Surfactant Counterion Research Articles

Mixed monolayers of polyelectrolytes and surfactants at the air–water interface

Dilute mixed solutions of non surface active anionic polymers (polyacrylamide and polystyrene sulfonate) and various surfactants have been studied with several methods: surface tension, ellipsometry, surface and bulk rheology. A strong synergistic lowering of the surface tension is found with cationic surfactants in the concentration range where no appreciable complexation of surfactant and polymer occurs in the bulk solution (as seen from viscosity measurements). A simple adsorption model, in which it is assumed that both polymer and surfactant counterions are expelled away from the surface, is in good agreement with the observations on polyacrylamide sulfonate. The behaviour of the more hydrophobic polystyrene sulfonate is very different and still not understood. The behaviour of this polymer with nonionic surfactants is also different. The connection of foam stability with surface and bulk complexation is discussed.

Counterion Effects on Hexadecyltrimethylammonium Surfactant Adsorption and Self-Assembly on Silica

Combining optical reflectometry and atomic force microscopy (AFM), we have studied the effects of the surfactant counterion on the adsorption isotherms, kinetics, and layer structure for cationic hexadecyltrimethylammonium (C16TA+) surfactants on negatively charged silica surfaces. The adsorption kinetics suggest that the adsorption mechanism changes at the critical micelle concentration (cmc). A change in mechanism is also suggested by differences observed in the state of interfacial self-assembly on either side of the cmc. Above the cmc, increasing the binding affinity of the counterion (from chloride to bromide) increased the surface excess concentration by approximately 60% and changed the structure of the adsorbed surfactant layer from aggregates with circular projections to wormlike micelles. The addition of 10 mM KCl or KBr increased the surfactant surface excess concentration for both counterions. Below the cmc, the counterion has only a small effect on the structure of the adsorbed layer, and the...

Influence of counterions on the interaction of pyridinium salts with model membranes.

The interaction of pyridinium salts (PS) with red blood cells and planar lipid membranes was studied. The aim of the work was to find whether certain cationic surfactant counterion influence its possible biological activity. The counterions studied were Cl-, Br-, I-, ClO4-, BF4- and NO3-. The model membranes used were erythrocyte and planar lipid membranes (BLM). At high concentration the salts caused 100% erythrocyte hemolysis (C100) or broke BLMs (CC). Both parameters describe mechanical properties of model membranes. It was found that the efficiency of the surfactant to destabilize model membranes depended to some degree on its counterion. In both, erythrocyte and BLM experiments, the highest efficiency was observed for Br-, the lowest for NO3-. The influence of all other anions on surfactant efficiency changed between these two extremities; that of chloride and perchlorate ions was similar. Some differences were found in the case of BF4- ion. Its influence on hemolytic possibilities of PS was significant while BLM destruction required relatively high concentration of this anion. Apparently, the influence of various anions on the destructive action of PS on the model membrane used may be attributed to different mobilities and radii of hydrated ions and hence, to different possibilities of particular anions to modify the surface potential of model membranes. This can lead to a differentiated interaction of PS with modified bilayers. Moreover, the effect of anions on the water structure must be taken into account. It is important whether the anions can be classified as water ordering kosmotropes that hold the first hydration shell tightly or water disordering chaotropes that hold water molecules in that shell loosely.

Micelle Formation of Sodium 1-Decanesulfonate and Change of Micellization Temperature by Excess Counterion

Aqueous solution properties of sodium 1-decanesulfonate have been studied at 308.2 K by electroconductivity, membrane potential, and photochemistry in order to determine the micellization parameters for the mass-action model: the micellization constant Kn, the micellar aggregation number n, and the number of counterions binding to a micelle m. The numerical values are ln Kn = 133, n = 26, and m = 16.6. These values account for the experimental concentrations of the surfactant ion and counterion to a concentration that is 3 times the critical micelle concentration (cmc). The micellization temperature (MT) or the Krafft temperature was determined to be 295.3 K from the temperature dependence of the cmc and of the aqueous solubility. The MT values shifted to higher temperature with increasing concentration of sodium chloride, and good agreement was found in the values evaluated from the difference between the aqueous solubility and the cmc in the presence of excess counterion.

Adsorption kinetics of ionic surfactants with detailed account for the electrostatic interactions: effect of the added electrolyte

We address the problem about the electro-diffusion-controlled adsorption from a submicellar solution of an ionic surfactant in the presence of a non-amphiphilic electrolyte (salt). We consider a disturbance of the surfactant adsorption layer, which is damped by the electro-diffusive transport of surfactant ions, counterions and coions. The evolution of the electric double layer with time is theoretically examined. The counterion adsorption (binding) is taken into account. Our purpose is to find the long-time asymptotic solution of the full electro-diffusion boundary problem. Assuming small perturbations the problem is linearized and a set of three differential operator equations is obtained. In the asymptotic case of long times these differential equations reduce to hypergeometric equations, whose solutions, fortunately, can be expressed in terms of elementary functions. As a result, explicit expressions for the characteristic relaxation times of the adsorptions of surfactant ions and counterions are obtained. Next we determine the characteristic relaxation time of surface tension, which is different from the adsorption relaxation time because of a specific contribution from the dynamic diffuse electric double layer. The thermodynamic parameters, entering the formula for the relaxation time, are expressed for various surfactant adsorption isotherms, those of Henry, Langmuir, Freundlich, Frumkin and van der Waals. The theory gives a quantitative interpretation of the experimental finding that the addition of salt accelerates the relaxation of the surface tension for the higher surfactant concentrations, but decelerates it for the lower surfactant concentrations. The results could be also useful for a theoretical interpretation of the processes in foams and emulsions under dynamic conditions.

Effects of reactive and non‐reactive counterion surfactants upon acid hydrolysis of hydroxamic acids

Effects of reactive and non‐reactive counterion surfactants upon acid hydrolysis of hydroxamic acids

Effects of reactive and non-reactive counterion surfactants upon acid hydrolysis of hydroxamic acids

Effects of reactive and non-reactive counterion surfactants upon acid hydrolysis of hydroxamic acids

Polymerization of Rodlike Micelles

A new polymerizable surfactant, cetyltrimethylammonium 4-vinylbenzoate (CTVB), has been prepared and investigated. In aqueous solution, highly entangled rodlike aggregates are formed at millimolar surfactant concentrations. Free-radical polymerization of the surfactant counterions results in nearly complete conversion to a stable, nonviscous solution. Small-angle neutron scattering reveals that the micellar diameter of 40 A is unchanged during the polymerization process and that the final polymerized micelle length (400−1200 A) is determined by the relative initiator concentration. In addition, the parent rodlike micellar aggregates rearrange into spherical micelles at elevated temperature, whereas the polymerized structures are insensitive to temperature changes or dilution.

Fluorescence Probing Investigations of the Mechanism of Formation of Organized Mesoporous Silica

Spectrofluorometry (pyrene and dipyrenylpropane emission spectra) and time-resolved fluorescence quenching have been used to study the effect of addition of a large excess of sodium hydroxide and sodium silicate on the properties of micelles of cetyltrimethylammonium bromide and chloride (CTAB and CTAC). The systems investigated had overall compositions similar to those used when preparing organized mesoporous silica. Our purpose was to test whether the surfactant counterions are effectively exchanged by silicate ions and micelles grow in the presence of silica. The results show the CTAC or CTAB micelle micropolarity is not affected by the presence of the additives. The pyrene lifetime measurements with the CTAB systems indicate that only a small fraction of micelle-bound bromide ions, about 16%, is exchanged by hydroxyl and silicate ions even when they are largely in excess. The presence of the two additives was found to increase very little the micelle aggregation number in CTAB- and CTAC-containing systems, indicating that there is hardly any micelle growth under the experimental conditions used. Last the micelle microviscosity is also little modified by the effect of the additives. These results call for a reexamination of some of the assumptions underlying models used to describe the synthesis of these extremely interesting materials.

Backward extraction of reverse micellar encapsulated proteins using a counterionic surfactant

The back-extraction of proteins encapsulated in AOT reverse micelles was performed by adding a counterionic surfactant, either TOMAC or DTAB. This novel backward transfer method gave higher backward extraction yields compared to the conventional method with high salt and high pH of the aqueous stripping solution. The protein activity was maintained in the resulting aqueous phase, which in this case had a near neutral pH and low salt concentration. A sharp decrease of the water content was observed in the organic phase corresponding to protein back-extraction using TOMAC. The backward transfer mechanism was postulated to be caused by electrostatic interaction between oppositely charged surfactant molecules, which lead to the collapse of the reverse micelles. The back-extraction process with TOMAC was found to be very fast; more than 100 times faster than back-extraction with the conventional method, and as much as 3 times faster than forward extraction. The formation of 1:1 complexes of AOT and TOMAC in the solvent phase was observed, and these hydrophobic complexes could be efficiently removed from the solvent using adsorption onto Montmorillonite in order for the organic solvent to be reused. A second cationic surfactant, DTAB, confirmed the general applicability of counterionic surfactants for the backward transfer of proteins.

Solvatochromism in aqueous micellar solutions: effects of the molecular structures of solvatochromic probes and cationic surfactants

Solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RB); 1-methyl-8-oxyquinolinium betaine (QB); sodium 1-methyl-8-oxyquinolinium betaine-5-sulfonate (QBS); and 1-methyl-3-oxypyridinium betaine (PB) was studied spectrophotometrically in micellar solutions of the following cationic surfactants: cetyltrimethylammonium chloride, cetyldimethylbenzylammonium chloride, dodecyltrimethylammonium chloride, and dodecyldimethylbenzylammonium chloride. Microscopic polarity of water at the (average) solubilization site of the solvatochromic probe, ET in kcal mol-1, was calculated from the position of the longest-wavelength absorption band of the probe. The visible spectrum of PB, the most hydrophilic probe, is not affected by surfactants because it is not included in the micellar pseudo phase. For the other three solvatochromic probes, calculated ET values depend on the structures of both the probe and the surfactant, namely, its headgroup and long-chain alkyl group. RB, the most hydrophobic probe, samples a much lower microscopic polarity than QB and QBS because it penetrates deeper into the cationic micelle. This conclusion has been confirmed by 1H NMR. Polarities measured by (zwitterionic) QB and (anionic) QBS differ because the latter probe exchanges with the surfactant counterion. Calculated ET values refer to micelle-bound probes and are, therefore, different from those reported in the literature, typically determined at [surfactant] ⩽0.05 mol L-1. Effective water concentrations at the solubilization sites of these solvatochromic probes has been calculated by using as references mixtures of water with each the following organic solvents: n-propanol and dioxane (RB); ethanol, n-propanol, acetonitrile and dioxane (QB and QBS).

Separation and purification of periplasmic cytochrome c553 using reversed micelles

Surfactant concentration, ionic strength, and pH were optimised for the selective separation and purification of periplasmic cytochrome c553 from recombinant E. coli TG2 cells using response surface methodology. Back-extraction was accomplished using counter-ionic surfactant addition. Optimum forward extraction conditions were: 65 mM bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT), 0.07 M NaCl, and pH 8.4, while the optimum back-extraction conditions were 80 mM trioctylmethylammonium chloride, 0.85 M KCl, and pH 9.62. In comparison to a conventionally purified sample using column chromatography (10 mg cytochrome c553 l−1 with a purity of 0.66), reverse micelles achieved the same concentration and similar purity (0.50) in only two simple steps.

Surface Tension and Surface Potential of Na n-Dodecyl Sulfate at the Air−Solution Interface: Model and Experiment

Surface potential vs concentration isotherms of Na n-dodecyl sulfate (SDDS) adsorbed at the air−solution interface, measured using the vibrating plate method at various concentrations of added salt, exhibit a pronounced minimum. The results of surface tension measurements indicate that the minimum occurs within the concentration range that corresponds to the transition from the Henry regime of adsorption for low surface coverages to the one typical for adsorption of amphiphiles at high surface coverages. We proposed a simple model of adsorption of ionic surfactants at air−fluid interfaces based on the assumption that surfactant headgroups and counterions can adsorb in the Stern layer at the same Helmholtz plane. The electric potential in the electric double layer was calculated according to the Gouy−Chapman model for the diffuse part of the double layer and a modified Stern model for the inner layer with corrections for the discrete charge effects. The total potential drop across the interface was assumed to consist of two contributions: (i) the potential drop in the diffuse and compact double layers, negative for n-alkyl sulfate ions adsorbed at the air−solution interface, and (ii) a positive contribution due to the effective dipole moment of adsorbed surfactant molecules attributed mainly to the terminal CH3 groups. Our model correctly describes the dependence of the surface tension and surface potential of SDDS solution on its concentration and the amount of added salt.

Block Ionomer Complexes from Polystyrene-block-polyacrylate Anions and N-Cetylpyridinium Cations

Insoluble nonstoichiometric and stoichiometric complexes were formed as a result of reaction between polystyrene-block-polyacrylate anions (PS-b-PAA-) and N-cetylpyridinium cations (C16Py+) in aqueous media. In the presence of water the nonstoichiometric complexes with low content of surfactant represent swollen materials that contain polystyrene fragments, PAA units neutralized by surfactant, and ionized PAA- chains capable of ion exchange reactions involving regular inorganic and surfactant counterions. In the solid state these complexes represent a special class of complex ionomer material containing a segregated crystalline domain from C16Py+ bound to the polyion, sodium−carboxylate ion pairs, and an amorphous polystyrene segment. Changing the polyion and surfactant ratio as well as lengths of the block ionomer segments can vary the microstructure of such materials and their properties.

Mechanisms of amino acid partitioning in cationic reversed micelles

The aim of this work is to discuss the mechanisms involved in amino acidsolubilization in cationic reversed micelles. A simple mechanism was assumedin which the amino acid solubilization is mediated by an ion-exchangeprocess between the amino acid and the surfactant counter ion neglecting theeffect of the reversed micellar structure. Based on this mechanism a simplemodel to predict equilibrium was developed and applied to the solubilizationof amino acids with different structures. It was found that solubilizationof hydrophilic and slightly hydrophobic amino acids can be described by anion-exchange mechanism and the amino acid equilibrium concentration can bedetermined for different experimental conditions using this model. However,solubilization of hydrophobic amino acids can not be described by a simpleion-exchange model. In this case hydrophobic contributions play an importantrole in amino acid solubilization and must be considered in the overallsolubilization process. This hydrophobic contribution was evaluated bydetermination of an interfacial partition coefficient. The overall aminoacid extraction was determined using distribution coefficients of all theamino acid forms and considering their dependence on ionic strength.

Adsorption Kinetics of Ionic Surfactants with Detailed Account for the Electrostatic Interactions

The problem of diffusion-controlled adsorption from a non-micellar solution of an ionic surfactant in the absence of added electrolyte is solved analytically for the case of small deviations from equilibrium. For that purpose the electro-diffusion equations of the transport of surfactant ions and counterions are combined with the Poisson–Boltzmann equation for the electrical field. The resulting set of equations is linearized and Laplace transform is applied. Analytical expression for the Laplace image of the adsorption is obtained in terms of elementary functions. Simple formulae for the short-time and long-time asymptotics of adsorption and surface tension relaxation are derived. To illustrate the effect of the electrostatic interactions we calculated the theoretical dependence of the characteristic relaxation time on the bulk surfactant concentration and surface potential for aqueous surfactant solutions in contact with various non-aqueous phases (air, heptane, decane, petroleum ether) and two surfactants: SDS and DTAB. The general trend is that the electrostatic effects decelerate the process of adsorption, as it could be expected. The derived exact analytical expressions quantifying these effects can be directly applied for the interpretation of experimental data for the kinetics of ionic surfactant adsorption. The reliability of our approach is verified through a comparison with other available theories.

Microemulsions as reaction medium for surfactant synthesis

Various microemulsion formulations were evaluated as reaction media for synthesis of the surface active compound decyl sulfonate from decyl bromide and sodium sulfite. The reaction rate was fast both in water-in-oil and in bicontinuous microemulsions based on nonionic surfactants. Two-phase systems with added phase transfer agent (quaternary ammonium salt or crown ether) were much less efficient. It is postulated that the low efficiency of the phase transfer agents in catalyzing the reaction is caused by strong ion pair formation between the product formed, decyl sulfonate and the phase transfer agent. To prove this point, decyl bromide was reacted with two other nucleophiles, sodium cyanide and sodium azide. Neither of these gave a reaction product that could form an ion pair with the phase transfer agent. With these reagents, phase transfer catalysis was almost as efficient as synthesis in a microemulsion. It was also demonstrated that the rate of decyl sulfonate formation in microemulsion could be increased further by the addition of a small amount of cationic surfactant. However, the choice of surfactant counterion is decisive concerning the effect on reaction rate. Whereas a small nonpolarizable ion such as acetate gives a considerable reaction rate increase, a large polarizable ion such as bromide slows down the reaction. Bromide is believed to interact so strongly with the interface that it prevents the reacting ion, sodium hydrogen sulfite, from reaching into the interfacial zone.

Counterion Effects on the Sorption of Cationic Surfactant and Chromate on Natural Clinoptilolite

We determined the effect of selected counterions (Cl-, Br-, and HSO4-) on the sorption of the cationic surfactant hexadecyltrimethylammonuium (HDTMA) on clinoptilolite zeolite and on the subsequent sorption of chromate by HDTMA−zeolite. The HDTMA sorption on the zeolite, as characterized by the Langmuir sorption maximum, followed the trend HDTMA-Br > HDTMA-Cl > HDTMA-HSO4 (208, 151, and 132 mmol/kg, respectively). The same counterion trend was observed for HDTMA sorption on KGA-1 kaolinite. Measurement of counterion sorption indicated that HDTMA-Br and HDTMA-Cl formed complete bilayers on the zeolite, whereas HDTMA-HSO4 showed less than full bilayer formation. Competitive sorption between HDTMA-Br and HDTMA-Cl on the zeolite also showed a preference for the Br- counterion. The counterion stabilization of HDTMA admicelles on the zeolite surface follows the same trends as the counterion stabilization of micelles in solution. Chromate sorption was also strongly influenced by the HDTMA−zeolite counterion, with chromate sorption maxima decreasing in the order HDTMA-HSO4 > HDTMA-Cl > HDTMA-Br (28, 16, and 11 mmol/kg, respectively). The sorption of chromate and other divalent anions on HDTMA−zeolite results from a combination of entropic, Coulombic, and hydrophobic effects, all of which are functions of the initial HDTMA counterion. In the design of surfactant-modified clays and zeolites for environmental applications, the strong influence of the surfactant counterion must be considered.

A New Approach to Micellization Parameters: Its Application to Sodium Dodecyl Sulfate Micelle

A new approach to determine the three micellization parameters for ionic surfactants, the micellization constant Kn, the micelle aggregation number n, and the number of counterions per micelle m, was developed by combination of electric conductivity and either surfactant ion or counterion concentration. The approach employed two slopes of the plots of specific conductivity against surfactant concentration below and above the critical micelle concentration and the mass action model of micelle formation. This approach can be used to determine the other two parameters when the n value is available. This method was applied to the aqueous solution of sodium dodecyl sulfate whose micellization parameters have been precisely determined and was found to be quite successful to obtain the three parameters.

Interaction of Magnesium and Cadmium Dodecyl Sulfates with Poly(ethylene Oxide) and Poly(vinylpyrrolidone): Conductance, Self-Diffusion and Fluorescence Probing Investigations

The interaction of magnesium and cadmium dodecyl sulfates (Cd(DS)2and Mg(DS)2) with poly(ethylene oxide) and poly(vinylpyrrolidone) (PEO and PVP) has been investigated in order to assess the effect of the surfactant counterion on the interaction. Electrical conductance, spectrofluorometry, and time-resolved fluorescence quenching were used to determine the critical micellar concentration (cmc), micelle ionization degree, surfactant binding range, and micelle aggregation numbers. Electrical conductance and cation self-diffusion measurements in solutions of NaCl, Cd(NO3)2, and Mg(ClO4)2were used to study the binding of the cations to the same polymers. The results show that the interaction between dodecyl sulfates with divalent counterions and PEO or PVP and the interaction between sodium dodecyl sufate and the same polymers are qualitatively similar. The surfactant aggregation begins at a concentration lower than the cmc of the surfactants investigated, the aggregation numbers of bound aggregates are lower than those for the corresponding free micelles and increase with surfactant concentration; and the bound aggregates are more ionized than the free micelles. The decrease in the aggregation number of the bound aggregates with increasing temperature was larger with PEO than with PVP, a result that can be explained in terms of the increasing hydrophobicity of PEO with temperature. In terms of bound charges per polymer repeat unit, the interaction of Cd2+and Mg2+ions with PEO or PVP is stronger than that of Na+ions with the same polymers. The interaction strength thus follows the same sequence as the counterion degree of binding to free micelles. However, the values of the free energy of transfer of a dodecylsulfate ion from a free micelle to a polymer-bound aggregate are not very different with Na+and divalent metal counterions, and are extremely small. The results obtained with the salt/polymer systems confirm the binding of cations by PEO and PVP in aqueous solutions.