Divalent Counterions Research Articles

Selective divalent/monovalent ion partitioning in cation exchange membranes

The selective partitioning of divalent and monovalent counterions through cation exchange membranes immersed in 2:1 and 1:1 binary salt solutions remains unclear, especially at the molecular level. We present a theoretical framework that quantifies these membranes’ sorption of divalent and monovalent ions, incorporating condensed and free variants to reconcile deviations from the ideal Donnan picture observed in published experimental data. This framework explores the membranes’ global and local selective ion sorption characteristics by assessing the effects of competitive ion condensations, where these counterions compete to bind to the fixed charge groups. First, we calculate the thermodynamic property—the global partition ratio—of the membranes immersed in these binary salt solutions, quantifying the partitioning between the binary salt solutions and the membranes. Our theoretical results successfully capture the published experimental partitioning data for membranes immersed in equimolar and non-equimolar 2:1 and 1:1 binary salt solutions. Furthermore, the Donnan potential of the membranes reciprocally changes with the total concentration of 2:1 and 1:1 external salt solutions. However, for the non-equimolar systems at a constant low total concentration, varying the binary solution concentration ratio of 2:1 to 1:1 salts maintains a strong electric potential intensity due to high ionic imbalance. Second, we quantify the local partitioning of the sorbed counterions that are condensed on the crosslinked polymeric chains and those that are mobile in the interstitial solution of the membrane, a variable that is difficult to access experimentally. Accordingly, we can fractionate the sorbed ions into condensed and free states, which mutually govern the membrane’s kinetic and thermodynamic (global and local partitioning) properties.

Unexpected Two-Stage Swelling of Weak Polyelectrolyte Brushes with Divalent Counterions.

We use particle-based, coarse-grained simulations to study the influence of divalent counterions on a weak polyelectrolyte brush. Our simulations show a profound influence of even small concentrations of divalent salt on the titration behavior of the brush, which is shown to be a combined effect of electrostatic interactions and the Donnan effect. Furthermore, we examine the partitioning of mono- and divalent counterions into the brush. We demonstrate the preferred uptake of divalent ions by the brush, which is further enhanced by electrostatic correlation effects. Finally, our simulations reveal a hitherto unobserved two-stage swelling of the brush as a function of the pH in the presence of divalent salt. This phenomenon arises as a consequence of charge regulation and ion partitioning.

Responses of assembled structures of block polyelectrolytes to electrostatic interaction strength.

In this paper, the responses of assembled behaviors of block polyelectrolytes (PEs) to the strength of electrostatic interactions are studied through molecular dynamic simulations. The results show that the assembled structures closely depend on the electrostatic strength. It should be noted that PE coacervation can outweigh the nucleation of hydrophobic blocks and invert the micelle structures at strong electrostatic strengths, leading to the formation of inverted micelles of PE cores and hydrophobic coronas. In the poor solvent condition for neutral block, diverse anisotropic micelles are presented; candy-like conventional micelles of hydrophobic cores and PE patches coexist with inverted candy-like micelles of PE cores and hydrophobic patches and with Janus micelles of semi-neutral aggregate and semi-PE cluster in the presence of divalent and trivalent counterions. The formation of conventional or inverted micelle is largely determined by the type of micellar fusion, which results from the nucleation competition between electrostatic correlation and hydrophobic interaction. The merge of micelles mediated by hydrophobic attraction leads to conventional hydrophobic cores, and the fusion induced by electrostatic correlations results in PE cores micelles. At strong electrostatic strengths, the PE chains exhibit rich conformations at trivalent counterions, ranging from a fully collapsed state to a rod-like state, and parallel alignment of PE chains is found.

Thickening supercritical CO2 at high temperatures with rod-like reverse micelles

Earlier studies demonstrated the ability of some fluorinated surfactants to form rod-like reverse micelles with the ability to thicken water/supercritical CO2 (scCO2) mixtures at temperatures below 45 ºC [Langmuir 26 (2010) 83–88. Soft Matter 8 (2012) 7044–7055. Colloids and Surfaces B, 168 (2018), 201–210.]. Such viscosity enhancement of scCO2 is known to increase sweep efficiency for oil recovery with CO2 flooding. However, temperatures of up to ∼100 ºC in conventional reservoirs are much higher than those employed in laboratory studies, and tend to weaken inter- and intra-molecular interactions between surfactant molecules, discouraging rod-like reverse micelle formation. With the aim of designing surfactants which form rod-like reverse micelles and thicken CO2 at high temperatures, this study examined phase behavior, nanostructures of reverse micelles and thickening ability of double ω-hydroperfluorocarbon-tail anionic surfactants in W/scCO2 mixtures at temperatures of 35 - 75 ºC and pressure of 80 - 400 bar with different water-to-surfactant molar ratios (W0). The measured CO2 viscosity increased by 1.9–2.2 × for double-chain surfactants M(di-HCF6)x (counterion Mx+ = Ni2+ and Co2+) at 40 mM, over the experimental temperature range. On the other hand, the shorter chain H(CF2)4CH2 twin-tail surfactants M(di-HCF4)x and Na(di-HCF6) gave only 1.1–1.5 × viscosity enhancements. The maximum thickening ability of M(di-HCF6)2 was at W0 = 10 in the W0 range of 5–20 75 ºC and 350 bar. High pressure and high temperature small-angle neutron scattering (SANS) was used determine the micellar structure in these systems, and rod micelles of aspect ratios of 4.5–6.5 were found. The results clearly suggest that ω-hydroperfluorohexyl-tails and divalent counterions induce the formation of rod-like reverse micelles in W/CO2 mixtures, even at high temperatures commensurate with in-reservoir conditions.

Coion exclusion properties of cation exchange membranes in 2:1 divalent salt solutions

The selectivity of ion exchange membranes hinges on their capacity to exclude coions. Yet, understanding how they reject coions in multivalent salt solutions remains challenging. This study presents a new molecular theory to understand coion exclusion. It examines counterion and coion condensation, including electrosteric effects. Supported by experimental data [Gokturk et al., Nat. Commun., 13 (2022)], we analyze cation exchange membrane potential in a 2:1 salt solution, revealing insights into coion selectivity and exclusion mechanisms. Notably, our theory deviates from earlier research by accommodating condensed divalent counterions that bind to one or two fixed-charged monomers. Additionally, we account for many-body electrostatic interactions, allowing the former to bind to sorbed coions. Our findings highlight the importance of coion condensation, especially in low-concentration salt conditions where coions are mainly condensed. This effect becomes less significant as the external solution concentration rises. These findings clarify the suitability of the ideal Donnan model in high-concentration salts, but its accuracy decreases in low-concentration situations. To summarize, our study emphasizes the need to account for counterion and coion condensation to understand membrane selectivity in low-concentration multivalent salt solutions.

The conformational phase diagram of charged polymers in the presence of attractive bridging crowders.

Using extensive molecular dynamics simulations, we obtain the conformational phase diagram of a charged polymer in the presence of oppositely charged counterions and neutral attractive crowders for monovalent, divalent, and trivalent counterion valencies. We demonstrate that the charged polymer can exist in three phases: (1) an extended phase for low charge densities and weak polymer-crowder attractive interactions [Charged Extended (CE)]; (2) a collapsed phase for high charge densities and weak polymer-crowder attractive interactions, primarily driven by counterion condensation [Charged Collapsed due to Intra-polymer interactions [(CCI)]; and (3) a collapsed phase for strong polymer-crowder attractive interactions, irrespective of the charge density, driven by crowders acting as bridges or cross-links [Charged Collapsed due to Bridging interactions [(CCB)]. Importantly, simulations reveal that the interaction with crowders can induce collapse, despite the presence of strong repulsive electrostatic interactions, and can replace condensed counterions to facilitate a direct transition from the CCI and CE phases to the CCB phase.

Divalent ion partitioning through dense ion exchange membranes

Understanding the mechanism behind the permeation of multivalent ions through ion exchange membranes remains an ongoing challenge. Here we address this knowledge gap by presenting a molecular theory to elucidate the influence of counterion condensation and electrosteric effects on the (multivalent) divalent counterion permeation through such membranes. Building upon published experimental data (Galizia et al., 2017), we analyze the energetic tendencies of divalent counterions to associate with the fixed charge groups and their entropic preferences to disassociate to gain translational freedom, allowing us to quantitatively assess the partitioning of divalent ions at two distinct levels. (i) The global partitioning between the external 2:1 salt solution and the membrane, and (ii) the local partitioning within the interstitial solution and the crosslinked polymeric chains within the membrane. Notably, our observations reveal that the global partitioning of divalent counterions does not differ when the membranes operate separately in magnesium chloride (MgCl2) and calcium chloride (CaCl2) solutions. However, at the local partitioning level, we identify a higher preference for calcium counterions to associate with the fixed charge groups than magnesium ions. This preference arises from calcium ions’ more favorable charge density (physical size), facilitating their association/pairing with the fixed charge groups. The key takeaway message from this work is that when dealing with multivalent ions, we must downlevel our investigation, looking at how the counterions relax between the interstitial solution and the crosslinked polymeric chains and, in return, comprehending how it ultimately controls the global transport property of these membranes (such as their Donnan potential and water uptake).

Sustainable power generation from salinity gradients by reverse electrodialysis: Influence of divalent ions

The performance of Reverse Electrodialysis (RED) stacks can be significantly degraded by the presence of divalent ions. To gain a deeper understanding of the influence of divalent ions, a RED model based on the Nernst-Planck framework was developed in this study. Using this model, a simulation study was performed when the feed solutions contain a mixture of NaCl and MgCl2. The impact of the divalent ion Mg2+ on RED performance was examined from the perspective of ion transport. The results indicate that divalent counterions decrease membrane permselectivity and membrane potential, while increasing membrane electrical resistance by affecting ion concentrations within the membrane. The uphill transport of divalent ions occurs only when the molar fraction of divalent ions in the feed solutions is relatively low. Specifically, when the feed solution contains only MgCl2 compared to only NaCl, the open-circuit voltage decreases by 36.2%, and the maximum gross power density decreases by 44.8%.

Effective Interactions between Double-Stranded DNA Molecules in Aqueous Electrolyte Solutions: Effects of Molecular Architecture and Counterion Valency.

A computational investigation of the effects of molecular topology, namely, linear and circular, as well as counterion valency, on the ensuing pairwise effective interactions between DNA molecules in an unlinked state is presented. Umbrella sampling simulations have been performed through the introduction of bias potential along a reaction coordinate defined as the distance between the centers-of-mass of pairs of DNA molecules, and effective pair interaction potentials have been computed by employing the weighted histogram analysis method. An interesting comparison can be drawn between the different DNA topologies studied here, especially with regard to the contrasting effects of divalent counterions on the effective pair potentials: while DNA-DNA repulsion in short center-of-mass distances decreases significantly in the presence of divalent counterion-ions (as compared to monovalent ions) for linear DNA, the opposite effect occurs for the DNA minicircles. This can be attributed to the fact that linear DNA fragments can easily adopt relative orientations that minimize electrostatic and steric repulsions by rotating relative to one another and by exhibiting more pronounced bending due to the presence of free ends.

Dilution and packing of anionic liquid surfactant in presence of divalent and trivalent counter-ions

Alkyl ether carboxylates are part of a new class of ionic liquids based on the Concept of Melting Point Lowering due to Ethoxylation (COMPLET) and possess unique aggregation behaviour due to the simultaneous anionic and non-ionic nature of the surfactant anion. Previous publications on H[C8E8c] (Polyoxyethylene(8) octyl ether carboxylic acid) and its monovalent salts have shown a variety of phases in aqueous systems with a transition from small spherical micelles at high water contents towards a phase of interdigitated head groups at low water contents. In the present paper, we examine the shorter homologue H[C8E5c] (Polyoxyethylene(5) octyl ether carboxylic acid) and its di- and trivalent transition and rare earth metal salts using X-ray scattering methods in the small- and wide-angle range (SWAXS). These measurements confirm the presence of spherical micelles in diluted aqueous systems which aggregate and whose head groups interdigitate at higher concentrations of the isotropic, non-birefringent ionic liquid. Swelling experiments yield two-dimensional swelling of the microstructure, and combined with the scattering pattern, the data infer prolate spheroidal direct (o/w) micelles with interdigitated head groups, which align to form short linear chains. These chains are offset to form a localised hexagonal close-like packing of the micelles. Most interesting, within this packing, the metal cations are confined into channels between the micelles in linear chains with ion-ion distances of 0.4–1.1 nm. In particular, in the water-free ionic liquid, this confinement may be responsible for some of the unique properties of these systems. It is suggested that the confinement of the cations leads to strong ion-ion aggregation that could be at the origin of the very peculiar magnetic properties of these simple one-component liquids, which contrasts with classical ferrofluids, in which magnetic nanoparticles are dispersed in another liquid.

Counterion-Dependent Material Properties of Phosphorylated Nanocellulose.

The material properties of cellulose nanofibers (CNFs) are governed by the surface chemical structure of the fibers. The chemical structure-property relationships for monovalent carboxylated CNFs are well understood. Here, we report the basic sheet properties of divalent phosphorylated CNFs with different phosphorus contents and counterion types. All examined sheet properties, including conditioned and wet tensile properties, electrical resistivities, and fire-retardant properties of the CNF sheets, were greatly enhanced by the counterion exchange from the initial sodium ions to calcium or aluminum ions. The phosphorus content had significant influences only on the conditioned tensile and fire-retardant properties. In comparison to CNF sheets with monovalent carboxy groups, the CNF sheets with divalent phosphate groups were superior in terms of their wet tensile properties and fire-retardant properties. Our research shows that the combination of the divalent phosphate introduction and counterion exchange provides a successful strategy for the practical application of CNF sheets as antistatic materials and flexible substrates for electronic devices.

Induced ion correlations lead to DNA attraction in manganese.

Divalent counterions show the ion specificity effect. In a free solution, Mn2+ ions can condense DNA into a disorder bundle of structures while the same valence Mg2+ ions dissolve DNA assemblies. Theoretical models were proposed to explain the Mn2+-induced condensation of DNA with limited success. Here, we employ all-atom molecular simulations to investigate the ion specificity effect on DNA-DNA interactions. An exhaustive sampling of inter-helical distance and angular orientations afforded by metadynamics simulations allowed us to construct the free energy surface of the DNA-DNA interactions. In excellent agreement with experiments, we observe that dsDNA strands show attraction in the presence of Mn2+, while they remain repulsive at the same ionic strength in MgCl2. To explain the disparate behavior of the two divalent cations, we investigate cation distributions and preferential binding sites. We observe significant differences in binding modes between Mn2+ and Mg2+. The manganese ions bind prevalently to the phosphate backbone, while magnesium ions occupy both backbone and the major groove of the DNA. We develop a simple like-charge attraction model based on the preferential binding sites derived from simulations. Our model explains the observed differences between the two cations successfully. It highlights the importance of induced ion correlations between adjacent pairs as the mechanism leading to attraction. Our results shed light into Mn2+ induced like charge attraction mechanism of DNA. The method can be extended to study other condensing agents.

Quaternary ammonium salts modification preparing charged Janus nanofiltration membrane for the simultaneous separation of divalent anions and cations

Simultaneous separation of divalent co-ions and counter-ions from mixed ions water environment was urgently needed to improve separation efficiency and reduce energy consumption. The charged Janus nanofiltration membranes (NFMs) came into being based on the Donnan effect, however, its practical application was still limited by complex preparation, low permeability or poor ion selectivity. Here, with carboxylated polysulfone (PSF–COOH) ultrafiltration membrane as the support layer, a newly discovered quaternary ammonium salt containing diamine group (ethidium bromide, EB) was used to modify pristine polyamide nanofilm to prepare a novel charged Janus NFM. Structural characterization and molecular dynamics simulation indicated that the difference in electrostatic force of PSF-COOH with piperazine (PIP) and EB resulted in a difference of at least 20 times between their diffusion rates and hence achieved their regular distributions in the oil-water interface. As a result, EB modification induced the structure heterogeneity (charge and morphology) of the polyamide layer, which allowed the selectivity of both monovalent/divalent anions and cations to be higher than 45, while ensuring a permeability of up to 56.2 L/m2⋅h. Moreover, other analogues of EB were applied and proved the generality of the mechanism for preparing charged Janus NFMs in this study, which represented a method for the scalable preparation of ultra-selective membranes with heterostructure for simultaneous separation of divalent co-ions and counter-ions.

Tuning Structure and Rheological Properties of Polyelectrolyte-Based Hydrogels through Counterion-Specific Effects

Tuning at will the properties of gel-forming systems is of key relevance for many biotechnological, agricultural, and biomedical applications. For polyelectrolyte-based gels, ion-specific effects can be an attractive way for this purpose. This study investigates the counterion-specific effect on the microscopic structure and the rheological properties of a physical hydrogel formed of ionene-type cationic polyelectrolytes. The focus is on two monovalent halide counterions (F– and Cl–) and a divalent counterion (SO42–). A strong counterion-specific effect appears within ionene-based gels. In the case of halide counterions, gelation is more effective for more weakly hydrated counterions. Indeed, strongly hydrated counterions maintain electrostatic repulsions between the chains and as a consequence gel formation is shifted toward higher concentrations (higher critical gelation concentration, CGC). The combination of the complementary small-angle X-ray and neutron scattering (SAXS and SANS) techniques reveals a strong contribution of ion–ion correlations in the structure of the gel network. Contrary to chloride gels, which present a single correlation length characterizing the distance between the cross-linking nodes, fluoride gels present an additional network of nodes. This is accompanied by a very rapid increase of the elastic modulus of fluoride gels, once CGC is reached. With divalent counterions, the gelation is even more remarkable with a lower CGC and a higher elastic modulus at equivalent polyelectrolyte concentrations. The presence of divalent counterions favors the association of chains, probably by a bridging effect. This evokes the “egg-box” model, and the characteristic scaling of the elastic modulus with reduced gel concentration confirms this. However, only a narrow concentration window for gel-forming exists for divalent counterions before precipitation takes over due to too strong attractive chain–chain interactions.

Ion-induced changes in DNA gels.

Small angle neutron scattering (SANS) measurements are reported for DNA gels under near physiological conditions in which the concentration of monovalent and divalent counter-ions and the pH are varied. The scattering intensity I(q) is described by a two-term equation, one due to osmotic concentration fluctuations and the other coming from static inhomogeneities frozen in by the cross-links. SANS in the low q range indicates the presence of large clusters and the size of which exceeds the resolution of the experiment. In the intermediate q-range, the intensity increases with the CaCl2 concentration and the slope approaches -1, corresponding to linear (rod-like) scatterers. In the highest q region, the scattering response is governed by the local chain geometry. Screening of electrostatic interactions by sodium chloride causes a moderate increase in the SANS intensity that is accompanied by an increase in the mesh size L of the network. Addition of calcium chloride, or a decrease in pH, produces similar trends, and ultimately leads to phase separation. The scattering intensity at q = 0, estimated from independent measurements of the osmotic pressure Π, is in excellent agreement with I(0) from the SANS measurements. Anomalous small angle X-ray scattering (ASAXS) measurements on the uncross-linked DNA show that the monovalent ion cloud is only weakly influenced by the addition of divalent ions. Conversely, the divalent counter-ion cloud tightly follows the contour of polymer chains.

Quantifying the coupled monovalent and divalent ions sorption in dense ion-exchange membranes

As a universal phenomenon in solution-membrane systems, the coupled competitive monovalent and divalent ions sorption significantly impacts the transport process in ion-exchange membranes. Often, the quantitative and qualitative relationships between them are unclear. Here, we present a lattice-based molecular model to capture the interplay effect of ion-pairing and electrostatic interaction on the competitive partitioning process. In particular, we utilize statistical mechanics to formulate how the monovalent and divalent counterions competitively pair with the fixed charge group in the membranes. To this end, the model can quantify (a) the counterions sorption and (b) the coion repulsion within the membranes, which, collectively, allow us to quantify the ion partitioning and the pairing site saturation. To showcase the capability of our model, we compare our results with published experiment data, where we achieve a relatively good agreement between them. We demonstrate that the competitive ion partitioning depends highly upon (i) the entropy change generated by the counterions pairing with the fixed charge group, (ii) the binding energy, and (iii) the osmotic pressure, and (iv) the electrostatic interaction. These different factors can explain the favorable sorption of divalent counterions in the cation-exchange membranes but monovalent counterions in the anion ones, as previously reported. Altogether, our theoretical approach provides an insight into the fundamental understanding of coupled competitive monovalent and divalent ions sorption within the dense ion-exchange membranes, further enhancing the knowledge of the competitive ion partitioning process.

Rheology of saturated micellar networks: Wormlike micellar solutions vs. bicontinuous micellar phases

The subject of this work is to investigate the rheological behavior of mixed micellar solutions (sodium laurylethersulfate and cocamidopropyl betaine) in the presence of Mg2+ divalent counterions. With the rise of salt concentration, the viscosity of micellar solutions increases to a high maximum followed by a steep decrease because of the initial growth and entanglement of wormlike micelles and a subsequent transition to branched micelles forming of a saturated micellar network. The proposed systematic rheological measurements show considerable variations in the rheological responses of the solutions when increasing the salt concentration. Rheological behavior and data are used to distinguish the micellar phases and to study the relation to micellar structures. The wormlike micellar solutions have a typical shear thinning behavior with a well-defined zero-shear viscosity, η0, described by the Cates reptation-reaction model or the augmented Maxwell model. Our data show that the power law dependence of η0 on the surfactant concentration is stronger than that reported in the literature and it is influenced of the added electrolytes. The branched micellar structures are characterized by the lower viscosities and larger elasticities, which follow the Maxwell model up to the intermediate values of the frequency of oscillations, however peculiar deviations from the Cole-Cole plot at large frequencies are detected. The isolated bicontinuous micellar phases are Newtonian fluids with viscosity 0.4–0.7 Pa.s independent on the salt concentration up to high shear rates. The threshold salt concentration ensuring the onset of the bicontinuous micellar phase is described by a simple empirical rule. These phases are characterized by large elasticities and not negligible yield stresses. The property of the bicontinuous micellar phases to form spontaneously oil-in-water nanoemulsions could find applications in drug delivery, extraction and separation processes, pharmaceutics production, etc.

Thermoresponsiveness of Carboxylated Polyallylamines Induced by Divalent Counterions as Ionic Effectors

We describe novel ionic effector-induced lower critical solution temperature (LCST)-type thermoresponsive polymers in an aqueous system. Carboxylated polyallylamines (carboxylated PAAs) are conjugates between polyallylamine and various carboxylic anhydrides possessing six-membered rings such as phthalic anhydride and 1-cyclohexene-1,2-dicarboxylic anhydride. These polymers showed LCST-type thermoresponsiveness in the presence of divalent metal ions (Ca2+, Mg2+, or Sr2+) in aqueous media, where a reversible phase transition between soluble and insoluble states was observed with a change in the temperature, although these polymers did not show thermoresponsiveness without divalent ions or with monovalent ions (Li+, Na+, or K+). The cloud points of the carboxylated PAAs could be precisely controlled by not only salt concentration, pH, salt species, and side-chain structures but also the mixing ratio of different salt species.

Nanofiltration of complex mixtures: The effect of the adsorption of divalent ions on membrane retention

In nanofiltration (NF), solution and membrane chemistry determine the membrane charge density. In multi-component electrolytes solutions, it is important to consider the interaction between ions and NF membranes. For instance, in the presence of divalent ions, charge reversal can take place due to the interaction of counterions with the charged functional groups of the membrane. This paper presents a model based on state-of-the-art mean-field theory that includes the effect of divalent counterions on the membrane charge density. By using a Langmuir equation with two model parameters, we consider the adsorption of divalent ions (e.g., Ca2+ and Mg2+) in the polyamide active layer of the membrane and calculate the effective membrane charge density. Two different sets of experimental data from literature are used to evaluate the model. Contrary to the statement of a recent study on NF, we show that mean-field theory can predict the rejection of all types of ions in a multi-component electrolyte solution when the effect of divalent counterions on the membrane charge density is included. Moreover, the results in this study reinforce the idea that adsorption of counterions plays a fundamental role in the performance of nanofiltration.

Synergism and foam stability in a binary mixture of tea saponin derived from Camellia oleifera Abel and Sodium Lauryl Ether Sulfate

Sodium Lauryl Ether Sulfate, AES, is an anionic surfactant containing ethylene oxide groups, which is widely used in various cosmetic. In this work we investigate the synergism and foam stability in a binary mixture of tea saponin (TS) and AES to promote the development of new formulations and reduce the use of synthetic surfactants in commercial products. The effect of TS addition on interfacial behavior was investigated by interfacial tension (γ) and foam properties, and the interfacial activity was analyzed on different fluid/fluid interfaces at different environments. The results showed that the mixtures of TS with AES displayed synergism in critical micelle concentration (cmc), such as the interaction parameter and thermodynamic parameter. Low concentration of TS addition not only caused the decrease of γcmc, but also improved the stability of the foam. The stability of foams has been interpreted on the basis of bubble distribution size and contact angle. Further study revealed that the interfacial activity of the AES/TS mixture was stable at high temperatures, in the presence of mono (Na+) and divalent (Ca2+ and Mg2+) counterions, and at a pH of 3–11.