Tensiometric and rheological investigations of single and mixed systems consisting of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions

The surface tension (σ), critical micelle concentration (CMC), surface excess (Γ), minimum area occupied by a surfactant molecule (A min), the viscosity and oscillatory rheological studies of aqueous solutions containing cocamidopropyl betaine (CAPB) and sodium dodecylbenzene sulfonate (SDBS) at molar fractions of 0.00, 0.25, 0.50, 0.75 and 1.00 and 25 °C were presented. CAPB and SDBS were not found to interact synergistically in water at any of the molar fractions studied. This is due to the fact that the critical micelle concentrations of these mixtures were higher than those predicted by Clint’s equation, indicating an antagonism that rarely occurs in mixed amphoteric/anionic surfactant systems. The minimum area occupied by a surfactant molecule (A min) was reduced in CAPB/SDBS mixtures compared to unmixed surfactants. In contrast, the viscosity of the mixed CAPB/SDBS system increased significantly from 1.0 mPa s to a maximum of 36.0 Pa s at higher CAPB mole fractions (0.5–0.8). The rheograms obtained from the oscillation measurements of the viscous CAPB/SDBS solutions are characteristic of wormlike micelles (WLMs) according to the Maxwell model. The results of this surprising binary CAPB/SDBS surfactant mixing system are presented and discussed.

Dilute hydrochloric acid induced switching from antagonism to synergism in the binary mixed systems of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS)

A synergism is reported in the binary mixtures of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS) at mole fractions of 0.25, 0.50, and 0.75, in dilute hydrochloric acid solutions (∼0.05 M) HCl, at 25 °C. Such synergism is confirmed by surface tension values, critical micelle concentration (CMC), and interaction parameter values according to Holland and Rubingh’s model and the satisfaction of Hua and Rosen conditions. Surface excess (Γ) at air/water interface and the minimum area occupied by surfactant molecule (A min) also indicated a reduction in the A min of mixed CAPB/SDBS compared to single surfactants that is due to the strong electrostatic interaction between surfactants heads. The composition of CAPB/SDBS mixed micelles, the interaction parameter (β) and the micellar activity coefficients (f 1 and f 2) were evaluated from the regular solution model for these binary amphoteric/anionic mixed systems. It is clear the CAPB surfactants are converted to the cationic form by charging in the presence of HCl, which enhances the interaction between the oppositely charged CAPB and SDBS heads and overcomes the unfavourable packing between the tails predicted in pure water. Such strong interaction was also reflected on the phase behavior of CAPB/SDBS binary mixed system. Accordingly, a switching from antagonism to synergism is reported for CAPB/SDBS binary mixed system in the presence of dilute hydrochloric acid.

Thermodynamics of complex chemical equilibria in surfactant mixtures

A thermodynamic approach was developed to predict the precipitation conditions of surfactants using the solubility product relationship between surfactant monomer concentrations, in order to calculate the monomer-precipitate equilibrium. This approach provides an explicit equation which predicts the amount of solid phase which forms in any surfactant mixture. All calculations of the total change in Gibbs energy (ΔG) were performed for concentrations of both surfactants that were below their CMC values. The elaborated ΔG-pH diagrams offer the possibility to determine the areas of thermodynamic stability of the solid phases depending on the chemical composition and acidity of the studied system. It was shown that with increasing concentration of the surfactant and the metal ion, the range of precipitate formation, either as slightly soluble salt or as slightly soluble acid, was extended by a few pH units in all cases.

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.

Synergistic interaction between sodium dodecyl benzene sulfonate (SDBS) and N,N-dimethyldodecan-1-amine oxide (DDAO) and their adsorption onto activated charcoal and Jordanian natural clay

Solutions of sodium dodecyl benzene sulfonate (SDBS) and N,N-dimethyldodecan-1-amine oxide (DDAO) with mole fractions of 0.00, 0.25, 0.50, 0.75 and 1.00 were prepared and their surface tension was measured as a function of total surfactant concentration. The critical micelle concentration (CMC) values of these mixed solutions were also determined. The minimum area occupied by a surfactant molecule at air/water interface was calculated for single and binary surfactant mixtures. A pronounced synergistic interaction between SDBS and DDAO was detected. The surface tension and CMC-values of SDBS/DDAO mixtures are significantly lower than those of the single surfactant. The mixed system of SDBS/DDAO exhibits a highly negative interaction parameter (β = −10.6) according to regular solution model, and is found to fulfill the condition of Hua and Rosen, indicting a strong synergistic interaction between the two surfactants. The contact angle measurements show the wettability of the surfactant mixture onto polyethylene substrate is higher than of the respective single surfactant. In addition, the adsorption of SDBS and DDAO or their mixtures on 1.0% activated carbon and 5.0% Jordanian natural clay (JNC), respectively, was investigated using the depletion method. The individual surfactants were found to adsorb to a considerable extent on activated carbon, and a slightly higher adsorption tendency was even measured for mixed SDBS/DDAO surfactant systems. Although no SDBS molecules adsorbed on JNC, adsorption was observed for solutions containing DDAO and SDBS/DDAO surfactants. The improvement in wettability and adsorption of SDBS/DDAO surfactants at the air/water and solid/water interfaces is directly related to the synergistic interaction between the two surfactants.

Pure and mixed aqueous micellar solutions of Sodium Dodecyl sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO): Role of temperature and composition

Aqueous mixtures of anionic and nonionic/cationic surfactants can form non-trivial self-assemblies in solution and exhibit macroscopic responses. Here, we investigate the micellar phase of pure and mixed aqueous solutions of Sodium Dodecyl Sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO) using a combination of Small Angle Neutron Scattering (SANS), Fourier-Transform Infrared Spectroscopy (FTIR) and rheological measurements. We examine the effect of temperature (0-60 °C), on the 20 wt% SDS micellar solutions with varying DDAO (⩽5 wt%), and seek to correlate micellar structure with zero-shear solution viscosity. SANS establishes the formation of prolate ellipsoidal micelles in aqueous solutions of pure SDS, DDAO and SDS/DDAO mixtures, whose axial ratio is found to increase upon cooling. Elongation of the ellipsoidal micelles of pure SDS is also induced by the introduction of the non-anionic DDAO, which effectively reduces the repulsive interactions between the anionic SDS head-groups. In FTIR measurements, the formation of elongated mixed ellipsoidal micelles is confirmed by the increase of ordering in the hydrocarbon chain tails and interaction between surfactant head-groups. We find that the zero-shear viscosity of the mixed surfactant solutions increases exponentially with decreasing temperature and increasing DDAO content. Significantly, a master curve for solution viscosity can be obtained in terms of micellar aspect ratio, subsuming the effects of both temperature and DDAO composition in the experimental range investigated. The intrinsic viscosity of mixed micellar solutions is significantly larger than the analytical and numerical predictions for Brownian suspensions of ellipsoidal colloids, highlighting the need to consider interactions of soft micelles under shear, especially at high concentrations.

Adsorption of Single and Mixed Surfactants onto Jordanian Natural Clay

The adsorption of sodium dodecyl sulfate (SDS) and N,N-dimethyldodecan-1-amine oxide (DDAO) and their mixtures from their solutions, onto Jordanian Natural Clay (JNC) was investigated at room temperature. The clay was firstly analyzed with X-ray fluorescence technique. The surface tension of the single and mixed surfactant solutions was then measured as a function of the total surfactant concentration and the critical micelle concentration (CMC) values of these solutions were determined. The interaction between SDS and DDAO as indicated by their interaction parameter was calculated according to the regular solution model. Adsorption of single surfactants onto 3 % wt/vol Jordanian Natural Clay, using the depletion method, was ultimately investigated. Although no absorbed SDS molecules were found onto the Jordanian Clay, but a pronounced adsorption tendency for DDAO and mixed SDS/DDAO molecules was obtained. The amounts of surfactant adsorbed onto 3 wt/vol% JNC follow the trend: SDS/DDAO > DDAO > SDS indicating the importance of synergistic interaction of surfactants on their adsorption behavior.