Adsorption of anionic surfactants on positively charged polystyrene particles II

In the case of cationic polystyrene latex, the adsorption of anionic surfactants involves a strong electrostatic interaction between both the particle and the surfactant, which may affect the conformation of the surfactant molecules adsorbed onto the latex-particle surface. The adsorption isotherms showed that adsorption takes place according to two different mechanisms. First, the initial adsorption of the anionic surfactant molecules on cationic polystyrene surface would be due to the attractive electrostatic interaction between both ionic groups, laying the alkyl-chains of surfactant molecules flat on the surface as a consequence of the hydrophobic interaction between these chains and the polystyrene particle surface, which is predominantly hydrophobic. Second, at higher surface coverage the adsorbed surfactant molecules may move into a partly vertical orientation with some head groups facing the solution. According to this second mechanism the hydrophobic interactions of hydrocarbon chains play an important role in the adsorption of surfactant molecules at high surface coverage. This would account for the very high negative mobilities obtained at surfactant concentration higher than 5×10^–7 M. Under high surface-coverage conditions, some electrophoretic mobility measurements were performed at different ionic strength. The appearance of a maximum in the mobility-ionic strength curves seems to depend upon alkyl-chain length. Also the effects of temperature and pH on mobilities of anionic surfactant-cationic latex particles have been studied. The mobility of the particles covered by alkyl-sulphonate surfactants varied with the pH in a similar manner as it does with negatively charged sulphated latex particles, which indicates that the surfactant now controls the surface charge and the hydrophobic-hydrophilic character of the surface.Dedicated to the memory of Dr. Safwan Al-Khouri IbrahimPresented at the Euchem Workshop on Adsorption of Surfactants and Macromolecules from Solution, Åbo (Turku), Finland, June 1989     

Assessment of the Surface Heterogeneity of Talc Materials

The hydrophobic and hydrophilic components of the surface of talc materials in aqueous solution were determined using ionic surfactants and their polar headgroup adsorption isotherms. The hydrophilic and hydrophobic surface areas are inferred from the amount of probe molecule adsorbed and the structure of the adsorbed layer. Natural dispersion of talc shows at 298 K a pH of 9.4 and the electrophoretic measurements indicate that the particles are negatively charged. The hydrophilic surface area is estimated from the adsorption of benzyltrimethylammonium ions (BTMA(+)) through electrostatic interactions as supported by the increase of divalent ions in the bulk phase and the decrease in the exothermic displacement enthalpy. It was also observed from the adsorption isotherm of benzene sulfonate anions that the density of positive surface sites is very low and is thus neglected. The adsorption of an anionic surfactant essentially occurs through dispersive interactions between the nonpolar organic tail of the molecule and the hydrophobic surface. Furthermore, some assumptions on the structure of dodecyl sulfate surfactant aggregates at the interface allow the hydrophobic part of the talc particles to be estimated. The cationic surfactant adsorption has been investigated and found to corroborate the hydrophilic and hydrophobic area values first obtained. Copyright 2001 Academic Press.     

Electrokinetic behavior and colloidal stability of polystyrene latex coated with ionic surfactants

This work is focused on analyzing the electrokinetic behavior and colloidal stability of latex dispersions having different amounts of adsorbed ionic surfactants. The effects of the surface charge sign and value, and the type of ionic surfactant were examined. The analysis of the electrophoretic mobility (mu(e)) versus the electrolyte concentration up to really high amounts of salt, much higher than in usual studies, supports the colloidal stability results. In addition, useful information to understand the adsorption isotherms was obtained by studying mu(e) versus the amount of the adsorbed surfactant. Aggregation studies were carried out using a low-angle light scattering technique. The critical coagulation concentrations (ccc) of the particles were obtained for different surfactant coverage. For latex particles covered by ionic surfactants, the electrostatic repulsion was, in general, the main contribution to the colloidal stability of the system; however, steric effects played an important role in some cases. For latices with not very high colloidal stability, the adsorption of ionic surfactants always improved the colloidal stability of the dispersion above certain coverage, independently of the sign of both, latex and surfactant charge. This was in agreement with higher mobility values. Several theoretical models have been applied to the electrophoretic mobility data in order to obtain different interfacial properties of the complexes (i.e., zeta potential and density charge of the surface charged layer).     

Adsorption of ethoxylated cationic surfactants on self-assembled monolayers of alkanethiols on gold using surface plasmon resonance detection

Adsorption of a series of ethoxylated cationic surfactants at model surfaces of alkanethiol self-assembled monolayers was studied by the surface plasmon resonance technique. Model surfaces were tailor-made by choosing alkanethiols or mixtures of alkanethiols with methyl, hydroxyl, carboxyl, and trimethylammonium groups in terminal position. The ethoxylated and quaternized cationic surfactants having from 2 to 18 oxyethylene units, showed a decrease in adsorbed amount with increasing oxyethylene chain length for both hydrophobic and hydrophilic surfaces. On a negatively charged surface, containing carboxylate groups, the surfactant with only two oxyethylene groups adsorbed strongly due to electrostatic attraction and the adsorption increased with increasing amount of surface carboxylate groups. This work shows the usefulness of self-assembled alkanethiols on gold as a tool for performing surfactant adsorption studies on surfaces with variable hydrophobicity and charge.     

Surfactant fouling of nanofiltration membranes: measurements and mechanisms.

Fouling of nanofiltration membranes is studied during filtration of aqueous surfactant solutions under different conditions. To this purpose, four typical nanofiltration membranes (Desal51HL, NF270, NTR7450 and NFPES10) and three typical surfactants (nonionic neodol, anionic SDBS and cationic cetrimide) are selected. Fouling is studied as a function of the surfactant concentration, with and without addition of an electrolyte (NaCl), at different pH and when filtering a mixture of surfactants. Adsorption experiments and hydrophobicity measurements (to study the orientation of the surfactants on the membrane surface) are also performed under the different conditions. The least membrane fouling is found for the anionic surfactant SDBS, while for the cationic surfactant cetrimide very low relative fluxes are observed. Neodol shows an intermediate degree of fouling. Both hydrophobic and electrostatic interactions (in the case of ionic surfactants) between the membrane surface and the surfactant explain the degree of adsorption and hence fouling, as membrane fouling is correlated with the amount of adsorbed surfactant. The difference between cetrimide and SDBS becomes especially visible when changing the pH: increasing the pH leads not only to an opposite orientation of the adsorbed surfactants, but also to an opposite trend in adsorbed amount and membrane fouling. This study permits selection of an optimal nanofiltration membrane to recycle wastewater containing surfactants in the carwash industry. The optimal choice would be a hydrophilic membrane with a low molecular weight cut-off and a small negative surface charge at neutral pH. Cationic surfactants in the wastewater should also be avoided as much as possible.     

十二烷基三甲基溴化铵在聚苯乙烯胶乳粒子上的吸附

实验测得C~1~2TAB在PS胶乳粒子表面的吸附等温线呈L型的二阶段吸附特征,这表明初始的C~1~2TA^+离子是将其季铵正电性头基吸引在PS链的负电性硫酸根端基上,并将碳氢链通过疏水相互作用吸附在PS链上。结合光子相关谱测得胶乳粒子流体力学半径R~H的变化,表明第I阶段围绕着这些初始吸附位的聚集吸附,产生平均聚集数为4.0的松散小聚集体,此时对应的浓度c/cmc=0.32是文献通常所指的临界表面胶团浓度csmc。其后的进一步聚集吸附最终生成了附着在PS链端基处且平均聚集数为19.5的球形吸附胶团。这一饱和吸附的结果增加了胶乳粒子在水溶液中的分散稳定性。     

Electrokinetic characterization of polystyrene–non-ionic surfactant complexes

An experimental study on the electrophoretic mobility (μ_e) of polystyrene particles after the adsorption of non-ionic surfactants with different chain lengths is described. Two sulphate latexes with relatively low surface charge densities (3.2 and 4.8 μC cm⁻²) were used as solid substrate for the adsorption of four non-ionic surfactants, Triton X-100, Triton X-165, Triton X-305 and Triton X-405, each one with 9–10, 16, 30 and 40 molecules of ethylene oxide (EO), respectively. The electrophoretic mobility of the polystyrene–non-ionic surfactant complexes was studied versus the amount of adsorbed surfactant (Γ). The presence of non-ionic surfactant onto particles surface seems to produce a slight shifting of the slipping plane because the mobilities of the different complexes display a very small decreasing. The increase in the number of EO chains in the surfactant molecule seems to operate as a steric impediment which decreases the number of adsorbed large surfactant molecules. The electrophoretic mobilities of the latex–surfactant complexes with maximum adsorption were measured versus the pH and ionic strength of the dispersion. While the different complexes showed a similar qualitative behaviour compared with that of the bare latex against the pH, the adsorption of the surfactant reduces the typical maximum in the μ_e−log[electrolyte].     

Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface

The adsorption behaviour of proteins and systems mixed with surfactants of different nature is described. In the absence of surfactants the proteins mainly adsorb in a diffusion controlled manner. Due to lack of quantitative models the experimental results are discussed partly qualitatively. There are different types of interaction between proteins and surfactant molecules. These interactions lead to protein/surfactant complexes the surface activity and conformation of which are different from those of the pure protein. Complexes formed with ionic surfactants via electrostatic interaction have usually a higher surface activity, which becomes evident from the more than additive surface pressure increase. The presence of only small amounts of ionic surfactants can significantly modify the structure of adsorbed proteins. With increasing amounts of ionic surfactants, however, an opposite effect is reached as due to hydrophobic interaction and the complexes become less surface active and can be displaced from the interface due to competitive adsorption. In the presence of non-ionic surfactants the adsorption layer is mainly formed by competitive adsorption between the compounds and the only interaction is of hydrophobic nature. Such complexes are typically less surface active than the pure protein. From a certain surfactant concentration of the interface is covered almost exclusively by the non-ionic surfactant. Mixed layers of proteins and lipids formed by penetration at the water/air or by competitive adsorption at the water/chloroform interface are formed such that at a certain pressure the components start to separate. Using Brewster angle microscopy in penetration experiments of proteins into lipid monolayers this interfacial separation can be visualised. A brief comparison of the protein adsorption at the water/air and water/n-tetradecane shows that the adsorbed amount at the water/oil interface is much stronger and the change in interfacial tension much larger than at the water/air interface. Also some experimental data on the dilational elasticity of proteins at both interfaces measured by a transient relaxation technique are discussed on the basis of the derived thermodynamic model. As a fast developing field of application the use of surface tensiometry and rheometry of mixed protein/surfactant mixed layers is demonstrated as a new tool in the diagnostics of various diseases and for monitoring the progress of therapies.     

THEORETICAL MODELING OF IONIC SURFACTANT ADSORPTION ON MINERAL OXIDE SURFACES

A model for the adsorption of ionic surfactants on oppositely charged solid surfaces of uniform charge density is developed. The model is based on the assumption that, on the solid surface, adsorbed surfactant monomers, monolayered and bilayered surfactant aggregates of different sizes and specifically adsorbing ions of added electrolyte constitute a mixture of hard discs. It means that only excluded area interactions between the surface discs are taken into account. To avoid a rapid two-dimensional condensation of the adsorbed surfactant the potential energy per molecule in the surface aggregates, which is a sum of chemical and electrostatic interactions, is assumed to decrease linearly with the increasing aggregate size. The electrostatic interactions of ionic species with the charged solid surface are described in terms of the Guy-Chapman theory of the double layer formation. The appropriate equations for adsorption isotherms of surfactant and electrolyte ions are derived and used to predict the experimental adsorption isotherms of DTAB on the precipitated silica at two different salt concentrations in the aqueous solution, On the basis of the obtained results the evolution of the adsorbed phase structure and the charge of silica particles with an increasing surface coverage is discussed.     

Adsorption/aggregation of surfactants and their mixtures at solid-liquid interfaces

Adsorption of surfactants and polymers at solid-liquid interfaces is used widely to modify interfacial properties in a variety of industrial processes such as flotation, ceramic processing, flocculation/dispersion, personal care product formulation and enhanced oil recovery. The behavior of surfactants and polymers at interfaces is determined by a number of forces, including electrostatic attraction, covalent bonding, hydrogen bonding, hydrophobic bonding, and solvation and desolvation of various species. The extent and type of the forces involved varies depending on the adsorbate and the adsorbent, and also the composition and other characteristics of the solvent and dissolved components in it. The influence of such forces on the adsorption behavior is reviewed here from a thermodynamics point of view. The experimental results from microcalorimetric and spectroscopic studies of adsorbed layers of different surfactant and polymer systems at solid-liquid interfaces are also presented. Calorimetric data from the adsorption of an anionic surfactant, sodium octylbenzenesulfonate, and a non-ionic surfactant, dodecyloxyheptaethoxyethylalcohol, and their mixtures on alumina, yielded important thermodynamic information. It was found that the adsorption of anionic surfactants alone on alumina was initially highly exothermic due to the electrostatic interaction with the substrate. Further adsorption leading to a solloid (hemimicelle) formation is proposed to be mainly an entropy-driven process. The entropy effect was found to be more pronounced for the adsorption of anionic-non-ionic surfactant mixtures than for the anionic surfactant alone. Fluorescence studies using a pyrene probe on an adsorbed surfactant and polymer layers, along with electron spin resonance (ESR) spectroscopy, reveal the role of surface aggregation and the conformation of the adsorbed molecules in controlling the dispersion and wettability of the system.     

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 surfactant adsorbed on encapsulation of fine inorganic powder with soapless emulsion polymerization

The encapsulation of fine inorganic powder was carried out with the soapless emulsion polymerization of methyl methacrylate in water in the presence of the powder, a layer of surfactant being adsorbed. The powder used was titanium dioxide. Surfactants added prior to the polymerization were sodium dodecyl sulfate, dodecyltrimethyl ammonium bromide, and polyoxyethylene sorbitan mono-oleate. The encapsulation state of the powder with polymer was closely related to the amount of surfactant adsorbed on the powder; and an amount of adsorption above a certain value was necessary for uniform encapsulation. Ionic surfactants were more useful than nonionic in the surfactants used, and could be adsorbed utilizing the electrostatic interaction between powder and the ionic end group. The combination of electric charges between the ionic end groups of surfactant and initiator was found to influence the molecular weight of capsulating polymer.     

Adsorption kinetics investigation for alkyltrimethylammonium bromides on ITO-aqueous interface

The adsorption of dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide onto indium-tin oxide (ITO) was studied by in-situ monitoring the capacitance and resistance of adsorbed layers for a series of concentrations of aqueous solutions. The experimental results suggested that the adsorption of both surfactants showed a three-region adsorption process and that the adsorption was driven by electrostatic attraction and hydrophobic interactions in two regions below critical micelle concentration (cmc). Above the cmc, the changes in electrical properties could be explained by the assumption that entire micelles were directly adsorbed onto the ITO surface and then underwent an adjustment process. The variations of water contact angle against surfactant concentration showed two distinct regions. The effect of high electrolyte concentration on the adsorbed layer structure was also discussed. The text was submitted by the authors in English.     

Modeling of confinement-induced phase transitions for surfactant layers on amphiphilic surfaces

A self-consistent field model is used to consider a solution of positively charged surfactants up to its critical micellization concentration adsorbing onto two surfaces in close proximity. Each surface mimics a polystyrene sulfonate interface; that is, hydrophobic properties are combined with a (fixed) negative charge. We observe large and sudden changes in adsorption as a function of separation, which are not normally considered when interpreting surface force measurements. The parameters are chosen such that the adsorbed surfactant layer is of a monolayer type when the surfaces are far apart. A typical interaction curve is presented for a fixed surfactant chemical potential, which is extracted from the set of adsorption isotherms each with a fixed slit width. When the slit width approaches the thickness of the two surfactant layers, a first-order phase transition takes place, which is driven by the unfavorable hydrophobic-water contacts. At the transition, the average orientation of the surfactants switches from a high concentration of tails at the surface to a bilayer configuration where tail profiles from both sides merge in the center. The headgroups are pulled slightly away from the surface. The interaction force jumps from a weak electrostatic repulsion at large distances (two effectively positively charged surface layers repel each other) to a strong electrostatic attraction at short distances (the central surfactant bilayer is attracted to the oppositely charged surfaces). The amount of adsorbed surfactants tend to decrease with decreasing distance between the surfaces but suddenly increases at the transition. Because of this, we anticipate that in surface force experiments, for example, there is a hysteresis associated with this transition: the forces and also the adsorbed amounts depend not only on the distance between the surfaces but also on the history if nonsufficient equilibration times are implemented.     

Restructuring of hydrophobic surfaces created by surfactant adsorption to mica surfaces

Hydrophobic surfaces created by the adsorption of a monolayer of surfactants, such as CTAB or DODAB, to mica display long-range mutual attraction when placed in water. Initially, this attraction was considered to be due to hydrophobic interaction, but more careful measurements using AFM showed that the surfactant monolayer undergoes rearrangements to produce charged patches on the surface; therefore, the nature of the long-range interaction is due to the electrostatic interaction between patches. The monolayer rearrangement depends on the nature of the surfactant and its counterion. To study possible monolayer rearrangements in molecular detail, we performed detailed molecular dynamics computer simulations on systems containing a monolayer of surfactants RN(CH(3))(3)(+)Cl(-) (R indicates a saturated hydrocarbon chain) adsorbed on a mica surface and immersed in water. We observe that when chain R is 18 carbons long the monolayer rearranges into a micelle but it remains a monolayer when the chain contains 24 carbons.     

Adsorption and adsolubilization of surfactants on titanium dioxides with functional groups

Adsorption of ionic surfactants on titanium dioxide with dodecyl chain groups or quaternary ammonium groups (XNm, where m is the carbon number of the alkyl chain, 4–16) was investigated. The adsorbed amount of cationic surfactants (dodecyltrimethylammonium bromide, DTAB; 1,2-bis(dodecyldimethylammonio)ethane dibromide, 2RenQ) on titanium dioxide with dodecyl chain groups increased with increasing concentration of the dodecyl chain due to hydrophobic interaction, where the adsorbed amounts of DTAB at saturation was considerably greater than those of 2RenQ. Adsorption of an anionic surfactant (sodium dodecyl sulfate, SDS) on XNm occurred mainly due to both electrostatic attraction force and hydrophobic interaction, depending on the alkyl chain length on XNm. On the other hand, adsorption of cationic surfactants, DTAC and 2RenQCl (their counter ions are chloride ions), on XNm was quite smaller compared with that of SDS due to electrostatic repulsion force. Adsolubilization of 2-naphthol in the surfactant-adsorbed layer on the titanium dioxides with the functional groups was also studied. The adsolubilized amounts of 2-naphthol on titanium dioxide with dodecyl chain groups were enhanced by adsorption of DTAB, but no distinct increase in the adsolubilization was observed by adsorption of 2RenQ. In the case of XNm, the amount of 2-naphthol adsorbed in the absence of surfactants increased with increasing alkyl chain length on XNm. Further, an appreciable increase in the adsolubilization of 2-naphthol on XNm with adsorption of 2RenQCl was observed. It was found from the admicellar partitioning coefficients that the adsolubilization of 2-naphthol preferably occurs on XNm by adsorption of SDS or 2RenQCl compared with that by DTAC. These differences in the adsolubilization were discussed by microproperties of the surfactant-adsorbed layers estimated using a spin probe.     

Controlled adsorption-desorption of cationic polymers on the surface of anionic latex particles

The complexation of anionic latex particles with two series of cationic copolymers is studied. The copolymers of the first series contain cationic and electroneutral (zwitter ion) hydrophilic units. The electrostatic adsorption of these copolymers on the surface of latex particles is accompanied by the formation of multiple salt bridges between cationic copolymer units and surface anionic groups. The dependence of ultimate adsorption on the molar fraction of cationic groups in copolymer α is described by a bell-shaped curve with a maximum at α = 0.05−0.10 and a long horizontal portion at α > 0.24. In terms of the adsorption theory of polyampholytes, such a pattern of the adsorption curve results from the compromise between the attraction of polymer chains to the surface induced by their polarization in the electric field of particles and the repulsion of like charged macromolecular units. The stability of complexes with the copolymers of the first series in water-salt media increases with an increase in α. The copolymers of the second series contain cationic and hydrophobic units. In this case, an increase in α is accompanied by a decrease in the amount of the adsorbed polymer throughout the studied α range (0.24–1). The complexes are stabilized not only via electrostatic interactions but also via hydrophobic interactions. A decrease in α decreases the role of electrostatics in stabilization of the complexes; however, this effect is compensated for by an increase in the number of hydrophobic contacts. This allows the stability of complexes to be preserved in concentrated water-salt solutions. The results of this study indicate that the stability of interfacial layers with the participation of cationic copolymers can be changed in a wide range by varying the ratio of ionic and electroneutral (hydrophilic or hydrophobic) comonomers in macromolecules.     

Depletion flocculation of latex dispersion in ionic micellar systems

The flocculation behavior of anionic and cationic latex dispersions induced by addition of ionic surfactants with different polarities (SDS and cetyltrimethylammonium bromide (CTAB)) have been evaluated by rheological measurements. It was found that in identical polar surfactant systems with particle surfaces of SDS + anionic lattices and CTAB + cationic lattices, a weak and reversible flocculation has been observed in a limited concentration region of surfactant, which was analyzed as a repletion flocculation induced by the volume-restriction effect of the surfactant micelles. On the other hand, in oppositely charged surfactant systems (SDS + cationic lattices and CTAB + anionic lattices), the particles were flocculated strongly in a low surfactant concentration region, which will be based on the charge neutralization and hydrophobic effects from the adsorbed surfactant molecules. After the particles stabilized by the electrostatic repulsion of adsorbed surfactant layers, the system viscosity shows a weak maximum again in a limited concentration region. This weak maximum was influenced by the shear rate and has a complete reversible character, which means that this weak flocculation will be due to the depletion effect from the free micelles after saturated adsorption.     

In situ AFM studies on self-assembled monolayers of adsorbed surfactant molecules on well-defined H-terminated Si(111) surfaces in aqueous solutions

The formation of self-assembled monolayers (SAMs) of adsorbed cationic or anionic surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium chloride (C8TAC), dodecyl trimethylammonium chloride (C12TAC), octadecyl trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium tetradecyl sulfate (SDS). The adsorbed surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0x10(-4) M C18TAC, whereas a slightly roughened layer was formed for 1.0x10(-4) M C8TAC and C12TAC. On the other hand, the addition of alcohols to solutions of 1.0x10(-4) M C8TAC, C12TAC, or SDS improved the molecular arrangement in the adsorbed surfactant layer. Similarly, the addition of a salt, KCl, also improved the molecular arrangement for both the cationic and anionic surfactant layers. Moreover, the adsorbed surfactant layer with a well-ordered structure was formed in a solution of mixed cationic (C12TAC) and anionic (SDS) surfactants, though each surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.