated here as m-s-m, has been investigated previously in several works, but the thermotropic behavior has not been well characterized. Only for 15-s-15 and 14-s-12 have thermotropic liquid crystals (Lc) been reported. In this work, for the first time and in contrast to previous reports, we observe thermotropic Lc formation for m-2-m geminis with m = 12, 14, 16, and 18, by means of polarizing microscopy and differential scanning calorimetry (DSC). Furthermore, we investigate mixtures of m-2-m and SDS, m-2-m Br2.2SDS, which exhibit crystal-to-crystal phase transitions at lower temperature and, at high temperature, smectic Lc phases. The transition temperatures and enthalpies for Lc phases, obtained by DSC, present clear trends upon increase of the chain lengths. Combining Langmuir film experiments, possible lamellar arrangements for the different phases are tentatively discussed.

Computer Simulations of Catanionic Surfactants Adsorbed at Air/Water Interfaces

Structural properties pertaining to the solvation of mixtures of dodecytrimethylammonium/dodecylsulfate adsorbed at water/air interfaces were studied using molecular dynamics techniques. Two different surfactant coverages, both in the submonolayer regime, were considered: an infinite-diluted catanionic pair and an equimolar mixture, at a surface concentration of 78.7 A2/headgroup. The most stable solvated structures for the single surfactant pair correspond to contact-head-ion-pairs (CHIP) at a distance close to 5 A. In addition, marginally stable solvent-separated-head-ion-pairs (SSHIP) at distances approximately 7 A were also observed. The mean free energy for the dissociation of CHIP was estimated to be approximately 1 kcal/mol. At finite surfactant concentrations, one observes a considerable degree of clustering between the amphiphiles, due to the strong Coulomb coupling between headgroups. The resulting spatial domains show asymmetric structures with linear dimensions comparable to the simulation box, suggesting the onset of percolative structures. The connectivity pattern of these domains was interpreted in terms of a simplified model consisting of two-dimensional charged Lennard-Jones spheres.

Selective determination of surface density of bromide ion through XAFS and its application to verification of a criterion of an ideal mixing of surfactant mixture

The mole fraction of chloride ion of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium chloride (DTAC) mixture in the adsorbed film XHC was estimated not only by the thermodynamic analysis of the surface tension data but also by analyzing the Br K-edge jump of the XAFS spectrum under the total reflection condition (TRXAFS method). The phase diagrams of adsorption (PDA) at several surface tensions from the two methods were in good agreement. On the basis of the PDA obtained, it was clearly shown that the criterion of an ideal mixing for the DTAB-DTAC system is not given by the linear relation between the total molality of surfactant mixture m and XHC, m = m0B + (m0C - m0B)XHC, but by the one between m2 and XHC, m2 = (m0B)2 + [(m0C)2 - (m0B)2]XHC. Furthermore, it was demonstrated that the theoretical approach that provides the latter relation draws a distinction between the criteria for an ionic surfactant mixture without a common ion and that for an ionic surfactant mixture with a common ion.

Observation of a Liquid−Gas Phase Transition in Monolayers of Alkyltrimethylammonium Alkyl Sulfates Adsorbed at the Air/Water Interface

The equilibrium adsorption layers of symmetric chain alkyltrimethylammonium alkyl sulfates (Cn+.Cn- for n = 8, 12) were investigated at the air/water interface by sum-frequency vibrational spectroscopy in the function of the bulk surfactant concentration. To ensure the surface purity of the solutions investigated, an improved version of the foam fractionation method was used for the purification of the constituent ionic surfactants and the surface purity of the solutions was also checked. In the monolayer of the C12+.C12- surfactant, a two-dimensional first-order gas/liquid phase transition was observed. At surfactant bulk concentrations just exceeding the concentration corresponding to the phase transition, the monolayer is conformationally disordered, liquidlike, but with increasing bulk surfactant concentration the conformational order of the monolayer increases. The SFG spectra of the C8+.C8- monolayer did not indicate the occurrence of phase transition at room temperature.

Effect of equimolar salt to decyltrimethylammonium decyl sulfate on vesicle formation and surface adsorption

The aqueous solution of mixture of sodium decyl sulfate (SDeS) and decyltrimethylammonium bromide (DeTAB) has been found to form equilibrium multilamellar vesicles (MLV) spontaneously. We measured the surface tension of the aqueous solution of 1:1 mixture of SDeS and DeTAB as a function of temperature T at various molalities m under atmospheric pressure. The surface density, the entropy of adsorption and the entropy of vesicle formation are evaluated and compared with those of the decyltrimethylammonium decyl sulfate (DeTADeS) aqueous solution system to investigate the role of small counterions in the mechanism of equilibrium vesicle formation. The saturated surface density Gamma (H,C ) vs T curve of the SDeS/DeTAB system sits below that of the DeTADeS system. Therefore, sodium and bromide ions are negatively adsorbed and nevertheless, they neutralize the electric charge of the decyl sulfate ion DeS(-) and the decyltrimethylammonium ion DeTA(+) to some extent to weaken the electrostatic attraction between the polar head groups in the adsorbed film. The net surfactant concentration required for vesicle formation was larger in the SDeS/DeTAB system. Hence, the electrostatic attraction between the polar head groups of the surfactant ions which is one of the major driving forces for vesicle formation is weakened by the presence of the counterions Na(+) and Br(-). Small but distinct changes in the surface density and the entropies of MLV formation of the SDeS/DeTAB system from those of the DeTADeS system were also found.

Thermodynamic Study on Vesicle Formation and Adsorption of Decyltrimethylammonium Decyl Sulfate

The surface tension of an aqueous solution of decyltrimethylammonium decyl sulfate (DeTADeS) was measured as a function of temperature T at various molalities &mcirc; under atmospheric pressure. DeTADeS has been found to form equilibrium multilamellar vesicles (MLV) spontaneously. The surface density, the entropies of adsorption, and the entropy of vesicle formation are evaluated. The mechanism of formation of equilibrium vesicles is investigated from the standpoint of thermodynamics and from the comparison of the results with those of the micelle-forming systems. From the relatively small change of the surface density Gamma;(H) on T at a given &mcirc;, the adsorbed film is implied to be tightly packed due to the strong electrostatic attraction between the polar headgroups. The energy change associated with adsorption from the vesicular state per mole of surfactant Delta(V)(H)u is positive in the entire temperature range; thus, the curved bilayer in MLV is energetically more favorable than the planar adsorbed film. From the negative values of the entropy of vesicle formation Delta(W)(V)s, it is concluded that vesicle formation is driven by enthalpy whereas micelle formation is mostly entropy driven. Copyright 2001 Academic Press.

Structure Effect on Nonideal Mixing of Alkyl Methyl Sulfoxide and Alkyldimethylphosphine Oxide in Adsorbed Film and Micelle

The surface tension of aqueous solutions of the octylmethylsulfoxide (OMS)-decyldimethylphosphine oxide (DePO) and decylmethylsulfoxide (DeMS)-octyldimethylphosphine oxide (OPO) mixtures was measured as a function of the total molality of the surfactants and the composition in the surfactants. Compositions in the adsorbed film and micelle were thermodynamically evaluated from the total molality vs composition and the critical micelle concentration (CMC) vs compositon curves, respectively. The phase diagrams of adsorption and micelle formation were drawn and compared with those of the DeMS-DePO mixture previously studied. A large gap in composition was observed between the adsorbed film and the micelle coexisting at the CMC and ascribed to the differences in geometric structure between them and the constituent molecules. Negative deviation from ideal mixings in adsorbed film and micelle was observed for the OMS-DePO mixture, whereas a positive one was observed for the DeMS-OPO. The structure effect on the nonideal mixings was clarified by using the excess thermodynamic functions of adsorption and micelle formation calculated from the activity coefficients in the adsorbed film and micelle. Copyright 1999 Academic Press.