Polyelectrolyte/surfactant mixtures at the air–solution interface

This review presents some of the recent developments in our understanding of the behaviour of polyelectrolyte/surfactant mixtures at the air–solution interface. The existence of a strong surface polyelectrolyte/surfactant interaction results in a complex pattern of surface adsorption. Recent studies, using a range of surface sensitive techniques, which include ellipsometry, neutron and X-ray reflectivity, surface tension and interfacial rheology, have considerably enhanced the understanding of their surface behaviour, which can be rationalized in terms of the competition between the formation of surface active polymer/surfactant complexes and solution polymer/surfactant micelle complexes.     

The impact of electrolyte on the adsorption of sodium dodecyl sulfate/polyethyleneimine complexes at the air-solution interface

The addition of electrolyte (0.1 M NaCl) is shown to have a significant impact upon the surfactant concentration and solution pH dependence of the adsorption of sodium dodecyl sulfate (SDS)/polyethyleneimine (PEI) complexes at the air-solution interface. Substantial adsorption is observed over a wide surfactant concentration range (from 10(-6) to 10(-)2 M), and over much of that range of concentrations the adsorption is characterized by the formation of surface multilayers. The surface multilayer formation is most pronounced at high pH and for PEI with a lower molecular weight of 2K, compared to the higher molecular weight of 25K. These results, obtained from a combination of neutron reflectivity and surface tension, highlight the substantial enhancement in surfactant adsorption achieved by the addition of a combination of the polyelectrolyte, PEI, and a simple electrolyte. Furthermore the effect of electrolyte on the pH dependence of the adsorption further highlights the importance of the hydrophobic interaction in surface surfactant/polyelectrolyte complex formation.     

Formation of complexes between alkyl polyglycosides and potato starch

The formation of complexes between soluble potato starch and three commercial alkyl polyglycosides has been studied by means of surface tension measurements at 37 degrees C. All surfactants assayed form complexes with starch, the quantity of bound surfactant being proportional to the amount of starch present in the solution. For all alkyl polyglycoside-starch systems tested, there is a direct proportional relationship between the bound and total surfactant concentrations, so that the formation of the surfactant-starch complex continues until the minimum surface tension is reached without detectable starch saturation prior to the occurrence of surfactant micelles. Binding isotherms and Scatchard plots support the idea that alkyl polyglycosides are bound to amylose by positive cooperative binding and to amylopectin by non-cooperative Langmuir-type binding.     

The role of electrolyte and polyelectrolyte on the adsorption of the anionic surfactant, sodium dodecylbenzenesulfonate, at the air-water interface

The role of the polyelectrolyte, poly(ethyleneimine), PEI, and the electrolytes NaCl and CaCl(2), on the adsorption of the anionic surfactant, sodium dodecylbenzenesulfonate, LAS, at the air-water interface have been investigated by neutron reflectivity and surface tension. The surface tension data for the PEI/LAS mixtures are substantially affected by pH and the addition of electrolyte, and are consistent with a strong adsorption of surface polymer/surfactant complexes down to relatively low surfactant concentrations. The effects are most pronounced at high pH, and this is confirmed by the adsorption data obtained directly from neutron reflectivity. However, the effects of the addition of PEI and electrolyte on the LAS adsorption are not as pronounced as previously reported for PEI/SDS mixtures. This is attributed primarily to the steric hindrance of the LAS phenyl group resulting in a reduction in the ion-dipole attraction between the LAS sulfonate and amine groups that dominates the interaction at high pH.     

Polyelectrolyte/surfactant mixtures in the bulk and at water/oil interfaces

Stabilization of emulsions by mixed polyelectrolyte/surfactant systems is a prominent example for the application in modern technologies. The formation of complexes between the polymers and the surfactants depends on the type of surfactant (ionic, non-ionic) and the mixing ratio. The surface activity (hydrophilic–lipophilic balance) of the resulting complexes is an important quantity for its efficiency in stabilizing emulsions. The interfacial adsorption properties observed at liquid/oil interfaces are more or less equivalent to those observed at the aqueous solution/air interface, however, the corresponding interfacial dilational and shear rheology parameters differ quite significantly. The interfacial properties are directly linked to bulk properties, which support the picture for the complex formation of polyelectrolyte/surfactant mixtures, which is the result of electrostatic and hydrophobic interactions. For long alkyl chain surfactants the interfacial behavior is strongly influenced by hydrophobic interactions while the complex formation with short chain surfactants is mainly governed by electrostatic interactions.     

Interfacial microgels formed by oppositely charged polyelectrolytes and surfactants. Part 2. Influence of surfactant chain length and surfactant/polymer ratio

The adsorption and complexation of polystyrene sulfonate (a highly charged anionic polyelectrolyte) and a series of cationic surfactants, alkyltrimethylammonium bromide, CnTAB, n = 8-16, at the air-water interface has been studied by combining surface tension and ellipsometry measurements. We find that increasing the chain length of the surfactant from 8 to 10 carbons leads to a sharp increase in adsorption of PSS/CnTAB complexes. When the surfactant tail length is further increased to 12 and 14 carbons, surface adsorption becomes less favored than macroscopic phase separation, resulting in a partial surface depletion. Furthermore, we find that when surface tensions are plotted against surfactant/monomer molar concentration ratio, all data collapse to a single curve. This result shows that the surfactant-polymer molar ratio, s/p, is a key parameter for tuning the surface activity of the complexes formed.     

Adsorption of oppositely charged polyelectrolyte/gemini surfactant mixtures at the air/water interface and the effects of NaBr: a surface tension study

The adsorption of mixed solutions containing an anionic polyelectrolyte, carboxymethylchitosan (CMCH), and cationic gemini surfactants, alkanediyl-bis-(dimethyldodecyl-ammonium bromide) (C₁₂-s-C₁₂, s?=?2, 6, 12), has been investigated by surface tension method. The oppositely charged polyelectrolyte and the surfactants co-adsorb at the surface to form highly surface-active complexes. Combining the surface tension data with the Gibbs equation, it is referred that the surface layers of the mixed solutions have the multi-level structure, which includes the sublayers beneath an outermost layer. The gemini surfactant spacer with different length takes different conformations in the surface layers. The salt (NaBr) effects on the adsorption of the mixtures have also been studied. The spacer length of C₁₂-s-C₁₂ influences the responses of CMCH/C₁₂-s-C₁₂ mixtures to the salt effects. The comprehensive salt effects depend on the competition between the salt-enhancing effect and the salt-weakening effect.     

Dilational viscoelasticity of anionic polyelectrolyte/surfactant adsorption films at the water–octane interface

In this article, the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water–octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. Polystyrene sulfonate sodium (PSS)/cationic surfactant hexadecanetrimethyl–ammonium bromide systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be explained well by electrostatic interaction. In the case of PSS/anionic surfactant sodium dodecyl sulfate (SDS) systems, the coadsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational elasticity. For PSS/nonionic surfactant TX100 systems, PSS may form a sub-layer contiguous to the aqueous phase with partly hydrophobic polyoxyethylene chain of TX100, which has little effect on the TX100 adsorption film and interfacial tension.     

Equilibrium and dynamic surface properties of long-chain quaternary ammonium hydroxides with different chain length and number

The equilibrium and dynamic surface tensions of five long-chain alkyl ammonium hydroxides (AAH) at the air/aqueous solution interface were investigated, and the effects of the length and number of alkyl chain on surface tensions had been discussed. With the increase of the length, the equilibrium surface tension (EST) increased from 28.65 to 40.52?mN/m. While, for the double chains at the critical micelle concentration (CMC), the EST decreased from 32.71 to 26.61?mN/m with the length increasing. In addition, the adsorption behaviors of the AAH were analyzed and the effective diffusion coefficients (D_eff) were calculated on basis of the Ward–Tordai equation. Moreover, the time required to attain the EST decreases with the increase of surfactant concentration. The longer the C–H chain is, the lower surface tension at initial concentration is. What’s more, the diffusion processing of the AAH to air/water interface mainly depends on the surfactant concentration, and the adsorption is controlled by diffusion mechanism in a dilute concentration, while under a high concentration the adsorption is controlled by mixed diffusion–kinetic mechanism.     

Adsorption of alkyl polyglucosides on the solid/water interface: equilibrium effects of alkyl chain length and head group polymerization

The equilibrium adsorption behavior of two n-alkyl-beta-D-glucosides (octyl (C8G1) and decyl (C10G1)) and four n-alkyl-beta-D-maltosides (octyl (C8G2), decyl (C10G2), dodecyl (C12G2), and tetradecyl (C14G2)) from aqueous solution on a titania surface, as measured by ellipsometry, has been investigated. The main focus has been on the effect of changes in the alkyl chain length and headgroup polymerization, but a comparison with their adsorption on the silica/water and air/water interfaces is also presented. Some comparison with the corresponding adsorption of ethylene oxide surfactants, in particular C10E6 and C12E6, is given as well. For all alkyl polyglucosides, the maximum adsorbed amount on titania is reached slightly below the critical micelle concentration (cmc), where it levels off to a plateau and the amount adsorbed corresponds roughly to a bilayer. However, there is no evidence that this is the actual conformation of the surfactant assemblies on the surface, but the surfactants could also be arranged in a micellar network. On hydrophilic silica, the adsorbed amount is a magnitude lower than on titania, corresponding roughly to a layer of surfactants lying flat on the surface. A change in the alkyl chain length does not result in any change in the plateau molar adsorbed amount at equilibrium; however, the isotherm slope for the alkyl maltosides increases with increasing chain length. Headgroup polymerization on the other hand affects the adsorbed amount. The alkyl glucosides start adsorbing at lower bulk concentrations than the maltosides and equilibrate at higher adsorbed amounts above the cmc. When compared with the ethylene oxide (EO) surfactants, it is confirmed that the EO surfactants hardly adsorb on titania, since the measured changes in the ellipsometric angles are within the noise level. They do, however, adsorb strongly on silica.     

表面活性剂对污泥脱水性能影响的机理研究:Gemini表面活性剂与聚电解质相互作用的分子动力学模拟

表面活性剂可以与污泥表面的胞外聚合物(EPS)吸附形成胶束,释放出自由水和结合水,从而达到改善污泥脱水性能的目的.本文采用粗粒化的分子动力学模拟方法,研究了Gemini表面活性剂与EPS形成复合物的过程和结构.聚电解质链的亲疏水性对吸附过程有显著影响,亲水聚电解质链与Gemini表面活性剂吸附的主要驱动力为静电吸引,Gemini表面活性剂头基吸附在链上,尾链朝向溶剂;疏水聚电解质链与Gemini表面活性剂吸附过程由静电作用与疏水作用共同促进,Gemini表面活性剂以平行于聚电解质链的构型存在.Gemini表面活性剂联结基团长度对吸附过程的影响甚微;聚电解质链的电荷密度对亲水聚电解质链的吸附产生协同作用,对疏水聚电解质链的吸附不产生作用.     

Dynamic surface properties of polyelectrolyte/surfactant adsorption films at the air/water interface: poly(diallyldimethylammonium chloride) and sodium dodecylsulfate

The dynamic surface elasticity, dynamic surface tension, and ellipsometric angles of mixed aqueous poly(diallyldimethylammonium chloride)/sodium dodecylsulfate solutions (PDAC/SDS) have been measured as a function of time and surfactant concentration. This system represents a typical example of polyelectrolyte/surfactant complex formation and subsequent aggregation on the nanoscale. The oscillating barrier and oscillating drop methods sometimes led to different results. The surface viscoelasticity of mixed PDAC/SDS solutions are very close to those of mixed solutions of sodium polystyrenesulfonate and dodecyltrimethylammonium bromide but different from the results for some other polyelectrolyte/surfactant mixtures. The abrupt drop in surface elasticity when the surfactant molar concentration approaches the concentration of charged polyelectrolyte monomers is caused by the formation of microparticles in the adsorption layer. Aggregate formation in the solution bulk does not influence the surface properties significantly, except for a narrow concentration range where the aggregates form macroscopic flocks. The mechanism of the observed relaxation process is controlled by the mass exchange between the surface layer and the flocks attached to the liquid surface.     

Adsorption of organic compounds onto polyelectrolyte immobilized-surfactant aggregates on cellulosic fibers

The adsorption of anionic surfactants with different hydrophobic chain lengths onto cellulose fibers pretreated with a cationic polyelectrolyte has been investigated. Five steps are involved in the adsorption process, which was ascribed to the formation of monolayer and bilayer surfactant aggregates. Electrostatic interaction between the residual surface charges followed by hydrophobic interaction among the alkyl chains are considered the main factors in the adsorption process. The adsorption of the anionic surfactant was found to greatly enhance the retention of organic compounds onto the polyelectrolyte-treated cellulose. The coadsorption phenomenon, which was dependent on the saturation level of the adsorbed surfactant, has been explained in terms of the accumulation of the organic solute on the hydrophobic core generated by the adsorbed layer.     

Effect of Molecular Weight on the Interfacial Dilational Viscoelasticity of Anionic Polyelectrolyte/Surfactant Systems

In this article, the effect of molecular weight on the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water-octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. PSS/cationic surfactant CTAB systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be well explained by electrostatic interaction. Molecular weight of PSS plays a crucial role in the nature of adsorption film. The complex formed by CTAB and higher molecular weight PSS, which has larger dimension and stronger interaction, results in higher dilational modulus at lower surfactant bulk concentration. In the case of PSS/anionic surfactant SDS systems, the co-adsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational modulus at lower surfactant bulk concentration. For PSS/nonionic surfactant T × 100 systems, PSS may form a sublayer contiguous to the aqueous phase, which has little effect on interfacial tension but slightly decreases dilational modulus.     

Effect of gemini surfactant additives on pour point depressant of crude oil

Efficiencies of cationic gemini surfactant additives in improving the pour point depressant of crude oil were investigated. The length of alkyl chain is a major factor affecting the improvement of the pour point depression. The adsorption behavior of these gemini surfactants at air/solution and oil/solution interfaces were investigated by measuring the surface tension and interfacial tension as functions of concentration. It is found that there is a good relation between surface properties especially interfacial tension of the gemini surfactants and their efficiency in depressing the pour point. Also, the surface parameters and free energies of micellization and adsorption confirm the decreasing and improving of pour point depression. Crystallization study in crude oil revealed the relationship between the structure and activity of gemini surfactant additives. It is found that the x-ray diffraction patterns of waxes with additives are remarkably different from those without additives. The mechanism of the depressants action has been suggested according the adsorption of each additive. Adsorption of the additive on the surface of the wax particles inhibits their growth and alters the crystal habits through micelle core. Pretreatment of the crude oil with pour point depressants has received the greatest acceptance due to its simplicity and economy.     

Adsorption of oppositely charged polyelectrolyte/surfactant complexes at the air/water interface: formation of interfacial gels

The adsorption and complexation of polystyrene sulfonate (a highly charged anionic polyelectrolyte) and dodecyltrimethylammonium bromide (a cationic surfactant) at the air-water interface can lead to interfacial gels that strongly influence foam-film drainage and stability. The formation and characteristics of these gels have been studied by combining surface tension, ellipsometry, and foam-film drainage experiments. Simultaneously, the solution electromotive force is measured and used to track the polymer-surfactant interactions in the bulk solution. We find that surface gelation occurs above the critical aggregation concentration in solution but before bulk precipitation of the polymer-surfactant complexes. Furthermore, we reveal that strong readsorption of polymer-surfactant complexes occurs during the resolubilization of the precipitated complexes at high surfactant concentrations (i.e., >critical micelle concentration). Seemingly overlooked in the past, this readsorption significantly influences the surface rheological properties and foam-film drainage of these systems.     

The influence of mixed cationic–anionic surfactants on the three-phase contact parameters in silica–solution systems

The formation of thin wetting films on silica surface from aqueous solution of (a) tetradecyltrimetilammonium bromide (C₁₄TAB) and (b) surfactant mixture of the cationic C₁₄TAB with the anionic sodium alkyl- (straight chain C₁₂–, C₁₄– and C₁₆–) sulfonates, was studied using the microscopic thin wetting film method developed by Platikanov. Film lifetimes, three-phase contact (TPC) expansion rates, receding contact angles and surface tension were measured. It was found that the mixed surfactants caused lower contact angles, lower rates of the thin aqueous film rupture and longer film lifetimes, as compared to the pure C₁₄TAB. This behavior was explained by the strong initial adsorption of interfacial complexes from the mixed surfactant system at the air/solution interface, followed by adsorption at the silica interface. The formation of the interfacial complexes at the air/solution interface was proved by means of the surface tension data. It was also shown, that the chain length compatibility between the anionic and cationic surfactants controls the strength of the interfacial complex and causes synergistic lowering in the surface tension. The film rupture mechanism was explained by the heterocoagulation mechanism between the positively charged air/solution interface and the solution/silica interface, which remained negatively charged.     

A Study of Dynamic Interfacial Properties as Related to Foaming Properties of Sodium 2,5-dialkyl Benzene Sulfonates

In this article, foaming properties and dynamic interfacial properties of a series of sodium 2,5-dialkyl benzene sulfonates in aqueous solutions were carried out to elucidate the relationship between foaming properties and dynamic interfacial properties. The properties of foams generated from bubbling air through different surfactant solutions were measured using a modified Bikerman device. The dynamic surface tension and surface dilational elasticity were obtained from an image analysis technique based on the oscillating bubble method. The surfactants molecular adsorption at the air/water interface was introduced with Rosen empirical equation and the rate of adsorption was determined from measurements of the dynamic surface tension. The surfactant with the longest alkyl chain shows the lowest dynamic surface activity, which lead to the lowest foam volume. The short ortho straight alkyl chain has little effect on the arrangement of molecules at the interface and the foam stability changes a little with the changing of the ortho alkyl chain length. The foam stability is correlated with both the higher surface dilational elasticity and the larger surface monolayer strength.     

Thermodynamics of surfactant-dye complex formation in aqueous solutions. Sodium alkyl sulfates and azo oil dye systems

Thermodynamics of surfactant-dye complex formation have been studied, in terms of equilibrium coefficient, using a spectrophotometer. The systems are 6 sodium alkyl sulfates, which have different alkyl chain lengths, and 4-phenylazo-1-naphthylamine. A pronounced spectral change in the dye solution occurs on addition of the surfactant; the change has a definite isosbestic point and a new absorption band at 535 nm because of surfactant-dye complex formation, which is caused by hydrophilic-hydrophilic interaction. As the alkyl chain length in the surfactant increases, the values of free energy change (negative) increase, while the value of enthalpy change (negative) increases and the value of entropy change (positive) decreases. The longer the alkyl chain length in surfactant increase, the more stable the surfactant-dye complex becomes.Surfactant-dye complex will form due to hydrophilic-hydrophilic interaction and will become more stable due to hydrophobic-hydrophobic interaction.     

Removal of solvent-based ink from printed surface of HDPE bottles by alkyltrimethylammonium bromides: effects of surfactant concentration and alkyl chain length

Three alkyltrimethylammonium bromides (i.e., dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromide or DTAB, TTAB, and CTAB, respectively) were used to remove a blue solvent-based ink from a printed surface of high-density polyethylene bottles. Either an increase in the alkyl chain length or the surfactant concentration was found to increase the deinking efficiency. Complete deinking was achieved at concentrations about 3, 8, and 24 times of the critical micelle concentration (CMC) of CTAB, TTAB, and DTAB, respectively. For CTAB, ink removal started at a concentration close to or less than its CMC and increased appreciably at concentrations greater than its CMC, while for TTAB and DTAB, significant deinking was only achieved at concentrations much greater than their CMCs. Corresponding to the deinking efficiency of CTAB in the CMC region, the zeta potential of ink particles was found to increase with increasing alkyl chain length and concentration of the surfactants, which later leveled off at some higher concentrations. Wettability of the surfactants on an ink surface increased with increasing alkyl chain length and concentration of the surfactants. Lastly, solubilization of ink binder in the surfactant micelles was found to increase with increasing alkyl chain length and surfactant concentration. We conclude that adsorption of surfactant on the ink pigment is crucial to deinking due to modification of wettability, zeta potential, pigment/water interfacial tension, and dispersion stability. Solubilization of binder (epoxy) into micelles is necessary for good deinking because the dissolution of the binder is required before the pigment particles can be released from the polymer surface.