Spontaneously formed nonequilibrium vesicles of cetyltrimethylammonium bromide and sodium octyl sulfate in aqueous dispersions

Innovative approaches to cationic and anionic (catanionic) amphiphiles self-assemblies: Synthesis, properties, and industrial applications

Meeting the contemporary demand for the development of functional, biocompatible, and environment friendly self-assembled structures using efficient, cost-effective, and energy-saving methods, the field of colloids has witnessed a surge in interest. Research into cationic and anionic (catanionic) surfactant combinations has gained momentum due to their distinct advantages and synergistic properties in this context. Catanionic self-assemblies have emerged as promising contenders for addressing these requirements. Catanionic self-assemblies possess high stability, adjustable surface charge, and low critical aggregation concentration. This comprehensive review article distinguishes between cationic/anionic non-equimolar and equimolar ratio mixing formation of high-salt catanionic self-assemblies known as catanionic mixture and salt-free counterparts, termed ion-pair amphiphiles, respectively. It explores diverse synthesis techniques, emphasizing the roles of solvents, salts, and pH conditions and covers both experimental and theoretical aspects of state-of-the-art catanionic self-assemblies. Additionally, the review investigates the development of multi-responsive catanionic self-assemblies using light, pH, temperature, and redox, responsive cationic/anionic amphiphiles. It provides an in-depth exploration of potential synergistic interactions and properties, underscoring their practical importance in a wide range of industrial applications. The review explores challenges like precipitation, stability and identifies knowledge gaps, creating opportunities in the dynamic catanionic self-assembly field. It aims to offer insights into the journey of catanionic self-assemblies, from inception to current status, appealing to a broad audience invested in their scientific and industrial potential.

Dynamic light scattering and transmission electron microscopy in drug delivery: a roadmap for correct characterization of nanoparticles and interpretation of results

In this focus article, we provide a scrutinizing analysis of transmission electron microscopy (TEM) and dynamic light scattering (DLS) as the two common methods to study the sizes of nanoparticles with focus on the application in pharmaceutics and drug delivery. Control over the size and shape of nanoparticles is one of the key factors for many biomedical systems. Particle size will substantially affect their permeation through biological membranes. For example, an enhanced permeation and retention effect requires a very narrow range of sizes of nanoparticles (50-200 nm) and even a minor deviation from these values will substantially affect the delivery of drug nanocarriers to the tumour. However, amazingly a great number of research papers in pharmaceutics and drug delivery report a striking difference in nanoparticle size measured by the two most popular experimental techniques (TEM and DLS). In some cases, this difference was reported to be 200-300%, raising the question of which size measurement result is more trustworthy. In this focus article, we primarily focus on the physical aspects that are responsible for the routinely observed mismatch between TEM and DLS results. Some of these factors such as concentration and angle dependencies are commonly underestimated and misinterpreted. We convincingly show that correctly used experimental procedures and a thorough analysis of results generated using both methods can eliminate the DLS and TEM data mismatch completely or will make the results much closer to each other. Also, we provide a clear roadmap for drug delivery and pharmaceutical researchers to conduct reliable DLS measurements.

Increasing the Resistance of Living Cells against Oxidative Stress by Nonnatural Surfactants as Membrane Guards

The importation of construction principles or even constituents from biology into materials science is a prevailing concept. Vice versa, the cellular level modification of living systems with nonnatural components is much more difficult to achieve. It has been done for analytical purposes, for example, imaging, to learn something about intracellular processes. Cases describing the improvement of a biological function by the integration of a nonnatural (nano)constituent are extremely rare. Because biological membranes contain some kind of a surfactant, for example, phospholipids, our idea is to modify cells with a newly synthesized surfactant. However, this surfactant is intended to possess an additional functionality, which is the reduction of oxidative stress. We report the synthesis of a surfactant with Janus-type head group architecture, a fullerene C₆₀ modified by five alkyl chains on one side and an average of 20 oxygen species on the other hemisphere. It is demonstrated that the amphiphilic properties of the fullerenol surfactant are similar to that of lipids. Not only quenching of reactive oxygen species (superoxide, hydroxyl radicals, peroxynitrite, and hydrogen peroxide) was successful, but also the fullerenol surfactant exceeds benchmark antioxidant agents such as quercetin. The surfactant was then brought into contact with different cell types, and the viability even of delicate cells such as human liver cells (HepG2) and human dopaminergic neurons (LUHMES) has proven to be extraordinarily high. We could show further that the cells take up the fullerenol surfactant, and as a consequence, they are protected much better against oxidative stress.

Characterization of fluorinated catansomes: a promising vector in drug-delivery

Catansomes, which are vesicles prepared from mixtures of oppositely charged surfactants, have been suggested as effective alternatives to phospholipid vesicles, i.e., liposomes, in applications such as drug-delivery. This is mainly due to their enhanced chemical and physical stability as well as to their relatively easy preparation, which is an advantage for large-scale productions. In this study we have investigated catansomes prepared from a perfluorinated anionic surfactant (sodium perfluorooctanoate) premixed with a hydrogenated cationic surfactant (dodecyltrimethylammonium bromide or 1-dodecylpyridinium chloride). The aim was to gain insights into the physicochemical properties of these systems, such as size, stability, surface charge, and membrane morphology, which are essential for their use in drug-delivery applications. The catansomes were mostly unilamellar and 100-200 nm in size, and were stable for more than five months at room temperature. After loading the catansomes with the fluorescent marker calcein, they were found to exhibit an appreciable encapsulation efficiency and a low calcein leakage over time. The addition of fatty acids to calcein-loaded catansomes considerably promoted the release of calcein, and the rate and efficiency of calcein release were found to be proportional to the fatty acid concentration and chain length. Our results prove the feasibility of utilizing catansomes as drug-delivery vehicles as well as provide a means to efficiently release the encapsulated load.

Effect of preparation method and cholesterol on drug encapsulation studies by phospholipid liposomes

Unilamellar liposomes, prepared from synthetic lipid mixture of DMPC and DMPG either by sonication or extrusion, were used to entrap water soluble and water insoluble molecules to investigate the efficacy of encapsulation by different liposome preparation methods. In the case of entrapment of hydrophilic protein cytochrome-C, the solutions were subjected to a series of ultrafiltration steps to eliminate any free protein outside the vesicles. It was observed that the protein could be encapsulated by the vesicles only if cholesterol was present in the bilayer. The release of cytochrome-C was observed spectrophotometrically upon vesicle-breakdown. The amount of protein encapsulated depended on the method of preparation and was found to be 10 times greater in extruded liposomes compared to those produced by sonication. Hydrophobic Vitamin E, on the other hand, could be encapsulated in the liposome bilayer, independently of the presence of cholesterol and the method of preparation. These fundamental results can be used to develop more efficient drug encapsulations and to have better understanding about their release.

Pharmaceutical applications for catanionic mixtures

Mixtures of oppositely charged surfactants, so called catanionic mixtures, are a growing area of research. These mixtures have been shown to form several different types of surfactant aggregates, such as micelles of various forms and sizes, and lamellar structures, such as vesicles. In this review, a short introduction to the field of catanionic mixtures is presented and the pharmaceutical possibilities offered by such mixtures are reviewed. There are several interesting ideas on how to apply catanionic mixtures to improve the delivery of, for example, drug compounds and DNA, or for HIV treatment.

Elucidating the assembled structure of amphiphiles in solution via cryogenic transmission electron microscopy

For the past twenty years, significant progress has been made in both developing cryogenic transmission electron microscopy (cryo-TEM) technology and understanding assembled behavior of amphiphilic molecules. Cryo-TEM can provide high-resolution images of complex fluids in a near state. Samples embedded in a thin layer of vitrified solvent do not exhibit artifacts that would normally occur when using chemical fixation or staining-and-drying techniques. Cryo-TEM has been useful in imaging biological molecules in aqueous solutions. Cryo-TEM has become a powerful tool in the study of -assembled structures of amphiphiles in solution as a complementary tool to small-angle X-ray and neutron scattering, light scattering, rheology measurements, and nuclear magnetic resonance. The application of cryo-TEM in the study of assembled behavior of amphiphilic block copolymers, hydrogels, and other complex soft systems continues to emerge. In this context, the usage of cryo-TEM in the field of amphiphilic complex fluids and self-assembled nano-materials is briefly reviewed, and its unique role in exploring the nature of assembled structure in liquid suspension is highlighted.

Highly efficient capture and long-term encapsulation of dye by catanionic surfactant vesicles

Vesicles formed from the cationic surfactant, cetyltrimethylammonium tosylate (CTAT) and the anionic surfactant, sodium dodecylbenzenesulfonate (SDBS), were used to sequester the anionic dye carboxyfluorescein. Carboxyfluorescein was efficiently sequestered in CTAT-rich vesicles via two mechanisms: encapsulation in the inner water pool and electrostatic adsorption to the charged bilayer. The apparent encapsulation efficiency (22%) includes both encapsulated and adsorbed fractions. Entrapment of carboxyfluorescein by SDBS-rich vesicles was not observed. Results show the permeability of the catanionic membrane is an order of magnitude lower than that of phosphatidylcholine vesicles and the loading capacity is more than 10 times greater.

Spontaneous formation of vesicles and dispersed cubic and hexagonal particles in amino acid-based catanionic surfactant systems

Mixed catanionic surfactant systems based on amino acids were investigated with respect to the formation of liquid crystal dispersions and the stability of the dispersions. The surfactants used were arginine-N-lauroyl amide dihydrochloride (ALA) and N(alpha)-lauroyl-arginine-methyl ester hydrochloride (LAM), which are arginine-based cationic surfactants; sodium hydrogenated tallow glutamate (HS), a glutamic-based anionic surfactant; and the anionic surfactants sodium octyl sulfate (SOS) and sodium cetyl sulfate (SCS). It is demonstrated that in certain ranges of composition there is a spontaneous formation of vesicular, cubic, and hexagonal structures. The solutions were characterized with respect to internal structure and size by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and turbidity measurements. Vesicles formed spontaneously and were found for all systems studied; their size distribution is presented for the systems ALA/SCS/W and ALA/SOS/W; they are all markedly polydisperse. The aging process for the system ALA/SOS/W was monitored both by turbidity and by cryo-TEM imaging; the size distribution profile for the system becomes narrower and the number average radius decreases with time. The presence of dispersed particles with internal cubic structure (cubosomes) and internal hexagonal structure (hexosomes) was documented for the systems containing ALA and HS. The particles formed spontaneously and remained stably dispersed in solution; no stabilizer was required. (Cubosome and hexosome are USPTO registered trademarks of Camurus AB, Sweden.) The spontaneous formation of particles and their stability, together with favorable biological responses, suggests a number of applications.

Reversible Micelle−Vesicle Conversion of Oleyldimethylamine Oxide by pH Changes

A preliminary study on the reversible micelle-vesicle conversion of oleyldimethylamine oxide [Kawasaki, H. et al. J. Phys. Chem. B. 2002, 106, 1524 ] is extended in the present study. In the presence of 0.01 M NaCl at a surfactant concentration of 0.05 M, a micelle-to-vesicle conversion with increasing degree of ionization alpha takes place in the following sequence: growth of fibrous micelle (alpha < 0.2), a fused network (alpha approximately 0.3), fibrous micelles + (perforated) vesicles (alpha = 0.4), and vesicles + lamellae (alpha = 0.5). Viscoelasticity correspondingly varies from the Maxwell-type behavior of the entangled network of fibrous micelles to the gel-like behavior of vesicle suspensions, via a fluid solution-like behavior of the fused network. This phase sequence is in contrast with the case of no added salt where no branching of micelles is observed, and long micelles and bilayers (vesicles + lamellae) coexist at alpha = 0.5. In water, a state of the lowest viscoelasticity occurs around alpha = 0.2 for both surfactant concentrations 0.05 and 0.15 M. Synergism between protonated and nonprotonated amine oxide headgroups is observed despite low ionic strengths. From the time course of the reversible micelle-vesicle conversion, vesicles seem to be formed from threadlike micelles within 25 h according to the shear moduli, while a longer conversion time is suggested by a flow property (viscosity). Shear thickening behavior is observed at alpha = 0.2 and 0.4 in 0.01 M NaCl but not in water.

Lamellar gels and spontaneous vesicles in catanionic surfactant mixtures

Caillé analysis of the small-angle X-ray line shape of the lamellar phase of 7:3 wt/wt cetyltrimethylammonium tosylate (CTAT)/sodium dodecylbenzene sulfonate (SDBS) bilayers shows that the bending elastic constant is kappa = (0.62 +/- 0.09)k(B)T. From this and previous results, the Gaussian curvature constant is kappa = (-0.9 +/- 0.2)k(B)T. For 13:7 wt/wt CTAT/SDBS bilayers, the measured bending elasticity decreases with increasing water dilution, in good agreement with predictions based on renormalization theory, giving kappa(o) = 0.28k(B)T. These results show that surfactant mixing is sufficient to make kappa approximately k(B)T, which promotes strong, Helfrich-type repulsion between bilayers that can dominate the van der Waals attraction. These are necessary conditions for spontaneous vesicles to be equilibrium structures. The measurements of the bending elasticity are confirmed by the transition of the lamellar phase of CTAT/SDBS from a turbid, viscoelastic gel to a translucent fluid as the water fraction is decreased below 40 wt %. Freeze-fracture electron microscopy shows that the gel is characterized by spherulite defects made possible by spontaneous bilayer curvature and low bending elasticity. This lamellar gel phase is common to a number of catanionic surfactant mixtures, suggesting that low bending elasticity and spontaneous curvature are typical of these mixtures that form spontaneous vesicles.

Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles

Mixtures of cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzene sulfonate (SDBS) in water form a fluid lamellar phase at < or = 40 wt % water but surprisingly turn into viscous gels at higher water fractions. The gels are characterized by spherulite and other bilayer defects consistent with a low bending elasticity, kappa approximately k(B)T, and a nonzero spontaneous curvature. Caillé analysis of the small-angle x-ray line shape confirms that for 7:3 wt:wt CTAT:SDBS bilayers at 50% water, kappa = 0.62 +/- 0.09 k(B)T and kappa = -0.9 +/- 0.2 k(B)T. For 13:7 wt:wt CTAT:SDBS bilayers, the measured bending elasticity decreases with increasing water dilution in good agreement with predictions based on renormalization theory, giving kappa(o) = 0.28 k(B)T. These results show that surfactant mixing is sufficient to make kappa approximately k(B)T, which promotes strong, Helfrich-type repulsion between bilayers that can dominate the van der Waals attraction. These are necessary conditions for spontaneous vesicles formed at even higher water fractions to be equilibrium structures.

Small-angle neutron scattering from mixtures of sodium dodecyl sulfate and a cationic, bolaform surfactant containing azobenzene

This paper reports on the microstructures formed in aqueous solutions containing mixtures of sodium dodecyl sulfate (SDS) and a photosensitive, bolaform surfactant, bis(trimethylammoniumhexyloxy)azobenzene dibromide (BTHA). By using quasi-elastic light scattering and small-angle neutron scattering, we determined that aqueous solutions containing SDS and the trans isomer of BTHA (0.1 wt % total surfactant, 15 mol % BTHA, 85 mol % SDS) form vesicles with average hydrodynamic diameters of 1350 +/- 50 angstroms and bilayer thicknesses of 35 +/- 2 angstroms. The measured bilayer thickness is consistent with a model of the vesicle bilayer in which the trans isomer of BTHA spans the bilayer. Upon illumination with UV light, the BTHA underwent photoisomerization to produce a cis-rich photostationary state (80% cis isomer). We measured this photoisomerization to drive the reorganization of vesicles into cylindrical aggregates with cross-sectional radii of 19 +/- 3 angstroms and average hydrodynamic diameters of 240 +/- 50 angstroms. Equilibration of the cis-rich solution in the dark at 25 degrees C for 12 h or illumination of the solution with visible light leads to the recovery of the trans-rich photostationary state of the solution and the reformation of vesicles, thus demonstrating the potential utility of this system as the basis of a tunable fluid.