Surfactant Tail Research Articles

A comparative study on the synthesis of a cationic gemini surfactant and a single tail surfactant-assisted ZnS quantum dots and their application to the degradation of methyl orange dye under visible light

In this work, the synthesis of cationic gemini surfactant, 1,3-bis(N,N-dimethyldohexylammonium bromide) (16–3-16) has been done and was used as a capping agent to prepare the ZnS quantum dots. Further, the synthesized ZnS quantum dots capped with gemini surfactant (16–3-16) and a single tail surfactant cetyltrimethylammonium bromide (CTAB) were characterized to evaluate the influence of surfactants on the structural, optical, and thermal properties of the ZnS quantum dots. From X-ray diffraction, the crystalline size of the gemini (16–3-16) capped ZnS quantum dots was smaller than the CTAB-capped ZnS quantum dots. The high-resolution transmission electron microscopy study shows that gemini-(16–3-16) capped ZnS quantum dots show highly confined average particle sizes of 2.87 nm compared to CTAB-capped ZnS 5.86 nm. The Brunauer-Emmett-Teller (BET) method measured the surface area, which showed that ZnS/16–3-16 had a larger surface area than the ZnS/CTAB quantum dots. Further, the synthesized ZnS quantum dots were examined by degrading an anionic dye, methyl orange, under visible light to determine their catalytic efficiency. Gemini surfactant-assisted ZnS quantum dots degraded (99 %) more efficiently than CTAB (64 %) in 110 min of reaction time. The rate constants for gemini (16–3-16) and CTAB-capped ZnS quantum dots were 0.03423 and 00638 min−1. Gemini surfactant-capped ZnS quantum dots demonstrate significantly improved photocatalytic efficiency compared to CTAB-capped ZnS quantum dots. The ZnS/16–3-16 quantum dot had good photocatalytic stability after three cycles and contributed to the degradation of MO dye by producing •OH radicals.

Gemini surfactant-engineered di-Schiff bases: Tailoring hydrophobicity to enhance catalytic and biological activities

This study explores the enhancement of catalytic performance of silver nanoparticles (AgNPs) using a series of gemini surfactants based on di-azomethine with varying hydrophobic tail lengths: DSGO, DSGD, and DSGH. By fine-tuning the molecular structure of these surfactants, we aim to develop more effective catalysts for purifying polluted water sources. The high surface energy of AgNPs often results in their aggregation, reducing their catalytic efficacy. However, increasing the surfactant tail length from 8 (DSGO) to 16 (DSGH) carbon atoms improves the stability and dispersibility of AgNPs, as confirmed by DLS, TEM, and UV analyses. DSGH, with its longer carbon tail, facilitates AgNPs synthesis with superior stability and smaller particle size, indicated by a higher zeta potential of +55.6 mV and a reduced size of 15.1 nm. This is attributed to the increased surface activity and hydrophobicity of the surfactants. As the tail length increases from 8 to 16, the investigated surfactant’s tendency for micellization and adsorption rises. Notably, DSGH exhibits the lowest critical micelle concentration (CMC) of 406 μM and the highest adsorption Gibbs free energy (ΔG°ads) of −59.08 kJ·mol−1 at 20 °C, enabling strong adsorption onto AgNPs. By modulating surfactant hydrophobicity, we effectively regulated AgNP catalytic activity. The DSGH/AgNPs composite shows enhanced catalytic activity, efficiently converting p-nitrophenol (p-NP) and methylene blue (MB) into less toxic species with apparent rate constants (kapp) of 0.368 and 0.537 min−1, respectively. Additionally, the AgNP-incorporated gemini surfactants exhibit promising antimicrobial activity against tested bacteria and fungi. Remarkably, the DSGD/AgNPs hybrid system surpasses commonly used commercial antimicrobial agents in efficacy. This study underscores the potential of manipulating surfactant structure to control the catalytic and antimicrobial activity of AgNPs.

Effects of dilution and age on perfume release from two mixed‐surfactant systems containing hydrotropes

AbstractThis study investigates the encapsulation and controlled release of perfume within micellar systems. We explored how perfume‐surfactant interactions and micelle structure influence fragrance delivery. Two mixed‐micelle systems, one with branched‐tail and the other with linear‐tail surfactants, were subjected to varying perfume concentrations, dilution, and time. Using small‐angle neutron scattering (SANS), gas chromatography‐mass spectrometry (GC‐MS), and statistical analysis, we determined that micelle size and shape are significantly affected by perfume content and dilution. While both systems exhibited similar trends, the branched‐tail system uniquely formed a lamellar phase at higher perfume levels. Our findings reveal a strong correlation between micelle geometry and headspace perfume concentration, highlighting the role of surfactant tail structure. This research provides fundamental insights into perfume‐micelle interactions, with potential applications in materials science and active ingredient delivery.

Determining the Role of Surfactant on the Cytosolic Delivery of DNA Cross-Linked Micelles.

Nucleic Acid Nanocapsules (NANs) are nucleic acid nanostructures that radially display oligonucleotides on the surface of cross-linked surfactant micelles. Their chemical makeup affords the stimuli-responsive release of therapeutically active DNA-surfactant conjugates into the cells. While NANs have so far demonstrated the effective cytosolic delivery of their nucleic acid cargo, as seen indirectly by their gene regulation capabilities, there remain gaps in the molecular understanding of how this process happens. Herein, we examine the enzymatic degradation of NANs and confirm the identity of the DNA-surfactant conjugates formed by using mass spectrometry (MS). With surface enhanced (resonance) Raman spectroscopy (SE(R)RS), we also provide evidence that the energy-independent translocation of such DNA-surfactant conjugates is contingent upon their release from the NAN structure, which, when intact, otherwise buries the hydrophobic surfactant tail in its interior. Such information suggests a critical role of the surfactant in the lipid disruption capability of the DNA surfactant conjugates generated from degradation of the NANs. Using NANs made with different tail lengths (C12 and C10), we show that this mechanism likely holds true despite significant differences in the physical properties (i.e., critical micelle concentration (CMC), surfactants per micelle, Nagg) of the resultant particles (C12 and C10 NANs). While the total cellular uptake efficiencies of C12 and C10 NANs are similar, there were differences observed in their cellular distribution and localized trafficking, even after ensuring that the total concentration of DNA was the same for both particles. Ultimately, C10 NANs appeared less diffuse within cells and colocalized less with lysosomes over time, achieving more significant knockdown of the target gene investigated, suggesting more effective endosomal escape. These differences indicate that the surfactant assembly and disassembly properties, including the number of surfactants per particle and the CMC can have important implications for the cellular delivery efficacy of DNA micelles and surfactant conjugates.

An emerging assay for rapid diagnosis of live Salmonella Typhimurium by exploiting aqueous/liquid crystal interface

The rapid and accurate identification of live pathogens with high proliferative ability is in great demand to mitigate foodborne infection outbreaks. Herein, we have developed an ultrasensitive image-based aptasensing array to directly detect live Salmonella typhimurium (S.T) cells. This method relies on the long-range orientation of surfactant-decorated liquid crystals (LCs) and the superiority of aptamers (aptST). The self-assembling of hydrophobic surfactant tails leads to a perpendicular/vertical ordered film at the aqueous/LC interface and signal-off response. The addition of aptST perturbed LCs’ ordering into a planar/tilted state at the aqueous phase due to electrostatic interactions between the surfactant with the aptST, and a signal-on response. Following the conformational switch of aptST in the presence of live S. typhimurium, a relative reversing signal-off response was observed upon the target concentration. This aptasensor could promptly confirm the presence of S. typhimurium without intricate DNA-extraction or pre-enrichment stats over a linear range of 1–1.1 × 106 CFU/mL and a detection limit of 1.2 CFU/mL within ∼30 min. These results were successfully validated using molecular and culture-based methods in spiked-milk samples, with a 92.61–104.61 % recovery value. Meanwhile, the flexibility of this portable sensing platform allows for its development and adoption for the precise detection of various pathogens in food and the environment.

Temperature response of sucrose palmitate solutions: Role of ratio between monoesters and diesters

HypothesisAqueous solutions of long-chain water-soluble sucrose ester surfactants exhibit non-trivial response to temperature variations, revealing a peak in viscosity around 40–50 °C. While previous investigations have explored the structures within sucrose stearate systems at various constant temperatures, a comprehensive understanding of the entire temperature dependence and the underlying molecular factors, contributing to this phenomenon is currently missing. ExperimentsTemperature dependent properties and supramolecular structures formed in aqueous solutions of commercial sucrose palmitate were examined using SAXS/WAXS, DSC, optical microscopy, rheological measurements, NMR, and cryo-TEM. FindingsThe underlying mechanism governing this unusual behavior is revealed and is shown to relate to the mono- to di-esters ratio in the solutions. Solutions primarily containing sucrose monoesters (monoesters molecules ≳ 98% of all surfactant molecules) exhibit behavior typical of nonionic surfactants, with minimal changes with temperature. In contrast, the coexistence of mono- and di-esters results in the formation of discrete monodisperse diester particles and a network of partially fused diester particles at low temperature. As the temperature approaches the diesters’ melting point, wormlike mixed micelles form, causing a viscosity peak. The height of this peak increases significantly with the diester concentration. Further temperature increase leads to fluidization of surfactant tails and formation of branched micelles, while excess diester molecules phase separate into distinct droplets.

Systematically varying zwitterionic-siloxane surfactant structure to affect interfacial properties, foam stability, and fire suppression

Zwitterionic siloxane-sulfobetaine (siloxaneSB) surfactants are developed to replace per- and poly-fluoroalkyl substances (PFAS) used currently in aqueous firefighting foams because PFAS pose significant threats to environment and human health. SiloxaneSB surfactant's head and tail structures are varied gradually, one step at a time, to vary hydrophilicity of the head and hydrophobicity, and oleophobicity of the tail. Identical methods and conditions are used across all the surfactants to correlate surfactant structure to interfacial and foam properties, and fire suppression. The results suggest that the amphiphilic balance of the siloxane surfactant have dramatic effects on CMC, interfacial tension, foam expansion ratio, foam degradation, heptane vapor permeation through a foam layer, and fire suppression but little effect on bubble size, coarsening, and liquid drainage. They show synergism between siloxaneSB and alkylpolyglycoside in gasoline fire suppression while trisiloxane-polyoxyethylene exhibited synergism in heptane fire suppression. A 28 ft2 gasoline pool-fire suppression results were consistent with the bench-scale findings for tetrasiloxaneSB and trisiloxane-polyoxyethylene formulations. Small changes to surfactant molecular structure can dramatically improve its amphiphilic balance, synergisms, fuel-foam interactions resulting in improved effectiveness of fluorine-free foams but depend very much on the fuel, unlike the fluorocarbon surfactants.

Self-Assembly of Lamellae-in-Lamellae by Double-Tail Cationic Surfactants.

The molecular structures of surfactants play a pivotal role in influencing their self-assembly behaviors. In this work, using simulations and experiments, an unconventional hierarchically layered structure in the didodecyldimethylammonium bromide (DDAB)/water binary system: lamellae-in-lamellae is revealed, a new self-assembly structure in surfactant system. This self-assembly structure refers to a lamellar structure with a shorter periodic length (inner lamellae) embedded in a lamellar phase with a longer periodic length (outer lamellae). The normal vectors of these two lamellar regions orient perpendicularly. In addition, it is observed that this lamellar-in-lamellar phase disappears when the two tails of the cationic surfactants become longer. The formation of the lamellar-in-lamellar architecture arises from multiple interacting factors. The key element is that the short tails of the DDAB surfactants enhance hydrophilicity and rigidity, which facilitates the formation of the inner lamellae. Moreover, the lateral monolayer of the inner lamellae provides shielding from the water and prompts the formation of the outer lamellae. These findings indicate that molecular structures and flexibility can profoundly redirect the hierarchical self-assembly behaviors in amphiphilic systems. More broadly, this work presents a new strategy to deliberately program hierarchical nanomaterials by designing specific surfactant molecules to act as tunable scaffolds, reactors, andcarriers.

Factors Influencing the Rheology of Methane Foam for Gas Mobility Control in High-Temperature, Proppant-Fractured Reservoirs

Gas-enhanced oil recovery (EOR) through huff-n-puff (HnP) is an important method of recovering oil from fracture-stimulated reservoirs. HnP productivity is hampered by fracture channeling, leading to early gas breakthroughs and gas losses. To mitigate these issues, foam-generating surfactants have been developed as a method of reducing injected gas phase mobility and increasing oil recovery. This work investigates foam generation and propagation by a proprietary surfactant blend in high-temperature, high-pressure, high-permeability, and high-shear conditions that simulate the environment of a proppant-packed fracture. Bulk foam tests confirmed the aqueous stability and foaming viability of the surfactant at the proposed conditions. Through several series of floods co-injecting methane gas and the surfactant solution through a proppant pack at residual oil saturation, the effects of several injection parameters on apparent foam viscosity were investigated. The foam exhibited an exceptionally high transition foam quality (>95%) and strong shear-thinning behavior. The foam viscosity also linearly decreased with increasing pressure. Another flood series conducted in an oil-free proppant pack showed that swelling of residual oil had no effect on the apparent foam viscosity and was not the reason for the inversely linear pressure dependency. An additional flood series with nitrogen as the injection gas was completed to see if the hydrophobic attraction between the methane and surfactant tail was responsible for the observed pressure trend, but the trend persisted even with nitrogen. In a previous study, the dependence of foam viscosity on pressure was found to be much weaker with a different foaming surfactant under similar conditions. Thus, a better understanding of this important phenomenon requires additional tests with a focus on the effect of pressure on interfacial surfactant adsorption.

Many-body dissipative particle dynamics simulations of micellization of sodium alkyl sulfates.

We present a study of micelle formation in alkyl sulfate surfactants using the simulation method of many-body dissipative particle dynamics (MDPD). We parametrise our model by tuning the intermolecular interactions in order to reproduce experimental values for the chemical potential and density at room temperature. Using this approach, we find that our model shows good agreement with experimental values for the critical micelle concentration (CMC). Furthermore, we show that our model can accurately predict CMC trends, which result from varying properties such as surfactant tail length and the salt concentration. We apply our model to investigate the effect of aggregation number on various micellar properties, such as the shape of individual micelles and the fraction of bound counterions. We show that micelles become aspherical at large aggregation numbers, in line with experimental predictions, and that longer tail surfactants are generally more spherical at all aggregation numbers compared to those which are shorter. We find excellent agreement between our simulations and experimental values for the degree of counterion binding, a factor that is crucial to accurately studying micellar shape, but one that is typically overlooked in the existing literature.

Investigation of the interfacial phenomena in the presence of nonionic surfactants and a silica nanoparticle at the n-decane-water interface: Insights from molecular dynamics simulation

In chemical enhanced oil recovery (EOR), the nanoparticles can be employed as conveyors to transfer the surfactants and impede the reduction of surfactant concentration. Here, a set of molecular dynamic simulations were carried out to examine the ability of hydrophobic silica nanoparticle (NP) to deliver the nonionic surfactant (Triethylene glycol monododecyl ether; C12E3) into the n-decane-water interface. Another set of simulations in which only surfactant molecules are initially located at the interface was performed in order to better characterize the influence of NP on the surfactant adsorption film.The NP exhibited high efficiency in delivering surfactant molecules to the n-decane-water interface. In addition, for the C12E3-NP and n-decane-NP, the Coulombic interactions are weaker than the van der Waals interactions confirming the nonpolar property of the silica nanoparticle. The density profiles indicate that as the concentration of surfactants increases at the interface, NP separates more from the surfactant layer. The interfacial tension (IFT) of n-decane-water systems containing only surfactants decreases as the surface coverage of the surfactants increases. Moreover, it was also proved that the intrinsic width of the interface increases with an increment in the surface coverage of surfactants. The surfactant molecules exhibit a great tendency to contribute as acceptors to the formation of hydrogen bonds with water molecules. With an increment in the surface coverage of surfactant molecules, the 2D radial distribution functions between the centers of mass (COMs) of surfactant tails show oscillations. Appeared oscillations represent the correlation between the surfactant tails. Presence of NP causes the oscillations start at lower surfactant coverage than without NP. The surfactant tails choose a random distribution at the low surface coverage of surfactants. In addition, they tend to be oriented partially perpendicular to the interface with an increase in surfactant surface coverage. Besides, NP causes the surfactant tails to orient more perpendicular to the interface than surfactants alone.

A “Limited Aggregation Model” to Predict the Size of Acrylamide‐Based Microgels Synthesized with Ionic Surfactants

AbstractThe size of acrylamide‐based microgels can be decreased by addition of ionic surfactants during the classical dispersion polymerization. Nevertheless, the mechanism of such syntheses is not well understood yet. Here, a “Limited Aggregation Model” is proposed by analogy with the limited coalescence mechanism occurring for Pickering emulsion stabilization. In such a model, nuclei aggregate until a constant, high enough surfactant surface coverage is reached, which ensures colloidal stability. Consequently, the total surface of the growing particles, linked to the inverse of their size, is linearly dependent on the surfactant concentration. This law is verified if the surfactant/polymer particle interaction is high enough to guarantee a “total adsorption” of the surfactants onto the particles. This simple model fits very well with all the data extracted from the literature, including very different synthesis conditions. Finally, it not only permits to predict the microgels size, but it is also an interesting tool to investigate the role of each synthesis parameter like initiator, solvent or polymer. For instance, it shows that the surfactant role is not linked to its charge, proving that a phenomenon complementary to the electrostatic repulsion, related to the surfactant tail, ensures the colloidal stability of the growing collapsed microgels.

The Characteristic Curvature (Cc) of Ionic Surfactants Assessed via Small-Angle X-ray Scattering.

The characteristic curvature (Cc), within the hydrophilic-lipophilic difference + net (Hn) - average (Ha) curvature (HLD-NAC) framework, is the dimensionless net curvature, -Hn·L (L is the surfactant's tail length parameter), that a surfactant acquires at the characteristic condition (T = 25 °C, no added cosurfactants, oil with an equivalent alkane carbon number (EACN) of zero and for ionic surfactants, a total salinity (S) of 1 g NaCl/100 mL). A recent article demonstrated the validity of the Cc concept, where Hn was assessed via oil and water solubilization radii. Here, we assess Hn from the characteristic length (ξ) obtained from the analysis of SAXS profiles of microemulsions produced at semicharacteristic conditions (characteristic condition but varying S). The predicted relationship, -L·Hn,semicharacteristic = Ccbi + bi·ln(S), was confirmed with the five ionic surfactants explored. The SAXS-assessed Cc (Cc = Ccbi/bi) values are consistent with those obtained from solubilization studies and phase inversion scans. The Cc-SAXS method provides a way to assess the hydrophobicity of ionic surfactants directly, avoiding the bias that could be introduced by cosurfactants in phase inversion studies and minimizing the impact of potential uncertainties in the surfactant volume to area ratio (vs/as) required to calculate the solubilization radii in the solubilization method.

Influence of water saturation on interlayer properties of HDTMA-, HDTMP-, and HDPy-modified montmorillonite organoclays

Surfactant-modified clay minerals, organoclays, are well-studied and used commercially for removing contaminants from water. Nevertheless, dry organoclay properties are typically used to understand and predict contaminant sorption. In this work, the effects of water saturation on two properties important to contaminant sorption were examined: organoclay interlayer space and surfactant alkyl tail conformational ordering. Montmorillonite was modified to 0.25–3.0 CEC using three surfactants: hexadecyltrimethylammonium (HDTMA), hexadecylpyridinium (HDPy), and hexadecyltrimethylphosphonium (HDTMP). At low surfactant-loadings (< 1 CEC and < 15% fOC), water saturation expanded the maximum organoclay interlayer by 4–8 Å, depending on the surfactant and its loading but had little effect on surfactant tail conformational ordering. At low surfactant loadings, water saturation likely promotes contaminant sorption relative to expectations based on dry organoclay properties, particularly for larger contaminants, due to a relaxation of the physical constraints of the interlayer. At higher surfactant loadings, HDTMA and HDTMP organoclays showed relatively small effects of water saturation, but effects were significant for HDPy organoclays. The maximum interlayer space of the HDPy organoclay at 20.7% fOC expanded 3-fold (+22.3 Å) upon hydration, which is attributed to a substantial rearrangement of the pyridinium headgroup. All HDPy organoclays with fOC > 20% experienced a notable increase in the alkyl tail conformational ordering upon hydration, becoming fully solid-like, even as negligible interlayer expansion occurred at the highest loadings. At high HDPy-loading water saturation would likely restrict sorption of contaminants relative to what might be expected based on dry properties, due to the energetic barrier of cavity formation in the more solid-like interlayer phase. This work provides evidence from three types of organoclays that water-saturated organoclay properties can differ substantially from those of dry organoclays. Characterization of organoclays under water-saturated conditions that are relevant to contaminant sorption will facilitate our understanding and improvement of organoclay performance.

A food safety gadget for acrylamide determination: Aptamer placement at surfactant-liquid crystal interface

A tag-free and sensitive aptasensor was introduced to quantify acrylamide based on pursuing the directional order of liquid crystal molecules (LCs) at the surfactant-decorated interface. The hydrophobic interaction of surfactant tails and LCs induced their well-ordered vertical design. The regular alignment of LCs was perturbed through the electrostatic interaction of the specific aptamer with the surfactant head, which altered the polarized texture of the aptasensor from dark to bright optical appearance. However, in the presence of acrylamide and its specific interaction with aptamer strand, LCs arranged vertically with a dark polarized texture. Using the designed aptasensor, acrylamide was detected over the ranges of 1 fmol L−1 to 1 nmol L−1 and 1 nmol L−1 to 50 μmol L−1 with the detection limit (LOD) of 0.28 fmol L−1 and the recovery value of 92.36–101.35%. Besides, the aptasensor successfully quantified acrylamide in the diverse real samples including decaffeinated coffee (36 μg kg−1), roasted coffee (159 μg kg−1), roasted date seeds (13–369 μg kg−1) and roasted quince (1556 μg kg−1). Acrylamide content of cocoa and instant date seed powder were less than LOD. The detection methodology described in this study provides a sensitive, facile, rapid and portable tool for LC-based sensing of acrylamide.

Effect of cationic surfactant tail length on the emulsion gel polymerization of styrene

Effect of cationic surfactant tail length on the emulsion gel polymerization of styrene

Synergism of Surfactant Mixture in Lowering Vapor-Liquid Interfacial Tension.

Through employing molecular dynamics, in this work, we study how a two-component surfactant mixture cooperatively reduces the interfacial tension of a flat vapor-liquid interface. Our simulation results show that in the presence of a given insoluble surfactant, adding a secondary surfactant would either further reduce interfacial tension, indicating a positive synergistic effect, or increase the interfacial tension instead, indicating a negative synergistic effect. The synergism of the surfactant mixture in lowering surface tension is found to depend strongly on the structure complementary effect between different surfactant components. The synergistic mechanisms are then interpreted with minimization of the bending free energy of the composite surfactant monolayer via cooperatively changing the monolayer spontaneous curvature. By roughly describing the monolayer spontaneous curvature with the balanced distribution of surfactant heads and tails, we confirm that the positive synergistic effect in lowering surface tension is featured with the increasingly symmetric head-tail distributions, while the negative synergistic effect is featured with the increasingly asymmetric head-tail distributions. Furthermore, our simulation results indicate that minimal interfacial tension can only be observed when the spontaneous curvature is nearly zero.

The effect of the number of hydrophobic tails of cationic ammonium surfactants on mesoporous TiO2 synthesized

Mesoporous titanium dioxide (TiO2) is one of the most studied mesoporous materials considering its special character and various applications. In the present work, mesoporous TiO2 was synthesized by a sol–gel method employing different hydrophobic tails of ammonium cationic surfactants templates. The prepared samples were characterized by various techniques. The XRD profiles confirmed that all samples crystallized into the TiO2 anatase phase. The crystallite size of all samples was found to vary in the range of 8.60 nm to 13.61 nm. The transition temperature of the anatase phase was increased to several Celsius degrees since TiO2 was fabricated with a template assistant. The surface area of the mesoporous TiO2 was increased in the range of 93 m2.g−1 (CTAB) − 116.8 m2.g−1 (MTAB). These values were larger than the TiO2 synthesized without a template (72 m2.g−1). The total pore volume was also increased between 0.1704 cm3.g−1 (CTAB) and 0.300 cm3.g−1 (MTAB), while the TiO2 synthesized without a template was only 0.161 cm3.g−1. Using CTAB and DDAB yield a uniform mesopore size distribution. MTAB tends to produce non-uniform pore of the mesoporous system. The soft-templating method opens up new possibilities for synthesizing mesoporous metal oxides.

Synergism and molecular mismatch in rhamnolipid/CTAC catanionic surfactant mixtures

Rhamnolipids represent a promising class of biosurfactants, whose industrial exploitation could potentially reduce the ecological impact of surfactant-based formulations. In this direction, the rational design of rhamnolipid mixtures with other surfactants constitutes a strategic field of research.In this work, we combine mono-rhamnolipid (mRha) with the cationic surfactant cetyltrimethylammonium chloride (CTAC). The study is conducted at pH = 7.1, so that the mRha carboxylic group is dissociated and catanionic mRha-CTAC mixtures form. The system is investigated across the entire composition range by surface tension and conductivity measurements, which show a strong synergism between the two surfactants in forming mixed aggregates, specifically in mRha-rich mixtures. No coacervation or precipitation is observed, due to the strong asymmetry between the surfactant molecular architectures, with mRha presenting a bulky headgroup and two relatively short tails compared to the single long tail of CTAC. Dynamic Light Scattering (DLS) and Small Angle Neutron Scattering (SANS) experiments show that, while the single surfactants predominantly form small micelles, their mixing results in the spontaneous formation of vesicles. Within the vesicle bilayer, the termini of the longer surfactant tails fold, assuming disordered conformations, as probed by Electron Paramagnetic Resonance (EPR) measurements.These results point to the rational design of rhamnolipid-based catanionic mixtures as a suitable tool to optimize their structural properties, constituting a solid basis for their practical applications.

Complexation between Dendritic Polyelectrolytes and Amphiphilic Surfactants: The Impact of Surfactant Concentration and Hydrophobicity

The solution structure of complexes composed of the cationic dendrimer, anionic surfactants, and their counterions is studied by molecular dynamics simulations using the bead–spring coarse-grained model. We identify that depending on the surfactant concentration and hydrophobicity the system exists in three structural regimes. In the first regime surfactant molecules are noncooperatively absorbed by the dendrimer. Here, the pervaded volume of the dendrimer and the bulk solution contain loosely distributed unimeric surfactants and counterions. In the second regime hydrophobic attractions between surfactant tails give rise to cooperative binding. The absorbed surfactants self-assembly into multichain micellar-like aggregates, whereas the bulk solution consists of separate unimers. In the third regime the surfactants form multichain aggregates both within the interior of the dendrimer and in the bulk. Our data indicate that self-assembly of the absorbed surfactants affects the dendrimer conformations as compared to its neutral counterpart. In the case of noncooperative binding the dendritic polyelectrolyte swells due to the osmotic pressure exerted by the absorbed unimers and counterions. In the regimes characterized by cooperative binding and pronounced self-aggregation of the encapsulated surfactants swelling of the dendrimer is suppressed. This is attributed to a reduction of the osmotic pressure inside the macromolecule due to the ion exchange phenomenon appearing as a result of strong Coulomb attractions between charged monomers of the dendrimer and the encapsulated micellar aggregates. We also find that aggregate formation affects the overall charge accumulated in the dendrimer. In particular, for the dendrimer loaded with massive aggregates, we report the charge inversion effect which transforms the cationic dendrimer into an anionic dendrimer–surfactant complex. Our results provide molecular insights into self-assembly of supramolecular complexes and controlled absorption of guest surfactant molecules in dendrimer-based host–guest systems.