Thermodynamics of Cationic and Anionic Surfactant Interaction

A Tale of Two Tails: Tail Ordering of Stoichiometric 1:1 DTAB:SDS Pairs Adsorbed at the Oil-Water Interface

Cationic:anionic surfactant mixtures adsorbed at an oil-water interface stabilize foams in the presence of oil, making them essential to the oil, gas, and firefighting industries. The oil tolerance of foams stabilized by surfactant mixtures, relative to pure (unmixed) cationic and anionic surfactants, results from the mixtures' enhanced flexibility in tailoring the physicochemical properties of the interface. To judiciously employ these mixtures, it is necessary to characterize the structure-function property relationship of their surfactant monolayers that lend to oil-tolerant/intolerant foams. In this work, we employ interfacial tensiometry and vibrational sum frequency spectroscopy to determine the composition (surfactant population and cationic:anionic ratio) and the structure (surfactant alkyl tail conformation) of monolayers prepared at the oil-water interface by 1:1 DTAB:SDS (dodecyltrimethylammonium bromide:sodium dodecyl sulfate) mixtures. We show that the interfacial surfactant density of 1:1 DTAB:SDS mixtures greatly exceeds that of pure DTAB and SDS at similar concentrations up to and beyond their respective critical micelle concentration. The enhanced interfacial adsorption of these mixtures is due to the adsorption of stoichiometric 1:1 DTAB:SDS surfactant pairs that form through the attractive electrostatic interactions between surfactant headgroups. We find that these paired surfactants preferentially adsorb at the interface, causing the interfacial DTAB:SDS ratio to be nearly 1:1. Additionally, we find that the SDS tail is more conformationally ordered than the DTAB tail, even though they are expected to be conformationally identical along the entire tail, since they are likely conjoined through van der Waals interactions. This leads to the conclusion that the surfactant pairs are in a staggered arrangement at the interface. These findings help to uncover molecular factors that contribute to the enhanced oil tolerance, and in some cases oil intolerance, of foams stabilized by cationic:anionic surfactant mixtures.

Marangoni flows triggered by cationic-anionic surfactant complexation

HYPOTHESIS

The gradients in surfactant distribution at a fluid-fluid interface can induce fluid flow known as the Marangoni flow. Fluid interfaces found in biological and environmental systems are seldom clean, where mixtures of various surfactants are present. The presence of multi-component surfactant mixtures introduces the possibility of interactions among constituents, which may impact Marangoni flows and alter flow dynamics.

EXPERIMENTS

We employed flow visualization, surface tension and reaction kinetic measurements, and numerical simulations to quantitatively investigate the Marangoni flows induced by the reacting surfactant mixtures. Different binary surfactant mixtures were utilized for comparative analysis.

FINDINGS

The impact of surfactant interactions on Marangoni flows is confirmed through the observation of diverse complex flow patterns that result from the combination of oppositely charged surfactants in varying composition ratios and concentrations. Unique flow patterns originate from the composition-dependent interfacial phenomena upon mixing surfactants. Our findings provide vital insights that could be used to guide the development of effective oil remediation or the spreading of waterborne pathogens in contaminated regions.

Evaluating the Extraction and Quantification of Marine Surfactants from Seawater through Solid Phase Extraction and Subsequent Colorimetric Analyses

Surfactants are amphiphilic molecules that adsorb to interfaces and affect the interfacial tension. Surfactants in seawater can impact gas-exchange, surface properties, and the composition and fate of sea spray aerosol. The accurate quantification of surfactants and their classes is crucial to constraining the effect of surfactants in seawater and their role in air-sea exchanges. Here, we evaluate and optimize a solid phase extraction (SPE) method paired with colorimetry and UV-vis spectroscopy to quantify the concentrations of anionic, cationic, and nonionic surfactants in seawater. We compare tandem SPE with two-step SPE and different elution volumes and evaluate the impact of different interferents. Improved extraction efficiencies were obtained with an 8 mL acetonitrile elution and with separate ENVI-18 and ENVI-Carb extractions, instead of tandem. With complex surfactant mixtures, the presence of anionic surfactants interfered with the quantification of cationic surfactants and caused underestimations of up to 83%. Using a two-step extraction and analyzing each seawater SPE extract separately during colorimetric quantification help avoid the effects of interferents and ensure more representative quantification of surfactants. With this method, average seawater surfactant concentrations ranged from 0.04 to 0.06 μM. At the highest concentrations, the class composition comprised 23% anionic, 21% cationic, and 56% nonionic surfactants.

Intrinsic Solubility of Ionizable Compounds from pKa Shift

Aqueous solubility of pharmaceutical substances plays an important role in small molecule drug discovery and development, with ionizable groups often employed to enhance solubility. Drug candidate compounds often contain ionizable groups to increase their solubility. Recognizing that the electrostatically charged form of the compound is much more soluble than the uncharged form, this work proposes a model to explore the relationship between the pKa shift of the ionizable group and dissolution equilibria. The model considers three forms of a compound: dissolved-charged, dissolved-uncharged, and aggregated-uncharged. It analyzes two linked equilibria: the protonation of the ionizable group and the dissolution-aggregation of the uncharged form, with the observed pKa shift depending on the total concentration of the compound. The active concentration of the aggregates determines this shift. The model was explored through the determination of the pKa shift and intrinsic solubility of specific compounds, such as ICPD47, a high-affinity inhibitor of the Hsp90 chaperone protein and anticancer target, as well as benzoic acid and benzydamine. The model holds the potential for a more nuanced understanding of intrinsic solubility and may lead to advancements in drug discovery and development.

Base-triggerable lauryl sarcosinate-dodecyl sulfate catanionic liposomes: structure, biophysical characterization, and drug entrapment/release studies

Equimolar mixtures of oppositely charged single-chain amphiphiles form a variety of phases, including vesicles. Such catanionic mixed lipid systems show high stability and exhibit versatile physicochemical properties. In the present study we have investigated the aggregation behaviour of lauryl sarcosinate hydrochloride (LS·HCl) in aqueous dispersion as well as its interaction with the anionic surfactant sodium dodecyl sulfate (SDS). The CMC of LS·HCl was estimated to be ∼5 mM by isothermal titration calorimetry (ITC) and fluorescence spectroscopy using pyrene as the fluorescent probe. Turbidimetric and ITC studies on the interaction of LS·HCl with SDS demonstrated that the two surfactants form an equimolar catanionic complex. The crystal structure of the lauryl sarcosinate-dodecyl sulfate (LS-DS) complex revealed that the complex is stabilized by classical N-H⋯O as well as C-H⋯O hydrogen bonds, besides the electrostatic attraction between LS (cation) and DS (anion) and dispersion interactions between the hydrocarbon chains. Differential scanning calorimetry studies revealed that the phase transition of the equimolar LS-DS complex is significantly reduced compared to the analogous LG-DS and LA-DS complexes in the fully hydrated state. Dynamic light scattering, atomic force microscopy and transmission electron microscopy studies demonstrated that the LS-DS catanionic complex forms stable medium-sized vesicles (diameter of ∼300-500 nm). In vitro studies with 5-fluorouracil and rhodamine 6G showed efficient entrapment and release of these two anti-cancer drugs in the physiologically relevant pH range of 6.0-8.0, but with contrasting pH dependences. These observations indicate that LS-DS catanionic vesicles may find application in designing drug delivery systems.

Thermodynamics of complex chemical equilibria in surfactant mixtures

A thermodynamic approach was developed to predict the precipitation conditions of surfactants using the solubility product relationship between surfactant monomer concentrations, in order to calculate the monomer-precipitate equilibrium. This approach provides an explicit equation which predicts the amount of solid phase which forms in any surfactant mixture. All calculations of the total change in Gibbs energy (ΔG) were performed for concentrations of both surfactants that were below their CMC values. The elaborated ΔG-pH diagrams offer the possibility to determine the areas of thermodynamic stability of the solid phases depending on the chemical composition and acidity of the studied system. It was shown that with increasing concentration of the surfactant and the metal ion, the range of precipitate formation, either as slightly soluble salt or as slightly soluble acid, was extended by a few pH units in all cases.

Environmental Assessment of Various End-of-Life Pathways for Treating Per- and Polyfluoroalkyl Substances in Spent Fire-Extinguishing Waters

Per- and polyfluoroalkyl substances (PFAS) are now thought to be far more prevalent in water bodies across the globe than previously reported. In particular, military bases, airports, and industrial sites are prone to contamination caused by runoff discharges from fire-extinguishing waters that contain PFAS such as aqueous film-forming foams (AFFF). These substances and their metabolites show a high degree of mobility as well as a low biotic and abiotic degradability; as a result, they are bioaccumulative and often migrate among the environmental compartments in addition to being toxic. As of now, there is no suitable end-of-life treatment process that is both technologically efficient and cost-effective for the handling of PFAS. Currently, the incineration of the collected extinguishing water at temperatures above 1100 °C is the recommended method for the disposal of PFAS to degrade material compounds. However, this method consumes extensive energy because it requires incineration of large quantities of water to treat a diluted fraction of PFAS. Aside from incineration, adsorption of PFAS on granulated activated carbon is one of the most widely used technologies, albeit with poor adsorption and often requiring very large downstream filtration systems. Finally, the application of functional precipitation agents using commercially available cationic surfactants is a novel approach (PerfluorAd^® [Cornelsen] process) that enables the effective precipitation of PFAS from the spent fire-extinguishing waters. Hence, the goal of the present study was to investigate the environmental impacts emanating from the proper treatment of spent fire-extinguishing water with the aforementioned 3 end-of-life treatment scenarios. A life cycle assessment was conducted for this purpose. The results show that the PerfluorAd process outperforms the other 2 treatment technologies across all environmental impact categories except for ozone depletion. Environ Toxicol Chem 2021;40:947-957. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Thermodynamics of Interactions Between Charged Surfactants and Ionic Poly(amino acids) by Isothermal Titration Calorimetry

Interactions between charges play a role in protein stability and contribute to the energetics of binding between various charged ligands. Ionic surfactants are charged molecules, whose interactions with proteins are still rather poorly understood despite their wide applications. Here, we show by isothermal titration calorimetry that cationic alkylammonium surfactants bind to negatively charged polyaspartate and polyglutamate homopolymers stoichiometrically, i.e., one surfactant molecule per charged amino acid. Similarly, negatively charged alkyl sulfates (e.g., sodium dodecyl sulfate) and alkane sulfonates bind stoichiometrically to positively charged polylysine, polyornithine, and polyarginine homopolymers. In these reactions, the interacting counterparts form ion pairs and the resulting electrostatically neutral complex coprecipitates from solution. The enthalpies and heat capacities are determined for various pairs of ionic surfactants and charged amino acid homopolymers. These results show the energetic contributions of ionic headgroups and the CH₂ group to surfactant interactions with proteins.

Study on surface properties of sodiumdodecyl sulfate and dodecyltrimethylammonium bromide mixed surfactants and their interaction with dyes

The antagonistic as well as synergetic interaction for dodecyltrimethylammonium bromide (DTAB) and sodiumdodecyl sulfate (SDS) mixed surfactants by using surface tension are investigated on the basis of the results obtained earlier, the efficiency of adsorption (pC₂₀), aggregation number (N), ΓΓmax, effective Gibbs free energy (ΔGeffo) and CMCC20 are calculated additionally with three different temperatures at T = 293.15, 298.15 and 303.15 K as the detailed surface properties. The binding constants and standard free energy change of SDS and DTAB mixture with the interaction of (2.5× 10⁻⁵ mol L^–1 of methyl orange, MO and methylene blue, MB) are carried out by using UV-Vis spectroscopy at room temperature by using different models. The closer values of the binding constants and standard free energy change for SDS and DTAB mixture with the interaction of MO and MB are included in our investigations.

Thermodynamic insights and molecular environments into catanionic surfactant systems: Influence of chain length and molar ratio

HYPOTHESIS

Imidazolium-based Ionic liquids as new generation cationic surfactants can provide designable alkyl chain length. In the catanionic surfactant systems, the alkyl chain lengths and molar ratios can greatly influence the interactions such as electrostatic and hydrophobic interaction. The variation in these interactions has a significant effect on the molecular environments of the self-assembly structure, and this process is always accompanied by the transition of aggregates and release or consumption of heat. Hence, it is of interest to study the relationship between intermolecular interactions, molecular environments, self-assembly structure and the change in energy of system in the catanionic surfactant mixed systems.

EXPERIMENTS

The enthalpy change ΔH of titrations the imidazolium-based into SDS micelle solution was studied to characterize the heat by using isothermal titration calorimetry (ITC) during the transitions of the aggregate structures. The corresponding self-assembly structure was monitored via cryogenic transmission electron microscopy (cryo-TEM). Employing proton magnetic resonance (¹H NMR), we focus on the interactions between imidazolium-based ILs and SDS based on the variations in the molecular environments of aggregates.

FINDINGS

Of these imidazolium-based ionic liquids/SDS system, the 1-octyl-3-methylimidazolium ([OMIM]Cl)/SDS system shows several features such as intense energy absorption and releasing processes, which indicate the formation of high entanglement wormlike micelles and vesicles. This is related to the formation of self-adjusting state between the SDS and [OMIM]Cl molecules due to the balance between the electrostatic interaction and hydrophobic interaction. Varying the alkyl chain length appears to cause significant differences to the molecular environments. From the molecular environments, three different models about the polarity of the catanionic surfactant molecules are used to explain the balance of the intermolecular interactions.

Self-assembly of sodium dodecylsulfate and dodecyltrimethylammonium bromide mixed surfactants with dyes in aqueous mixtures

The micellar property of mixed surfactant systems, cationic (dodecyltrimethylammonium bromide, DTAB) and anionic (sodium dodecylsulfate, SDS) surfactants with variable molar ratios in aqueous system has been reported by using surface tension and conductivity measurements at T = 293.15, 298.15 and 303.15 K. DTAB concentrations are varied from 1.0 × 10^-4 to 3 × 10^-4 mol l^-1 in 1.0 × 10^-2 mol l^-1 SDS solution while the SDS concentration is varied from 1.0 × 10^-3 to 1.5 × 10^-2 mol l^-1 in approximately 5.0 × 10^-3 mol l^-1 DTAB, so that such concentrations of DTAB-SDS (DTAB-rich) and SDS-DTAB (SDS-rich) solutions were chosen 3 : 1 ratio. The critical micellar concentration, as well as surface and thermodynamic properties for DTAB-rich and SDS-rich solutions, were evaluated by the surface tension (γ) and conductivity (κ) methods. The pseudo phase separation model was coupled with the dissociated Margules model for synergism. The Krafft temperature behaviour and optical analysis of mixed surfactants are studied using conductivity and UV-Vis spectroscopy, respectively. The dispersibility and stability of DTAB-rich and SDS-rich solutions with and without dyes (2.5 × 10^-5 mol l^-1 of methyl orange and methylene blue) are carried out by using UV-Vis spectroscopy and dynamic light scattering.

An Interaction of Anionic- and Cationic-Rich Mixed Surfactants in Aqueous Medium through Physicochemical Properties at Three Different Temperatures

The mixed micellization of aqueous binary mixtures of DTAB-rich and SDS-rich surfactants, comprising sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) is studied in aqueous solution by using the physicochemical properties (PCPs) at three different temperatures (T = 293.15, 298.15, and 303.15 K) and P=0.1  MPa. The DTAB concentration is varied from 0.0001 to 0.03 M/mol·L−1 in the ∼0.01 M/mol·L−1 SDS solution, while the concentration of SDS is varied from 0.001 to 0.015 M/mol·L−1 in the ∼0.005 M/mol·L−1 DTAB. The stable formulations have been obtained by employing the DTAB-rich and SDS-rich surfactants solutions in 3 : 1 ratio. Therefore, different phases and aggregated states formed in the ternary combinations of DTAB/SDS/H2O have been identified and described. The calculated PCPs have been utilized for determining the nature of the solute-solvent interaction (SLS0I). With increasing surfactants concentration, the polarisation of the solution also increases along with an increase in relative viscosity (ηr), viscous relaxation time (τ), and surface excess concentration (Γmax). However, the surface area of the molecule (Amin), hydrodynamic volume (Vh), and hydrodynamic radius (Rh) decrease along with an increase in surfactants concentration.

Thermodynamic selectivity of functional agents on zeolite for sodium dodecyl sulfate sequestration

This study proposes a thermodynamic approach to effectively select functional agents onto zeolite for sodium dodecyl sulfate (SDS) sequestration in greywater reuse. We combine isothermal titration calorimetry (ITC) and quantum chemistry simulation (QCS) to identify the interactions between SDS and agents at the molecular level. Three potential agents, cetyl trimethyl ammonium bromide (CTAB), N,N,N-trimethyltetradecan-1-aminium bromide (C14TAB), and 14-hydroxy-N,N,N-trimethyltetradecan-1-aminium bromide (C14HTAB), differ in carbon chain length and hydrophilic groups. The ITC titration of SDS with CTAB released the highest heat, followed by those with C14TAB and C14HTAB, as was the same trend for the amounts of SDS adsorbed by the respective functionalized-zeolites. Results suggest that the favorable SDS sorption occurred at the bilayer CTAB-zeolite is driven by enthalpy as similar as the SDS…CTAB interaction found, regardless of the contribution from electrostatic and/or hydrophobic behaviors, while the declined sorption is entropy-driven via the predominant hydrophobic interaction onto the monolayer CTAB-zeolite. The data presented here interpret the nature of molecularly thermodynamic quantities and enable the manipulation of sorption capacity optimization.

Rosin Surfactant QRMAE Can Be Utilized as an Amorphous Aggregate Inducer: A Case Study of Mammalian Serum Albumin

Quaternary amine of diethylaminoethyl rosin ester (QRMAE), chemically synthesized biocompatible rosin based cationic surfactant, has various biological applications including its use as a food product additive. In this study, we examined the amorphous aggregation behavior of mammalian serum albumins at pH 7.5, i.e., two units above their isoelectric points (pI ~5.5), and the roles played by positive charge and hydrophobicity of exogenously added rosin surfactant QRMAE. The study was carried out on five mammalian serum albumins, using various spectroscopic methods, dye binding assay, circular dichroism and electron microscopy. The thermodynamics of the binding of mammalian serum albumins to cationic rosin modified surfactant were established using isothermal titration calorimetry (ITC). It was observed that a suitable molar ratio of protein to QRMAE surfactant enthusiastically induces amorphous aggregate formation at a pH above two units of pI. Rosin surfactant QRMAE-albumins interactions revealed a unique interplay between the initial electrostatic and the subsequent hydrophobic interactions that play an important role towards the formation of hydrophobic interactions-driven amorphous aggregate. Amorphous aggregation of proteins is associated with varying diseases, from the formation of protein wine haze to the expansion of the eye lenses in cataract, during the expression and purification of recombinant proteins. This study can be used for the design of novel biomolecules or drugs with the ability to neutralize factor(s) responsible for the aggregate formation, in addition to various other industrial applications.

Thermodynamics of Ion Pair Formations Between Charged Poly(Amino Acid)s

Electrostatic interactions between the positively and negatively charged amino acids in proteins play an important role in macromolecular stability, binding, and recognition. Numerous amino acids in proteins are ionizable and may exist in negatively (e.g., Glu, Asp, Cys, Tyr) or positively (e.g., Arg, Lys, His, Orn) charged form dependent on pH and their pKas. In this work, isothermal titration calorimetry was used to determine the average standard values of thermodynamic parameters (the Gibbs free energy, enthalpy, entropy, and the heat capacity) of interaction between the positively charged amino acid homopolymers (polyarginine, polylysine, and polyornithine) and the negatively charged homopolymers (polyaspartic and polyglutamic acids). These values are of potential use in the computational models of interacting proteins and other biological macromolecules. The study showed that oppositely charged poly(amino acid)s bound each other with the stoichiometry of one positive to one negative charge. Arginine bound to the negatively charged amino acids with exothermic enthalpy and higher affinity than lysine. This result also suggests that positive charges in proteins should not be considered entirely equivalent if carried by lysine or arginine. The difference in binding energy of arginine and lysine association with the negatively charged amino acids was attributed to the enthalpy of the second ionic hydrogen bond formation between the guanidine and carboxylic groups. Despite the favorable enthalpic contribution, all such ion pair formation reactions were largely entropy-driven. Consistent with previously observed ionic interactions, the positive heat capacity was always observed during the amino acid ion pair formation.

Surfactant selection principle for reducing critical micelle concentration in mixtures of oppositely charged gemini surfactants

Cationic quaternary ammonium gemini surfactants C(n)H(2n+1)(CH3)2N(+)CH2CHCHCH2(CH3)2N(+)C(n)H(2n+1)2Br(-) (C(n)C4C(n), n = 12, 8, 6) with alkyl spacers, C(n)H(2n+1)(CH3)2N(+)CH2CHOHCHOHCH2(CH3)2N(+)C(n)H(2n+1)2Br(-) (C(n)C4(OH)2C(n), n = 12, 8, 6, 4) with two hydroxyl groups in alkyl spacers, and cationic ammonium single-chain surfactants C(n)H(2n+1)(CH3)2N(+)Br(-) (C(n)TAB, n = 12, 8, 6) have been chosen to fabricate oppositely charged surfactant mixtures with anionic sulfonate gemini surfactant C12H25N(CH2CH2CH2SO3(-))CH2CH2CH2(CH3)2N(CH2CH2CH2SO3(-))C12H252Na (C12C3C12(SO3)2). Surface tension, electrical conductivity, and isothermal titration microcalorimetry (ITC) were used to study their surface properties, aggregation behaviors, and intermolecular interactions. The mixtures of C12C3C12(SO3)2/C(n)C4(OH)2C(n) (n = 12, 8) and C12C3C12(SO3)2/C12C4C12 show anomalous larger critical micelle concentration (CMC) than C12C3C12(SO3)2, while the mixtures of C12C3C12(SO3)2/C(n)C4(OH)2C(n) (n = 6, 4), C12C3C12(SO3)2/C(n)C4(OH)2C(n) (n = 6, 4), and C12C3C12(SO3)2/C(n)TAB (n = 12, 8, 6) exhibit much lower CMC than C12C3C12(SO3)2. The results indicate that strong hydrophobic interactions between the alkyl chains assisted by strong electrostatic attractions between the headgroups and hydrogen bonds between the spacers lead to the formation of less surface active premicellar aggregates in bulk solution, resulting in the increase of CMC. If these interactions are weakened or inhibited, less surface active premicellar aggregates are no longer formed in the mixtures, and thus the CMC values are reduced. The work reveals that the combination of two surfactants with great self-assembling ability separately may have strong intermolecular binding interactions; however, their mixtures do not always generate superior synergism properties. Only moderate intermolecular interaction can generate the strongest synergism in CMC reduction.