Aggregation behavior of sodium dioctylsulfosuccinate in aqueous ethylene glycol medium. A case of hydrogen bonding between surfactant and solvent and its manifestation in the surface tension isotherm

Tuning the Mechanical and Thermal Properties of Hydroxypropyl Methylcellulose Cryogels with the Aid of Surfactants

The mechanical and thermal properties of cryogels depend on their microstructure. In this study, the microstructure of hydroxypropyl methylcellulose (HPMC) cryogels was modified by the addition of ionic (bis (2-ethylhexyl) sodium sulfosuccinate, AOT) and non-ionic (Kolliphor^® EL) surfactants to the precursor hydrogels (30 g/L). The surfactant concentrations varied from 0.2 mmol/L to 3.0 mmol/L. All of the hydrogels presented viscous behavior (G″ > G′). Hydrogels containing AOT (c > 2.0 mmol/L) led to cryogels with the lowest compressive modulus (13 ± 1 kPa), the highest specific surface area (2.31 m²/g), the lowest thermal conductivity (0.030 W/(m·°C)), and less hygroscopic walls. The addition of Kolliphor^® EL to the hydrogels yielded the stiffest cryogels (320 ± 32 kPa) with the lowest specific surface area (1.11 m²/g) and the highest thermal conductivity (0.055 W/(m·°C)). Density functional theory (DFT) calculations indicated an interaction energy of −31.8 kcal/mol due to the interaction between the AOT sulfonate group and the HPMC hydroxyl group and the hydrogen bond between the AOT carbonyl group and the HPMC hydroxyl group. The interaction energy between the HPMC hydroxyl group and the Kolliphor^® EL hydroxyl group was calculated as −7.91 kcal/mol. A model was proposed to describe the effects of AOT or Kolliphor^® EL on the microstructures and the mechanical/thermal properties of HPMC cryogels.

Robust Method to Determine Critical Micelle Concentration via Spreading Oil Drops on Surfactant Solutions

The spreading of a liquid on another is often encountered in oil spills and coatings and is also of industrial relevance in pharmaceuticals and petrochemicals. In this study, the spreading of oil drops on aqueous solutions containing cationic, anionic, and nonionic surfactants over a wide range of surfactant concentrations is investigated. The spreading behavior quantified by measuring the time evolution of the projected area of the oil lens reveals the occurrence of a maximum, which is strongly dependent on the concentration of the surfactant in the aqueous solution. Our experiments show that this dependence is different at concentrations above and below the critical micelle concentration (CMC) of the surfactant and can be captured by two straight lines of different slopes. Interestingly, these two straight lines intersect at a concentration that coincides with the CMC of the surfactants in solution. We find that this behavior is universal as shown by performing experiments with different types of surfactants, their purity, and other system variables. Thus, we propose a method to unambiguously determine the CMC of surfactant solutions compared to the conventional techniques. The proposed method is simple, versatile, and applicable for the determination of CMC of both ionic and nonionic surfactants.

Altering the X-ray Scattering Contrast of Triton X-100 Micelles and Its Trapping in a Supercooled Solvent

The structure of core-shell micelles formed by nonionic surfactant Triton X-100 (TX-100) in a supercooled glucose-urea melt is investigated by contrast variation small-angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and HR-TEM. Cooling a molten mixture of glucose-urea (weight ratio of 3:2) to room temperature yields a supercooled solvent without crystallization that can be used for trapping micelles of TX-100. By use of a combination of water and glucose-urea mixture at different proportions as solvent for micellization, the scattering length density (SLD) of the solvent can be tuned to match the shell contrast of the micelles. A systematic analysis of SAXS and SANS data with different SLD of solvent permits a quantitative evaluation of electron density profile of micelles in different matrices. The core of TX-100 micelles shows significant swelling in glucose-urea melt, as compared to that in water. The dimension and morphology of micelles were evaluated by scattering techniques and HR-TEM. Dynamic light scattering (DLS) studies suggest that, unlike micelles in water, the diffusion of micelles in supercooled glucose-urea melt decreased by several orders of magnitude.

Interaction between a Nonsteroidal Anti-inflammatory Drug (Ibuprofen) and an Anionic Surfactant (AOT) and Effects of Salt (NaI) and Hydrotrope (4-4-4)

Ibuprofen (IBF), 2-(4-isobutylphenyl) propionic acid, is a surface-active, common nonsteroidal anti-inflammatory drug (NSAID), and it possesses a high critical micelle concentration (cmc) compared to that of conventional surfactants. The interactions of this common NSAID with an anionic surfactant, sodium octyl sulfosuccinate, were studied by tensiometric, fluorimetric, and calorimetric measurements to investigate this system as a possible model drug-delivery system for an NSAID like IBF, particularly in a high-dose regime for IBF. The interactions between the drug and the surfactant were modeled using a regular solution theory approach in the presence and absence of a model electrolyte (sodium iodide) and a novel nonaromatic, gemini hydrotrope, tetramethylene-1,4-bis( N, N-dimethyl- N-butylammonium)bromide (4-4-4). Both the simple and the hydrotropic electrolyte were shown to have an effect on the solution properties (aggregation parameters, interfacial properties, and thermodynamics of aggregate formation) of the drug-surfactant mixtures and on the interaction between the drug and the surfactant. Surface charges of all self-assembled systems were estimated from ζ-potential measurements, whereas density functional theory calculations showed the interaction energy comparison among all of the binary and ternary combinations. All of these results were interpreted in terms of how altering the subtle balance of hydrophobic and electrostatic forces can significantly improve the ability of these self-assembled systems to transport drug molecules.

Spontaneous wormlike micelles formed in a single-tailed zwitterionic surface-active ionic liquid aqueous solution

Wormlike micelles were successfully fabricated by the self-assembly of a single-tailed zwitterionic surface-active ionic liquid (SAIL), 3-(1-hexadecyl-3-imidazolio) propanesulfonate β-naphthalene sulfonate (C₁₆IPS-Nsa), in aqueous solutions without any additives. With increasing zwitterionic SAIL concentration, spontaneous transition from micelles to wormlike micelles and then a hexagonal phase occurs. Interestingly, the wormlike micelles are closed, stretched and directionally arranged, which was rarely found in previous research studies. This kind of wormlike micelle is useful to serve as novel soft template in the synthesis of functional materials. ¹H NMR illustrated that the introduced π-π stacking and hydrophobic interactions originated from the hydrophobic aromatic counterions are responsible for the structural transformation. Density functional theory (DFT) simulated the optimum configuration of C₁₆IPS-Nsa and calculated the interaction energy of C₁₆IPS-Nsa-H₂O is 39.6 kJ mol^-1. This work paves the way for regulating the self-assembly structures of amphiphiles through changing specific weak interactions.

Aggregation behavior of zwitterionic surface active ionic liquids with different counterions, cations, and alkyl chains

The effects of anionic type, cationic structure and alkyl chain length are illustrated to regulate the self-assembly of zwitterionic SAILs.

Solvent and Substituent Effects on the Aggregation Behavior of Surface-Active Ionic Liquids with Aromatic Counterions and the Dispersion of Carbon Nanotubes in their Hexagonal Liquid Crystalline Phase

The aggregation behavior of surface-active ionic liquids (SAILs) 1-dodecyl-3-methylimidazolium m- and p-hydroxybenzoate (m-C12mimHB and p-C12mimHB) in water and ethylammonium nitrate (EAN) was investigated. Surface tension measurements indicate that the cmc values of SAILs in EAN are much higher than those in water, resulting from the weaker solvophobic effect of EAN, and the stronger stability of SAILs/EAN complexes proven by DFT calculations. Compared to 1-dodecyl-3-methylimidazolium salicylate (C12mimSal), the effect of substituent position leads to weaker interactions between aromatic counterions and headgroups. The hexagonal liquid crystal (H1) phase formed by C12mimHB in water or EAN at a higher concentration was determined by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheology techniques. Structural parameters estimated from SAXS curves suggest that the higher SAILs concentration or temperature leads to a smaller lattice parameter (a0) and a denser arrangement of cylinders. For C12mimHB, the formation of the H1 phase in H2O is easier than that in EAN. Furthermore, compared to C12mimSal, C12mimHB exists over a broad region of the hexagonal liquid crystalline (H1) phase, which is due to the different position of the substituents on the aromatic ring of counterions. Therefore, the H1 phase of the lypotropic liquid crystals (LLCs) formed in the C12mimHB/H2O system exhibits excellent performance in uniformly dispersing multiwalled carbon nanotubes (MWCNTs). Increasing the concentration of MWCNTs results in a larger lattice parameter (a0) value, indicating the integration of MWCNTs within the cylinders of the H1 phase. The rheological measurement results demonstrate that MWCNTs/LLCs composites are highly viscoelastic, and the presence of MWCNTs obviously strengthens the apparent viscosity of the H1 phase.

Experimental and DFT studies on the aggregation behavior of imidazolium-based surface-active ionic liquids with aromatic counterions in aqueous solution

Two imidazolium-based surface-active ionic liquids with aromatic counterions, namely, 1-dodecyl-3-methylimidazolium salicylate (C12mimSal) and 1-dodecyl-3-methylimidazolium 3-hydroxy-2-naphthoate (C12mimHNC), were synthesized, and their aggregate behavior in aqueous solutions was systematically explored. Surface tension and conductivity measurements indicate that both C12mimSal and C12mimHNC show superior surface activity compared to the common imidazolium-based SAIL with the same hydrocarbon chain length, 1-dodecyl-3-methylimidazolium bromide (C12mimBr). This result demonstrates that the incorporation of aromatic counterions favors the formation of micelles. C12mimHNC displays a higher surface activity than C12mimSal, resulting from the different hydrophobicities of the counterions. In comparison with C12mimBr, C12mimSal not only can form hexagonal liquid-crystalline phase (H1) in aqueous solution, but also exhibits a broad region of cubic liquid-crystalline phase (V2) at higher concentration. As for the C12mimHNC/H2O system, a lamellar liquid-crystalline (L(α)) phase was observed. These lyotropic liquid crystals (LLCs) were characterized by polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS). Structural parameters calculated from SAXS patterns suggest that a higher concentration of the SAIL leads to a denser arrangement whereas a higher temperature results in the opposite effect. The rheological results manifest that the formed H1 phase in the C12mimSal/H2O system exhibits an impressive viscoelastic behavior, indicated by a modulus (G' and G″) that is 1 order of magnitude higher than that of C12mimBr. Density functional theory (DFT) calculations reveal that C12mimSal has a more negative interaction energy with a water molecule and the Sal(-) counterion presents a stronger electronegativity than the HNC(-) counterion. The specific phase behavior of the C12mimSal/H2O and C12mimHNC/H2O systems can be attributed to the strong synergic interaction between the imidazolium cation and the aromatic counterion, including electrostatic attraction, hydrophobic interaction, and especially π-π interaction.

Micellization and related behavior of sodium dodecylsulfate in mixed binary solvent media of tetrahydrofuran (Tf) and formamide (Fa) with water: a detailed physicochemical investigation

The detailed aggregation behavior of sodiumdodecyl sulfate (SDS) in tetrahydrofuran (Tf)-water (W) and formamide (Fa)-water (W) media at varied volume percent compositions has been investigated. Surface tension (ST), conductance (Cond), viscosity (Visc), isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR) and small angle neutron scattering (SANS) were used in this study. The presence of nonaqueous solvents affected the critical micelle concentration (CMC) of SDS, the counter-ion binding of the micelle, the energetics of the air/water interfacial adsorption and micellization of the amphiphiles in the bulk, the ion-association (ion-pair, triple-ion, quadruple, etc. between Na(+) and DS(-) ions) as well as the weakly soluble (aggregation less) amphiphile solution. Tf has been observed to produce a "dead zone" or "non-micelle formation zone" in the mixed Tf-W domain of 10-40 vol%. Fa influenced the SDS aggregation up to 70 vol%, at higher proportions (below the Krafft temperature (K(T))), instead of the micelle, "randomly arranged globular assembly" (RAGA) was formed. The correlation of the standard free energy of micellization (ΔG(m)(0)) with different solvent parameters (1) dielectric constant (ε), (2) viscosity (η0), (3) Reichardt parameter (E(T)(30)), (4) Gordon parameter (G), and (5) Hansen-Hildebrand hydrogen bonding parameter (δ(h)) has been attempted. It has been found that δ(h) produced a master correlation between ΔG(m)(0) and δ(h) for different binary mixtures such as Tf-W, Fa-W, Ml-W and Dn-W.

Micellization of sodium laurylethoxysulfate (SLES) and short chain imidazolium ionic liquids in aqueous solution

In the present study the interactions between an anionic surfactant sodium laurylethoxysulfate (SLES) and three short chain imidazolium (1-butyl-3-methylimidazolium) based ionic liquids (bmim-octyl SO4, bmim-methyl SO4 and bmim-BF4) in aqueous solution have been investigated. Generally when a surfactant is dissolved in a hydrophilic ionic liquid aqueous solution the critical micelle concentration (cmc) obtained is attributed to the surfactant because the ionic liquid (IL) is considered to be only a cosolvent. However, some short hydrophilic ionic liquids posses surface activity in aqueous solution and behave like a surfactant. In that case mixed aggregates between surfactant and ionic liquid can be formed. The three SLES/IL systems here studied have been treated as typical binary surfactant mixtures in aqueous solution. Surface tension measurements have revealed that mixed aggregates and monolayers of surfactant and ionic liquid instead of single surfactant are responsible for the surface active properties of these aqueous solutions. From the Regular Solution Theory, negative interaction parameters (β) for mixed aggregates and monolayers have been found for all SLES/IL mole ratios indicating synergism between the anionic surfactant and the ionic liquids.

Self-assembly, surface activity and structure of n-octyl-β-D-thioglucopyranoside in ethylene glycol-water mixtures

The effect of the addition of ethylene glycol (EG) on the interfacial adsorption and micellar properties of the alkylglucoside surfactant n-octyl-β-d-thioglucopyranoside (OTG) has been investigated. Critical micelle concentrations (cmc) upon EG addition were obtained by both surface tension measurements and the pyrene 1:3 ratio method. A systematic increase in the cmc induced by the presence of the co-solvent was observed. This behavior was attributed to a reduction in the cohesive energy of the mixed solvent with respect to pure water, which favors an increase in the solubility of the surfactant with EG content. Static light scattering measurements revealed a decrease in the mean aggregation number of the OTG micelles with EG addition. Moreover, dynamic light scattering data showed that the effect of the surfactant concentration on micellar size is also controlled by the content of the co-solvent in the system. Finally, the effect of EG addition on the microstructure of OTG micelles was investigated using the hydrophobic probe Coumarin 153 (C153). Time-resolved fluorescence anisotropy decay curves of the probe solubilized in micelles were analyzed using the two-step model. The results indicate a slight reduction of the average reorientation time of the probe molecule with increasing EG in the mixed solvent system, thereby suggesting a lesser compactness induced by the presence of the co-solvent.