Counterion release from adsorbed highly charged polyelectrolyte: an electrooptical study

The adsorption of sodium poly(4-styrene sulfonate) on oppositely charged beta-FeOOH particles is studied by electrooptics. The focus of this paper is on the release of condensed counterions from adsorbed polyelectrolyte upon surface charge overcompensation. The fraction of condensed Na+ counterions on the adsorbed polyion surface is estimated according to the theory of Sens and Joanny and it is compared with the fraction of condensed counterions on nonadsorbed polyelectrolyte. The relaxation frequency of the electrooptical effect from the polymer-coated particle is found to depend on the polyelectrolyte molecular weight. This is attributed to polarization of the layer from condensed counterions on the polyion surface, being responsible for creation of the effect from particles covered with highly charged polyelectrolyte. The number of the adsorbed chains is calculated also assuming counterion condensation on the adsorbed polyelectrolyte and semiquantative agreement is found with the result obtained from the condensed counterion polarizability of the polymer-coated particle. Our findings are in line with theoretical predictions that the fraction of condensed counterions remains unchanged due to the adsorption of highly charged polyelectrolyte onto weakly charged substrate.     

Counterion condensation on charged spheres, cylinders, and planes

We use the framework of counterion condensation theory, in which deviations from linear electrostatics are ascribed to charge renormalization caused by collapse of counterions from the ion atmosphere, to explore the possibility of condensation on charged spheres, cylinders, and planes immersed in dilute solutions of simple salt. In the limit of zero concentration of salt, we obtain Zimm-Le Bret behavior: a sphere condenses none of its counterions regardless of surface charge density, a cylinder with charge density above a threshold value condenses a fraction of its counterions, and a plane of any charge density condenses all of its counterions. The response in dilute but nonzero salt concentrations is different. Spheres, cylinders, and planes all exhibit critical surface charge densities separating a regime of counterion condensation from states with no condensed counterions. The critical charge densities depend on salt concentration, except for the case of a thin cylinder, which exhibits the invariant criticality familiar from polyelectrolyte theory.     

Influence of surfactant counterions during electrophoretic particle deposition

This study demonstrates the influence of a cationic surfactant on colloidal particle electrodeposition (migration and adhesion). Three cetyltrimethylammonium salts (CTA+) with various counterions (bromide, chloride, and hydrogenosulphate) were studied. Particle transport toward the electrode was driven by the electrophoretic force. Once particles reached the electrode, a wide variety of behaviors were observed, depending on surfactant concentration and counterions: particles would stick permanently or slide along the electrode surface, remain or detach upon potential switching, act as nuclei for aggregate growth, or produce a homogeneous particle film. The experimental results also demonstrate the specific influence of surfactant counterions on the deposited film morphology.     

Structure of adsorption layers of ionic surfactants at the solid/liquid interface

A large number of experimental results of different surfactant adsorption systems (mainly on the silicas) obtained from both equilibrium and kinetic studies under different conditions are interpreted by a model of small individual surface aggregates. The adsorption model is contrasted with the influences of various factors, including electrostatic interaction, hydrophobic interaction, concentrations, types of coions, types of counterions, surfactant structure, alkyl chain length, types of head groups, neutral electrolytes, pH, adsorbent structure, porosity, surface charge density, and surface polarity.Dedicated to Frau Professor Dr. Elsa Ullmann on the occasion of her 80th birthday     

Complexation of ferric oxide particles with pectins of different charge density

The effect of polyelectrolyte charge density on the electrical properties and stability of suspensions of oppositely charged oxide particles is followed by means of electro-optics and electrophoresis. Variations in the electro-optical effect and the electrophoretic mobility are examined at conditions where fully ionized pectins of different charge density adsorb onto particles with ionizable surfaces. The charge neutralization point coincides with the maximum of particle aggregation in all suspensions. We find that the concentration of polyelectrolyte, needed to neutralize the particle charge, decreases with increasing charge density of the pectin. The most highly charged pectin presents an exception to this order, which is explained with a reduction of the effective charge density of this pectin due to condensation of counterions. The presence of condensed counterions, remaining bound to the pectin during its adsorption on the particle surface, is proved by investigation of the frequency behavior of the electro-optical effect at charge reversal of the particle surface.     

Electro-optics of colloid-polyelectrolyte complexes: counterion release from adsorbed macromolecules

Complexation between sodium carboxymethylcellulose (NaCMC) and ellipsoidal particles of oppositely charged beta-FeOOH is studied using electric light scattering and electrophoresis. We focus on the complex behavior for overcharging of the particle surface. In this case, the fraction of condensed counterions on NaCMC is found to remain unchanged during polymer adsorption onto a weakly charged particle surface. Using the theory of Sens and Joanny, we evaluate the fraction of condensed counterions and compare it with results for nonadsorbed NaCMC. The polarization of condensed counterions along the chain of the adsorbed macromolecule is proved to create the electro-optical effect in suspensions stabilized by NaCMC adsorption.     

Counterion condensation in hydrosols of single-crystalline detonation nanodiamond particles obtained by air annealing of their agglomerates

The pattern of previously recorded dependences of the specific surface charge and electrophoretic mobility of monodisperse detonation nanodiamond particles on pH of aqueous KCl solutions suggests that counterions are condensed on the particle surface. Counterion condensation is considered in terms of the Levin model, and the experimental ratios between the densities of the electrokinetic and surface charges of dispersed particles, as well as the fractions of condensed counterions, are calculated as depending on pH and KCl concentration in nanodiamond hydrosols. The obtained dependences lead to the conclusion that counterion condensation on the surface of detonation nanodiamond particles does indeed take place.     

Dynamics of foams of ethoxylated ionic surfactant in the presence of micelles and multivalent ions

Foams produced from surfactant solutions containing micelles of the anionic surfactant sodium polyoxyethylene-2 sulfate and counterions of different valence (aluminium, calcium or sodium) are investigated. For this purpose an experimental setup consisting of a glass column and units for detection of pressure, flow and frequency is constructed. Blowing gas bubbles in the surfactant solution at a constant gas pressure produces the foam. Simultaneous monitoring of the bubble volume and frequency relates the foam growth rate to the dynamic surface tension of the surfactant solution. The foam growth rate plotted versus the gas flow rate exhibits a break point at about 80 mL/min, attributed to the transition from regime of bubbles (at lower flow rates - monodisperse foam) to jet regime (at higher flow rates - polydisperse foam). Due to the high surfactant concentration, the foam is stable and its height is linearly increasing with the time. Two types of experiments are carried out. (i) At a constant counterion concentration and variable surfactant concentration, the rate of foam growth increases initially with increasing of the surfactant concentration reaching a plateau at higher concentrations. The foams of pure surfactant grow always slower than the foams with added aluminium ions. (ii) At a constant surfactant concentration and variable counterion concentration, the rate of foam growth exhibits a maximum. It corresponds to number of aggregated surfactant monomers nearly equal to the number of charges provided by the counterions, for example when one aluminium ion binds three surfactant monomers in a micelle. The point of maximum coincides with the transition from small spherical micelles to large cylindrical ones. This transition affects also the micelle lifetime, which is related to the ability of releasing monomers by a micelle in order to supply the bubble surface with surfactant. In support to this hypothesis, the maximum foam growth is found corresponding to lower dynamic surface tension allowing the generation of a large number smaller in size bubbles. The results for the foam growth agree in some extent with the data from independent measurements on the liquid drainage from wet foams.     

抗衡离子对硫酸盐表面活性剂气/液界面性质影响的分子动力学模拟

采用全原子分子动力学方法研究了抗衡离子为第一主族离子(Li⁺、Na⁺、K⁺、Rb⁺和Cs⁺)的十二烷基硫酸盐表面活性剂的气/液界面性质. 通过分析体系中各组分的密度分布曲线, 考察表面活性剂单分子层在界面的聚集形态, 并利用径向分布函数分析了表面活性剂极性头基与抗衡离子间的相互作用. 研究结果表明: 随着抗衡离子半径的增大, 不同体系的界面水层厚度依次增加, 表面活性剂极性头基与抗衡离子形成的Stern和扩散层厚度也相应增加. 但表面活性剂吸附层的抗衡离子缔合度以及体系表面张力却随抗衡离子半径的增大而减小. 研究表明抗衡离子的差异对十二烷基硫酸盐表面活性剂气/液界面性质有很大影响.     

Effect of equimolar salt to decyltrimethylammonium decyl sulfate on vesicle formation and surface adsorption

The aqueous solution of mixture of sodium decyl sulfate (SDeS) and decyltrimethylammonium bromide (DeTAB) has been found to form equilibrium multilamellar vesicles (MLV) spontaneously. We measured the surface tension of the aqueous solution of 1:1 mixture of SDeS and DeTAB as a function of temperature T at various molalities m under atmospheric pressure. The surface density, the entropy of adsorption and the entropy of vesicle formation are evaluated and compared with those of the decyltrimethylammonium decyl sulfate (DeTADeS) aqueous solution system to investigate the role of small counterions in the mechanism of equilibrium vesicle formation. The saturated surface density Gamma (H,C ) vs T curve of the SDeS/DeTAB system sits below that of the DeTADeS system. Therefore, sodium and bromide ions are negatively adsorbed and nevertheless, they neutralize the electric charge of the decyl sulfate ion DeS(-) and the decyltrimethylammonium ion DeTA(+) to some extent to weaken the electrostatic attraction between the polar head groups in the adsorbed film. The net surfactant concentration required for vesicle formation was larger in the SDeS/DeTAB system. Hence, the electrostatic attraction between the polar head groups of the surfactant ions which is one of the major driving forces for vesicle formation is weakened by the presence of the counterions Na(+) and Br(-). Small but distinct changes in the surface density and the entropies of MLV formation of the SDeS/DeTAB system from those of the DeTADeS system were also found.     

Counterion condensation and release in micellar solutions

Counterion condensation and release in micellar solutions are investigated by direct measurement of counterion concentration with ion-selective electrode. Monte Carlo simulations based on the cell model are also performed to analyze the experimental results. The degree of counterion condensation is indicated by the concentration ratio of counterions in the bulk to the total ionic surfactant added, alpha< or =1. The ionic surfactant is completely dissociated below the critical micelle concentration (cmc). However, as cmc is exceeded, the free counterion ratio alpha declines with increasing the surfactant concentration and approaches an asymptotic value owing to counterion condensation to the surface of the highly charged micelles. Micelle formation leads to much stronger electrostatic attraction between the counterion and the highly charged sphere in comparison to the attraction of single surfactant ion with its counterion. A simple model is developed to obtain the true degree of ionization, which agrees with our Monte Carlo results. Upon addition of neutral polymer or monovalent salts, some of the surfactant counterions are released to the bulk. The former is due to the decrease of the intrinsic charge (smaller aggregation number) and the degree of ionization is increased. The latter is attributed to competitive counterion condensation, which follows the Hefmeister series. This consequence indicates that the specific ion effect plays an important role next to the electrostatic attraction.     

Effect of counterions on surface and foaming properties of dodecyl sulfate

The influence of counterions of surfactant on interfacial properties is studied by measuring foamability, foam stability, equilibrium and dynamic surface tension, and surface viscosity. The surfactant chosen is anionic dodecyl sulfate with various counterions, Li(+), Na(+), Cs(+), and Mg(++). Surface tension measurements show a decrease in the following order: LiDS > NaDS > CsDS > Mg(DS)(2). Foamability done using shaking method shows similar order as surface tension, i.e., LiDS > NaDS > CsDS > Mg(DS)(2). This has been explained in terms of the differences in micellar stability and diffusion of monomers. This is further confirmed by our dynamic surface tension results, which show the same order as equilibrium surface tension (i.e., LiDS > NaDS > CsDS > Mg(DS)(2)) at low bubble frequencies but the order is LiDS > NaDS = Mg(DS)(2) > CsDS at high bubble frequencies. Foam stability measurements were done at concentrations below and above cmc to elucidate the role of micelles. It was found that there is no significant change in foam stability when counterions are changed for surfactant concentration values below the cmc, but at concentration above cmc the foam stability of CsDS and Mg(DS)(2) are much greater than LiDS and NaDS indicating presence of stable micelles are essential to high foam stabilities. Surface viscosity measurements correlated well with the foam stability trends and gave the following order LiDS < NaDS < CsDS < Mg(DS)(2), indicating that the molecules of CsDS and Mg(DS)(2) are tightly packed at the air/water interface.     

Presence or absence of counterion specificity in the interaction of alkylammonium surfactants with alkylacrylamide gels

Patterns in the interaction of cationic surfactants with nonionic polymer gels, which were inferred from a recent study from our laboratory, are confirmed by measurements of a series of alkylammonium surfactants with different counterions with a series of alkyl acrylamide gels of increasing hydrophobicity. Two swelling patterns were observed: Either the swelling continued above the surfactant critical micelle concentration (cmc) and the maximum swelling differed for different counterions and increased in the order of Br-相似文献   

Electrokinetic potential of multilayer carbon nanotubes in aqueous solutions of electrolytes and surfactants

The effect of electrolytes with single-, double- and triple-charged counterions, as well as cationic (cetyltrimethylammonium bromide) and anionic (sodium dodecyl sulfate) surfactants, on the electrokinetic potential of multiwall carbon nanotubes prepared via catalytic pyrolysis of propylene in a gas phase according to the CCVD technology has been studied. It has been shown that the influence of different electrolytes on the electrophoretic mobility of the nanotubes does not differ essentially from their effect on the behavior of well-studied inorganic dispersed particles; i.e., the dependence of the absolute values of the ζ-potential on the concentration of a 1: 1 electrolyte passes through a maximum and the introduction of double-charged counterions dramatically reduces the ζ-potential, while the addition of triple-charged cations and a cationic surfactant causes charge reversal of the nanotube surface. The adsorption of sodium dodecyl sulfate in a neutral medium increases the negative ζ values; this effect evens out in the presence of an alkali due to a rise in the electrostatic repulsion between surfactant anions and nanotube surface, which bears a high negative charge resulting from the dissociation of surface functional groups.     

Electrophoretic properties of dodecyltrimethylammonium bromide micelles in KBr solution

Charge in ionic micelles determines the trends of their stability and their practical applications. Charge can be calculated from zeta potential (zeta) measurements, which, in turn, can be obtained by Doppler microelectrophoresis. In this study, the electrophoretic properties of dodecyltrimethylammonium bromide (DTAB) in KBr aqueous solution (0-6 mM) were determined by Doppler microelectrophoresis. At very low surfactant concentrations (up to 6 mM), zeta potential was quite constant and due to the ionized monomers (DTA+). Above 6 mM, zeta potential increased to a maximum at surfactant concentrations still below the critical micellar concentration (CMC). This increase could be explained by a formation of nonmicellar aggregates of DTAB. Then, above the CMC, zeta potential underwent an abrupt reduction, which was dependent qualitatively and quantitatively on KBr concentration, and which could be due to an increase of the number of counterions adsorbed on the micelle surface. Calculation of effective micellar charge from zeta potential gave the surface charge density. Comparing this value with the theoretical, obtained from geometrical considerations, a fraction of 0.29 of charged micellar headgroups was obtained when DTAB was in aqueous solution, which is consistent with the value obtained by conductivity measurements.     

Simulation of phase equilibria in lamellar surfactant systems

The coexistence of two lamellar liquid crystalline phases has been investigated by means of Monte Carlo simulations. The surfaces of the negatively charged bilayers formed by the surfactant molecules are modeled as planar infinite walls with a uniform surface charge density. Water is treated as a dielectric continuum, and only electrostatic interactions are considered. The counterions are mono- and divalent point ions, and their ratio is allowed to vary. Monovalent counterions lead to a repulsive osmotic pressure at all separations, while an attractive region exists when the counterions are divalent. In the latter case, one would expect a phase separation to take place, although it is not observed experimentally due to the limited stability of the lamellar phase at high water content. In a system with mixed counterions, however, the osmotic pressure exhibits a van der Waals loop under such conditions that two phases can coexist. A phase diagram is constructed, and the agreement with experimental data is excellent.     

The impact of multivalent counterions, Al3+, on the surface adsorption and self-assembly of the anionic surfactant alkyloxyethylene sulfate and anionic/nonionic surfactant mixtures

The impact of multivalent counterions, Al(3+), on the surface adsorption and self-assembly of the anionic surfactant sodium dodecyl dioxyethylene sulfate, SLES, and the anionic/nonionic surfactant mixtures of SLES and monododecyl dodecaethylene glycol, C(12)E(12), has been investigated using neutron reflectivity, NR, and small angle neutron scattering, SANS. The addition of relatively low concentrations of Al(3+) counterions induces a transition from a monolayer to well-defined surface bilayer, trilayer, and multilayer structures in the adsorption of SLES at the air-water interface. The addition of the nonionic cosurfactant, C(12)E(12), partially inhibits the evolution in the surface structure from monolayer to multilayer interfacial structures. This surface phase behavior is strongly dependent upon the surfactant concentration, solution composition, and concentration of Al(3+) counterions. In solution, the addition of relatively low concentrations of Al(3+) ions promotes significant micellar growth in SLES and SLES/C(12)E(12) mixtures. At the higher counterion concentrations, there is a transition to lamellar structures and ultimately precipitation. The presence of the C(12)E(12) nonionic cosurfactant partially suppresses the aggregate growth. The surface and solution behaviors can be explained in terms of the strong binding of the Al(3+) ions to the SLES headgroup to form surfactant-ion complexes (trimers). These results provide direct evidence of the role of the nonionic cosurfactant in manipulating both the surface and solution behavior. The larger EO(12) headgroup of the C(12)E(12) provides a steric hindrance which disrupts and ultimately prevents the formation of the surfactant-ion complexes. The results provide an important insight into how multivalent counterions can be used to manipulate both solution self-assembly and surface properties.     

Aggregation and adsorption of sodium dioctylsulfosuccinate in aqueous ammonium chloride solution: role of mixed counterions

The critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (AOT) was determined at 25 °C from surface tension and fluorescence methods in aqueous NH(4)Cl solution for assessing the influence of mixed counterions on the special counterion binding behavior (SCB) of AOT. The SCB of AOT refers to a sudden twofold increase in the value of the counterion binding constant (β) in aqueous medium when the concentration (c(*)) of the added 1:1 sodium salt is about 0.015 mol kg(-1), and it has been tested so far for sodium ion only. In the presence of sodium and ammonium mixed counterions also the SCB of AOT exist, but with lower c(*) (0.009 mol kg(-1) NH(4)Cl). Synergism in the cmc occurs due to mixed counterions. In the case of inorganic counterions, unlike the case with organic counterions, the cmc is dependent on the total counterion concentration in solution and negligibly on the specific type of counterion. Na(+) and NH(4)(+) bind almost equally to the micelle in the region of low β (below c(*)), but in the region of high β (above c(*)) NH(4)(+) binds predominantly. It has been shown that the theoretical expression for the surface excess of ionic surfactant+electrolyte system containing a single counterion can also be used to evaluate the surface excess in the presence of mixed counterions if the two counterions are considered to undergo Henry-type adsorption at the air-solution interface.