Binding of counterions by micelles of ionic surfactants

The dependence of the concentration of free counterions on the overall concentration of an ionic surfactant is analyzed in terms of the quasi-chemical theory of micellization. The degree of binding of counterions by micelles of sodium dodecyl sulfate is determined from the results of potentiometric measurements performed using a sodium-selective electrode. It is disclosed that experimental and theoretical data are in satisfactory agreement, provided that the model used allows for the association of surface-active ions in a solution, with this association reducing the average coefficient of sodium dodecyl sulfate activity in an intermicellar medium.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 213–217.Original Russian Text Copyright © 2005 by Podchasskaya, Usyarov.     

Binding characteristics of ionic surfactants with human hair

Binding isotherms of two types of ionic surfactants, C_mH_{2m +1}SO₄Na (m = 8,10,12) and C_nH_{2n + 1}N⁺(CH₃)₃C1 (n=10, 12), to human hair in aqueous solutions were examined to clarify effects of hydrophobic and electrostatic interaction of ionic surfactants with hair. The binding isotherms of anionic surfactants showed cooperativity with discontinuously increasing shapes, while the binding isotherms of cationic surfactants showed a Langmuir-type, regardless of the difference of a hair condition.The calculated free energy change (— G@#@) for binding, obtained from Klotz' plots, suggests that the binding processes are governed mainly by a hydrophobic interaction, and bound surfactants probably expose their alkyl chains to the aqueous phase, since no-G was observed with the increase of m or n and values of enthalpy change(H) were positive or zero.     

Binding of uranyl by humic acid

The binding of tracer level UO₂⁺² to a soil humic acid was measured by a solvent extraction technique. The binding is interpreted as involving only the carboxylate groups of the humate and both 1:1 and 1:2 UO₂⁺²: CO₂⁻ binding is observed. Estimates based on these values indicate that uranyl complexing by humic and/or fulvic materials is not significant in sea water but may play a role in fresh water systems. Retention of uranyl from ground water by soil humics would be strong.     

Binding of calcium by humic acid

The binding of radiotracer ⁴⁵Ca to humic acid was studied by a solvent extraction technique. The dependence of binding on pH and temperature was determined. For an ionic medium of 0.1 M (NaClO₄), the binding constant varied from 2.25 ± 0.04 at pH 3.9 to 3.32 ± 0.04 at pH 5.0. Thermodynamic parameters of binding calculated from the temperature coefficient indicated that a large, positive entropy change accounts for the favorable free energy of complexation.     

Binding of solvent molecules by micelles of ionic surfactants

The distribution of molecules between the free (intermicellar) and the micelle-bound states is found from the results of selective measurements of their self-diffusion coefficients in micellar solutions of ionic surfactants, i.e., sodium dodecylsulfate and cetyltrimethylammonium bromide.     

Binding constant of thorium with gray humic acid

Humic acid sample was separated from the bottom sediments of Lake Quarun, in Egypt. It was purified and characterized by elemental analysis, potentiometric titration, IR, UV-visible and ¹³C NMR spectroscopies. The product of humic acid was very low (0.009%), gray in color and has low carboxylate capacity (2.4 meq/g). The first derivative of the titration curve indicated one maximum only, which implies one kind of carboxylate groups. The binding constant of ²³⁴Th with Lake Quarun humic acid was determined by solvent extraction. Only one parameter, β ₁, was required to fit the binding as a function of carboxylate concentration: the Th⁴⁺ bound to the carboxylate sites in the gray humic acid forming 1:1 complex only. The binding constant increased with the degree of ionization and with the pK_a of the humic acid.     

Binding of surfactants to hydrophobically modified polymers

Recent progress in the understanding of the binding of surfactants to hydrophobically modified polymers (HMP), and the consequences of such binding, is reviewed. HMP are water-soluble polymers onto which low proportions of hydrophobic sidechains (hydrophobes) have been grafted. In an aqueous environment, the HMP hydrophobes associate among themselves and with added surfactant molecules into micelle-like aggregates. An HMP may therefore be considered as a ‘modified surfactant’, and the binding of surfactants to HMP is analogous to the mixed micellisation in mixed surfactant solutions. The binding isotherm gives the concentration of free (monomeric) surfactant and the stoichiometry of the HMP/surfactant complex at different total compositions. In mixtures involving ionic surfactants, it is found that the free surfactant often dominates, and gives important contributions to the ionic strength. Characteristic properties of HMP/surfactant mixtures may be related to stoichiometries of the mixed complexes. Thus, the maximum in solution viscosity, which is commonly found in HMP/surfactant mixtures, occurs at a similar hydrophobe stoichiometry (ratio of bound surfactant to HMP hydrophobe) for many different systems, although the total concentrations of surfactant at the maximum may vary by orders of magnitude, depending on the surfactant cmc. The solubility of a complex of oppositely charged HMP and surfactant is related to the charge stoichiometry of the complex. The phase separation/redissolution phenomena occurring in the bulk solution influence the HMP adsorption to surfaces and the forces between surfaces with adsorbed HMP.     

Adsorption of ionic surfactants

Two adsorption models for ionic surfactants based on the Frumkin equation are examined to describe the measured surface tension isotherms of a series of alkali dodecylsulphates. In the model A the number of optimization parameters is reduced by additional modeling. The adsorption of counter-ions in the Stern layer is described via forming of ionic bonds, which free energy is significantly higher than that obtained by the model B. Concurrently the lateral interactions on the water/air interface are also found to be orders of magnitude stronger. Thus, the values of the adsorption parameters are more realistic, which supports the model A as a more relevant one.     

Binding of hexadecylammonium surfactants to water-soluble neutral polymers

The binding of hexadecyltrimethylammonium bromide and hexadecyldimethyl-2-hydroxyethylammonium bromide to neutral polymers was measured by a potentiometric titration method using surfactant selective electrodes. Binding to poly(vinyl alcohol) was slightly cooperative, while that to poly(ethylene oxide) lacked the co-operativity. Poly(vinyl pyrrolidone) did not bind them at all. Binding affinity as estimated by a distribution coefficient of the cationic surfactants between the bulk and polymer phases is about 2 orders of magnitude smaller than that of anionic sodium dodecyl sulfate. The heat of binding was estimated from the temperature dependence of the distribution coefficient and found to be endothermic. It is imagined that the cationic surfactants are simply partitioned between the aqueous bulk phase and the polymer coil phase which is regarded as aqueous organic mixed solvent.     

Fluoride binding to humic acid

Fluoride (F^–) binding to humic acid has been measured as a function of pH (5–6.6). The pH dependent binding is attributed to the anion being trapped within the large structure (territorial bound) but is not bound to a particular functional group (site bound). Studying fluoride binding provides insight to cation, anion and neutral species interactions with humic acid.     

Interactions between polyampholytes and ionic surfactants

The interactions of two partially charged ampholytic terpolymers [consisting of acrylamide, sodium 2-acrylamido-2-methylpropanesulphonate, and 2-(methacryloyloxyethyl)trimethylammonium chloride segments with molar compositions 80/12/08 and 80/08/12] and two fully charged ampholytic copolymers (containing only the two latter comonomers with molar compositions of 80/20 and 50/50), with cationic surfactants [tetradecyl- trimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB)] and the anionic surfactant sodium dodecylsulphate (SDS), are investigated. The studies include phase behaviour (swelling, solubilisation, precipitation), viscometry, electrical conductivity, and potentiometry (bromide ion and surfactant ion-specific electrodes). The 80/08/12 and 80/12/08 polyampholytes swell in water and are solubilised in the presence of cationic or anionic surfactants above a particular surfactant concentration that is proportional to the polymer concentration. The polyampolyte 80/20 is soluble in water but precipitates in the presence of TTAB, whereas 50/50 is insoluble in water and in the presence of TTAB, but is solubilised upon addition of SDS. The results indicate that TTAB binds to 80/12/08 with little or no cooperativity. Solubilisation appears to be the result of the increasing polyelectrolyte character of the polyampholyte upon neutralisation of its charged sites by bound surfactant ions of opposite charge. The binding of TTAB by the 50/50 polyampholyte is very weak and non-cooperative. In contrast, 80/20 binds TTAB cooperatively, much like a true polyelectrolyte-surfactant system of opposing charges. In particular, the binding is characterised by the existence of a critical aggregation concentration. A partial phase diagram for this system has been determined from the TTA⁺-electrode potential data. The behaviour of true polyelectrolytes and polyampholytes, with respect to their interaction with surfactants, is discussed. Received: 22 July 1998 Accepted: 14 September 1998     

Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK _a,1 ^int=6.7±0.2 and logK _a,2 ^int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK ₁ = 3.7±0.1 and pK ₂ = 6.6±0.1 and the total acidity was 4.56 mmol g⁻¹. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10⁻⁵ mol dm⁻³) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm⁻³ NaNO₃. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm⁻³ salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed. Received: 20 July 1999 /Accepted in revised form: 20 October 1999     

Binding on strong polyelectrolytes of mixed ionic and nonionic surfactants below their critical micelle concentration observed by fluorescence

The binding of mixed surfactants of cationic cetyltrimethylammonium bromide (CTAB) and nonionic octaethylene glycol monododecyl ether (C ₁₂E ₈) on anionic polyelectrolyte poly[2-acrylamido-2-methylpropanesulfonic acid (PAMPS)] and fluorophore-labeled copolymers containing about 40 mol% of AMPS was investigated at different mole fractions, Y , of CTAB in the surfactant mixture. The excimer emission of the cationic probe 1-pyrenemethylamine hydrochloride (PyMeA·HCl), nonradiative energy transfer (NRET) between pyrene and naphthalene labels and I ₁/ I ₃ of the pyrene label were determined by varying the total surfactant concentration, c _Surf. The I _E/ I _M value of PyMeA·HCl firstly increases and then decreases to 0 with c _Surf, showing a maximum on every curve. The critical aggregation concentration of the mixed surfactants determined from the I _E/ I _M maximum decreased from 5×10 ^-5 to 1×10 ^-5 mol/l as Y increased from 0.1 to 0.50, and then leveled off as Y increased up to unity. And at least 5×10 ^-6 mol/l CTAB was required for the mixed surfactants to bind on the PAMPS cooperatively. Equimolar binding of CTAB on AMPS was formed at I _E/ I _M=0 when Y =0.25, while at Y =0.1 some CTAB molecules in the mixed micelle were directed to the water phase without binding with AMPS. Both the intramolecular and the intermolecular NRET increased and then decreased with c _Surf, having a maximum on each curve corresponding to the equimolar binding of CTAB and AMPS so long as Y >0, indicating the coiling of the chain and interchain aggregation upon bound surfactants. The I _Py/ I _Np value at the maximum decreased with decreasing Y because more nonionic surfactant C ₁₂E ₈ participated into the polyelectrolyte-mixed surfactant complexes together with bound CTAB.     

Preparation of substituted ionic carbohydrate polymers and their interactions with ionic surfactants

The formation of micelles of hexadecyltrimethylammonium chloride (CTAC) and sodium dodecylsulfate (SDS) in aqueous solutions containing charged polysaccharides was studied by steady-state and time-resolved fluorescence measurements using pyrene as a photophysical probe. Micropolarity studies using the I₁/I₃ ratio of the vibronic emission bands of pyrene and the behaviour of the I_E/I_M ratio between the excimer and monomer emissions show the formation of hydrophobic domains. The interactions between the polyelectrolytes and surfactants of opposite charge lead to the formation of induced pre-micelles at surfactant concentrations lower than the critical micellar concentration (cmc) of the surfactants. At similar concentrations, the I_E/I_M ratio shows a peak. This aggregation process is assumed to be due to electrostatic attractions. At higher surfactant concentrations, near the critical micellar concentration, micelles with the same properties as those found in pure aqueous solution are formed. On the other hand, systems containing polyelectrolytes and surfactants of the same charge do not show this behaviour at low concentrations. The presence of long alkyl chains bound to the polyelectrolytes also induces the formation of free micelles at concentrations somewhat below the aqueous cmc.     

A simple adsorption model for ionic surfactants

In the past, few theoretical attempts have been made to describe quantitatively the adsorption of ionic surfactants at liquid interfaces. Well-known adsorption isotherms due to Frumkin or Hill–de Boer cannot respond to the specific electrostatic and geometric properties of the surfactant molecules. Our approach is based on a combination of the Gouy–Chapman theory with a modified Frumkin isotherm. The modification implies that the system is free to choose an optimal head group area and an optimal arrangement of the surfactant molecules in the interface as a function of bulk concentration. Interaction energies between neighbouring adsorbed surfactant molecules and between surfactant and water molecules are taken into consideration. The minimum of the Gibbs free energy of the system is equivalent to a minimal interfacial tension. Thus, the thermodynamically stable isotherm can be obtained as the lower envelope of the family of σ versus ln c isotherms resulting from different choices of the model parameters, including the area per molecule. According to the Gibbs equation, the Γ versus ln c adsorption isotherm is obtained as the derivative of this envelope. By variation of the model parameters, the envelope of the calculated adsorption isotherms can be fitted to experimental data of the interfacial tension versus bulk concentration. A computer program is used to calculate the σ versus c and the Γ versus ln c curves as well as to fit the parameters. Received: 28 October 1999/Accepted: 8 February 2000     

A new model to study the phase transition from microstructures to nanostructures in ionic/ionic surfactants mixture

The phase behavior and aggregate structures of mixtures of the oppositely charged surfactants cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) are explored at high dilution by pulsed field gradient stimulated echo (PFG-STE) NMR. The aggregation numbers and hydrodynamic radii of vesicles and mixed micelles were determined by a combination of viscosity and self-diffusion coefficient measurements. The average size of the mixed micelles was larger than that of micelles containing uniformly charged head groups. Analysis of the variations of the self-diffusion coefficient and viscosity with changing concentration of CTAB or SDS in the cationic-rich and anionic-rich regions revealed a phase transition from vesicles to mixed micelles. Differences in the lengths of the CTAB and SDS hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and precipitate phase), and vesicles form spontaneously over a wide range of compositions in both cationic-rich and anionic-rich solutions. The results obtained from conductometry measurements confirmed this transition. Finally, according to the capacitor model, a new model was developed for estimating the surface potentials and electrostatic free energy (g(elec)). Then we investigated the variations of electrostatic and transfer free energy in phase transition between mixed micelle and vesicle.     

Interaction of ionic surfactants with cornea-mimicking anionic liposomes

The interaction of surface-active molecules with lipid bilayers is ubiquitous both in biological systems and also in several technological applications. Here we explore the interaction of ionic surfactants with liposomes whose composition mimics the ocular epithelia. In this study, liposomes with a composition mimicking ocular epithelia are loaded with calcein dye above the self-quenching concentration. The liposomes are then exposed to surfactants, and the rate of dye leaked from the liposomes due to the interaction of surfactants is measured. Both cationic and anionic surfactants at various concentrations and ionic strengths are explored. Results show that the liposome bilayer permeability to the dye increases on exposure to the surfactants, leading to the release of the dye trapped in the core. However, the dye release stops after a finite time, suggesting a transient increase in permeability followed by healing. The leakage profiles exhibit two different timescales for the cationic surfactant but only one timescale for the anionic surfactant. The total dye leakage increases with surfactant concentration, and at a given concentration, the dye leakage is significantly higher for the cationic surfactants. The timescale for the healing decreases with increasing surfactant concentration, and increasing ionic strength increases the dye leakage for the anionic surfactant. These results show that the surfactant binding to the lipid bilayer increases the permeability while the bilayers heal likely because of the surfactant jump from the outer to the inner leaflet and/or rearrangement into tighter aggregates.     

A calorimetric study of three long-chain ionic surfactants

Differential enthalpies of solution in water of crystalline sodium dodecyl sulphate (SDS), sodium p-octylbenzenesulphonate (SOBS), and hexadecyltrimethylammonium bromide (CTAB) have been measured as a function of concentration at temperatures between 25 and 50 °C. The concentration change was small in the experiments and the results give a good approximation to the partial molar enthalpy content of the surfactant in the monomer and micellar states relative to the crystalline state. The enthalpies of dissolution to give monomers showed a strong, linear increase with temperature for SDS and SOBS and a nearly linear increase for CTAB, while the enthalpy of dissolution to give micelles was constant between 25 and 50 °C for the first two surfactants and only slowly increased for CTAB. Partial molar heat capacities were derived for monomeric and micellar SDS and SOBS. The large positive partial molar heat capacities of the monomeric surfactants are characteristic for hydrophobic solutes and the large heat capacity change for micelle formation arises from the loss of hydrophobic hydration in the formation of micelles.Results of microtitration experiments at 25 ° C show that the micelle formation of CTAB is not a simple aggregation process, but indicate a secondary process taking place closely after the critical micelle concentration (CMC).     

Solubilities of ionic surfactants in aqueous polymer solutions

Summary Solubility of ionic surfactants in water have been found to increase with added amount of nonionic hydrophilic polymers when both are capable of forming watersoluble complexes. Experiments were carried out with sodium hexadecyl sulfate (SHS) and hexadecyl amine hydrochloride (HAC) below theKrafft points of the respective surfactants. The solubility increment of the surfactants brought about by the polymers depends upon the kind of polymers, their molecular weight, concentration and temperature. The results have been interpreted on the basis of the amount of surfactants bound to the polymers and by the properties of the polymer-surfactant complexes formed. It is noteworthy that the anionic surfactant (SHS) can be bound to polymers much more extensively than the cationic surfactant (HAC).

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Zusammenfassung Die L?slichkeit von ionischen Benetzern in Wasser w?chst mit dem zugefügten Betrag an nicht-ionischen hydrophilen Polymeren, wenn beide zusammen wasserl?sliche Komplexe büden k?nnen. Versuche wurden mit Natrium-hexadecyl-sulfat (SHS) und Hexadecylamin-hydrochlorid (HAC) ausgeführt, und zwar unter denKrafft-Punkten der bezüglichen Benetzer. Das L?slichkeitsinkrement der Benetzer, gegeben durch die Polymeren, h?ngt von der Art des Polymeren, dessen Molekulargewicht, Konzentration und Temperatur ab. Die Ergebnisse werden interpretiert auf der Grundlage des Betrages an Benetzer, der an das Polymere gebunden ist, und durch die Eigenschaften des Komplexes Polymer/Benetzer. Es ist bemerkenswert, da? der anionische Benetzer (SHS) viel st?rker an das Polymere gebunden werden kann als der kationische Benetzer (HAC).