Rheological study of the shape transition of block copolymer-nonionic surfactant mixed micelles

Structural and therapeutic properties of salicylic acid-solubilized Pluronic solutions and hydrogels

Salicylic acid (SA) finds extensive applications in the treatment of rheumatic and skin diseases because of its analgesic, anti-inflammatory and exfoliating properties. As it is lipophilic in nature, there is a need for appropriate delivery systems to harness these properties for different applications. Herein, we examined the suitability of Pluronic P123/F127 micellar systems as delivery media by investigating the structural, flow and antimicrobial properties of P123/F127-SA solutions and hydrogels using DLS, SANS, rheological and zone inhibition measurement techniques. SA modulates the aggregation characteristics of these surfactant systems and brings about spherical-to-worm-like micelle-to-vesicular structural transitions in the hydrophobic Pluronic P123 system, a spherical-to-worm-like micellar transition in the mixed P123/F127 system and an onset of inter-micellar attraction in the hydrophilic Pluronic F127 system. SA-solubilized systems of both hydrophobic and hydrophilic Pluronics inhibit the growth of Gram-positive and Gram-negative bacteria with comparable MIC values. This suggests that the interaction of SA molecules with the bacterial cell membrane remains unobstructed upon encapsulation in Pluronic micelles. F127 hydrogel-based SA formulations with rheological properties suitable for topical applications and up to 15% SA loading were prepared. These will be useful SA ointments as F127 is an FDA-approved excipient for topical drug delivery applications. The results indicate that Pluronics remain effective as delivery agents for SA and exhibit interesting structural polymorphism upon its solubilization.

Cellulose nanocrystal and Pluronic L121-based thermo-responsive composite hydrogels

Cellulose nanocrystal (CNC) is a promising sustainable material with its biocompatibility, high aspect ratio, and mechanical strength. CNC-based systems have potential applications in various fields including biosensors, packaging, coating, energy storage, and pharmaceuticals. However, turning CNC into smart systems remains a challenge due to the lack of stimuli-responsiveness, limitation in compatibility with hydrophobic matrices, and their agglomeration tendency. In this work, a thermo-responsive nanocomposite system is constructed with CNCs and polymersome forming Pluronic L121 (L121), and its phase behavior and mechanical properties are investigated in detail. Two different CNC concentration (4 % and 5 %) is studied by changing the L121 concentration (1-20 %) to understand the effect of unimers and polymersomes on the CNC network. At dilute L121 concentrations (1-5 %), the composite system becomes softer but more fragile below the transition temperature. However, it becomes much stronger at higher L121 concentrations (10-20 %), and a gel network is obtained above the transition temperature. Interestingly, the elastically reinforced CNC gels exhibit greater resistance to microstructural breakdown at large strains due to the soft and deformable nature of the large polymersomes. It is also found that the gelation temperature for hydrogels is tunable with increasing L121 concentration, and the nanocomposite hydrogels displayed thermo-reversible rheological behavior.

Modulation of Excited State Proton Transfer Dynamics of a Lactim-Lactam Tautomeric System in Different Block Copolymer-Surfactant Aggregates

The proton transfer (PT) process in 1-(2-hydroxy-5-chloro-phenyl)-3,5-dioxo-1H-imidazo-[3,4-b]isoindole (ADCL) has been studied in three different copolymer-surfactant supramolecular assemblies prepared in aqueous 1% P123 triblock copolymer micellar solution with varying concentrations of surfactants (sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and triton-X-100 (TX 100)). The aim of the present study is to monitor the modulation of the PT process by changing the degree of micellar hydration inside the P123 micelle with the addition of the three different surfactants (two ionic and one non ionic), that is, in P123-surfactant aggregates. Besides, a comparative study has been done with these results with those in water, pure P123 micellar medium and three different surfactants medium. The micropolarity measurement and time-resolved fluorescence anisotropic measurements have been performed to evaluate the binding location of the probe (ADCL) in the three different copolymer-surfactant supramolecular assemblies. It is found that the micropolarity at the binding site of the molecule in the various environments largely influences the PT rate of ADCL. The PT rate is found to be the slowest in the P123 medium and in P123-surfactant aggregates the rate becomes faster as the micropolarity around the binding locations of the molecule in these aggregates is higher in comparison to that in P123 micelle.

Colloidal micelles of block copolymers as nanoreactors, templates for gold nanoparticles, and vehicles for biomedical applications

Target drug delivery methodology is becoming increasingly important to overcome the shortcomings of conventional drug delivery absorption method. It improves the action time with uniform distribution and poses minimum side effects, but is usually difficult to design to achieve the desire results. Economically favorable, environment friendly, multifunctional, and easy to design, hybrid nanomaterials have demonstrated their enormous potential as target drug delivery vehicles. A combination of both micelles and nanoparticles makes them fine target delivery vehicles in a variety of biological applications where precision is primarily required to achieve the desired results as in the case of cytotoxicity of cancer cells, chemotherapy, and computed tomography guided radiation therapy.

Interaction of normal and reverse pluronics (L44 and 10R5) and their mixtures with anionic surfactant sodium N-dodecanoylsarcosinate

Interaction of monomeric normal L44 [(PEO)10(PPO)23(PEO)10]) and reverse 10R5 [(PPO)8(PEO)22(PPO)8] pluronics designated as L and R, respectively, and their mixtures with the anionic surfactant sodium N-dodecanoylsarcosinate (SDDS) in aqueous medium has been studied by tensiometry, conductometry, calorimetry and dynamic light scattering methods. The critical aggregation concentration (CAC), concentration of maximum binding (CS), and the extended critical micelle concentration (CMCe) of SDDS resulted from the interaction have been presented. L has shown the formation of CAC, CS and CMCe, whereas R has shown the absence of CAC but the presence of both CS and CMCe. Micellization of SDDS is an endothermic process whereas the formation of CAC and CMCe is exothermic events, the formation of CS is endothermic. The hydrodynamic diameter (Dh) and zeta potential of L and R, and their SDDS interacted species have been determined. Dh decreased with increasing SDDS concentration for L whereas it was invariant for R. Mixed L and R in varied proportions have been also examined in the solution state to probe into their mutual interaction, and interaction with SDDS. The formation of CAC of their mixtures was absent; CMCe formation was exothermic while that of CS was endothermic like their individuals.

Structural and rheological studies on growth of salt-free wormlike micelles formed by star-type trimeric surfactants

We investigated the growth mechanisms of wormlike micelles formed by star-type trimeric surfactant (3C(12)trisQ) with a hydrocarbon chain length of 12 in an aqueous solution. A 3C(n)trisQ molecule consists of three hydrocarbon chains and three hydrophilic groups connected by spacer chains, where n is the carbon number in the hydrocarbon chain. Our recent studies showed that the aggregates formed by 3C(12)trisQ exhibited sphere-to-rod transition and the growth of wormlike micelles in an aqueous solution in the absence of salt. We performed small-angle neutron scattering (SANS) and rheological measurements and investigated the aggregation behavior of 3C(12)trisQ with various surfactant volume fractions. All SANS profiles for the 3C(12)trisQ indicated peak-profiles in the q range of 0.02 Å(-1) < q < 0.05 Å(-1), where the magnitude of the scattering vector q is defined by q = 4π sin(θ/2)/λ (λ and θ represent the wavelength and scattering angle, respectively). These peaks were attributed to repulsive interparticle interactions between the micelles. The volume fraction dependence of the SANS peak-position was in agreement with the rheological behavior. These results suggest that 3C(12)trisQ shows sphere-to-rod transition and can produce wormlike micelles in the absence of salt. To determine the structural parameters quantitatively, model-fitting analysis was performed using a charged cylindrical or charged ellipsoidal particle scattering function. The radius, length, and number of water molecules per surfactant molecule (n(w)) inside the micelles were evaluated. The length increased and the n(w) value decreased with increasing φ, indicating that the growth of a wormlike micelle accompanies the extrusion of water from the micelle. The end-cap energies of star-type trimeric, gemini, and monomeric surfactants were evaluated from φ dependence of zero-shear viscosity. We found that wormlike micelles formed by 3C(12)trisQ exhibited a higher end-cap energy than gemini surfactant.

Star-shaped trimeric quaternary ammonium bromide surfactants: adsorption and aggregation properties

Novel star-shaped trimeric surfactants consisting of three quaternary ammonium surfactants linked to a tris(2-aminoethyl)amine core were synthesized. Each ammonium had two methyls and a straight alkyl chain of 8, 10, 12, or 14 carbons. The adsorption and aggregation properties of these tris(N-alkyl-N,N-dimethyl-2-ammoniumethyl)amine bromides (3C(n)trisQ, in which n represents alkyl chain carbon number) were characterized by equilibrium and dynamic surface tension, rheology, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) techniques. 3C(n)trisQ showed critical micelle concentrations (CMC) 1 order of magnitude lower than that of the corresponding gemini surfactants with an ethylene spacer and the corresponding monomeric surfactants. The logarithm of the CMC decreased linearly with increasing hydrocarbon chain length for 3C(n)trisQ. The slope of the line, which is well-known as Klevens equation, was larger than those of the monomeric and gemini surfactants; however, considering the total carbon number in the chains, the slope was shallower than the monomeric and was close to the gemini. Through the results such as surface tensions at the CMC (32-34 mN m(-1)) and the parameters of standard free energy, CMC/C(20) and pC(20), it was found that 3C(n)trisQ could adsorb densely at the air/water interface despite the strong electrostatic repulsion between multiple quaternary ammonium headgroups. Moreover, dynamic surface tension measurements showed that the kinetics of adsorption for 3C(n)trisQ to the air/water interface was slow because of their bulky structures. Furthermore, the results of rheology, SANS, and cryo-TEM determined that 3C(n)trisQ with n = 10 and 12 formed ellipsoidal micelles at low concentrations in solution and the structures transformed to threadlike micelles with very few branches for n = 12 as the concentration increased, but for n = 14 threadlike micelles formed at relatively low concentrations.

How PEO-PPO-PEO triblock polymer micelles control the synthesis of gold nanoparticles: temperature and hydrophobic effects

Aqueous micellar solutions of F68 (PEO(78)-PPO(30)-PEO(78)) and P103 (PEO(17)-PPO(60)-PEO(17)) triblock polymers were used to synthesize gold (Au) nanoparticles (NPs) at different temperatures. All reactions were monitored with respect to reaction time and temperature by using UV-visible studies to understand the growth kinetics of NPs and the influence of different micellar states on the synthesis of NPs. The shape, size, and locations of NPs in the micellar assemblies were determined with the help of TEM, SEM, and EDS analyses. The results explained that all reactions were carried out with the PEO-PPO-PEO micellar surface cavities present at the micelle-solution interface and were precisely controlled by the micellar assemblies. Marked differences were detected when predominantly hydrophilic F68 and hydrophobic P103 micelles were employed to conduct the reactions. The UV-visible results demonstrated that the reduction of gold ions into nucleating centers was channeled through the ligand-metal charge-transfer complex (LMCT) and carried out by the surface cavities. Excessive hydration of the surface cavities in the case of F68 micelles produced a few small NPs, but their yield and size increased as the micelles were dehydrated under the effect of increasing temperature. The results concluded that the presence of well-defined predominantly hydrophobic micelles with a compact micelle-solution interfacial arrangement of surface cavities ultimately controlled the reaction.

Structural characterization of nonionic mixed micelles formed by C12EO6 surfactant and P123 triblock copolymer

A structural characterization of mixed micelles formed in aqueous solution by the PEO-PPO-PEO triblock copolymer P123 and the nonionic surfactant C(12)EO(6) was carried out using various techniques, including ultralow shear viscosimetry, depolarized dynamic light scattering (VH-DLS), depolarized static light scattering (VH-SLS), and small-angle X-ray scattering (SAXS). The sphere-to-rod transition of the mixed micelles was studied in a diluted regime (P123 concentrations ranging from 0.5 to 10 wt %) at C(12)EO(6)/P123 molar ratios of 2.2, 3.2, 6.0, and 11 as well as for the pure C(12)EO(6). The data from VH-SLS and viscosimetry displayed a sharp increase in the intensity and viscosity, respectively, at the sphere-to-rod transition, and the results from the two methods were in accordance. In both techniques, an increased transition temperature with increasing content of C(12)EO(6) (in the molar ratio regime from 2.2 to 11) was observed. SAXS was used as the main technique, and a thorough structural characterization was performed, where indirect Fourier transformation (IFT) and generalized indirect Fourier transformation (GIFT) were employed in the analysis procedure of the SAXS data. The p(r) functions obtained from the IFT (employed at low P123 concentrations, i.e., 1.0 and 2.0 wt %) and GIFT (employed above 2.0 wt %) analyses revealed increased inhomogeneities in the mixed micelles when the molar ratio was increased. This suggested that the C(12)EO(6) organized themselves at the interface between the PPO core and the PEO corona of the P123 micelles, with the C(12) alkyl chain stretching into the hydrophobic core and the EO(6) part residing in the hydrophilic corona. The structure factor parameters obtained with GIFT for a molar ratio of 2.2 at a P123 concentration of 5.0 wt % showed radius values smaller than what was estimated from the p(r) functions. This was explained by an interpenetration of the PEO chains from one mixed micelle into a neighboring one. VH-DLS was performed on the mixed micelles at a temperature 3 degrees C above the transition temperature and at a molar ratio of 2.2. From the analyzed data, the average length L of the rods was estimated to be 102 nm.

Association of a hydrophobically modified polyelectrolyte and a block copolymer followed by fluorescence techniques

By using absorption and fluorescence (steady-state and time-resolved) techniques, the interaction between a poly(acrylic acid) (PAA), randomly grafted with pyrene (Py) units (PAAMePy55), and a triblock copolymer of poly(ethylene oxide) and poly(propylene oxide) (EO(20)PO(68)EO(20), P123) was investigated. From the fluorescence data, it is shown that upon addition of P123 a decrease of the (pyrene-pyrene, Py-Py) intramolecular association, i.e., a decrease of dynamic and static excimer formation, is observed. Time-resolved fluorescence data reveal the existence of two types of monomers (monomers that are able to form excimer, MAGRE, and isolated monomers) and two excimers. Addition of P123 causes also an increase of the amount of isolated Py monomers. The overall fluorescence data suggest that the PAAMePy55 and the P123 block copolymer associate strongly at low pH, leading to the formation of P123 micelles surrounded by one PAAMePy55 chain, where the pyrene groups are located at the PPO/PEO interface of the P123 micelles. Steady-state fluorescence results also showed that an excess of P123 micelles in solution is required for the association to occur. At high pH (pH 9 and above) the situation is less clear. The steady-state (particularly in the I(1)/I(3) ratio) and time-resolved fluorescence results indicate a contact between the pyrene groups and PEO, which then would imply that there may be an interaction, but much weaker than at low pH.

Effect of ethanol on the micellization and gelation of pluronic p123

In certain applications copolymer P123 (E21P67E21) is dissolved in water-ethanol mixtures, initially to form micellar solutions and eventually to gel. For P123 in 10, 20, and 30 wt % aqueous ethanol we used dynamic light scattering from dilute solutions to confirm micellization, oscillatory rheometry, and visual observation of mobility (tube inversion) to determine gel formation in concentrated solutions and small-angle X-ray scattering (SAXS) to determine gel structure. Except for solutions in 30 wt % aqueous ethanol, a clear-turbid transition was encountered on heating dilute and concentrated micellar solutions alike, and as for solutions in water alone (Chaibundit et al. Langmuir 2007, 23, 9229) this could be ascribed to formation of wormlike micelles. Dense clouding, typical of phase separation, was observed at higher temperatures. Regions of isotropic and birefringent gel were defined for concentrated solutions and shown (by SAXS) to have cubic (fcc and hcp) and hexagonal structures, consistent with packed spherical and elongated micelles, respectively. The cubic gels (0, 10, and 20 wt % ethanol) were clear, while the hex gels were either turbid (0 and 10 wt % ethanol), turbid enclosing a clear region (20 wt % ethanol), or entirely clear (30 wt % ethanol). The SAXS profile was unchanged between turbid and clear regions of the 20 wt % ethanol gel. Temperature scans of dynamic moduli showed (as expected) a clear distinction between high-modulus cubic gels (G'max approximately 20-30 kPa) and lower modulus hex gels (G'max<10 kPa).