Surfactant Head Groups Research Articles

Effect of Headgroup Variation on the Self-Assembly of Cationic Surfactants with Sulfonatocalix[6]arene.

The effect of headgroup variation on the association of supramolecular amphiphiles composed of 4-sulfonatocalix[6]arene (SCX6) and cationic surfactant possessing tetradecyl substituent was studied in aqueous solutions at pH 7. When the surfactant contained hydrophilic trimethylammonium, pyridinium, or 1-methylimidazolium headgroup, highly reversible temperature-responsive nanoparticle-supramolecular micelle transformation could be attained at appropriately chosen component mixing ratios and NaCl concentrations. In these cases, the substantial negative molar heat capacity change (ΔCp) rendered nanoparticle formation strongly endothermic at low temperature, whereas the assembly to supramolecular micelle was always accompanied by enthalpy gain. The ΔCp values became less negative when the charge density and the hydrophilic character of the surfactant headgroup diminished. The association of the more hydrophobic 6-methoxyquinolinium and quinolinium surfactants with SCX6 did not lead to supramolecular micelle formation because the self-assembly into nanoparticles was highly exothermic.

Insight into Water Structure at the Surfactant Surfaces and in Microemulsion Confinement.

Interactions with surfactant molecules can significantly alter the structure of interfacial water. We present a comparative study of water-surfactant interactions using two different spectroscopic approaches: water at planar surfactant monolayers by sum frequency generation (SFG) spectroscopy and interfacial water confined in reverse micelles formed by the same surfactants using IR absorption spectroscopy. We report spectral features in the OH-stretching region (3200-3700 cm-1) that are observed in both IR and SFG spectra, albeit with different relative amplitudes, for ionic surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) and nonionic surfactant polyoxyethylene(4)lauryl ether (Brij L-4) reverse micelles in hexane and the corresponding monolayers at the air/water interface. A prominent feature in the SFG spectra of the OH stretch at 3560 cm-1 is attributed to water molecules that have a weak donor hydrogen bond to the surfactant headgroup. The same feature is observed in the IR spectra of reverse micelles after deconvoluting the interfacial versus bulk spectral contributions. We performed an orientational analysis of these water molecules utilizing the polarization-dependent SFG spectra, which shows an average tilt angle of the OH stretch of surfactant-bound water molecules of ∼155° with respect to the surface normal.

Interaction of the interleukin 8 protein with a sodium dodecyl sulfate micelle: A computer simulation study.

Molecular simulations were carried out to study the sodium dodecyl sulfate (SDS) surfactant with the interleukin 8 (IL8) protein as a model to investigate the influence of amphiphilic molecules on proteins. Simulations for an SDS micelle with an IL8 protein show that both aggregates, which were initially separated, eventually approach each other to form a single complex. The results showed that the protein was attached to the SDS micelle by the charged positive amino acids whereas less contacts were observed for the negatively charged amino acids. Structural protein properties, such as amino acid contacts and pair correlation functions were conducted between the micelle and the protein groups and they showed greater interactions between the surfactant headgroups and the positively charged residues in the protein. Moreover, hydrogen bonds were also calculated between both structures and a greater number of bonds among the SDS headgroups and the charged positive amino acids in the protein was found. Graphical Abstract Attachment of the interleukin 8 protein with a sodium dodecyl sulfate (SDS) micelle is given by the charged positive amino acids as indicated by the interaction of those amino acids with the SDS headgroups.

In-situ shear-banding quantification of surfactant solutions in straight microfluidic channels

Surfactant molecules aggregate into various micellar morphologies, depending on temperature, concentration, formulation, and flow. Micellar solutions are known to undergo shear-banding when subjected to strong shear rates, as the fluid spontaneously divides itself into bands of high and low-shear rate, both under the same applied shear stress. This phenomenon occurs because of the complex structure of micellar solutions, which undergo phase transitions upon applied flow, changing the viscosity accordingly. Here, we study shear-banding of micellar solutions in one of the simplest microfluidic geometries, a straight planar channel with rectangular cross section. Four solutions with similar zero-shear viscosity and nonlinear rheological response, but different structures are compared to investigate the flow-structure relation and its impact on shear-banding. Micellar solutions are prepared by adding different amounts of the same organic salt, sodium salicylate, to surfactant molecules with different headgroups, i.e., cetyltrimethylammonium bromide and cetylpyridinium chloride. From spatially resolved microparticle image velocimetry and flow-induced birefringence measurements, the shear rate and shear stress profiles developed on the xy-plane of a planar microchannel are obtained from a series of volumetric flow rates. Based on these profiles, in-situ rheological parameters, such as the local viscosity, are calculated by applying the stress-optical rule. The local response in a microfluidic channel is compared to the bulk rheological response in a rotational rheometer and clear correlations are found especially for the stress plateau region, the fingerprint of shear-banding. Based on the local rheological characterization of these micellar solutions, the development and growth of shear-bands is observed and quantified. The role of salt concentration and surfactant headgroup on the resulting micellar morphology is discussed, as well as its impact on the development of shear-banding.

Wettability modification of lignite by adsorption of alkyltrimethylammonium bromides with different alkyl chain length

ABSTRACTTo overcome the problem of moisture re-adsorption of dried lignite with common evaporation drying methods, a set of linear alkyl quaternary ammonium surfactants (C10TAB, C12TAB, and C16TAB) were used to modify lignite surface, and the effects of alkyl chain length on the adsorption characteristics of surfactants and wettability of lignite surface were evaluated. Pseudo-first and pseudo-second-order kinetic models and Langmuir model were, respectively, used to simulate adsorption kinetics and adsorption isotherms. The results showed that three surfactants gradually formed double-layer adsorption on lignite surface and the loading of surfactant increased with the length of alkyl chain. X-ray photoelectron spectroscopy analysis showed that C3H9N+ moiety of the surfactants would preferentially interact with O˭C‒O groups of lignite. Results of wetting heating and moisture re-adsorption showed that three surfactants obviously decreased hydrophilicity and restrained moisture re-adsorption of lignite, but with the formation of double-layer adsorption, hydrophilic headgroups of surfactant faced outward, which caused increase in hydrophilicity of lignite. As a result of two opposite effects of surfactant chain length on lignite wettability, the effect of C12TAB on decreasing hydrophilicity was the best among the three chosen surfactants.

Control of human skin wettability using the pH of anionic surfactant solution treatments

The outermost layer of human skin, or stratum corneum, acts as a protective barrier between underlying living tissue and the external environment. The wettability of this tissue layer can influence spreading of chemicals and the adhesion of pathogenic microorganisms. We show in this article that the wettability of isolated human stratum corneum can be controlled through treatment with solutions of the anionic surfactant, sodium lauryl sulfate, buffered to different pH values. Relative to control treatments with the buffer solution alone, surfactant solution treatments under acidic conditions cause delipidated stratum corneum to become more hydrophobic. In contrast, alkaline conditions cause the stratum corneum to become more hydrophilic; irrespective of lipid composition. This transition is consistent with a reorientation of bound surfactants at the tissue interface. Under acidic conditions, electrostatic binding of negatively charged surfactant head groups with positively charged keratin in the stratum corneum would increase tissue hydrophobicity due to the exposed hydrophobic tails. However, a hydrophobic based attraction of the apolar surfactant tails to the stratum corneum surface under alkaline conditions would leave the hydrophilic surfactant head groups exposed, causing increased tissue hydrophilicity. Changes in wettability with pH become diminished when lipids ordinarily found in stratum corneum are present, suggesting the lipids partially inhibit surfactant binding. Profilometry studies of the tissue topography highlight that surfactant induced changes in stratum corneum surface roughness cannot account for the observed changes in wettability.

Conformational transition of κ-casein in micellar environment: Insight from the tryptophan fluorescence

Intrinsically disordered proteins (IDPs) are under intense analysis due to their structural flexibility and importance in biological functions. Minuscule modulation in the microenvironment induces significant conformational changes in IDPs, and these non-native conformations of the IDPs often induce aggregation and cause cell death. Changes in the membrane composition often change the microenvironment, which promote conformational change and aggregation of IDPs. κ-Casein, an important milk protein, belongs to the class of IDPs containing net negative charges. In this present work, we have studied the interaction of κ-casein with cetyltrimethyl ammonium bromide (CTAB), a positively charged surfactant, utilizing various steady state fluorescence, time-resolved fluorescence and circular dichroism spectroscopy. Our results clearly indicate that κ-casein undergoes at least two conformational transitions in presence of various concentrations of CTAB. The intrinsically disordered κ-casein assumes a partially folded conformation at lower concentration of CTAB, which adopts an unstructured conformation at higher concentration of CTAB. The partially folded conformation of κ-casein at a lower CTAB concentration might be induced by the favorable electrostatic interaction between the positively charged surfactant headgroup and net negative charges of the protein, whereas surfactant nature of CTAB is being pronounced at higher concentration of CTAB.

Low frequency sonic waves assisted cloud point extraction of polyhydroxyalkanoate from Cupriavidus necator.

Low frequency sonic waves, less than 10kHz were introduced to assist cloud point extraction of polyhydroxyalkanoate from Cupriavidus necator present within the crude broth. Process parameters including surfactant system variables and sonication parameters were studied for their effect on extraction efficiency. Introduction of low frequency sonic waves assists in the dissolution of microbial cell wall by the surfactant micelles and release of cellular content, polyhydroxyalkanoate granules released were encapsulated by the micelle core which was confirmed by crotonic acid assay. In addition, sonic waves resulted in the separation of homogeneous surfactant and broth mixture into two distinct phases, top aqueous phase and polyhydroxyalkanoate enriched bottom surfactant rich phase. Mixed surfactant systems showed higher extraction efficiency compared to that of individual Triton X-100 concentrations, owing to increase in the hydrophobicity of the micellar core and its interaction with polyhydroxyalkanoate. Addition of salts to the mixed surfactant system induces screening of charged surfactant head groups and reduces inter-micellar repulsion, presence of ammonium ions lead to electrostatic repulsion and weaker cation sodium enhances the formation of micellar network. Addition of polyethylene glycol 8000 resulted in increasing interaction with the surfactant tails of the micelle core there by reducing the purity of polyhydroxyalkanoate.

Thermoresponsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant.

We put forward a simple protocol to prepare thermoresponsive Pickering emulsions. Using hydrophilic silica nanoparticles in combination with a low concentration of alkyl polyoxyethylene monododecyl ether (C12En) nonionic surfactant as emulsifier, oil-in-water (o/w) emulsions can be obtained, which are stable at room temperature but demulsified at elevated temperature. The stabilization can be restored once the separated mixture is cooled and rehomogenized, and this stabilization-destabilization behavior can be cycled many times. It is found that the adsorption of nonionic surfactant at the silica nanoparticle-water interface via hydrogen bonding between the oxygen atoms in the polyoxyethylene headgroup and the SiOH groups on particle surfaces at low temperature is responsible for the in situ hydrophobization of the particles rendering them surface-active. Dehydrophobization can be achieved at elevated temperature due to weakening or loss of this hydrogen bonding. The time required for demulsification decreases with increasing temperature, and the temperature interval between stabilization and destabilization of the emulsions is affected by the surfactant headgroup length. Experimental evidence including microscopy, adsorption isotherms, and three-phase contact angles is provided to support the mechanism.

Effects of Hydrotropic Salt on the Nanoscopic Dynamics of DTAB Micelles.

Effects of a hydrotropic salt, sodium salicylate (NaSal), on the dynamic behavior of cationic dodecyltrimethylammonium bromide (DTAB) micelles as studied using dynamic light scattering (DLS) and quasielastic neutron scattering (QENS) techniques are reported here. DLS study showed that the addition of NaSal leads to a decrease in the apparent diffusion coefficient of the whole micelle indicating micellar growth. QENS data analysis suggested that observed dynamics involves two distinct motions, lateral motion of the surfactant over the curved micellar surface and localized segmental motion of the surfactant. It is found that the addition of NaSal slows down the lateral motion of DTAB while the localized segmental motion of the DTAB chain is not affected much. An atomistic molecular dynamics (MD) simulation was performed to gain further insight into the underlying phenomena. MD simulation results are found to be consistent with the experimental observations. MD simulation revealed that location of the salicylate ions on the micellar surface and their strong electrostatic association with their oppositely charged surfactant headgroup are the major factors in slowing down the lateral motion of the DTAB molecule. In the present work, a quantitative description of the effects of NaSal on the nanoscopic dynamics of DTAB micelles and its correlation with the microstructure of the micelle is provided.

Two-phase hollow fiber-liquid microextraction based on reverse micelle for the determination of quercetin in human plasma and vegetables samples

A new two-phase hollow fiber-liquid microextraction based on reverse micelle as sensitive and efficient method for the extraction of quercetin in human plasma, onion and tomato samples was developed for the first time. The extraction is based on the solubilization of analyte in the aqueous core of the reverse micelle of cationic cethyltrimethyl ammonium bromide surfactant, CTAB, which are present in the organic phase and are used as acceptor phase in hollow fiber-liquid phase microextraction. Experimental results show that the electrostatic interaction between the oppositely charged surfactant head group present in the reverse micelles and the analyte molecule has a key role in the extraction. Extraction parameters including the organic phase, pH, surfactant concentrations, stirring rate, extraction time, temperature and ionic strength were optimized. The optimum condition is obtained when the variables are set to: pH= 7.5, organic solvent= 1-octanol, CTAB concentration= 7mmolL−1, stirring rate= 900rpm at room temperature and extraction time= 30min. Under the mentioned condition, the calibration range was found to be linear over 0.5–1000ngmL−1 with the limit of detection of 0.1ngmL−1. The extraction recovery of 97.4%, with relative standard deviation of 3.5%, was obtained via tree replicated measurements on a 15ngL−1 quercetin standard solution. Finally, the newly developed method was successfully used for the preconcentration and determination of quercetin in plasma, onion and tomato samples.

Head-group effect of surfactants of cationic type in interaction with propoxylated sodium salt of polyacrylic acid in aqueous solution

Polyacrylic acid (PAA) partially (50mol.%) neutralized by sodium hydroxide was propoxylated by propylene oxide (PO) obtaining terpolymer (PNPAA). The conductivity and tensiometry measurements were used to study an interaction of PNPAA with ionic-liquid surfactants of cationic type–nonylethylolammonium bromides in aqueous solutions of different concentrations at 25°C. From the results of these measurements, critical aggregation concentrations, critical micelle concentrations and the effective degree of counterion binding were determined. Depending on the nature of head-group of the cationic surfactant, the polyelectrolyte-surfactant interaction was studied and its impact on colloidal-chemical parameters of the polyelectrolyte/surfactant complexes was clarified. Petroleum-collecting properties of the obtained polyelectrolyte/surfactant complexes have been studied in various waters of a different mineralization degree.

Interfacial Properties and Monolayer Collapse of Alkyl Benzenesulfonate Surfactant Monolayers at the Decane-Water Interface from Molecular Dynamics Simulations.

The molecular structure of a surfactant molecule is known to have a great effect on the interfacial properties and the type of nanostructures formed. In this work, we have performed molecular dynamics simulations on six isomers of an alkyl benzenesulfonate surfactant to investigate the effect of the degree and position of aromatic substitution on the interfacial properties and on the collapse of the surfactant monolayer at a decane-water interface. The surface pressure of the monolayers was shown to increase with increasing surface coverage, until some of the monolayers become mechanically unstable and form large undulations. Shifting the primary alkyl chain of the surfactant from the para to the meta position was found to significantly affect the orientation of the surfactant head groups, while the attachment position of the benzene ring along the primary alkyl chain plays a greater role in the orientation of the surfactant tails. In general, to the extent considered in this work, our results suggest that additional alkyl substitution and meta substitution of the primary alkyl chains increase both the effectiveness and efficiency of the surfactants, and accelerate the onset of monolayer collapse. The interface was found to consist of an inner Helmholtz layer of partially dehydrated counterions in contact with the surfactant head groups, an outer Helmholtz layer of hydrated counterions, and a diffuse layer. The di- and trisubstituted surfactants formed nearly spherical swollen micelles encapsulating pure decane, which effectively solubilizes decane in water as a microemulsion. The monosubstituted surfactants formed elongated buds that protrude from the interface, but did not detach from the monolayer. To our knowledge, the role of aromatic substitution on interfacial properties has not been investigated by molecular simulations previously. The results from this work could provide insights to design improved surfactants by exploiting aromatic substitution to encapsulate material for drug delivery and other applications.

Influence of bile salt on vitamin E derived vesicles involving a surface active ionic liquid and conventional cationic micelle

This study has been actually performed with the aim to develop vitamin E derived vesicles individually from a surface active ionic liquid (1-Hexadecyl-3-Methylimidazolium chloride ([C16mim]Cl)) and a common cationic amphiphile (benzyldimethylhexadecylammonium chloride (BHDC)) and also to investigate their consequent breakdown in presence of bile salt molecule. From this study, it is revealed that the rotational motion of coumarin 153 (C153) molecule is hindered as the vitamin E content is increased in the individual micellar solution of [C16mim]Cl and BHDC. The extent of enhancement in rotational relaxation time is more pronounced in case of [C16mim]Cl-vitamin E solutions than in the BHDC-vitamin E vesicular aggregates which confirms the greater rigidity of the former vesicular system than the later one. Moreover, the effect of bile salt in the vitamin E forming vesicular assemblies have also been unravelled. It is found that the large area occupancy by the steroidal backbone of the bile salt plays a crucial role towards the enlargement of the average surfactant head group area. This results in disintegration of the vesicles composed of vitamin E and consequently, vesicles are transformed into mixed micellar aggregates. From the anisotropy measurement it is found that the rotational motion of C153 is more hindered in the [C16mim]Cl/BHDC-NaCh mixed micelles compared to that inside the individual vesicles. The fluorescence correlation spectroscopic (FCS) study also confirms that the mixed micelles have a more compact structure than that of the [C16mim]Cl-vitamin E and BHDC-vitamin E vesicles. Altogether, the micelle to vesicle transition involving any vitamin and their disruption by bile salt would be an interesting investigation both from the view point of basic colloidal chemistry and towards the generation of new drug delivery vehicle due to their unique microenvironment. Therefore, in future, these systems can be utilised as vehicle for the transport and as well as delivery of drugs and as probable reactor in nanomaterial synthesis.

Low-symmetry sphere packings of simple surfactant micelles induced by ionic sphericity

Supramolecular self-assembly enables access to designer soft materials that typically exhibit high-symmetry packing arrangements, which optimize the interactions between their mesoscopic constituents over multiple length scales. We report the discovery of an ionic small molecule surfactant that undergoes water-induced self-assembly into spherical micelles, which pack into a previously unknown, low-symmetry lyotropic liquid crystalline Frank-Kasper σ phase. Small-angle X-ray scattering studies reveal that this complex phase is characterized by a gigantic tetragonal unit cell, in which 30 sub-2-nm quasispherical micelles of five discrete sizes are arranged into a tetrahedral close packing, with exceptional translational order over length scales exceeding 100 nm. Varying the relative concentrations of water and surfactant in these lyotropic phases also triggers formation of the related Frank-Kasper A15 sphere packing as well as a common body-centered cubic structure. Molecular dynamics simulations reveal that the symmetry breaking that drives the formation of the σ and A15 phases arises from minimization of local deviations in surfactant headgroup and counterion solvation to maintain a nearly spherical counterion atmosphere around each micelle, while maximizing counterion-mediated electrostatic cohesion among the ensemble of charged particles.

Modeling micelle formation and interfacial properties with iSAFT classical density functional theory.

Surfactants reduce the interfacial tension between phases, making them an important additive in a number of industrial and commercial applications from enhanced oil recovery to personal care products (e.g., shampoo and detergents). To help obtain a better understanding of the dependence of surfactantproperties on molecular structure, a classical density functional theory, also known as interfacial statistical associating fluid theory, has been applied to study the effects of surfactant architecture on micelle formation and interfacial properties for model nonionic surfactant/water/oil systems. In this approach, hydrogen bonding is explicitly included. To minimize the free energy, the system minimizes interactions between hydrophobic components and hydrophilic components with water molecules hydrating the surfactant head group. The theory predicts micellar structure, effects of surfactant architecture on critical micelle concentration, aggregation number, and interfacial tension isotherm of surfactant/water systems in qualitative agreement with experimental data. Furthermore, this model is applied to study swollen micelles and reverse swollen micelles that are necessary to understand the formation of a middle-phase microemulsion.

Red-emitting cationic fluorophore as a probe for anionic surfactants

A water soluble N-methylpyridinium dye (DABP) was synthesized and investigated as a fluorescence probe for the detection of anionic surfactants. DABP shows strong solvatochromism with emission at 685 nm in water. The probe-surfactant interaction induces distinct color changes and optical properties. In the presence of anionic surfactants such as SDS, AOT and zwitterionic surfactant such as CHAPS, a multi-fold intensity enhancement is observed but not with positively charged surfactant, CTAB indicating the selectivity for negatively charged surfactants. This is ascribed to the electrostatic attraction between the negatively charged surfactant head group and the positively charged pyridinium ring of the diarylbutadiene chromophore. Such distinguishable emission behavior may be of interest for designing fluorogenic sensors for targeted detection.

Solvent bar microextraction using a reverse micelle containing extraction phase for the determination of warfarin from human plasma by high-performance liquid chromatography

For the first time, solvent bar microextraction using a reverse micelle RMM-SBME as extraction phase was developed for extraction of warfarin in human plasma sample. The reverse micelle of a cationic surfactant (hexadecyltrimethylammonium bromide) in 1-octanol was applied as supported and acceptor phase in SBME. In this process, the analyte was extracted from aqueous samples into a reverse micelle impregnated and also filled in the wall pores and lumen the polypropylene hollow fiber membrane by the electrostatic interactions between the analyte and the surfactant head groups. The influences of fundamental parameters affecting extraction efficiency of warfarin including sample solution pH, surfactant concentration and extraction time were studied using central composite design optimization method. The analysis of extracted analyte was performed by high performance liquid chromatography with ultra violet detector.Under the optimum condition, the sample containing 40ngmL−1 of warfarin was extracted with extraction recovery of 97.8% that pertained to enrichment factor of 260. A good linearity over a concentration range 1–600ngmL−1 with correlation coefficient value of 0.9989 was provided by the current method. Limits of detection and inner and inter-day relative standard deviation were 0.3ngmL−1 and less than 4.4%, respectively. This method was compared to conventional SBME, and illustrated higher extraction efficiency. The method needs minimal sample preparation time since the porous hollow fiber membrane prevents from being extracted potential interfering materials, acts as a filter, and any extra cleanup was not required. The method with bar of 3cm and acceptor phase volume of 8.5μL is environmentally green/friendly method. The proposed method was successfully applied for the preconcentration and determination of warfarin in plasma sample.

Multiscale Modeling of the Effects of Salt and Perfume Raw Materials on the Rheological Properties of Commercial Threadlike Micellar Solutions

We link micellar structures to their rheological properties for two surfactant body-wash formulations at various concentrations of salts and perfume raw materials (PRMs) using molecular simulations and micellar-scale modeling, as well as traditional surfactant packing arguments. The two body washes, namely, BW-1EO and BW-3EO, are composed of sodium lauryl ethylene glycol ether sulfate (SLEnS, where n is the average number of ethylene glycol repeat units), cocamidopropyl betaine (CAPB), ACCORD (which is a mixture of six PRMs), and NaCl salt. BW-3EO is an SLE3S-based body wash, whereas BW-1EO is an SLE1S-based body wash. Additional PRMs are also added into the body washes. The effects of temperature, salt, and added PRMs on micellar lengths, breakage times, end-cap free energies, and other properties are obtained from fits of the rheological data to predictions of the "Pointer Algorithm" [ Zou , W. ; Larson , R.G. J. Rheol. 2014 , 58 , 1 - 41 ], which is a simulation method based on the Cates model of micellar dynamics. Changes in these micellar properties are interpreted using the Israelachvili surfactant packing argument. From coarse-grained molecular simulations, we infer how salt modifies the micellar properties by changing the packing between the surfactant head groups, with the micellar radius remaining nearly constant. PRMs do so by partitioning to different locations within the micelles according to their octanol/water partition coefficient POW and chemical structures, adjusting the packing of the head and/or tail groups, and by changing the micelle radius, in the case of a large hydrophobic PRM. We find that relatively hydrophilic PRMs with log POW < 2 partition primarily to the head group region and shrink micellar length, decreasing viscosity substantially, whereas more hydrophobic PRMs, with log POW between 2 and 4, mix with the hydrophobic surfactant tails within the micellar core and slightly enhance the viscosity and micelle length, which is consistent with the packing argument. Large and very hydrophobic PRMs, with log POW > 4, are isolated deep inside the micelle, separating from the tails and swelling the radius of the micelle, leading to shorter micelles and much lower viscosities, leading eventually to swollen-droplet micelles.

The effect of betaine on the foam stability: Molecular simulation

Zwitterionic betaines are widely used as foam boosters due to these can enhance the stability of foam films. In this paper, mechanistic insights of betaine to improve the stability of alkyl-polyoxyethylene carboxylate (AEC) foam are provided by molecular simulation. In the simulation, we observe the electropositive nitrogen atoms in betaine interact with the electronegative sulfur atoms, an electrostatic structure is formed at the air/water interface. Interaction energies of the mixed surfactants are calculated by the quantum chemistry methods. The calculations show betaine-AEC and betaine–betaine possess attractive interaction, and that AEC–AEC has repulsion to each other. In the other words, the repulsion between the headgroups of anionic surfactants is relaxed by betaine. Additionally, the influence of concentration of betaine on the stability of foam films is also simulated. The RDF and coordination numbers show that the electrostatic structures become denser with the increasing concentration of betaine. Therefore, entry barrier is enhanced accordingly. The SMD simulation also demonstrates the same variation tendency of entry barrier. The simulation details provide vital supplements to experiments.