Ali Reza Tehrani-Bagha

Solubilization of Hydrophobic Dyes in Surfactant Solutions

In this paper, the use of surfactants for solubilization of hydrophobic organic dyes (mainly solvent and disperse dyes) has been reviewed. The effect of parameters such as the chemical structures of the surfactant and the dye, addition of salt and of polyelectrolytes, pH, and temperature on dye solubilization has been discussed. Surfactant self-assemble into micelles in aqueous solution and below the concentration where this occurs—the critical micelle concentration (CMC)—there is no solubilization. Above the CMC, the amount of solubilized dye increases linearly with the increase in surfactant concentration. It is demonstrated that different surfactants work best for different dyes. In general, nonionic surfactants have higher solubilization power than anionic and cationic surfactants. It is likely that the reason for the good performance of nonionic surfactants is that they allow dyes to be accommodated not only in the inner, hydrocarbon part of the micelle but also in the headgroup shell. It is demonstrated that the location of a dye in a surfactant micelle can be assessed from the absorption spectrum of the dye-containing micellar solution.

Solubilization of two organic dyes by cationic ester-containing gemini surfactants

Solubilization of two different types of organic dyes, Quinizarin with an anthraquinone structure and Sudan I with an azo structure, has been studied in aqueous solutions of a series of cationic gemini surfactants and of a conventional monomeric cationic surfactant, dodecyltrimethylammonium bromide (DTAB). Surfactant concentrations both above and below the critical micelle concentration were used. The concentration of solubilized dye at equilibrium was determined from the absorbance of the solution at λ(max) with the aid of a calibration curve. The solubilization power of the gemini surfactants was higher than that of DTAB and increased with increasing alkyl chain length. An increase in length of the spacer unit resulted in increased solubilization power while a hydroxyl group in the spacer did not have much effect. Ester bonds in the alkyl chains reduced the solubilization power with respect to both dyes. A comparison between the absorbance spectra of the dyes in micellar solution with spectra in a range of solvents of different polarity indicated that the dye is situated in a relatively polar environment. One may therefore assume that the dye is located just below the head group region of the micelle. Attractive π-cation interactions may play a role for orienting the dye to the outer region of the micelle.

Cationic ester-containing gemini surfactants: adsorption at tailor-made surfaces monitored by SPR and QCM.

Adsorption of a series of ester-containing cationic surfactants at a surface containing 90% methyl groups and 10% carboxyl groups was studied by two surface analysis techniques, surface plasmon resonance (SPR) and quartz crystal microbalance (QCM). Such a surface, which is at the same time hydrophobic and negatively charged, is of interest as a model for many polymeric surfaces. Two different types of ester gemini surfactants and their monomeric counterparts were included together with nonester containing surfactants of similar structure. The results show that the gemini surfactants give the same adsorbed amount at the surface as the monomeric surfactants when compared at the same bulk concentration normalized to the critical micelle concentration (cmc) in bulk. Since the cmc of the geminis is around 20 times lower than the cmc of the corresponding monomeric surfactants, the gemini surfactants are much more effective in covering the surface. The two techniques gave similar relative values but the QCM values were always higher than those from SPR, which is due to the former method taking also adsorbed water into account. The adsorption, as measured by both methods, was found to follow closely the Langmuir adsorption model.

Cationic ester-containing gemini surfactants: Determination of aggregation numbers by time-resolved fluorescence quenching

The micellar aggregation number of a series of ester-containing gemini surfactants has been determined with steady state and with time-resolved fluorescence quenching. The latter method gave values of aggregation number about twice those obtained with the former method. It was found that the length of the spacer was the most important factor affecting the aggregation number. The length and the nature of the surfactant alkyl chains were of less importance in spite of the fact that the length of the alkyl chains strongly affects the solution properties of the unimers.

Mesoscopically Ordered Bone‐Mimetic Nanocomposites

A sustainable approach that highly mimics bone-material deposition is reported to produce mechanically stable, degradable composites with nanostructures resembling that of natural bone. Molecular self-assembly combining intermolecular crosslinking leads to resilient matrices possessing long-range ordered aqueous domains, inside which moderately aligned poorly crystalline apatite is converted from the transient amorphous calcium phosphate phase.

Cationic gemini surfactants with cleavable spacer: Chemical hydrolysis, biodegradation, and toxicity

The paper describes synthesis and characterization of a new type of cationic gemini surfactant, which has dodecyl tails and a spacer that contains an ester bond. The nomenclature used to describe the structure is 12Q2OCO1Q12, with Q being a quaternary ammonium group and the numbers indicating the number of methylene or methyl groups. Due to the close proximity to the two quaternary ammonium groups, the ester bond is very stable on the acid side and very labile already at slightly alkaline conditions. The hydrolysis products are two single chain surfactants (i.e. 12Q2OH and 12Q1COOH) which are less surface active than the intact gemini surfactant. 12Q2OCO1Q12 was found to be readily biodegradable, i.e. it gave more than 60% biodegradation after 28 days. This is interesting because similar gemini surfactants but with ester bonds in the tails instead of the spacer, have previously been found not to be readily biodegradable. The gemini surfactant was found to be toxic to aquatic organisms (ErC50 value of 0.27 mg/l), although less toxic than the two hydrolysis products.

Growth Behavior, Geometrical Shape, and Second CMC of Micelles Formed by Cationic Gemini Esterquat Surfactants

Micelles formed by novel gemini esterquat surfactants have been investigated with small-angle neutron scattering (SANS). The growth behavior of the micelles is found to differ conspicuously depending on the length of the gemini surfactant spacer group. The gemini surfactant with a long spacer form rather small triaxial ellipsoidal tablet-shaped micelles that grow weakly with surfactant concentration in the entire range of measured concentrations. Geminis with a short spacer, on the other hand, form weakly growing oblates or tablets at low concentrations that start to grow much more strongly into polydisperse rodlike or wormlike micelles at higher concentrations. The latter behavior is consistent with the presence of a second CMC that marks the transition from the weakly to the strongly growing regime. It is found that the growth behavior in terms of aggregation number as a function of surfactant concentration always appear concave in weakly growing regimes, while switching to convex behavior in strongly growing regimes. As a result, we are able to determine the second CMC of the geminis with short spacer by means of suggesting a rather precise definition of it, located at the point of inflection of the growth curve that corresponds to the transition from concave to convex growth behavior. Our SANS results are rationalized by comparison with the recently developed general micelle model. In particular, this theory is able to explain and reproduce the characteristic appearances of the experimental growth curves, including the presence of a second CMC and the convex strongly growing regime beyond. By means of optimizing the agreement between predictions from the general micelle model and results from SANS experiments, we are able to determine the three bending elasticity constants spontaneous curvature, bending rigidity, and saddle-splay constant for each surfactant.

Micelle growth of cationic gemini surfactants studied by NMR and by time-resolved fluorescence quenching

The micelle growth of a series of five cationic gemini surfactants has been investigated by time-resolved fluorescence quenching (TRFQ) and by two NMR techniques, line width analysis and diffusometry. The surfactant series was designed such that the effect of a number of variables could be assessed: length of the spacer unit, presence of ester bonds in the tails close to the head groups, and presence of a hydroxyl group in the spacer. For the gemini with long spacer, the micelles remained relatively small in size upon an increase of the concentration. The gemini surfactants with short spacer, on the other hand, showed a considerable micellar growth as the concentration was raised. It is of particular interest that the relatively simple line width analysis of one dimensional (1)H NMR spectra gave qualitatively the same results as the more sophisticated TRFQ and NMR diffusometry techniques.

Spontaneous Formation of Nanocubic Particles and Spherical Vesicles in Catanionic Mixtures of Ester-Containing Gemini Surfactants and Sodium Dodecyl Sulfate in the Presence of Electrolyte

Self-assembly of pure ester-containing cationic gemini surfactants, dodecyl esterquat, and dodecyl betainate geminis, and cation-rich catanionic mixtures of them with sodium dodecyl sulfate (SDS) were investigated using surface tension, electrical conductivity, dynamic light scattering (DLS), transmission electron microscopy (TEM) and cyclic voltammetry (CV) measurements in the absence and presence of KCl. Different physicochemical properties such as the critical micelle concentration (CMC), degree of counterion dissociation (αdiss), interfacial properties, morphology of aggregates, and interparticle interaction parameters were determined. Both geminis formed micelles in the absence of KCl, and mixing with SDS did not change the morphology; just a growth in micelle size was observed. However, the aggregation behavior of these geminis with respect to the position of the ester bond in the alkyl chain appeared completely different in the presence of KCl. Esterquat gemini formed cubic nanoparticles (or cobosomes) in the presence of [KCl] = 0.05 M and transformed into spherical micelles upon increasing the surfactant concentration. By contrast, betainate gemini formed vesicles in the presence of [KCl] = 0.05 M and subsequently converted to micelles as the surfactant concentration increased. The morphology of esterquat gemini (in the presence of 0.05 M KCl) after mixing with SDS changed from cubic nanoparticles (or cobosomes) to cylindrical nanoparticles coexistent with cobosomes. Betainate gemini remained vesicular upon mixing with SDS, and no dramatic structural change of aggregates took place. The morphology changes of aggregates upon mixing with SDS were explained from calculating the interactions between two gemini surfactants and SDS on the basis of regular solution theory.

Epoxy Resin Monomers with Reduced Skin Sensitizing Potency

Epoxy resin monomers (ERMs), especially diglycidyl ethers of bisphenol A and F (DGEBA and DGEBF), are extensively used as building blocks for thermosetting polymers. However, they are known to commonly cause skin allergy. This research describes a number of alternative ERMs, designed with the aim of reducing the skin sensitizing potency while maintaining the ability to form thermosetting polymers. The compounds were designed, synthesized, and assessed for sensitizing potency using the in vivo murine local lymph node assay (LLNA). All six epoxy resin monomers had decreased sensitizing potencies compared to those of DGEBA and DGEBF. With respect to the LLNA EC3 value, the best of the alternative monomers had a value approximately 2.5 times higher than those of DGEBA and DGEBF. The diepoxides were reacted with triethylenetetramine, and the polymers formed were tested for technical applicability using thermogravimetric analysis and differential scanning calorimetry. Four out of the six alternative ERMs gave polymers with a thermal stability comparable to that obtained with DGEBA and DGEBF. The use of improved epoxy resin monomers with less skin sensitizing effects is a direct way to tackle the problem of contact allergy to epoxy resin systems, particularly in occupational settings, resulting in a reduction in the incidence of allergic contact dermatitis.