Micellar Interactions in Nonionic/Ionic Mixed Surfactant Systems

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.

Cooperative Adsorption of Nonionic Triton X-100 and Dodecyldimethylamine Oxide Surfactant Mixtures at the Hydrophilic Silica-Water Interface Studied by Total Internal Reflection Raman Spectroscopy and Multivariate Curve Resolution

The adsorption of dimethyldodecylamine oxide (DDAO) and Triton X-100 (TX) as single components and mixed systems at the silica-water interface has been studied using total internal reflection (TIR) Raman spectroscopy combined with multivariate curve resolution (MCR). In this study, the mixtures of DDAO and TX indicate minimal synergism in the bulk solution; however, the cooperative adsorption behavior on the silica surface is shown with various mixtures of DDAO (up to 1.3 mM) and TX (up to 1.1 mM). Adding the DDAO (up to 0.3 mM) to TX solution, the surface excess of TX shows 30% enhancement, from 1.2 to 1.8 μmol m^-2. Adding the DDAO also shifts the TX adsorption isotherms, resulting in the Gibbs free energy change of -2.87 ± 0.73 kJ mol^-1. This free energy change is interpreted as the decrease in surface energy when the silica surface charged sites are screened by the DDAO adsorbed layer. Alternatively, when a DDAO solution contains a small amount of TX molecules, i.e., < 30 μM, its adsorption on the silica surface quickly equilibrates. In addition, the formation of a more ordered liquid-crystalline adsorbed layer, as in the case of single-component DDAO adsorption, is not observed.

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.

Interactions between surfactants and the skin - Theory and practice

One of the primary causes of skin irritation is the use of body wash cosmetics and household chemicals, since they are in direct contact with the skin, and they are widely available and frequently used. The main ingredients of products of this type are surfactants, which may have diverse effects on the skin. The skin irritation potential of surfactants is determined by their chemical and physical properties resulting from their structure, and specific interactions with the skin. Surfactants are capable of interacting both with proteins and lipids in the stratum corneum. By penetrating through this layer, surfactants are also able to affect living cells in deeper regions of the skin. Further skin penetration may result in damage to cell membranes and structural components of keratinocytes, releasing proinflammatory mediators. By causing irreversible changes in cell structure, surfactants can often lead to their death. The paper presents a critical review of literature on the effects of surfactants on the skin. Aspects discussed in the paper include the skin irritation potential of surfactants, mechanisms underlying interactions between compounds of this type and the skin which have been proposed over the years, and verified methods of reducing the skin irritation potential of surfactant compounds. Basic research conducted in this field over many years translate into practical applications of surfactants in the cosmetic and household chemical industries. This aspect is also emphasized in the present study.

Tuning the position of head groups by surfactant design in mixed micelles of cationic and carbohydrate surfactants

HYPOTHESIS

Emerging applications of carbohydrate/cationic surfactant mixtures require not only synergistic mixing, but also accessible sugar headgroups at the exterior of micelles. A previous study showed that the glucoside headgroups of octyl-β-d-glucopyranoside aggregate at the interior of mixed micelles with equimolar cetyltrimethylammonium bromide rather than mixing with trimethylammonium groups at the corona. The current study tests the hypothesis that structural characteristics of the surfactants (the relative lengths of the alkyl tails and the type of linker) can be tuned to shift the carbohydrate groups to micelle surfaces.

EXPERIMENTS

The structural arrangement of 30 mM equimolar mixed micelle solutions in D₂O is investigated using NMR. The dynamics in different regions are probed using ¹H spin-lattice (T₁) and spin-spin (T₂) relaxation measurements, and relative positioning by nuclear Overhauser effect spectroscopy (NOESY). Additional micellar properties are determined using solvatochromic fluorescent probes.

FINDINGS

Matching surfactant alkyl tail lengths is found ineffective at "pushing out" the carbohydrate headgroups due to a large mismatch in interactions between the headgroups and D₂O. However, inserting a novel polar triazole group between the carbohydrate head group and the hydrophobic tail (e.g. in n-octyl-β-d-xylopyranoside) using click chemistry is able to "pull out" the carbohydrate, thus giving accessible sugar moieties at the surface of mixed micelles.

The flotation and adsorption of mixed collectors on oxide and silicate minerals

The analysis of flotation and adsorption of mixed collectors on oxide and silicate minerals is of great importance for both industrial applications and theoretical research. Over the past years, significant progress has been achieved in understanding the adsorption of single collectors in micelles as well as at interfaces. By contrast, the self-assembly of mixed collectors at liquid/air and solid/liquid interfaces remains a developing area as a result of the complexity of the mixed systems involved and the limited availability of suitable analytical techniques. In this work, we systematically review the processes involved in the adsorption of mixed collectors onto micelles and at interface by examining four specific points, namely, theoretical background, factors that affect adsorption, analytical techniques, and self-assembly of mixed surfactants at the mineral/liquid interface. In the first part, the theoretical background of collector mixtures is introduced, together with several core solution theories, which are classified according to their application in the analysis of physicochemical properties of mixed collector systems. In the second part, we discuss the factors that can influence adsorption, including factors related to the structure of collectors and environmental conditions. We summarize their influence on the adsorption of mixed systems, with the objective to provide guidance on the progress achieved in this field to date. Advances in measurement techniques can greatly promote our understanding of adsorption processes. In the third part, therefore, modern techniques such as optical reflectometry, neutron scattering, neutron reflectometry, thermogravimetric analysis, fluorescence spectroscopy, ultrafiltration, atomic force microscopy, analytical ultracentrifugation, X-ray photoelectron spectroscopy, Vibrational Sum Frequency Generation Spectroscopy and molecular dynamics simulations are introduced in virtue of their application. Finally, focusing on oxide and silicate minerals, we review and summarize the flotation and adsorption of three most widely used mixed surfactant systems (anionic-cationic, anionic-nonionic, and cationic-nonionic) at the liquid/mineral interface in order to fully understand the self-assembly progress. In the end, the paper gives a brief future outlook of the possible development in the mixed surfactants.

Trimethylsilyl hedgehogs – a novel class of super-efficient hydrocarbon surfactants

Presented here are the results for a novel class of hydrocarbon surfactants, termed trimethylsilyl hedgehogs (TMS-hedgehogs), due to the presence of silicon in the tails.

Hyperbranched hydrocarbon surfactants give fluorocarbon-like low surface energies

Two series of Aerosol-OT-analogue surfactants (sulfosuccinate-type di-BCnSS and sulfoglutarate-type di-BCnSG) with hyperbranched alkyl double tails (so-called "hedgehog" groups, carbon number n = 6, 9, 12, and 18) have been synthesized and shown to demonstrate interfacial properties comparable to those seen for related fluorocarbon (FC) systems. Critical micelle concentration (CMC), surface tension at the CMC (γCMC), and minimum area per molecule (Amin) were obtained from surface tension measurements of aqueous surfactant solutions. The results were examined for relationships between the structure of the hedgehog group and packing density at the interface. To evaluate A and B values in the Klevens equation for these hedgehog surfactants, log(CMC) was plotted as a function of the total carbon number in the surfactant double tail. A linear relationship was observed, producing B values of 0.20-0.25 for di-BCnSS and di-BCnSG, compared to a value of 0.31 for standard double-straight-tail sulfosuccinate surfactants. The lower B values of these hedgehog surfactants highlight their lower hydrophobicity compared to double-straight-tail surfactants. To clarify how hydrocarbon density in the surfactant-tail layer (ρ(layer)) affects γCMC, the ρ(layer) of each double-tail surfactant was calculated and the relationship between γCMC and ρ(layer) examined. As expected for the design of low surface energy surfactant layers, ρ(layer) was identified as an important property for controlling γCMC with higher ρ(layer), leading to a lower γCMC. Interestingly, surfactants with BC9 and BC12 tails achieved much lower γCMC, even at low ρ(layer) values of <0.55 g cm(-3). The lowest surface energy surfactant studied here was di-BC6SS, which had a γCMC of only 23.8 mN m(-1). Such a low γCMC is comparable to those obtained with short FC-tail surfactants (e.g., 22.0 mN m(-1) for the sulfosuccinate-type FC-surfactant with R = F(CF2)6CH2CH2-).

Mixed Micelles Containing Sodium Laurate: Effect of Chain Length, Polar Head Group, and Added Salt

We studied the mixing behavior of binary mixtures of alkylglycosides (CnGly); i.e. n-Octyl β-D-glucopyranoside, n-Decyl β-D-glucopyranoside and n-Decyl β-D-maltoside in combination with sodium laurate (NaL), and N,N-dimethyldodecylamine oxide in combination with NaL. The critical micelle concentration (CMC) data were obtained as functions of the composition based on equilibrium surface tension measurements. The cmc values with and without the addition of salt systems were then analyzed according to the regular solution model developed by Rubingh for mixed micelles. For the added salt systems, we applied Maeda's formulation for ionic/nonionic mixed micelles. For NaL/CnGly mixed systems, an increase in the hydrophilicity of the polar head group of CnGly resulted in a strong interaction between NaL and CnGly. In addition, introducing ionic species to the added salt systems stabilized the nonionic micelles.

Synergism in Mixed Anionic–Amphoteric Surfactant Solutions: Influence of Anionic Surfactant Chain Length

The influence of chain length on the composition of mixed micelles in binary mixtures of N,N-dimethyldodecylamine oxide (DDAO) and sodium decyl-, sodium dodecyl- and sodium tetradecylsulfate (abbreviated as SDeS, SDS and STS, respectively) has been determined at 30°C. From the surface tension measurements, the critical micelle concentration (cmc) data were measured as a function of mixing composition. Cmc-values were then analyzed according to regular solution model. The composition of mixed micelles, the interaction parameter (β) and the activity coefficients were evaluated from the regular solution model for the all anionic-amphoteric mixed systems. The interaction parameter values indicated a synergistic interaction between DDAO and the three anionic surfactants at all mole fractions to be due to an overall attractive interaction in the mixed micelles. The strength of the interaction between the amphoteric surfactant and the sodium alkyl sulfate in three mixed systems obeys the following order: SDeS/DDAO > SDS/DDAO > STS/DDAO suggesting that the decrease in the length of anionic surfactant alkyl chain results in a stronger interaction with DDAO. On the other hand, the viscosity data reveals a viscosity increasing trend of STS/DDAO > SDS/DDAO > SDeS/DDAO.

Titration of mixed micelles containing a pH-sensitive surfactant and conventional (pH-Insensitive) surfactants: a regular solution theory modeling approach

We present a thermodynamic theory to model the hydrogen-ion titration of mixed micelles containing a pH-sensitive surfactant and any number of conventional (pH-insensitive) surfactants. In particular, a simple expression is derived for the pKm, a parameter analogous to the pKa of simple acids, which describes the deprotonation equilibrium of the micellized pH-sensitive surfactant. The pseudophase approximation and regular solution theory (RST) are used to relate the pKm to (1) the pKa of the surfactant monomers, (2) the critical micelle concentrations (cmc's) of the protonated and deprotonated forms of the pH-sensitive surfactant, (3) the composition of the mixed micelle, and (4) parameters characterizing pairwise interactions between the surfactant molecules in the mixed micelle. Micellar titrations can be used to determine the magnitude of these interaction parameters. Conversely, knowledge of the cmc's and the interaction parameters allows the prediction of the pKm, which can then be used to calculate the micelle composition and surface charge as a function of solution pH. In addition, we have found that, in the context of RST, multicomponent surfactant mixtures are equivalent to a binary surfactant mixture of the pH-sensitive surfactant and a single effective surfactant whose interactions with the pH-sensitive surfactant are an average of those in the multicomponent surfactant mixture. We also discuss the experimental uncertainty in the pKm measurements. To account for the increased uncertainty in the pKm data at extreme micelle compositions, a weighted regression is proposed for the analysis of experimental titration data characterized by widely varying uncertainties. The theory presented here is validated using micellar titration data from the literature for several pH-sensitive surfactants in solutions containing 0.1 M salt. In most cases, the parameters extracted from an analysis of the titration data agree with the cmc and interaction parameters obtained by other means. One notable exception is the surfactant tetradecyldimethylamine oxide (C14DAO), which appears to have concentration-dependent interactions due to extensive growth of cylindrical micelles. Micellar titrations were also conducted on binary surfactant mixtures containing the pH-sensitive surfactant dodecyldimethylamine oxide (C12DAO) and either the cationic surfactant dodecyltrimethylammonium bromide (C12TAB) or the nonionic surfactant dodecyl octa(ethylene oxide) (C12E8). The theory provides a reasonable description of the experimental titration data at all surfactant mixing ratios, although a larger discrepancy is found in the C12DAO/C12E8 system, in which C12E8 interacts preferentially with the protonated, cationic form of C12DAO. Interestingly, C12TAB was also observed to interact preferentially with the protonated, cationic form of C12DAO, although the preference is much weaker than that in the C12DAO/C12E8 system.

The role of the surfactant head group in the emulsification process: binary (nonionic-ionic) surfactant mixtures

Dilute emulsions of dodecane in water were prepared under constant flow rate conditions with binary surfactant systems. The droplet size distribution was measured as a function of the mixed surfactant composition in solution. The systems studied were (a) the mixture of anionic sodium dodecyl sulfate (SDS) with nonionic hexa(ethyleneglycol) mono n-dodecylether (C12E6) and (b) the mixture of cationic dodecyl pyridinium chloride (DPC) with C12E6. At a constant concentration of SDS or DPC surfactant in solution (below the CMC) the mean emulsion droplet size decreases with the increase in the amount of C12E6 added to the solution. However, a sharp break of this droplet size occurs at a critical concentration and beyond this point the mean droplet size did not significantly change upon further increase of the C12E6. This point was found to corresponded to the CMC of the mixed surfactant systems (as previously determined from microcalorimetry measurements) and this result suggested the mixed adsorption layer on the emulsion droplet was similar to the surfactant composition on the mixed micelles. The emulsion droplet size as a function of composition at the interface was also studied. The mean emulsion droplet size in SDS-C12E6 solution was found to be lower than that in DPC-C12E6 system at the equivalent mole fraction of ionic surfactant at interface. This was explained by the stronger interactions between sulphate and polyoxyethylene head groups at the interface, which facilitate the droplet break-up. Counterion binding parameter (beta) was also determined from zeta-potential of dodecane droplets under the same conditions and it was found that (beta) was independent of the type of the head group and the mole fraction of ionic surfactant at interface.