Effect of urea on the aggregation behavior of gemini surfactants and their mixed micelles with Pluronic L64

Salt induced micellization conduct in PEO-PPO-PEO-based block copolymers: a thermo-responsive approach

The nanoscale self-assembly behavior in ethylene oxide (EO) and propylene oxide (PO)-based block copolymers (BCPs) commercially available as Pluronics®: L44 (PEO10-PPO23-PEO10) and F77 (PEO53-PPO34-PEO53) is put forth in aqueous solution and in the presence of sodium salts NaCl and Na2SO4. The moderate hydrophilicity of L44 is attributed to its low molecular weight PPO segment, while the high percentage of PEO content in F77 contributes to its extreme hydrophilicity. The impact of sodium salts (NaCl and Na2SO4) on the self-assembly is investigated to understand their influence and role in micellization, by employing various physicochemical techniques such as phase behavior conduct, calorimetry, tensiometry, scattering, and spectral analysis. The results indicate that at a low temperature range of 20-30 °C, Pluronics® solutions with a concentration of 10% w/v remain molecularly dissolved as individual units called unimers (Gaussian chain), which have a hydrodynamic size (Dh) of approximately 4-6 nm. Additionally, loose clusters of a few hundred nanometers in size are also observed. Though, at higher concentrations of BCPs and in the presence of salt or elevated temperatures, the examined micellar structures exhibit a higher degree of organization i.e., spherical or ellipsoidal in terms of size and shape. Also, the solubilization enhancement of a hydrophobic dye called orange OT within the examined micellar system is also undertaken using a spectral approach.

Rationalizing the Design of Pluronics-Surfactant Mixed Micelles through Molecular Simulations and Experiments

Aqueous systems comprising polymers and surfactants are technologically important complex fluids with tunable features dependent on the chemical nature of each constituent, overall composition in mixed systems, and solution conditions. The phase behavior and self-assembly of amphiphilic polymers can be changed drastically in the presence of conventional ionic surfactants and need to be clearly understood. Here, the self-aggregation dynamics of a triblock copolymer (Pluronics L81, EO₃PO₄₃EO₃) in the presence of three cationic surfactants (with a 12C long alkyl chain but with different structural features), viz., dodecyltrimethylammonium bromide (DTAB), didodecyldimethylammonium bromide (DDAB), and ethanediyl-1,2-bis(dimethyldodecylammonium bromide) (12-2-12), were investigated in an aqueous solution environment. The nanoscale micellar size expressed as hydrodynamic diameter (D_h) of copolymer-surfactant mixed aggregates was evaluated using dynamic light scattering, while the presence of a varied micellar geometry of L81-cationic surfactant mixed micelles were probed using small-angle neutron scattering. The obtained findings were further validated from molecular dynamics (MD) simulations, employing a simple and transferable coarse-grained molecular model based on the MARTINI force field. L81 remained molecularly dissolved up to ∼20 °C but phase separated, forming turbid/translucent dispersion, close to its cloud point (CP) and existed as unstable vesicles. However, it exhibited interesting solution behavior expressed in terms of the blue point (BP) and the double CP in the presence of different surfactants, leading to mixed micellar systems with a triggered morphology transition from unstable vesicles to polymer-rich micelles and cationic surfactant-rich micelles. Such an amendment in the morphology of copolymer nanoaggregates in the presence of cationic surfactants has been well observed from scattering data. This is further rationalized employing the MD approach, which validated the effective interactions between Pluronics-cationic surfactant mixed micelles. Thus, our experimental results integrated with MD yield a deep insight into the nanoscale interactions controlling the micellar aggregation (Pluronics-rich micelles and surfactant-rich micelles) in the investigated mixed system.

Obtaining of Mesoporous Aluminosilicates with High Hydrothermal Stability by Composite Organic Templates: Utility and Mechanism

We have reported the synthesis of mesoporous aluminosilicates (MAs) with high hydrothermal stability via assembly of basic characteristic structure units of typical microporous zeolite Y. In spite of this, high consumption of organic template and H₂O remains a major obstacle to its industrial application. Herein, a facile and effective strategy called "composite templates" was employed to decrease significantly the amount of P123 and H₂O. In this method, composite micelles of P123/poly(vinyl alcohol) (PVA) could be more easily dispersed in the solution due to the lowering of water's surface tension caused by the free hydroxyl groups of PVA. Moreover, the improved assembly ability of composite micelles in high concentration solution leads to the synthesis of hydrothermally stable MAs with 45% decrease of organic template P123 and 81% that of water amount. It was found that by the introduction of composite templates, the textural properties such as the surface area of materials, volume of pore, size of pore, and thickness were enlarged simultaneously. Meanwhile, this article presented an understanding into the assembly of composite micelles in the process of synthesis of MAs.

Structure and Interactions in Perfluorooctanoate Micellar Solutions Revealed by Small-Angle Neutron Scattering and Molecular Dynamics Simulations Studies: Effect of Urea

The self-assembly of surfactants in aqueous solution can be modulated by the presence of additives including urea, which is a well-known protein denaturant and also present in physiological fluids and agricultural runoff. This study addresses the effects of urea on the structure of micelles formed in water by the fluorinated surfactant perfluoro-n-octanoic acid ammonium salt (PFOA). Analysis of small-angle neutron scattering (SANS) experiments and atomistic molecular dynamics (MD) simulations provide consensus strong evidence for the direct mechanism of urea action on micellization: urea helps solvate the hydrophobic micelle core by localizing at the surface of the core in the place of some water molecules. Consequently, urea decreases electrostatic interactions at the micelle shell, changes the micelle shape from prolate ellipsoid to sphere, and decreases the number of surfactant molecules associating in a micelle. These findings inform the interactions and behavior of surface active per- and polyfluoroalkyl substances (PFAS) released in the aqueous environment and biota.

Mechanisms associated with the effects of urea on the micellar structure of sodium dodecyl sulphate in aqueous solutions

We used simulations by Molecular Dynamics to characterize the mechanism whereby the variations in the urea concentration modifies the micellar structure of sodium dodecyl sulfate monomers in water. From a self-assembled micellar system, we observed that increasing urea concentration leads to a decrease in aggregation number. Likewise, when increasing urea concentration, the micelles increase their nonpolar surface exposed to solvent, while the polar surface exposed to solvent decreases. This rearrangement process of SDS micelles in presence of urea is mainly due to the fact that the ions of Na⁺ that stabilize the micellar structure increase its interaction with urea. In this process, the SDS hydrophilic head and Na⁺ ions increases its solvation by urea, destabilizing micellar structure and exponing the hydrophobic core to the solvent.

Dielectric Behavior and Phase Behavior of Block Copolymer PEO13-PPO30-PEO13 Aqueous Solution

Dielectric spectroscopy can be applied to study the structure and dynamics of block polymer. In this work, dielectric measurements of block copolymer Pluronic L64 solution are carried out in the frequency range between 40 Hz and 110 MHz with variable temperatures and concentrations. We analyze the phase behavior of the PEO₁₃-PPO₃₀-PEO₁₃ (Pluronic L64) aqueous system according to the concentration/temperature-dependence of direct current conductivity. The result indicates the sensitivity of the phase behavior and conductivity of the Pluronic L64 solution to temperature. Besides, two relaxations were observed: relaxation 1 (0.5 MHz) is related to the gelation process, while relaxation 2 (5 MHz) is caused by the interface polarization. On the basis of relaxation 2, the volume fraction and permittivity of the particle were calculated. The formations of the block copolymer micelle and gel are monitored successfully by the temperature/concentration-dependence of the dielectric parameters and the volume fraction.

A comparative study and evaluation of sulfamethoxazole adsorption onto organo-montmorillonites

Three organo-montmorillonites were prepared using surfactants, and their adsorption behaviors toward sulfamethoxazole (SMX) were investigated. The surfactants used were cetyltrimethyl ammonium bromide (CTMAB), 3-(N,N-dimethylhexadecylammonio) propane sulfonate (HDAPS) and 1,3-bis(hexadecyldimethylammonio)-propane dibromide (BHDAP). The properties of the organo-montmorillonites were characterized by X-ray diffraction, scanning electron microscopy and N2 adsorption-desorption isotherm measurements. Results showed that the interlayer spacing of montmorillonite was increased and the surface area as well as the morphology were changed. Batch adsorption experiments showed that the surfactant loading amount had a great effect on the adsorption of SMX. The adsorption process was pH dependent and the maximum adsorption capacity was obtained at pH3 for HDAPS-Mt, while CTMAB-Mt and BHDAP-Mt showed a high removal efficiency at 3-11. The adsorption capacity increased with the initial SMX concentration and contact time but decreased with increasing solution ionic strength. Kinetic data were best described by the pseudo second-order model. Equilibrium data were best represented by the Langmuir model, and the Freundlich constant (n) indicated a favorable adsorption process. The maximum adsorption capacity of SMX was 235.29 mg/g for CTMAB-Mt, 155.28 mg/g for HDAPS-Mt and 242.72 mg/g for BHDAP-Mt. Thermodynamic parameters were calculated to evaluate the spontaneity and endothermic or exothermic nature. The adsorption mechanism was found to be dominated by electrostatic interaction, while hydrophobic interaction played a secondary role.