Cationic Dodecyltrimethylammonium Bromide Research Articles

Ultra-stable Pickering liquid crystal-in-water emulsions decorated with thermoresponsive soft microgels and their response to aqueous analytes

This work presents an extensive investigation of the Pickering stabilization of liquid crystal (LC)-in-water emulsions using soft nanometer-sized colloidal poly(N-isopropylacrylamide) (PNIPAM) microgel particles and the responsivity of the PNIPAM microgel-stabilized Pickering LC droplet-based assembly towards amphiphilic analytes. The evolution of size and stability of LC droplets with microgel concentration are found to be dependent on the emulsifier-regime. Despite microgel coating, the LC-water interface remains accessible to model analytes such as anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB). The analytes can pass through the interfacial pores and/or meshes of porous microgels to induce a bipolar-to-radial ordering transition within the droplets. The dose-response behaviour is largely influenced by the initial microgel concentration, type of analytes and interfacial layer (charge), and the number of droplets exposed to the analytes. The PNIPAM microgel-coated LC droplets notably exhibited enhanced sensitivity under conditions promoting electrostatic attractive interactions between the interfacial microgel layer and analytes. Overall, the results highlight the potential of Pickering LC-in-water emulsions for reliable and quantitative analysis of aqueous analytes, thus opening up new avenues toward the development of droplet-based optical sensors.

Colloidal interactions between nanochitin and surfactants: Connecting micro- and macroscopic properties by isothermal titration calorimetry and rheology

This study elucidates the intricate interactions between chitin nanocrystals (ChNC) and surfactants of same hydrophobic tail (C12) but different head groups types (anionic, cationic, nonionic): sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and polyoxyethylene(23)lauryl ether (Brij-35). Isothermal Titration Calorimetry (ITC) and rheology are used to study the complex ChNC-surfactant interactions in aqueous media, affected by adsorption, self-assembly and micellization. The ITC results demonstrate that the surfactant head group significantly influences the dynamics and nature of the involved phenomena. Cationic DTAB's reveal minimal interaction with ChNC, non-ionic Brij-25's interact moderately at low concentrations driven by hydrophobic effects while SDS's interacts strongly and show complex interaction patterns that fall across four distinct regimes with SDS addition. We attribute such behavior to initiate through electrostatic attraction and terminate in surfactant micelle formation on ChNC surfaces. ITC also elucidates the impact of ChNC concentration on key parameters including critical aggregation concentration (CAC) and saturation concentration (C2). Dynamic rheological analysis indicates the molecular interactions translate to non-linear variations in the elastic modulus (G') upon SDS addition mirroring that observed in ITC experiments. Such a direct correlation between molecular interactions and macroscopic rheological properties provides insights to aid in the creation of nanocomposites with tailored properties.

Tensiometry-based sensing of aggregation and of evaporation behavior of a volatile amphiphile in mixed solutions with ionic and nonionic surfactants

Mixed surfactants solutions containing volatile amphiphiles, e.g. fragrances, are envisaged to exhibit correlations between aggregation behavior, molecular interactions, composition of the mixed adsorbed layers and the concentration of the volatile component in the vapors. These issues are addressed with tensiometric methods by considering dynamic and steady-state interfacial adsorption-evaporation behavior at both sides of air-liquid interface in mixed solutions of geraniol and micelle-forming ionic and non-ionic surfactants. Pendant drop tensiometry proved to be a sensitive method in assessing the concentration of geraniol in the vapors above mixed solutions. Differences in the release of the fragrance from the mixed interfacial layers are explained by competitive contributions of specific barrier mechanisms and concentration-dependent aggregation in bulk solutions. Synergetic effect in reducing surface tension was evident for mixtures of geraniol with anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB) surfactants. Dynamic anti-synergetic effect in SDS-geraniol mixtures is attributed to strong aggregation behavior, which was found to hinder the evaporation of geraniol. Measurements of the headspace concentration indicated weak interactions between geraniol and micelles of non-ionic highly surface active Brij-35 surfactant. The presented results extend the traditional analytical toolbox for evaluating factors which affect the mass transfer of volatile organic amphiphiles between aqueous and gas phases.

Effects of static magnetic field on the surface tension of surfactant solutions

AbstractThe magnetic field (MF) effects resulting from water or solution treatments are still of significant interest. However, a relatively small number of papers have been published dealing with the pure surfactant solutions alone. On the other hand, surfactants are applied in many industrial processes as well as in everyday life and are also present in different waste waters. Therefore, it seemed interesting to investigate whether some effects would appear after the MF treatment of pure aqueous surfactant solutions. In the earlier published papers after MF action the changes in water evaporation rate and the surface tension were found for both the cationic dodecyltrimethylammonium bromide (DTAB) and anionic sodium dodecyl sulfate (SDS) solutions. The gradient MF originated from three connected ring magnets inside which the solutions were placed. The objective of this paper is to study whether using the same magnets but in a different position the effects of the MF are observed. The given surfactant solution in a closed vessel was placed in an uniform MF when the magnets were in the “lying” position. The investigated solutions on the magnet surface remained for 24 h. In this paper, despite SDS and DTAB also the cationic hexadecyltrimethylammonium bromide (CTAB) and nonionic Triton X‐100 solutions were applied. It appeared that in the uniform MF the surface tension of cationic and anionic surfactant solutions changes. However, larger changes were observed in the gradient MF. Generally, the changes of surface tension depend on the surfactant kind and its concentration. Stronger MF influence was found for the cationic surfactant and almost no changes were observed for nonionic Triton X‐100.

Effect of nonionic and anionic surfactant on ecotoxicity and micellization behaviors of dodecyl trimethyl ammonium bromide (DTAB)

Mixtures of surfactants are extensively present in industrial applications for better performance and lower cost. The toxicity of cationic dodecyl trimethyl ammonium bromide (DTAB) against Photobacterium phosphoreum was investigated in the presence of other common surfactants, such as nonionic fatty alcohol-polyoxyethlene ether (AEO), and anionic sodium dodecyl benzene sulfonate (SDBS) as well as sodium dodecyl sulfate (SDS). The mechanism of the combined toxicity was explored based on the micellization behaviors and the characteristics of mixed micelles in surfactant systems. The toxicity of cationic DTAB was significantly higher than that of anionic SDBS, SDS and nonionic AEO, and was not affected by the combination with AEO or SDBS. However, the toxicity of the DTAB/SDS mixture was remarkably lower than that of DTAB alone. Scanning electron microscopy (SEM) images verified that the mixing of DTAB and SDS indeed hindered their action against the bacteria. The micellization behavior of binary mixtures showed an antagonistic effect in DTAB/AEO and DTAB/SDBS systems, but a synergistic effect in DTAB/SDS systems. Thus, the synergism facilitated the formation of mixed micelles and greatly reduced the toxicity of DTAB/SDS mixtures. Furthermore, the formation of mixed micelles in DTAB/SDS systems not only decreased the electrostatic effect of DTAB with the bacteria but also inhibited the interaction of surfactant molecules with bioactive protein, resulting in the lowest toxicity of DTAB/SDS mixtures with mass ratios of 2:3 and 1:4. These results can provide a reference for the development of highly efficient and environmentally friendly composite surfactants.

Exploring cationic polyelectrolyte-micelle interaction via excited-state proton transfer. Signatures of probe transfer.

Excited-state proton transfer (ESPT) is a sensitive tool for the delicate monitoring of structural reorganization, hydration level, and confinement within surfactant and polymer assemblies. Here, we investigate the interaction of a cationic polyelectrolyte, poly(diallyl dimethylammonium chloride) (PDADMAC), with micelles of differently charged surfactants using 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) as an ESPT probe. We used three surfactants: anionic sodium dodecyl sulfate (SDS), cationic dodecyl trimethylammonium bromide (DTAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), possessing the same alkyl (dodecyl) chain but varying headgroup charges. The fluorescence of HPTS residing initially within the micellar medium modulates differently in the presence of PDADMAC. For the anionic SDS and cationic DTAB micelles, the emission spectrum of HPTS does not alter significantly; however, for SB12 micelles, the emission spectrum undergoes a strong modulation upon adding the polyelectrolyte. The emission intensities quench strongly at a low concentration of PDADMAC but recover at a higher concentration. The emission intensity ratio of the two emission bands also changes significantly, implying strong modulation of the ESPT process with varying PDADMAC concentrations. The time-resolved area normalized emission spectra (TRANES) disclose single isoemissive points in the SB12 micelle at low and high concentrations of PDADMAC but two different isoemissive points (one characteristic of the SB12 micelle at 500 nm and another characteristic of the PDADMAC interface at 480 nm) in the mixed assembly at an intermediate concentration. Detailed analysis suggests that the polyelectrolyte can enforce the transfer of the anionic probe HPTS from the zwitterionic micelle to the PDADMAC assembly above a specific PDADMAC concentration. The transfer of the molecular probe between two assemblies resembles a drug sequestration event, and the study reveals necessary emission signatures.

Influence of n-alcohols on aqueous DTAB micelles studied by ultrasonic analysis

Abstract The influence of chain length of n-alcohols such as 1-butanol, 1-pentanol, 1-hexanol and 1-heptanol on cationic dodecyl trimethyl ammonium bromide (DTAB) micelles has been investigated. The effect of concentration was determined at alcohol concentrations of (10, 20, 30, 40 and 50) mM and at temperatures of 298.15 K, 303.15 K, 308.15 K and 313.15 K using ultrasonic velocity, density, viscosity and conductivity measurements. To study molecular interactions in micelles of various mixtures of DTAB and n-alcohols by using acoustical parameters, such as adiabatic compres-sibility (β ad), intermicellar free length (L f ), acoustic impedance (Z), molar volume (V M ) have been calculated by using ultrasonic velocity (U) and density (ρ). With the help of the trends observed when varying these parameters, the molecular interactions and thus the micellar growth of mixed systems of DTAB and n–alcohol were discussed. Viscosity data such as absolute viscosity, viscous relaxation time, oil solubilization, foam stability and conductance data complemented the observed ultrasonic data.

Adsorption and Aggregation Behavior of Mixtures of Quaternary-Ammonium-Salt-Type Amphiphilic Compounds with Fluorinated Counterions and Surfactants.

The surface adsorption and aggregation behavior of a mixture of quaternary-ammonium-salt-type amphiphilic monomeric compounds (C4 FSA, C8 FSA, and C4 NTf2) or gemini compounds (C10-2-C4 FSA) and various surfactants (nonionic hexaoxyethylene dodecyl ether (C12EO6), anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (C12TAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C12Sb)) was investigated. Both types of compounds contained alkyl chains of nonidentical lengths that used bis(fluorosulfonyl)imide (FSA-) or bis(trifluoromethanesulfonyl)imide (NTf2-) as counterions. The mixtures were analyzed for surface tension, viscosity, electrical conductivity, and pyrene fluorescence, in addition to evaluation by cryogenic transmission electron microscopy, small-angle X-ray scattering, and dynamic light scattering. Our results showed that the surface tension depended on the surfactant structure. For the mixture of C8 FSA and SDS, as the SDS concentration increased, the surface tension first decreased and became constant at the critical micelle concentration (CMC). In this concentration range, C8 FSA and SDS were approximately equimolar (2.5 mmol dm-3), the mixture adsorbed efficiently at the air-water interface, and vesicles and linear-type micelles were formed in the solution owing to the decreased electrostatic repulsion between the hydrophilic groups. As the SDS concentration further increased, the surface tension increased and reached another constant value. The C8 FSA at the interface was replaced by SDS and the aggregates transformed into spherical micelles. The surface tension plot of the mixture of the amphiphilic compounds and C12Sb showed a minimum at the CMC. The lowest CMC and surface tension were observed for C10-2-C4 FSA, indicating that the gemini compounds offer excellent adsorption and orientation at the air-water interface. It was revealed that the quaternary-ammonium-salt-type amphiphilic compounds in this study acted as ionic liquids on their own and as surfactants in aqueous solution. Further, they could improve the surface activity of conventional ionic surfactants.

Interfacial complexation of a neutral amphiphilic ‘tardigrade’ co-polymer with a cationic surfactant: Transition from synergy to competition

HypothesisNeutral amphiphilic PEG-g-PVAc co-polymer (a “tardigrade” polymer consisting of a hydrophilic polyethylene glycol, PEG, backbone with hydrophobic polyvinyl acetate, PVAc, grafts) can form complexes at the air–water interface with cationic dodecyltrimethylammonium bromide (DTAB) via self-assembly. Compared to anionic SDS, cationic DTAB headgroups are expected to interact strongly with the negatively charged OH– groups from the partial dissociation of the PVAc grafts. We anticipate a transition from synergistic to competitive behaviour, which is expected to be dependent on the surfactant structural characteristics and concentration. ExperimentsDTAB/PEG-g-PVAc mixtures were investigated using a combination of dynamic and equilibrium surface tension measurements, neutron reflectivity (NR) at the air–water interface, and foaming tests. We varied the concentrations of both the DTAB (0.05 to 5 critical micelle concentration, cmc) and that of PEG-g-PVAc (0.2 and 2 critical aggregation concentration, cac). FindingsOur results show that the interfacial interactions between DTAB and PEG-g-PVAc were both synergistic and antagonistic, depending sensitively on the surfactant concentration. At DTAB concentrations below its cmc, a pronounced cooperative adsorption behaviour was likely driven by the hydrophobic interactions between the DTAB tail and the PVAc grafts and the attraction between the DTAB headgroups and the partially dissociated –O- groups in the partially hydrolysed PVAc grafts, forming a mixed layer. This synergistic adsorption behaviour transitioned to a competitive adsorption behaviour at DTAB concentrations above its cmc, leading to polymer-surfactant partition, forming a “hanging” polymer layer underlying a surfactant monolayer at the interface. We postulate that DTAB/PEG-g-PVAc complexation in the bulk contributed to partial depletion of the mixture from the interface. We therefore consider this polymer/surfactant system to be a moderately interacting system at the air–water interface. No discernible differences in the foaming behaviour were observed between the DTAB/PEG-g-PVAc systems and the pure surfactant. Our results suggest that surfactant headgroup characteristics (particularly charges) were crucial in determining the structure and composition of polymer-surfactant complexes at the air–water interface, as well as the foamability and foam stability, whilst the coexistence of the synergistic and competitive adsorption behaviour is attributed to the unique architecture of the tardigrade polymer with amphiphilicity and partial charge, facilitating different surfactant-polymer interactions at different DTAB concentrations.

Thermodynamics of the micellization of quaternary based cationic surfactants in triethanolamine-water media: a conductometry study

Abstract The effect of triethanolamine, a solvent with wide technical and industrial benefit on the micellization of an aqueous mixture of cationic surfactants, dodecyltrimethylammonium bromide (DETAB) and hexadecyltrimethylammonium bromide (HATAB) was studied to examining the stability of the mixed micelles at 298.1, 303.1, 308.1 and 313.1 K using the electrical conductance method. The values of the critical micelle concentration (C*) were found to decrease with an increase in the concentration of triethanolamine (TEA). The values of the free energy of micellization (ΔGm) were negative at a particular temperature, and the extent of spontaneity was discovered to increase when the concentration of TEA was increased. However, an increase in temperature was observed to have a negative linear relationship with the spontaneity of the process. The formation of the mixed micelles was an exothermic process, and it was also TEA and temperature-dependent with a trend similar to those observed in the free energy of micellization (ΔGm). The degree of disorderliness of the system was also found to be entropy driven at a higher concentration of TEA. The synergistic interaction between the molecules of DETAB–HATAB in the presence of TEA (0.4% v/v) and the spontaneity of the system was at the maximum at 0.1:0.9 mol fraction ratio and the energetics of the system was discussed based on hydrophobic–solvophobic interaction of the monomers in TEA at elevated temperatures.

Tuning of micelle adsorption on nanoparticles by combination of surfactants

The interaction of anionic silica nanoparticles with nonionic decaethylene glycol mono-dodecyl ether (C12E10) and ionic surfactants [both anionic sodium dodecyl sulfate (SDS) and cationic dodecyl trimethyl ammonium bromide (DTAB)] has been studied by small-angle neutron scattering and dynamic light scattering. The nonionic and cationic surfactant micelles are adsorbed on nanoparticles, whereas no adsorption of SDS surfactant micelles on nanoparticles is observed. The adsorption of C12E10 micelles provides additional steric stability to nanoparticles. However, the adsorption of cationic micelles leads to micelles-mediated fractal aggregation of nanoparticles. In the case of SDS surfactant, nanoparticles and micelles coexist in the solution. Furthermore, the adsorption behavior of surfactant micelles on nanoparticles has been tuned using a combination of nonionic and ionic surfactants. The combination of nonionic C12E10 with anionic SDS makes surfactant micelles to desorb from nanoparticles, whereas the combination of nonionic C12E10 with cationic DTAB leads to fractal aggregation of nanoparticles. The systematic transitions of micelle adsorption to desorption on nanoparticles with a C12E10–SDS mixed surfactant system and the aggregation of nanoparticles in a C12E10–DTAB mixed surfactant system as a function of ionic surfactant (SDS or DTAB) concentration have been examined. The micelles desorption from nanoparticles follows an exponential decay behavior with an increase in SDS concentration in C12E10–SDS, whereas the aggregate size shows an exponential growth with DTAB in C12E10–DTAB. The electrostatic interactions between nanoparticles and surfactant micelles are found to be dominating for tuning these transitions.

Modifications in the nanoparticle-protein interactions for tuning the protein adsorption and controlling the stability of complexes

We report the pathways to suppress or enhance the protein adsorption on nanoparticles and thereby control the stability of the nanoparticle-protein complexes with the help of selective additives. This has been achieved by tuning the electrostatic interaction between the nanoparticles and proteins, in the presence of surfactant and multivalent counterions. The preferential binding of the proteins with the surfactant and multivalent ions induced charge reversibility of nanoparticles can lead to adsorption of an otherwise non-adsorbing protein and vice versa. The findings are demonstrated for anionic silica nanoparticles and two globular proteins [lysozyme (cationic) and bovine serum albumin (BSA) (anionic)] as model systems, in the presence of two ionic surfactants [anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB)], and ZrCl4 as multivalent salt. Small-angle neutron scattering with the unique advantage of contrast variation has been used to probe the role of individual components in the multi-component system. It is shown that the non-adsorbing behavior of BSA with silica nanoparticles changes into adsorbing in the presence of oppositely charged DTAB surfactant, whereas the strong adsorbing behavior of lysozyme on nanoparticles modifies to be non-adsorbing in the presence of oppositely charged SDS surfactant. The presence of multivalent counterions (ZrCl4) leads the charge reversal of the nanoparticles, transforming the lysozyme from adsorbing to non-adsorbing, and no significant change in the behavior of BSA. The results presented can find potential applications in the field of nanobiotechnology.

Unusual Maximum in the Adsorption of Aqueous Surfactant Mixtures: Neutron Reflectometry of Mixtures of Zwitterionic and Ionic Surfactants at the Silica-Aqueous Interface.

The adsorption of two zwitterionic surfactants, dodecyldimethylammonium propanesulfonate (C12PS) and dodecyldimethylammonium carboxybetaine (C12CB), and of their mixtures with the cationic dodecyltrimethylammonium bromide (C12TAB) and the anionic sodium dodecylsulfate (SDS) at the silica-water interface has been studied by neutron reflection (NR). The total adsorption, the composition of the adsorbed layer, and some structural information have been obtained over a range of concentrations from below the critical micelle concentration (CMC) to about 30× the mixed CMC. The adsorption behavior has been considered in relation to the previously measured micellar equilibrium of these mixtures in their bulk solutions and their adsorption at the air-water interface. C12CB adsorbs cooperatively close to its CMC to form an almost complete bilayer on its own, whereas C12PS adsorbs more weakly in a fragmented bilayer structure. Although SDS does not normally adsorb at the silica-water interface, SDS adsorbs strongly and cooperatively with C12PS at fractional SDS compositions up to about 0.5. This cooperativity is lost when the adsorbed fraction of SDS rises above about 0.5. At this point, adsorption drops sharply, creating an unusual maximum in the variation of adsorption with a total concentration above the mixed CMC. Neither the increase in cooperativity nor the subsequent decline in adsorption results directly from variations of the independently determined monomer concentrations in the bulk solution. The adsorption maximum is predominantly the effect of strong cooperative interaction, possibly accompanied by partial segregation of SDS within the layer, followed by charge repulsion from the surface. Although the solution aggregation and adsorption at the A-W interface are similar for SDS with C12CB, the addition of SDS to C12CB at the silica-water interface promotes the opposite behavior to that of SDS with C12PS, and SDS simply disrupts the cooperative binding of C12CB. Unlike SDS, the cationic surfactant C12TAB adsorbs on silica. It therefore coadsorbs at the SiO2-W interface with either C12CB or C12PS. However, in neither case is there any pronounced cooperativity and, even though the presence of C12TAB might be expected to favor adsorption, the adsorption is generally unexpectedly low.

Interaction of a bovine serum albumin (BSA) protein with mixed anionic-cationic surfactants and the resultant structure.

The interaction of a bovine serum albumin (BSA) protein with the mixture of anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB) has been investigated by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Both SDS and DTAB as individuals interact electrostatically as well as hydrophobically with BSA and form connected protein-decorated micelle like complexes in the aqueous solution, in which the well-defined surfactant micelles are organized along the randomly distributed unfolded polypeptide chain of the protein. The protein-surfactant interaction has been tuned by adding different molar mixtures of SDS and DTAB in BSA aqueous solution. It is found that a lower molar fraction of either surfactant in the protein-mixed surfactant complexes results in the formation of a connected protein-decorated micelle structure similar to those of pure surfactants. As the molar fraction of one of the surfactants in the mixture approaches the equimolar fraction, the structure formed by the protein-mixed surfactant is very different from the connected protein-decorated micelle like structure. Different microstructures of BSA-mixed surfactant complexes are formed, mostly governed by the structure of mixed surfactants arising from the strong electrostatic interaction of oppositely charged components. In this case, unfolded proteins wrap the structures of mixed surfactants around their surface. Along with the connected protein-decorated micelle like structure, rod-like and bilayer vesicles of protein-surfactant complexes are formed at different molar fractions of mixed surfactants.

What happens when pesticides are solubilised in binary ionic/zwitterionic-nonionic mixed micelles?

HypothesisSurfactants have been widely used as adjuvants in agri-sprays to enhance the solubility of pesticides in foliar spray deposits and their mobility through leaf cuticles. Previously, we have characterised pesticide solubilisation in nonionic surfactant micelles, but what happens when pesticides become solubilised in anionic, cationic and zwitterionic and their mixtures with nonionic surfactants remain poorly characterised. ExperimentsTo facilitate characterisations by SANS and NMR, we used nonionic surfactant hexaethylene glycol monododecyl ether (C12E6), anionic sodium dodecylsulphate (SDS), cationic dodecyltrimethylammonium bromide (DTAB) and zwitterionic dodecylphosphocholine (C12PC) as model adjuvant systems to solubilise 3 pesticides, Cyprodinil (CP), Azoxystrobin (AZ) and Difenoconazole (DF), representing different structural features. The investigation focused on the influence of solubilisates in driving changes to the micellar nanostructures in the absence or presence of electrolytes. NMR and NOESY were applied to investigate the solubility and location of each pesticide in the micelles. SANS was used to reveal subtle changes to the micellar structures due to pesticide solubilisation with and without electrolytes. FindingsUnlike nonionic surfactants, the ionic and zwitterionic surfactant micellar structures remain unchanged upon pesticide solubilisation. Electrolytes slightly elongate the ionic surfactant micelles but have no effect on nonionic and zwitterionic surfactants. Pesticide solubilisation could alter the structures of the binary mixtures of ionic/zwitterionic and ionic/nonionic micelles by causing elongation, shell shrinkage and dehydration, with the exact alteration being determined by the molar ratio in the mixture.

Protein Microgel-Stabilized Pickering Liquid Crystal Emulsions Undergo Analyte-Triggered Configurational Transition.

Herein, we report a novel approach that involves Pickering stabilization of micometer-sized liquid crystal (LC) droplets with biocompatible soft materials such as a whey protein microgel (WPM) to facilitate the analysis of analyte-induced configurational transition of the LC droplets. The WPM particles were able to irreversibly adsorb at the LC-water interface, and the resulting WPM-stabilized LC droplets possessed a remarkable stability against coalescence over time. Although the LC droplets were successfully protected by a continuous network of the WPM layer, the LC-water interface was still accessible for small molecules such as sodium dodecyl sulfate (SDS) that could diffuse through the meshes of the adsorbed WPM network or through the interfacial pores and induce an LC response. This approach was exploited to investigate the dynamic range of the WPM-stabilized LC droplet response to SDS. Nevertheless, the presence of the unadsorbed WPM in the aqueous medium reduced the access of SDS molecules to the LC droplets, thus suppressing the configuration transition. An improved LC response to SDS with a lower detection limit was achieved after washing off the unadsorbed WPM. Interestingly, the LC exhibited a detection limit as low as ∼0.85 mM for SDS within the initial WPM concentration ranging from 0.005 to 0.1 wt %. Furthermore, we demonstrate that the dose-response behavior was strongly influenced by the number of droplets exposed to the aqueous analytes and the type of surfactants such as anionic SDS, cationic dodecyltrimethylammonium bromide (DTAB), and nonionic tetra(ethylene glycol)monododecyl ether (C12E4). Thus, our results address key issues associated with the quantification of aqueous analytes and provide a promising colloidal platform toward the development of new classes of biocompatible LC droplet-based optical sensors.

Probing the Molecular Structure of Coadsorbed Polyethylenimine and Charged Surfactants at the Nanoemulsion Droplet Surface.

Nanoemulsions, nanoscale oil droplets dispersed in an aqueous medium, can be stabilized by polymer-surfactant (PS) mixtures, making them ubiquitous in commercial, industrial, and pharmaceutical applications. It is well-known that the presence of PS layers coadsorbed at the droplet surface plays a significant role in droplet stability and functionality; however, little is understood about the molecular nature of this coadsorption. Such insights are especially important for application in drug delivery where physiological conditions can vary the environmental pH and significantly impact stabilization. Hence, the focus of this study examines the surface properties of ∼300 nm nanoemulsions stabilized by the coadsorption of polyethylenimine (PEI) and charged alkyl surfactants sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB). PEI is a common charge-tunable polymer used in nanocarrier templates. This study employs vibrational sum frequency scattering spectroscopy, coupled with ζ-potential and surface pressure measurements performed as a function of varying concentrations and pH. The surface specific spectroscopic results reported herein reveal that PEI adsorption and molecular ordering is influenced by both electrostatic and hydrophobic interactions. While the degree of PEI adsorption is stronger in the presence of anionic SDS than cationic DTAB, for both surfactants, PEI is molecularly disordered in acidic conditions and adopts a persistent net ordering as the solution pH becomes more basic. Both surfactants also display degrees of interfacial conformational ordering that is altered by the presence of the coadsorbed polymer. These results demonstrate the molecular-level diversity in PEI behavior at the droplet interface and provide insight into how such behavior can be controlled to yield nanocarrier technology with specific functions and enhanced efficacy.

Reciprocal effects of multi-walled carbon nanotubes and oppositely charged surfactants in bulk water and at interfaces

In this work we made the complementary use of nuclear magnetic resonance spectroscopy, dynamic light scattering, electric conductivity and surface tension methods to study the micellization of cationic surfactants cetyl trimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB), as well as anionic surfactant sodium dodecyl sulfate (SDS) in the presence of multi-walled carbon nanotubes (CNT). It was shown that the CNTs is effectively dispersed in water medium due to the adsorption of surfactant ions at CNT surface. The NMR spectral analysis (chemical shifts and line broadening) specified that surfactants interact with CNTs surface both by alkyl chains and by charged head groups. For cationic CTAB and DTAB this interaction occurs mainly through methylene (CH2)n groups of alkyl radicals, at the same time, the anionic SDS interacts with the CNTs surface by terminal methyl groups (CH3). Interaction specificity of cationic and anionic surfactants leads to their different structural organization to CNT surface. Experimental observations make it possible to propose the model where the attraction of cationic DTAB and CTAB positively charged head groups with negatively charged CNT surface results to the orientation of cationic surfactants along CNT surface. In contrast to positively charged DTAB and CTAB the repulsion of SDS with negatively charged head group from CNT surface and contacted SDS terminal groups results in the normal orientation of anionic surfactant to CNT surface. The consistency of proposed orientation model arrived from conductivity study of surfactant-CNT dispersions. According to conductivity data, the non-micellized cationic surfactants follow complete dissociation in the presence of CNT no matter are they free or bound to CNT. On the contrary the absence of complete dissociation of anionic surfactant in the pre-micellar systems may be the result of SDS ordering on CNT surface and forming the charged surface with double electric layer binding the portion of counter-ions.

Insights into the stability of polytetrafluoroethylene aqueous dispersion: Role of surfactant

Polytetrafluoroethylene (PTFE) has attracted significant interest from both industry and academia owing to its remarkable properties such as wear and corrosion resistance, low surface energy and low dielectric constant. High stability and high solid content (~60%) are primary criteria for PTFE aqueous dispersions toward preparing various end-use articles; however, they are difficult to be achieved simultaneously. Additionally, the stabilization mechanism has not been clearly unraveled and understood till date. Herein, four types of surfactants including cationic dodecyltrimethylammonium bromide (DTAB), anionic sodium dodecylsulfate, zwitterionic lauryl betaine (LB), and nonionic lauryl alcohol polyoxyethylene Brij 30, are used to stabilize 60% PTFE dispersions from the as-polymerized dilute mixture. The stabilities of these dispersions are quantitatively and objectively characterized for the first time using a Turbiscan analyzer based on multiple light scattering technology. While DTAB causes flocculation, the other three surfactants can formulate concentrated dispersions. The dispersion with LB loses the stability in a short time, whereas Brij 30-stabilized dispersion exhibits high stability due to the formation of a network of wormlike micelles. It is determined that the instability of surfactant-stabilized dispersion stems from particle sedimentation rather than coalescence or aggregation. The size of dispersed particle and dispersion viscosity play dominant roles in controlling the particle settling rate, thereby affecting the stability. This study affords useful insights into the stability of concentrated PTFE dispersion, which can guide the selection of suitable surfactants to prepare PTFE dispersions with desired stabilities.

Comparing the efficiency of pure and mixed cationic and nonionic surfactants used in enhanced oil recovery by mesoscopic simulations

The decrease of the interfacial tension between water and oil by addition of nonionic and cationic surfactants known to be optimal for enhanced oil recovery is studied here using dissipative particle dynamics numerical simulations. Three nonionic surfactants are modeled: Neodol nonylphenol ethoxylate, Neodol Propoxate, and nonylphenol ethoxylate, and the cationic dodecyl trimethyl ammonium bromide (DTAB). To model DTAB accurately the electrostatic interactions are included explicitly. The interfacial tension is calculated as a function of surfactant concentration, at constant temperature and volume. DTAB is found to be more efficient in lowering the interfacial tension than the nonionic surfactants, at the same concentration. Our simulations show that their different performance is due to their adsorption mechanisms at the interface. The nonionic surfactants adsorb mostly as monolayers, while DTAB adsorbs in multilayers. An equation for the interfacial tension as a function of surfactant concentration equivalent to Szyszkowski’s equation is proposed for multilayer adsorption at the water-oil interface. The two – dimensional potentials of mean force calculated at the water-oil interface reveal a homogeneous interface when DTAB is present. This is in contrast with the nonionic surfactants, whose potentials of mean force show separate regions of hydrophobic and hydrophilic regions at the interface. Lastly, synergistic combinations of the cationic and nonionic surfactants are tested, yielding optimal mixtures that can reduce more the interfacial tension at lower surfactant concentrations. It is argued that our conclusions are useful to researchers designing new surfactants not only for enhanced oil recovery but also for other soft matter applications.