Ellipsoidal Micelles Research Articles

Visible Light-Responsive Composition-Dependent Morphology and Cargo Release in Mixed Micelles of Dendron Amphiphiles.

2,2-Bis-(methylol)propionic acid-based second-generation polyester dendron amphiphile (T-D) containing visible light-responsive donor-acceptor Stenhouse adduct (DASA) as hydrophobic tails is synthesized. Micelles of T-D amphiphile and its mixed micelles of varying compositions with nonresponsive dendron amphiphile containing lauryl groups are prepared in aqueous solution. In transmission electron microscopy and atomic force microscopy analyses, T-D amphiphiles show rice grain-like ellipsoidal micelles as the predominant morphology. Mixed micelles display a composition-dependent morphology gradient such that the morphology changes from rice grain like to mixed to completely spherical with decreasing content of the T-D amphiphile. Complete morphology change to spherical micelles and partial reversal to ellipsoidal micelles, finally leading to ill-defined aggregates, are observed when the T-D amphiphile micelles are subjected to visible light-dark storage photoswitching cycles. Small-angle neutron scattering (SANS) analysis of 1 wt.% micellar solution in THF:water (10:90) reveals only a minor change in shape and size upon photoirradiation, and the data could be fitted to spherical or ellipsoidal model. Release of hydrophobic dye from mixed micelles is tuned by the content of the photoresponsive amphiphile. Cellular uptake and visible light-triggered release of hydrophobic drug from mixed micelles are demonstrated using MDA-MB-231 cells, suggesting their applicability for photoresponsive drug delivery.

Dissipative particle dynamics simulations on the self-assembly of rod-coil asymmetric diblock molecular brushes bearing responsive side chains.

The self-assembly behaviors of rod-coil asymmetric diblock molecular brushes (ADMBs) bearing responsive side chains in a selective solvent are investigated via dissipative particle dynamics simulations. By systematically varying the polymerization degree, copolymer concentration, and side chain length, several morphological phase diagrams were constructed. ADMB assemblies exhibited a rich variety of morphologies, including cylindrical micelles, spherical micelles, nanowires, polyhedral micelles, ellipsoid micelles, and large compound micelles. The structures of the representative nanowires were analyzed in detail. A kinetics study revealed that the one-dimensional growth of nanowires follows the step-growth polymerization mechanism. Besides, by calculating the local order parameter of the rigid chains, we found that increasing the lengths of A and C side chains can promote the ordered arrangement of the rigid chains. Moreover, the rod-to-coil conformation transitions were simulated to explore the stimuli-responsive behaviors of ADMBs with responsive rigid side chains. The simulation results indicated that the volume of the assemblies expanded without the support of the rigid chains. The present work not only provides a comprehensive understanding of the self-assembly behaviors of ADMBs but also provides meaningful theoretical support for the development of novel molecular brush materials.

Worm-like Micelle Formation of Sugar-Based Nonionic Surfactant with Multibranched Methyl Chains.

The aggregation properties of sugar-based surfactants with methyl groups at multiple positions on the linear alkyl chain are an understudied topic. Therefore, in this study, we performed a structural analysis of aggregates formed by two types of sugar-based nonionic surfactants with multibranched methyl chains, 3,7,11-trimethyldodecyl maltoside (bC15Mal), as well as with a linear chain, dodecyl-β-d-maltoside (C12Mal), in aqueous solutions using rheological and small-angle X-ray scattering techniques. bC15Mal showed an excellent performance in lowering surface tension (28.5 mN m-1), exhibiting efficient adsorption and orientation at the air/water interface. bC15Mal formed rod-like micelles at a concentration of 10 mmol dm-3 in aqueous solutions and transitioned to worm-like micelles at 50 mmol dm-3, followed by gel solutions at 700 mmol dm-3. As the surfactant concentration increased, the length of the worm-like micelles decreased, and the mesh size decreased, indicating that the strings were densely packed. Conversely, C12Mal formed ellipsoidal micelles over a wide range of concentrations from 50 to 700 mmol dm-3. The major axis of the core and the overall axis of the ellipsoidal micelles decreased with increasing concentrations. Thus, it was clear that the introduction of methyl groups at positions 3, 7, and 11 of the dodecyl chain of the sugar-based surfactant resulted in a unique aggregation behavior, that is, the formation of aggregates with high-order structures at low concentrations.

Ellipsoidal micelle formation of quaternary ammonium salt-based gemini surfactants: structural analysis through small-angle X-ray scattering.

Gemini surfactants exhibit better adsorption and aggregation properties than those of monomeric surfactants. However, to enhance the functional properties of gemini surfactants, the effect of spacer structures on their aggregation behavior must be elucidated. Small-angle X-ray scattering (SAXS) has been performed to study the aggregate structures of monomeric surfactants, but its application has not been expanded to the analysis of gemini surfactants. Therefore, in this study, we performed a structural analysis of aggregates formed by quaternary ammonium salt-based gemini surfactants with different spacer structures in aqueous solutions through viscosity, dynamic light scattering (DLS), and SAXS measurements. We also investigated the effects of the spacer structure and surfactant concentration on the aggregation behavior. The DLS results indicated that gemini surfactants with spacers containing cyclic structures, such as diethylene and triethylene chains, formed small micelles (several nanometers in size) at the limiting concentration for dissolution in water. In contrast, gemini surfactants with nitrogen and oxygen atoms at the center of the spacer formed ellipsoidal micelles at low concentrations, as shown by SAXS results. The core and overall radii of the minor and major axes of the ellipsoidal micelles decreased with increasing surfactant concentration and were larger for spacers with two ethylene chains connected to central nitrogen and oxygen atoms than for spacers with rigid diethylene and ethylene chains connected to a central nitrogen atom.

Salt-Induced Linker Dehydration Modulates Micellar Structure in Ether-Linked Sulfate Surfactants.

Ether-linked surfactants are widely used in formulations such as liquid soaps, but despite their ubiquity, it is unclear how n-ethylene glycol linkers in surfactants, such as sodium lauryl n-(ethylene glycol) sulfate (SLEnS), influence micellar packing in the presence of NaCl. In the present work, we probe the structure and hydration of ether linkers in micelles comprising monodisperse SLEnS surfactants using contrast-variation small-angle neutron scattering (CV-SANS) and small-angle X-ray scattering (SAXS). Using SAXS, changes in micellar structure were observed for SLEnS (n = 1, 2, or 3) arising from the extent of ethoxylation. Scattering profiles indicated a clear transition from elongated cylindrical micelles to shorter ellipsoidal micelles with increasing ethoxylation. With CV-SANS, micellar structure and linker geometries of SLE3S were able to be resolved, indicating that a change in micellar architecture is modulated by dehydration of the tri(ethylene glycol) linker, offering new insights into the role of water and ions in the self-assembly of this key class of surfactants.

5-Fluoro uracil mediated transformation of surface-active ionic liquid based micellar nanoaggregates into pH-responsive vesicular nanoaggregates as promising drug delivery vehicle

Surface-active ionic liquids (SAILs) are poised as significant contenders in sustained and targeted drug delivery due to their membrane-like structures and their ability to transport drug molecules regardless of their polarity. In order to design a simple system that an encapsulate the anticancer drug and release it in a sustained manner, herein we designed vesicular nano-aggregates through synergistically interacting chemotherapeutic drug 5-Fluoro Uracil (5-FU) with the biocompatible SAILs, Choline Oleate ([Ch][Ol]) and Tetramethyl guanidine Oleate ([TMG][Ol]). The intention for designing the system is to offers less complexity and improved drug loading efficiency and targeted and sustained release. The molecular interactions have been proven by computational approach and the size of the nanoaggregates was determined using techniques such as Dynamic Light Scattering (DLS), Förster Resonance Energy Transfer (FRET) and Small-Angle Neutron Scattering (SANS). SANS fitting data and TEM images confirmed the transition of prolate ellipsoid micelles at [5-FU] = 0 mM to uni-lamellar ([5-FU] = 1 mM) and multi-lamellar vesicles ([5-FU] = 2 mM). These vesicular nano-aggregates are studied for their stability against temperature, pH-response and ionic strength of solution, revealing stability up to 60 °C, robust at physiological pH 7.4 and remain stable to exposure to Na+ and Ca2+ ions at concentrations up to 180 mM and 50 mM, respectively. The vesicular nano-aggregates ([Ch][Ol]/5-FU and [TMG][Ol]/5-FU) when exposed to physiological pH (i.e., 7.4) released 64.8 % and 61.2 % drug release whereas when the same systems were exposed to cancerous pH (5.2 and 6.7) released 95.9 % and 84.5% (pH 5.2 and 6.7) and 94.8 % and 81.2 % (pH 5.2 and 6.7) for the [Ch][Ol]/5-FU and [TMG][Ol]/5-FU systems, respectively. Results for the studied system encompasses them as a promising candidate for drug release at specific sites. The anticipated research might pave the way for the development of a biocompatible and stimuli-responsive drug carrier that would allow for the localized delivery of an anticancer drug.

Self-assembling properties of mono and di-rhamnolipids characterized using small-angle X-ray scattering

Rhamnolipids are glycolipid surfactants composed by a hydrophilic head of either one (mono-RL) or two (di-RL) rhamnose moieties coupled to hydroxyaliphatic chains that can be of different lengths. In spite of their importance in different fields of applications, as bioremediation processes for instance, self-aggregation physico-chemical properties of RLs are not unique. This because a variety of aggregates morphologies (shape and size) can either exist or coexist in aqueous dispersion due to mono-RL:di-RL molar ratio, hydrophobic tails length, pH and the presence of co-surfactants and additives. Recently, a theorethical approach reported the self-assembling morphologies of either pure mono or di-RL in aqueous environment, predicting the formation of spherical to ellipsoidal micelles to worm-like and disk-like aggregates depending on RL concentration and fatty acid chain length. In order to add new information to those previously available, the present work investigated the self-assembling properties of mono-RL-C10-C10 and di-RL-C10-C10 separately in aqueous dispersion by small angle X-Ray scattering (SAXS). A novel approach was applied to the data analysis coupling the scattering length density profiles of the RLs chemical groups and Monte Carlo simulations. Such an approach allowed us to infer about the preferred mono-RL and di-RL conformations that fit better in the self-assembling morphologies. In this way, we show that mono-RL-C10-C10 self-assembles into lamella-like aggregates coexisting with 30 % of multi-lamella aggregates (circa of 5 closed stacked lamella) from a concentration ranging from 10 to 50 mM, with hydrophobic thickness of about 12 Å, a hydrated polar head thickness of 10 Å, and an area per glycolipid of 76 Å2. On the other hand, di-RL prefers to self-associate into flexible cylinder-like aggregates, from 70 mM to 110 mM concentration, with hydrophobic radius on the order of 7.5 Å, a hydrated polar shell of 21.5 Å, with hydropobic/polar interface of 110 Å2 per glycolipid. Interestingly, the parameters obtained from the best fitting to the experimental data associated to the volume fraction distribution of the chemical groups within the aggregates revealed that the hydrophobic chains are more disordered in mono-RL planar aggregates than in di-RL worm-like aggregates, as well as the hydration properties. Further, the addition of 100 mM NaCl in di-RL aqueous dispersion leads to the formation of longer worm-like aggregates. Taking together, this work opens a new avenue regarding characterization of biosurfactants self-assembling properties by using SAXS, also contributing to prepare more efficient biosurfactant dispersions depending on the desired applications in industrial sectors and bioremediation.

Concentration Dependent Asymmetric Synergy in SDS-DDAO Mixed Surfactant Micelles.

We investigate the structure and interactions of a model anionic/amphoteric mixed surfactant micellar system, namely, sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO), employing SANS, FTIR, DLS, and pH measurements, in the range 0.1-100 mM total surfactant concentration and 0-100% DDAO. Increasing surfactant concentration is found to elongate the prolate ellipsoid micelles (RPolar ∼ 25-40 Å), accompanied by up to a 6-fold increase in micellar charge. The surfactant synergy, in terms of micellar charge and size, diffusion coefficient, solution pH, and headgroup interactions, was found to vary with concentration. At lower concentrations (≤50 mM), the SDS-DDAO ratio of maximum synergy is found to be asymmetric (at 65-85% DDAO), which is rationalized using regular solution theory, suggesting an equilibrium between Na+ dissociation, DDAO protonation, and counterion concentration. At higher concentrations, maximum synergy shifts toward the equimolar ratio. Overall, our study expands and unifies previous reports, providing a comprehensive understanding for this model, synergetic mixed micellar system.

Structural Analysis of Aggregates Formed by Linear- and Star-type Quaternary Ammonium Salt-Based Trimeric Surfactants Using Rheology and Small-Angle X-ray Scattering.

We performed a structural analysis of aggregates formed by two types of trimeric surfactants based on quaternary ammonium salts─linear-type 3Cnlin-s-Q and star-type 3Cntris-s-Q─featuring varying alkyl chain lengths (n) and spacer chain lengths (s) in aqueous solutions. We performed rheology, dynamic light scattering, and small-angle X-ray scattering measurements on the trimeric surfactants and investigated the effects of the alkyl chain length, spacer chain length, spacer skeleton structure, and surfactant concentration on their aggregation behavior. Linear-type 3C12lin-3-Q transitioned from gel solutions to worm-like micelles at high concentrations, and 3C14lin-3-Q became gel solutions over a wide range of concentrations. In contrast, all other studied surfactants formed ellipsoidal micelles. The minor and major axes of the ellipsoidal micelles formed by liner-type 3Cnlin-3-Q increased with the increasing alkyl chain length. As the spacer chain length of 3Cnlin-s-Q increased from 3 to 6, and as the spacer skeleton expanded from linear-type 3Cnlin-s-Q to star-type 3Cntris-s-Q, the surfactants formed ellipsoidal micelles without the formation of aggregates with a high-order structure, demonstrating this behavior over a broad concentration range.

Glycyrrhizic acid aggregates seen from a synthetic surfactant perspective.

Bio- or plant-based surfactants are a sustainable and renewable alternative to replace synthetic chemicals for environmental, drugs and food applications. However, these "green" surfactants have unique molecular structures, and their self-assembly in water might lead to complex morphologies and unexpected properties. The micellization of saponin molecules, such as glycyrrhizic acid (GA), differs significantly from those of conventional synthetic surfactants, yet these differences are often overlooked. Saponins self-assemble in complex hierarchical helical morphologies similar to bile salts, rather than the expected globular, ellipsoidal and wormlike micelles. Here, we review two potential routes for molecular self-assembly of GA, namely kinetics of crystallization and thermodynamic equilibrium, focusing on their structure as a function of concentration. Some uncertainty remains to define which route is followed by GA self-assembly, as well as the first type of aggregate formed at low concentrations, thus we review the state-of-the-art information about GA assembly. We compare the self-assembly of GA with conventional linear surfactants, and identify their key similarities and differences, from molecular and chemical perspectives, based on the critical packing parameter (CPP) theory. We expect that this work will provide perspectives for the unclear process of GA assembly, and highlight its differences from conventional micellization.

Microscopic Analysis of the Water/Glycerol/EO30PS System in Bulk and on a Solid Substrate.

Surfactant systems are often employed in cosmetic formulations where they dry on skin as a surface, thereby becoming increasingly concentrated systems. To better understand this drying process, we focused on the difference of self-assembled structures of the water/glycerol/polyoxyethylene (30) phytosteryl ether (EO30PS) system in bulk and on a solid substrate because the interaction between the substrate and the surfactant may have a substantial effect on the self-assembly, which may be related to the bulk structure but in detail may also differ strongly from the bulk situation. In bulk, small-angle neutron scattering (SANS) experiments showed that with increasing loss of water, the degree of ordering increases but changes of the aggregate structure are rather small. The results indicate that ellipsoidal micelles of EO30PS are densely packed and simply become more ordered in bulk during the drying process. On the other hand, neutron reflectometry revealed that EO30PS molecules adsorb onto a Si surface in the form of bilayers and analysis indicates that at a high concentration (c = 20 wt %), there are on average two bilayers (a double bilayer) on the Si substrate. The adsorbed membrane structure of EO30PS is rather thin with respect to its hydrophobic part, indicating tilted molecules, containing only some solvent, and being not highly ordered. These experimental results then allow for a much deeper understanding of the structural properties of practical formulations as they are applied, for instance, in cosmetic lotions.

Investigating the Influence of Aromatic Counterions on the Micellar Structure and Aggregation Behavior of Morpholinium-Based Surface-Active Ionic Liquids in an Aqueous Solution.

Two morpholinium-based surface-active ionic liquids (SAILs) with aromatic counterions were synthesized, namely, n-dodecyl-n-methylmorpholinium salicylate [C12mmor][Sal] and n-dodecyl-n-methylmorpholinium 3-hydroxy-2-naphthoate [C12mmor][3-h-2-n], and explored their aggregation behavior in aqueous solutions systematically. Electrical conductivity, small-angle neutron scattering (SANS), surface tension (ST), and UV-vis spectroscopy measurements were employed to determine various thermodynamic, micellar, and interfacial parameters, like the degree of counterion binding (β), critical micelle concentration (CMC), minimum area per molecule (Amin), surface excess concentration (Γmax), standard Gibbs free energy of adsorption (ΔGad0), aggregation number (Nagg), standard Gibbs free energy of micellization (ΔGm0), standard enthalpy of micelle formation (ΔHm0), and the standard entropy of micellization (ΔSm0) in an aqueous solution. Incorporating the aromatic counterions favors significantly excellent micellization properties over conventional halogenated SAILs such as [C12mmor][Br]. SANS analysis revealed that upon changing the counterion from salicylate to 3-hydroxy-2-naphthoate, the structure changed from prolate ellipsoidal micelles to large unilamellar vesicles. Also, increasing the concentration in the case of [C12mmor][Sal] resulted in a lower aggregation number.

New Lyotropic Complex Fluid Structured in Sheets of Ellipsoidal Micelles Solubilizing Fragrance Oils.

New lyotropic, fragranced, viscoelastic fluid with a complex structure is obtained from fragranced microemulsions by the addition of a fatty acid. Nonhomogeneous mixing of an appropriate nonionic surfactant, a fatty acid, and a fragrance oil led to the formation of anisotropically shaped and highly oriented micelles in aqueous solution. The nano- and microstructures, and consequently the viscosity, are controlled by the balance of fatty acids used as a cosurfactant and fragrance molecules, which partly behave as a cosurfactant and partly segregate in the micelles of the hydrophilic nonionic surfactant. The transition from isotropic microemulsion to a more structured viscoelastic solution is characterized by X-ray scattering and rheological methods. Considering our X-ray scattering results, we propose a structure composed of planar sheets of ellipsoidal micelles arranged in a lamellar type of stacking. The complex structured, low viscous, transparent fluid is capable of solubilizing a fragrance inside the ellipsoidal micelles, as well as retaining microparticles containing fragrance, without the addition of a polymeric thickener or another gelator. These features allow the creation of a 2-in-1 fragrance-solubilizing liquid product compatible with all types of home and body care consumer products.

Small-angle x-ray scattering investigation of the integration of free fatty acids in polysorbate 20 micelles

A critical quality attribute for liquid formulations is the absence of visible particles. Such particles may form upon polysorbate hydrolysis resulting in release of free fatty acids into solution followed by precipitation. Strategies to avoid this effect are of major interest for the pharmaceutical industry. In this context, we investigated the structural organization of polysorbate micelles alone and upon addition of the fatty acid myristic acid (MA) by small-angle x-ray scattering. Two complementary approaches using a model of polydisperse core-shell ellipsoidal micelles and an ensemble of quasiatomistic micelle structures gave consistent results well describing the experimental data. The small-angle x-ray scattering data reveal polydisperse mixtures of ellipsoidal micelles containing about 22–35 molecules per micelle. The addition of MA at concentrations up to 100 μg/mL reveals only marginal effects on the scattering data. At the same time, addition of high amounts of MA (>500 μg/mL) increases the average sizes of the micelles indicating that MA penetrates into the surfactant micelles. These results together with molecular modeling shed light on the polysorbate contribution to fatty acid solubilization preventing or delaying fatty acid particle formation.

Physiochemical insight into the solution behavior of cationic gemini surfactant in water and ethanol–water systems

AbstractSurfactants in water and both alcohol‐water mixed solutions are used extensively in a host of industrial applications. This work presents the solution behavior and micellar transition of a cationic gemini surfactant (GS): N,N′‐dihexadecyl‐N,N,N′,N′‐tetramethyl‐N,N′‐ethanediyl‐diammonium dibromide (16‐2‐16) in water and mixed water‐ethanol media. Phase behavior for 16‐2‐16 in the ethanol–water system was investigated at ambient temperature. The rheological data obtained for these systems at varying alcohol concentrations showed that the system viscosity (η) decreased with as the ethanol concentration increased. Small‐angle neutron scattering (SANS) was used to probe the structural details of the cationic micelles as a function of ethanol concentration and temperature. The scattering data inferred a structural transition from unilamellar vesicles (ULV) through rod‐like micelles to ellipsoidal micelles occurs that is dependent on the solvent composition and temperature indicating the behavior of ethanol molecules as a cosolvent in the process of micelle breaking. The plausible physicochemical interactions in the 16‐2‐16‐ethanol mixed system were further investigated using a computational simulation study employing density functional theory (DFT)/B3LYP (Gauss View 5.0.9) utilizing a 3‐21G basis set.

Comparative study of the co-assembly behaviour of 3-chloro-4-hydroxy-phenylazo dyes with DTAB.

The co-assembly of three one-fold negatively charged 3-chloro-4-hydroxy-phenylazo dyes (Yellow, Blue and Red) with the cationic surfactant dodecyltrimethylammoniumbromide (DTAB) was studied to probe dye-DTAB binding stoichiometry and assembly morphology. For each dye, phase separation was observed above a given dye : DTAB ratio with the ratio depending on the dye. While Yellow and DTAB showed liquid/liquid phase separation above Yellow : DTAB = 1 : 1.67, crystalline dye-DTAB complexes were observed for Blue-DTAB and Red-DTAB above Blue : DTAB = 1 : 2.56 and Red : DTAB = 1 : 2.94 respecively. In homogeneous solution, UV/vis spectroscopic investigations suggest stochiometries of Yellow : DTAB = 1 : 2, Blue : DTAB = 1 : 3 and Red : DTAB = 1 : 4. It was concluded, that Yellow exhibits the highest dye : DTAB binding stoichiometry in both, dye-surfactant complexes in the 2-phase region and in solution, whereas the lowest dye : DTAB binding stoichiometry was observed for Red-DTAB in both cases. The observed stoichiometries are inversely correlated to the impact dye addition has on the morphology of DTAB micelles. Generally, addition of dye to DTAB micelles leads to a reduction in spontaneous curvature of these micelles and to the formation of triaxial ellipsoidal or cylindrical micelles from oblate ellipsoidal DTAB micelles. At a DTAB concentration of 30 mM and a dye concentration of 5 mM, this effect was most pronounced for Red and least pronounced for Yellow, whilst Blue showed an intermediate effect.

Universal Character of Breaking of Wormlike Surfactant Micelles by Additives of Different Hydrophobicity.

Wormlike surfactant micelles are widely used in various applications including fracturing technology in oil industry, template synthesis of different nanoobjects, micellar copolymerization of hydrophilic and hydrophobic monomers, and so forth. Most of those applications suggest the solubilization of different additives in the micelles. The present paper is aimed at the comparative study of the effect of the solubilization of hydrophobic (n-decane and 1-phenylhexane) and hydrophilic (N-isopropylacrylamide and acrylamide) substances on the rheological properties and structure of the micelles using several complementary techniques including rheometry, small angle neutron scattering, dynamic light scattering, and diffusion ordered NMR spectroscopy. For these studies, mixed micelles of potassium oleate and n-octyltrimethylammonium bromide containing the excess of either anionic or cationic surfactants were used. It was shown that hydrophobic additives are completely solubilized inside the micelles being localized deep in the core (n-decane, 1-phenylhexane) or near the core/corona interface (1-phenylhexane). At the same time, only a small fraction of hydrophilic additives (14% of N-isopropylacrylamide and 4% of acrylamide) penetrate the micelles being localized at the corona area. Despite different localization of the additives inside the micelles, all of them induce the breaking of wormlike micelles with the formation of either ellipsoidal microemulsion droplets (in the case of hydrophobic additives) or ellipsoidal surfactant micelles (in the case of hydrophilic additives). The breaking of micelles results in the drop of viscosity of the solution up to water value. The main result of this paper consists in the observation of the fact that for all the additives under study, the dependences of the viscosity on the volume fraction of additive lie on the same master curve being shifted along the volume fraction axis by a certain factor depending on the hydrophobicity of the added species. Those data are quite useful for various applications of wormlike surfactant micelles suggesting the solubilization of different additives inside them.

Light Responsiveness and Assembly of Arylazopyrazole-Based Surfactants in Neat and Mixed CTAB Micelles.

The self-assembly of an arylazopyrazole-based photosurfactant (PS), based on cetyltrimethylammonium bromide (CTAB), and its mixed micelle formation with CTAB in aqueous solution was investigated by small angle neutron and X-ray scattering (SANS/SAXS) and UV-vis absorption spectroscopy. Upon UV light exposure, PS photoisomerizes from E-PS (trans) to Z-PS (cis), which transforms oblate ellipsoidal micelles into smaller, spherical micelles with larger shell thickness. Doping PS with CTAB resulted in mixed micelle formation at all stoichiometries and conditions investigated; employing selectively deuterated PS, a monotonic variation in scattering length density and dimensions of the micellar core and shell is observed for all contrasts. The concentration- and irradiance-dependence of the E to Z configurational transition was established in both neat and mixed micelles. A liposome dye release assay establishes the enhanced efficacy of photosurfactants at membrane disruption, with E-PS exhibiting a 4-fold and Z-PS a 10-fold increase in fluorescence signal with respect to pure CTAB. Our findings pave the way for external triggering and modulation of the wide range of CTAB-based biomedical and material applications.

Volumetric determination of reverse micelle structural properties and the validity of commonplace approximations

Molecular volumes within reverse micelles are important in determining many aggregate properties. Molecular volumes also depend upon differences in molecular environment and thereby might also be capable of detecting changes in molecular structure and interactions within an aggregate. However, approximations are typically used in their place along with approximations of aggregate radius, aggregation number, surfactant tail length, and reverse micelle shape particularly for Aerosol OT systems. Consequently, volumetric measurements have not been thoroughly tested as a measure of aggregate properties. Volumetric methods do not require the commonly used approximations. Therefore, comparison with previous approaches will demonstrate volumetric measurements’ utility in measuring aggregate structural properties and provide a useful assessment of the applicability of these commonplace approximations. Partial molar volumes of water and surfactant, evaporation rate, and reverse micelle radius are measured over the w0 = 0–40 range for water/Aerosol OT/isooctane solutions. These results are used to calculate how aggregation number, surfactant layer thickness, head group area, and water droplet structure change with aggregate radius. Volumetric measurements reveal a surfactant layer structural transition at w0 = 5, an aggregate shape change at w0 = 25, and reproduced the general trends in structural properties. Important variations from approximated methods are noted. Molecular volume and aggregate radius approximations were found valid in many but not all situations. However, approximations of aggregation number and tail length created important miscalculations of some properties when treating reverse micelles as spherical. It was found that consideration of an ellipsoidal reverse micelle shape for w0 < 10 could account for previously identified changes in surfactant layer thickness and head group area that were based on a spherical geometry.

Small Angle X-Ray Scattering Data Analysis and Theoretical Modelling for the Size and Shape Characterization of Drug Delivery Systems Based on Vitamin E TPGS Micelles

We developed a simple two-dimensional/two-components theoretical model that describes the structure and functionality of a VitE-TPGS system of micelles assuming a hydrophobic inner core and an outer hydrated hydrophilic shell. We then conceptually applied the developed methodology to a simple system of VitE-TPGS micelles unloaded and loaded with an active pharmaceutical ingredient, eltrombopag, to verify if the model could reliably monitor the size change of the micelle upon loading. The fit of laboratory Small Angle X-Ray Scattering data against such model allows us to extract absolute values of the micelles size under a spherical shape hypothesis as well as the distribution within the system between components and level of hydration. The intensity scale of the SAXS experimental data needs to be normalized to a reference standard (pure water) to get absolute scattered intensities. The mathematical model which has been developed under a general hypothesis of ellipsoidal micelles, is applied to our experimental data under the simplified spherical assumption, which suitably fits our experimental data.