Static and dynamic light-scattering studies on micellar solutions of alkyldimethylbenzylammonium chlorides

Formation and structural features of micelles formed by surfactin homologues

Surfactin, a group of cyclic lipopeptides produced by Bacillus subtilis, possesses surfactant properties and is a promising natural and biologically active compound. In this study, we present a comprehensive characterization of surfactin, including its production, chromatographic separation into pure homologues (C₁₂, C₁₃, C₁₄, C₁₅), and investigation of their physicochemical properties. We determined adsorption isotherms and interpreted them using the Gibbs adsorption equation, revealing that the C₁₅ homologue exhibited the strongest surface tension reduction (27.5 mN/m), while surface activity decreased with decreasing carbon chain length (32.2 mN/m for C₁₂). Critical micelle concentration (CMC) were also determined, showing a decrease in CMC values from 0.35 mM for C₁₂ to 0.08 mM for C₁₅. We employed dynamic light scattering (DLS), transmission electron microscopy (TEM), and density functional theory (DFT) calculations to estimate the size of micellar aggregates, which increased with longer carbon chains, ranging from 4.7 nm for C₁₂ to 5.7 nm for C₁₅. Furthermore, aggregation numbers were determined, revealing the number of molecules in a micelle. Contact angles and emulsification indexes (E₂₄) were measured to assess the functional properties of the homologues, showing that wettability increased with chain length up to C₁₄, which is intriguing as C₁₄ is the most abundant homologue. Our findings highlight the relationship between the structure and properties of surfactin, providing valuable insights for understanding its biological significance and potential applications in various industries. Moreover, the methodology developed in this study can be readily applied to other cyclic lipopeptides, facilitating a better understanding of their structure-properties relationship.

Combination of a protic ionic liquid-like surfactant and biocompatible solvents to generate environmentally friendly anionic reverse micelles

imim–DEHP, a versatile protic IL-like surfactant to formulate aqueous RMs in biocompatible non-polar solvents.

Revisiting the colloidal fundamentals of water-dispersible polyesters: interactions and self-assembly of polymer nanoaggregates in water

Water-dispersible sulfopolyesters are a major class of film-forming and solution-modifying polymers, which are routinely used in applications such as inks, adhesives, coatings, and personal care products. Since these polyesters are designed to be used as waterborne dispersions, understanding their colloidal interactions in dispersions is critical for their application. By using a range of commercially available water-dispersible sulfopolyesters as a model system, we investigated the relationship between their molecular composition, colloidal interactions, and phase equilibria. We established how these polyesters undergo different molecular configurations and nanoaggregated states, depending on the nature of the liquid medium. For example, the polyesters are in a solvated molecular form in certain organic solvents, whereas they self-assemble into compact nanoaggregates in water. We found that the interactions of these nanoaggregates follow the classical DLVO theory of critical colloidal coagulation where the stability of these nanoparticles is extremely sensitive to multivalent electrolytes (i.e., C_crit ∝ z^-6). By using static, dynamic, and electrophoretic light scattering, we correlate their nanoscale intermolecular and interparticle interactions with corresponding macroscale phase behavior in both organic medium and water, based on the theoretical framework of second virial coefficients. We present a model for nanoaggregate formation in water based on the critical surface charge density of these nanoparticles. Such fundamental understanding of colloidal interactions could be used to efficiently control and improve the colloidal stability and film-formation ability of these polyesters and may enable the design of novel high-performance surfactant-free waterborne dispersion systems.

Improvement of the amphiphilic properties of a dialkyl phosphate by creation of a protic ionic liquid-like surfactant

Imim-DEHP, an interesting protic IL-like surfactant to create RMs in aliphatic and aromatic non-polar solvents and unilamellar vesicles in water.

Self-Assembly and Critical Aggregation Concentration Measurements of ABA Triblock Copolymers with Varying B Block Types: Model Development, Prediction, and Validation

The dissipative particle dynamics (DPD) simulation technique is a coarse-grained (CG) molecular dynamics-based approach that can effectively capture the hydrodynamics of complex systems while retaining essential information about the structural properties of the molecular species. An advantageous feature of DPD is that it utilizes soft repulsive interactions between the beads, which are CG representation of groups of atoms or molecules. In this study, we used the DPD simulation technique to study the aggregation characteristics of ABA triblock copolymers in aqueous medium. Pluronic polymers (PEG-PPO-PEG) were modeled as two segments of hydrophilic beads and one segment of hydrophobic beads. Tyrosine-derived PEG5K-b-oligo(desaminotyrosyl tyrosine octyl ester-suberate)-b-PEG5K (PEG5K-oligo(DTO-SA)-PEG5K) block copolymers possess alternate rigid and flexible components along the hydrophobic oligo(DTO-SA) chain, and were modeled as two segments of hydrophilic beads and one segment of hydrophobic, alternate soft and hard beads. The formation, structure, and morphology of the initial aggregation of the polymer molecules in aqueous medium were investigated by following the aggregation dynamics. The dimensions of the aggregates predicted by the computational approach were in good agreement with corresponding results from experiments, for the Pluronic and PEG5K-oligo(DTO-SA)-PEG5K block copolymers. In addition, DPD simulations were utilized to determine the critical aggregation concentration (CAC), which was compared with corresponding results from an experimental approach. For Pluronic polymers F68, F88, F108, and F127, the computational results agreed well with experimental measurements of the CAC measurements. For PEG5K-b-oligo(DTO-SA)-b-PEG5K block polymers, the complexity in polymer structure made it difficult to directly determine their CAC values via the CG scheme. Therefore, we determined CAC values of a series of triblock copolymers with 3-8 DTO-SA units using DPD simulations, and used these results to predict the CAC values of triblock copolymers with higher molecular weights by extrapolation. In parallel, a PEG5K-b-oligo(DTO-SA)-b-PEG5K block copolymer was synthesized, and the CAC value was determined experimentally using the pyrene method. The experimental CAC value agreed well with the CAC value predicted by simulation. These results validate our CG models, and demonstrate an avenue to simulate and predict aggregation characteristics of ABA amphiphilic triblock copolymers with complex structures.

How the cation 1-butyl-3-methylimidazolium impacts the interaction between the entrapped water and the reverse micelle interface created with an ionic liquid-like surfactant

The behavior of the interfacial water entrapped in reverse micelles (RMs) formed by the ionic liquid-like surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT) dissolved in benzene (or chlorobenzene) was investigated using noninvasive techniques such as dynamic light scattering (DLS), static light scattering (SLS), FT-IR and (1)H NMR. The DLS and SLS results reveal the formation of discrete spherical and non-interacting water droplets stabilized by the bmim-AOT surfactant. Moreover, since the droplet size increases as the W0 (W0 = [water]/[surfactant]) value increases, water interacts with the RM interface. From FT-IR and (1)H NMR data, a weaker water-surfactant interaction in bmim-AOT RMs in comparison with the RMs created by sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) is detected. Consequently, there are less water molecules interacting with the interface in bmim-AOT RMs, and their hydrogen bond network is not completely disrupted as they are in Na-AOT RMs. The results show how the nature of the new cation impacts the interaction between the entrapped water and the RM interface, modifying the interfacial water structure in comparison with the results known for Na-AOT.

Applications of polymer micelles for imaging and drug delivery

Polymeric micelles, self-assembling nano-constructs of amphiphilic copolymers, are widely considered as convenient nano-carriers for a variety of applications, such as diagnostic imaging, and drug and gene delivery. They have demonstrated a variety of favorable properties including biocompatibility, longevity, high stability in vitro and in vivo, capacity to effectively solubilize a variety of poorly soluble drugs, changing the release profile of the incorporated pharmaceutical agents, and the ability to accumulate in the target zone based on the enhanced permeability and retention effect. Moreover, additional functions can be imparted to the micelle-based delivery systems by engineering their surface for specific applications. Various targeting ligands can be attached for cell or intracellular accumulation at a site of interest. Also, the chelation or incorporation of imaging moieties into the micelle structure enables in vivo biodistribution studies. Moreover, pH-, thermo-, ultrasound-, enzyme- and light-sensitive block-copolymers allow for controlled micelle dissociation and triggered drug release in response to the pathological environment-specific stimuli and/or externally applied signals. The combination of these approaches can further improve specificity and efficacy of micelle-based drug delivery to promote the development of smart multifunctional micelles.

Investigation of various types of inverse micelles in nonpolar liquids using transient current measurements

Transient current measurements are used to characterize a wide variety of charge carriers in nonpolar liquids. The transient current method allows us to obtain both the concentration and mobility of charge carriers and therefore also the hydrodynamic radius using Stokes' law. In this article, five different surfactants in dodecane are investigated: OLOA11K, Solsperse13940, Span80, Span85, and AOT. We show that different types of currents are observed depending on the size of the inverse micelles. For large inverse micelles such as for OLOA11K, Solsperse13940, and Span80, the measurement of the transient current is straightforward because of the low steady-state current level. However, for small inverse micelles such as AOT and Span85, the current from the generation of charges is much larger such that high voltages, a small distance between the electrodes, and dielectric coatings on the electrodes are required to measure the signal related to the initially present charged inverse micelles. The estimated hydrodynamic radii of AOT and Span85, the two smallest inverse micelles, are in good agreement with the values reported in the literature. The comparison of the transient currents with simulations indicates that the dynamics of the charge transport are well-understood.

Effect of the cationic surfactant moiety on the structure of water entrapped in two catanionic reverse micelles created from ionic liquid-like surfactants

The behavior of water entrapped in reverse micelles (RMs) formed by two catanionic ionic liquid-like surfactants, benzyl-n-hexadecyldimethylammonium 1,4-bis-2-ethylhexylsulfosuccinate (AOT-BHD) and cetyltrimethylammonium 1,4-bis-2-ethylhexylsulfosuccinate (AOT-CTA), was investigated by using dynamic (DLS) and static (SLS) light scattering, FTIR, and (1)H NMR spectroscopy techniques. To the best of our knowledge, this is the first report in which AOT-CTA has been used to create RMs and encapsulate water. DLS and SLS results revealed the formation of RMs in benzene and the interaction of water with the RM interface. From FTIR and (1)H NMR spectroscopy data, a difference in the magnitude of the water-catanionic surfactant interaction at the interface is observed. For the AOT-BHD RMs, a strong water-surfactant interaction can be invoked whereas for AOT-CTA this interaction seems to be weaker. Consequently, more water molecules interact with the interface in AOT-BHD RMs with a completely disrupted hydrogen-bond network, than in AOT-CTA RMs in which the water structure is partially preserved. We suggest that the benzyl group present in the BHD(+) moiety in AOT-BHD is responsible for the behavior of the catanionic interface in comparison with the interface created in AOT-CTA. These results show that a simple change in the cationic component in the catanionic surfactant promotes remarkable changes in the RMs interface with interesting consequences, in particular when using the systems as nanoreactors.

Adsorption of the disinfectant benzalkonium chloride on montmorillonite. Synergistic effect in mixture of molecules with different chain lengths

The biocide benzalkonium chloride (BAC) is a mix of cationic alkylbenzyldimethylammonium surfactants having different alkyl chain lengths. A comparative study of adsorption on the phyllosilicate clay montmorillonite of two of these surfactants, with alkyl chains having respectively 12 C atoms (BAC-12) and 14 C atoms (BAC-14), and a mixture of both surfactants is presented in this work. Adsorption isotherms were performed for individual surfactants and for a 1:1 mixture BAC-12+BAC-14. The adsorption was investigated in an ample concentration range that covers almost seven orders of magnitude in concentrations (from 1 nM to 10 mM), range that includes environmentally relevant concentrations. Quantification of BAC was performed by HPLC-UV and LC-MS and the results were completed with powder X-Ray diffraction. The adsorption of both surfactants leads to adsorption isotherms with two well differentiated steps. The first step corresponds almost exclusively to a cation exchange process, and the binding constant is very similar for both surfactants. The second step of the isotherms is observed at higher concentrations and adsorption is mainly driven by lateral interactions between surfactant molecules. The binding constant of this step is larger for BAC-14 than for BAC-12. Adsorption from a BAC-12+BAC-14 mixture shows a synergistic behaviour, possibly due to a better packing arrangement in the interlayer. Calculations show that in natural systems silicate clays are major sorbents of BAC at low concentrations whereas binding to humic acid is predominant at high concentrations.

The UV Spectroscopic Determination of Two Critical Micelle Concentrations of Domiphen Bromide in Solutions with KBr and Calculations of the Packing Parameter

Three different forms of the surface-active domiphen cations were detected by the UV spectroscopy in aqueous solutions of domiphen bromide, eventually with KBr additive. In sufficiently dilute solutions there are free domiphen cations, which aggregate into two types of micelles at two distinct concentrations, interpreted as the first and second critical micelle concentration, cmc and cmc2. From the logarithmic dependences of cmc and cmc2 on the bromide concentrations the corresponding degrees of counterion binding were estimated. Conformation analysis of the domiphen cation was performed and the packing parameters of the lowest energy conformers were calculated. The observed spectral shifts and the calculated packing parameters suggest that two conformers of domiphen cation with different arrangement of the phenoxyethyl group are responsible for the respective formation of two types of micelles.

Adsorption of cationic surfactants on covered hanging mercury drop electrode surface of variable area

Cetyldimethylbenzylammonium chloride (CDBACl) or cetyltrimethylammonium bromide (CTAB) is preadsorbed on mercury and used as substrate. The adsorptive stripping voltammetry with the two-step procedure is used. The mercury droplet with the preadsorbed surfactant is expanded in aqueous solutions of KCl, KBr, CTAB, CDBACl, or cetylethyldimethylammonium bromide (CEDAB). The surface area was increased from 0.0022cm(2) up to 0.0571cm(2). The surfactant molecules are maintained close to each other and in the vicinity of the electrode by the applied electric field. The expanding of the droplets resulted in a reorientation of the adsorbed molecules depending on the surfactant surface concentration. In some cases, condensed films were observed. Differences were noticed in the adsorption and desorption potential region. A linear increase in the capacitance current with the surface area was found in all cases up to a maximum increase in the surface area. Partly disorganized films were also observed. In some cases, defects were noticed during expansion. In one case, fractal structure was observed.

Adsorption of cetyltrimethylammonium bromide and/or cetyldimethylbenzylammonium chloride on partly covered hanging mercury drop electrode

Adsorbed cetyldimethylbenzylammonium chloride (CDBACl) or cetyltrimethylammonium bromide (CTAB) on mercury is used as template for the adsorption of CTAB, CDBACl, or their equimolar mixture at 20 °C. Adsorptive stripping voltammetry with the two step procedure is used. The results are compared with previously published results on the adsorption of CTAB and CDBACl on mercury and then transferred in base electrolyte. A surfactant is preadsorbed. The adsorption of the second does not remove away from the mercury the first one, as evidenced by the capacitance measurements and the repeated scans. The surfactants were maintained close to each other and in the vicinity of the electrode by the applied electric field. In all cases studied, there was a decrease in the capacitance in the potential range -0.8 to -1 V to very low capacitance values forming condensed film. Mixed films and synergy effects were observed. The already adsorbed CTAB on mercury did not permit the desorption-reorientation peaks of CDBACl. Shifts of the capacitance peaks were observed to more positive potentials and were attributed to the occurrence of a slow change in the organization of the monolayer. The electrical state of the preadsorbed surfactant would be of critical importance in the formation of the various structures. The results suggested that the ordering and arrangement of molecules could be controlled by appropriate selection of templates.

Condensed film formation of binary mixtures of cetyltrimethylammonium bromide and cetyldimethylbenzylammonium chloride in a wide potential region at the mercury/electrolyte interface

The adsorption of binary mixtures of cetyltrimethylammonium bromide (CTAB) and cetyldimethylbenzylammonium chloride (CDBACl) at the mercury/electrolyte interface was studied in various electrolyte systems. The optimum surfactant concentration and electrolyte ratio was searched for, to obtain the formation of a condensed film at as wide a potential range as possible at the highest temperature possible. The optimum conditions found were 2 x 10(-4) M CTAB and 2 x 10(-4) M CDBACl in 0.07 M KF and 0.03 M KBr at 2 degrees C. The capacitances vs time curves were used for the reconstruction of isochronous capacitance vs potential curves. These curves showed that in that system the condensed film was formed in the potential range from -0.4 to -1.9 V vs Ag/AgCl in less than 220 s. The stability of this film, following the removal of the mercury drop from the solution, was also studied.

Effect of the deaeration on the adsorption of some cationic surfactants at the mercury/electrolyte solution interface

The effect of the deaeration on the adsorption of three cationic surfactants cetyldimethylbenzylammonium chloride (CDBACl), cetyltrimethylammonium bromide (CTAB) and octadecyltrimethylammonium bromide (OTAB) at the mercury/electrolyte solution interface is studied. The deaeration is studied using either nitrogen or helium and the effect of deaeration process and time is also studied. In all cases an effect of the deaeration time is found which is mainly observed at potentials where a condensed film is formed. Capacity-time curves at the potentials where the film is formed show a nucleation and growth mechanism with induction time that depends on the deaeration time. The deaeration slows down the kinetics of the film formation but does not change the equilibrium capacitance value of the film. The decrease of the dissolved gas from the water that perturbs its structure is perhaps the main reason for the behaviour observed during the adsorption of these surfactants.

On the adsorption of cetyldimethylbenzylammonium chloride at the mercury/electrolyte solution interface

The adsorption of cetyldimethylbenzylammonium chloride (CDBACl) on the hanging mercury electrode is studied in various supporting electrolytes at various temperatures from 1 to 50 degrees C. A condensed film with low capacitance is formed at negative potentials at transition temperatures below approximately 40 degrees C. The decrease of the temperature favors the film formation, and increases the width of the capacitance pit, while its value remains practically constant. Hysteresis phenomena are also observed during different scan directions. Capacitance-time curves at the potentials where the film is formed show in some cases a nucleation and growth mechanism with induction time and studied by the Avrami formulation. At high temperatures an increase of the capacitance with time is observed depending on the CDBACl concentration and slightly on the electrolyte used, and is attributed to the formation of hemimicelles. At high negative potentials a second narrow region with lower capacitance values is observed. This is easily observed at very high temperatures, while it is absent at lower temperatures. It depends upon the concentration of CDBACl and the electrolyte used. The results are different from those obtained for the adsorption of cetyltrimethylammonium bromide on mercury, indicating the importance of interaction between the hydrophobic chains.