Adsorption Of Mixed Surfactants Research Articles

Synergistic Adsorption and Molecular Arrangement of Mixed Surfactants at the Air/Water Interface

The self-assembly of mixed surfactants at the air/water interface plays a crucial role in mineral flotation separation. This study aimed to investigate the molecular arrangement and synergistic adsorption of mixed surfactants at the air/water interface using molecular dynamics (MD) simulation and surface tension measurement. The stability of mixed surfactant solutions was also measured using dynamic light scattering and ζ potential. We investigated the mixture of octylhydroxamic acid (OHA) and dodecanol (DOD) surfactants at different weight ratios. The surface activity of the OHA/DOD mixture was characterized in detail, including the relative concentration distribution, structural properties, and radial distribution function. The MD simulations revealed that OHA/DOD was inserted into the water as a headgroup at the air/water interface, and the carbon chain extended into the air. A strong synergistic effect was observed between the two surfactants, and a dense monolayer film was formed at the air/water interface, demonstrating higher surface activity. With an increase in the DOD ratio, the hydrogen bond between the OHA headgroup and water molecules became stronger. The order of surface activity of the mixture was OHA:DOD = 1:1 > OHA:DOD = 2:1 > OHA:DOD = 4:1. In the temperature range from 24 to 30 °C, the surface tension decreased and the critical micelle concentration increased with increasing temperature. The addition of DOD changed the conformation of the mixed surfactants, which was beneficial to the aggregation of OHA/DOD complexes. When OHA:DOD = 1:1, the average particle size was the largest and the formed micelles were the most stable. The experimental results are consistent with the surface tension and simulation results.

Self-assembly of the surfactant mixtures on graphene in the presence of electrolyte: a molecular simulation study

The insolubility of graphene nanosheets in aqueous media has been a limitation for the practical applications of this unique material. The non-covalent functionalization of graphene with mixed surfactants is an effective procedure for preparation of stable graphene dispersions. Physical adsorption of surfactants onto graphene surfaces is a significant stage in the dispersion of graphene nanosheets in the aqueous environment. Studies have shown that the presence of electrolyte greatly affects the adsorption process of pure surfactants on solid surfaces. Examination of mixed surfactants adsorption on graphene in the presence of electrolyte will help to better understand the mechanism of molecular interactions between the graphene nanosheets and mixed surfactants. Therefore, in this study, we used molecular dynamics simulations to investigate the adsorption of mixed surfactants on graphene in the presence of electrolyte and to study the morphology of assemblies formed from mixed surfactants on graphene. We investigate the effects of electrolyte concentration and surface coverage of surfactants' mixture on the adsorption process and structural changes in the assemblies formed on graphene nanosheets. We have found that increasing the ionic strength of electrolyte reinforces stretching of surfactants adsorbed on graphene toward the aqueous phase, and this leads to a clear volume expansion in the structure of graphene-surfactant mixture hemi-spherical micelles. In fact, screening effect of electrolyte ions on electrostatic repulsion between charged head-groups made molecules approach each other, leading to a more compact assembly of surfactants on graphene surface. As hemi-spherical micelles of mixed surfactants-graphene expanded, the steric repulsion between these micelles also increased, which in turn, inhibited the re-aggregation of graphene sheets covered with surfactants.

Interfacial Dilational Viscoelasticity of Adsorption Layers at the Hydrocarbon/Water Interface: The Fractional Maxwell Model

In this communication, the single element version of the fractional Maxwell model (single-FMM or Scott–Blair model) is adopted to quantify the observed behavior of the linear interfacial dilational viscoelasticity. This mathematical tool is applied to the results obtained by capillary pressure experiments under low-gravity conditions aboard the International Space Station, for adsorption layers at the hydrocarbon/water interface. Two specific experimental sets of steady-state harmonic oscillations of interfacial area are reported, respectively: a drop of pure water into a Span-80 surfactant/paraffin-oil matrix and a pure n-hexane drop into a C13DMPO/TTAB mixed surfactants/aqueous-solution matrix. The fractional constitutive single-FMM is demonstrated to embrace the standard Maxwell model (MM) and the Lucassen–van-den-Tempel model (L–vdT), as particular cases. The single-FMM adequately fits the Span-80/paraffin-oil observed results, correctly predicting the frequency dependence of the complex viscoelastic modulus and the inherent phase-shift angle. In contrast, the single-FMM appears as a scarcely adequate tool to fit the observed behavior of the mixed-adsorption surfactants for the C13DMPO/TTAB/aqueous solution matrix (despite the single-FMM satisfactorily comparing to the phenomenology of the sole complex viscoelastic modulus). Further speculations are envisaged in order to devise combined FMM as rational guidance to interpret the properties and the interfacial structure of complex mixed surfactant adsorption systems.

Synergisms in Binary Mixtures of Anionic and pH‐Insensitive Zwitterionic Surfactants and Their Precipitation Behavior with Calcium Ions

AbstractThis work aims to investigate synergy in anionic and zwitterionic surfactant mixtures, as they result in better interfacial properties and micellization behavior. Various mixtures of the pH‐insensitive zwitterionic surfactant 3‐(decyldimethylammonio) propanesulfonate (Zwittergent 3–10) and sodium dodecylsulfate (SDS) were prepared in aqueous solution at a range of pH values between 2 and 13. The thermodynamic parameters during mixed surfactant adsorption at the air/water interface are obtained and the results show the mixed surfactant systems having superior properties to the constituent surfactants. Experimentally, the mixed surfactant solutions clearly improve the surface activities by lowering the critical micelle concentration (CMC) and lowering the surface tension at the air/water interface. The synergisms are investigated through the interaction parameters estimated from regular solution theory that is used to quantitatively describe the nonideality of surfactant mixtures. High negative interaction parameters are obtained from these surfactant mixtures. Experimental precipitation phase boundaries of SDS in the presence of CaCl2 were also investigated in mixtures containing pH‐insensitive zwitterionic surfactant at different pH levels from 2 to 13 and SDS mole fractions of 0.25, 0.50, 0.75, and 1.00. Changes in the precipitation phase boundaries are due to the changes in the speciation or activities of the major components both below and above the CMC. As a result, the precipitation phase boundaries are pH dependent. In addition, mixed micellization and counterion binding to the micelle also change the precipitation phase boundary above the CMC. The activity‐based solubility product of calcium dodecylsulfate is also determined from the precipitation phase boundaries below the CMC. X‐ray diffraction patterns and SEM images confirm that only calcium dodecylsulfate precipitates in the soap scum for all pH and surfactant compositions studied.

Dispersion of Carbon Nanotubes Using Mixed Surfactants: Experimental and Molecular Dynamics Simulation Studies

The ability of cationic-rich and anionic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) for dispersing of carbon nanotubes (CNTs) in aqueous media has been studied through both the experimental and molecular dynamics simulation methods. Compared to the pure CTAB and SDS, these mixtures are more effective with the lower concentrations and more individual CNTs, reflecting a synergistic effect in these mixtures. The synergistic effects observed in mixed surfactant systems are mainly due to the electrostatic attractions between surfactant heads. In addition, the surface charge related to the colloidal stability of mixed surfactant-covered nanotubes has been characterized by means of ζ-potential measurements. The results indicate that the hydrophobic interactions between surfactant tails also give rise to the higher adsorption of surfactant molecules. Furthermore, molecular dynamics (MD) simulations have been performed to provide insight about the structure of surfactant aggregates onto nanotubes and to attempt an explanation of the experimental results. The MD simulation results indicate that the random and disordered adsorption of mixed surfactants onto carbon nanotubes may be preferred for a low surfactant concentration. Our research may provide experimental and theoretical bases for using mixed surfactants to disperse CNTs, which can open an avenue for new applications of mixed surfactants.

Manipulating Interfacial Polymer Structures through Mixed Surfactant Adsorption and Complexation

The effects of a nonionic alcohol ethoxylate surfactant, C(13)E(7), on the interactions between PVP and SDS both in the bulk and at the silica nanoparticle interface are studied by photon correlation spectroscopy, solvent relaxation NMR, SANS, and optical reflectometry. Our results confirmed that, in the absence of SDS, C(13)E(7) and PVP are noninteracting, while SDS interacts strongly both with PVP and C(13)E(7) . Studying interfacial interactions showed that the interfacial interactions of PVP with silica can be manipulated by varying the amounts of SDS and C(13)E(7) present. Upon SDS addition, the adsorbed layer thickness of PVP on silica increases due to Coulombic repulsion between micelles in the polymer layer. When C(13)E(7) is progressively added to the system, it forms mixed micelles with the complexed SDS, reducing the total charge per micelle and thus reducing the repulsion between micelle and the silica surface that would otherwise cause the PVP to desorb. This causes the amount of adsorbed polymer to increase with C(13)E(7) addition for the systems containing SDS, demonstrating that addition of C(13)E(7) hinders the SDS-mediated desorption of an adsorbed PVP layer.

The effects of the addition of the polyelectrolyte, poly(ethyleneimine), on the adsorption of mixed surfactants of sodium dodecylsulfate and dodecyldimethylaminoacetate at the air–water interface

The effects of the addition of the polyelectrolyte, poly(ethyleneimine), PEI, on the adsorption of the mixed surfactants of sodium dodecylsulfate, SDS, and dodecyldimethylaminoacetate, dodecyl betaine, at the air–water interface have been investigated using neutron reflectivity and surface tension. In the absence of PEI the SDS and dodecyl betaine surfactants strongly interact and exhibit synergistic adsorption at the air–water interface. The addition of PEI, at pH 7 and 10, results in a significant modification of the surface partitioning of the SDS/dodecyl betaine mixture. The strong surface interaction at high pH (pH 7 and 10) between the PEI and SDS dominates the surface behavior. For solution compositions in the range 20/80–80/20 mol ratio dodecyl betaine/SDS at pH 7 the surface composition is strongly biased towards the SDS. At pH 10 a similar behavior is observed for a solution composition of 50/50 mol ratio dodecyl betaine/SDS. This strong partitioning in favor of the SDS at high pH is attributed to the strong ion–dipole attraction between the SDS sulfate and the PEI imine groups. At pH 3, where the electrostatic interactions between the surfactant and the PEI are dominant, the dodecyl betaine more effectively competes with the SDS for the interface, and the surface composition is much closer to the solution composition.

Investigation of spreading of surfactant mixtures

Binary surfactant mixtures have been much studied over the years for both great practical and theoretical importance. However, the mechanisms different nonionic surfactant types follow during aggregation into mixed micelles, and probably while forming a mixed surfactant adsorption layer are very poorly understood. Furthermore, the explanation for non-ideal behaviour of trisiloxane surfactant mixtures is still lacking. We have investigated binary mixtures of two commercial superspreaders, Silwet L-77 ® and Additive 67 ®, with conventional Triton X-100 ® surfactant. Observed non-ideal behaviour has been proven to be largely dependent on both total concentration of the binary surfactant mixture and the hydrophobicity of the substrate which it is spreading on. In addition to that, synergistic and antagonistic effects have been revealed and explanations for the observed strange behaviour suggested, with steric problems due to differences in geometry of the surfactant molecules being the key factor. We also showed that present impurities in commercial surfactants can play a major role in the performance of binary surfactant mixtures and that for certain applications mixtures of trisiloxanes with conventional surfactant can be more efficient than trisiloxanes alone.

Physicochemical Properties and Surface Tension Prediction of Mixed Surfactant Systems: Triton X‐100 with Dodecylpyridinium Bromide and Triton X‐100 with Sodium Dodecylsulfonate

Dependences of the surface tension of aqueous solutions of ionic (dodecylpyridinium bromide, sodium dodecylsulfonate) and nonionic (Triton X‐100) surfactants and their mixtures on total surfactant concentration and solution composition were studied, and the surface tension of the mixed systems were predicted using different Miller's model. It was found that how to select the model for calculation of ω is corresponding to the degree of the deviation from the ideality during the adsorption of mixed surfactants. The compositions of micelles and adsorption layers at air‐solution interface as well as parameters (βm, βads) of headgroup‐headgroup interaction between the molecules of ionic and nonionic surfactants were calculated based on Rubingh model. The parameters (B1) of chain‐chain interaction between the molecules of ionic and nonionic surfactants were calculated based on Maeda model. The free energy of micellization calculated from the phase separation model (ΔG 2 m ), and by Maeda's method (ΔG 1 m ) agree reasonably well at high content of nonionic surfactant. The excess free energy ΔG ads E and ΔG m E (except α=0.4) for TX‐100/SDSn system are more negative than that TX‐100/DDPB system. These can be probably explained with the EO groups of TX‐100 surfactant carrying partial positive charge.

Calculation of the molecular exchanging energy of binary surfactants system on the surface monolayer of aqueous solution

By using the binary anionic/cationic surfactants system CH3(CH2)nOSO3−/CH3(CH2)nN+(CH3)3 as an example, the molecular exchanging energy (ɛ) of adsorption on the surface monolayer of aqueous solution has been studied. ɛ can be obtained with two methods. One is from the relationship between ɛ and the molecule interaction parameter (β). This relationship is founded by considering that the adsorption of mixed surfactants on the surface monolayer of solution satisfies the dimensional crystal model condition under which β can be obtained by testing the surface tension of solution. The other is directly from the molecular structure of surfactants with the Lennard-Jones formula. The results for the studied system show that these two methods coincide well.

Mixed Solutions of Anionic and Zwitterionic Surfactant (Betaine): Surface-Tension Isotherms, Adsorption, and Relaxation Kinetics

Here, we present experimental surface-tension isotherms of mixed solutions of two surfactants, sodium dodecyl sulfate (SDS) and cocoamidopropyl betaine (Betaine), measured by means of the Wilhelmy plate method. The kinetics of surface-tension relaxation exhibits two characteristic time scales, which have been distinguished to determine correctly the equilibrium surface tension. The transition from the zwitterionic to the cationic form of Betaine is detected by surface-tension measurements. Synergistic dependence of the critical micellization concentration on the composition of the surfactant blend is established. The experimental surface-tension isotherms are fitted by means of the two-component van der Waals model, and an excellent agreement between theory and experiment was achieved. Having determined the parameters of the model, we calculated different properties of the mixed surfactant adsorption layer at various concentrations ofSDS, Betaine, and salt. Such properties are the adsorptions ofthe two surfactants, the surface dilatational elasticity, the occupancy of the Stern layer by bound counterions, the surface electric potential, and so forth. In particular, the addition of a small amount of Betaine to SDS significantly increases the surface elasticity. The results could be further applied to predict the thickness and stability of foam films or the size of the rodlike micelles in the mixed solutions of SDS and Betaine.

Co-adsorption of quaternary ammonium compounds—nonionic surfactants on solid–liquid interface

Adsorption of mixed surfactants on the solid substrates has received scant attention in the past. However, in recent times, there have been a few investigations, which have been overviewed. Experimental results of co-adsorption and parallel zeta potential studies using mixed quaternary ammonium compounds CTAB, TTAB and nonyl phenyl ethoxylates (NP13, NP20, NP30) have been reported in this presentation. Three solid substrates, quartz, polystyrene and PTFE were chosen for investigation. ‘S’ shaped adsorption isotherms were obtained in case of all the three substrates. The presence of nonionics had varied effect on the adsorption of CTAB/TTAB depending on concentration. The zeta potential studies, in general, support the adsorption behaviour. However, the change on the solid substrate is influenced predominantly by the positively charged CTAB. Presence of premicellar aggregates on the solid surface is indicated. The nature of solid substrate appears to be of secondary importance.

The composition of mixed surfactants and cationic polymer/surfactant mixtures adsorbed at the air-water interface

The use of specular neutron reflection and small-angle neutron scattering (SANS) to determine the composition of mixed surfactants and polymer/surfactant mixtures, absorbed at the air-water interface, in micelles and polymer or micellar complexes, is described. For the characterisation of adsorption at the air-water interface by neutron reflectivity, the emphasis is predominantly on the concentration regime where the surface is in equilibrium with a bulk aggregated phase; that is, for concentrations greater than the cmc of the mixture. This is a regime that is difficult to access by standard experimental techniques. The SANS measurements of micelle composition are an important complement to the reflectivity measurements in the understanding of mixed surfactant adsorption. Recent results on the anionic/non-ionic surfactant mixture of sodium dodecyl sulphate (SDS) and hexaethylene monododecyl ether (C 12E 6) in the absence and presence of the cationic polymer and on the non-ionic mixture of monododecyl/triethylene glycol (C 12E 3/monododecyl octaethylene glycol (C 12E 8) illustraate the power and advantages of neutron scattering techniques.

Entropy of adsorption of mixed surfactants from solutions onto the air/water interface

The partial molar entropy change for mixed surfactant molecules adsorbed from solution at the air/water interface has been investigated by surface thermodynamics based upon the experimental surface tension isotherms at various temperatures. Results for different surfactant mixtures of sodium dodecyl sulfate and sodium tetradecyl sulfate, decylpyridinium chloride and sodium alkylsulfonates have shown that the partial molar entropy changes for adsorption of the mixed surfactants were generally negative and decreased with increasing adsorption to a minimum near the maximum adsorption and then increased abruptly. The entropy decrease can be explained by the adsorption-orientation of surfactant molecules in the adsorbed monolayer and the abrupt entropy increase at the maximum adsorption is possible due to the strong repulsion between the adsorbed molecules.

Adsorption dynamics of single and binary surfactants at the air/water interface

Models of diffusion-controlled adsorption of mixed surfactants at the air/water interface reveal a rich variety of interesting behavior. The models predict the following. A maximum in the dynamic adsorption concentration for the less surface-active surfactant can occur if this component has a higher initial bulk concentration. The maximum is even more pronounced for small diffusion lengths, at which the dynamic subsurface concentration of the fast-adsorbing component can temporarily exceed that of the bulk. If an impurity is more surface-active than the main component, then small amounts of the impurity can drastically alter the adsorption behavior of the latter, as shown in some examples. These phenomena can be partly explained in terms of the timescales of adsorption for single surfactants described by the Henry's law or Langmuir isotherms, and partly by the competition of the components for surface sites. Since dynamic surface concentration data are not yet available for binary surfactants, the model predictions are used to calculate dynamic surface tensions and compare them with dynamic tension data. The results have implications for the dynamic and equilibrium selectivities of foam fractionation processes and for the dynamics of free-surface flows.

The structure of mixed surfactant monolayers at the air-liquid interface, as studied by specular neutron reflection

The specular reflection of neutrons is now finding widespread application in the study of a range of problems in surface chemistry. In recent studies, using hydrogen-deuterium contrast variation, it has been possible in the investigation of surfactant adsorption at the air-liquid interface to determine both the surfactant surface excess and the detailed surface structure of the surfactant molecules. The authors present results that extend these studies to the adsorption of mixed surfactants at the air-liquid interface. For the mixed system sodium dodecyl sulphate and dodecanol, they have determined the surface excess and surface structure of each component in the mixed surface layer.