Xilian Wei

Wormlike micelles and gels reinforced by hydrogen bonding in aqueous cationic gemini surfactant systems

The viscoelastic properties of the wormlike micellar solutions of 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecylammonium bromide) and 2-hydroxyl-propanediyl-α,ω-bis(dimethyltetradecylammonium bromide), abbreviated as 12-3(OH)-12 and 14-3(OH)-14, respectively, have been investigated using steady state and frequency sweep rheological measurements. For comparison, the wormlike micellar solutions of propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (referred to as 12-3-12) were also examined on an identical level. Steady state rheological measurements revealed that at low concentrations, the zero-shear viscosity of the 12-3(OH)-12 system was far higher than that of the 12-3-12 system. From dynamic rheological data, the characteristic parameters were drawn out using the Maxwell fluid model and the viscoelastic rheological behavior was discussed according to the living polymer model proposed by Cates et al. The results showed that 12-3(OH)-12 formed longer wormlike micelles than 12-3-12. With increasing the alkyl tail length, 14-3(OH)-14 produced higher viscoelasticity than 12-3(OH)-12 and even formed a gel-like solid at 50 mmol L−1. These were attributed to the role of the intermolecular hydrogen bonding which occurred between the hydroxyl substituted spacers of m-3(OH)-m, where m represents the number of carbon atoms in the alkyl tail. Increase of the alkyl tail length was favorable for enhancement of the hydrogen bonding interaction and thus greatly promoted the micellar growth.

Ionic liquid crystals of quaternary ammonium salts with a 2-hydroxypropoxy insertion group

A 2-hydroxypropoxy group was inserted at the junction between the headgroup and the alkyl chain of quaternary ammonium salts. This group promotes a bent configuration for the alkyl chain relative to the ammonium headgroup, as compared to those without the 2-hydroxypropoxy group. At the same time, the insertion of this group weakens the N+–X− ionic interactions and hydrogen bonding interactions within the ionic layers and, therefore, allows the compounds to have a wide temperature range of mesophase. The molecular structures were determined by single crystal X-ray diffraction, in order to have a better understanding of the influence of the 2-hydroxypropoxy group on the packing arrangement and the mesomorphic properties. Their mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray powder diffraction and rheological measurements. Its important to note that anions have a profound effect on the phase properties of these liquid-crystalline compounds, such as phase transition temperatures, crystallisation rate, layer spacing and structural strength.

Crystalline structures and mesomorphic properties of gemini diammonium surfactants with a pendant hydroxyl group.

A series of homologous crystals of gemini diammonium surfactants (GDAS) containing one hydroxyl substituted methylene spacer are prepared. The crystal structures of these compounds, namely [C(n)H(2)(n)(+1)-N(+) (CH(3))(2)-CH(2)CH(OH)CH(2)-N(+)(CH(3))(2)-C(n)H(2)(n)(+1)], are determined by single-crystal X-ray diffraction techniques, in order to have a better understanding of the structure relation between the solid and the mesophase. The hydroxyl groups enhance the hydrogen bonding interaction between neighboring compounds, and therefore the packing of GDAS in the solid state is arranged to form a herringbone-like mode. To the best of our knowledge, this is the first GDAS crystal with a herringbone-like structure. Their mesomorphic properties are investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray powder diffraction, and rheological measurement. These compounds have relatively low melting points and form thermotropic mesophases over a broad temperature range, as compared to those without a hydroxyl substituted at the spacer. The rheological behavior of the smectic phases clearly reveals that hydrogen bonds exert a significant effect on the high values of moduli and viscosity. Moreover, the melting point and rheological parameters increase, conforming to the length of alkyl chains.

Formation and properties of wormlike micelles in solutions of a cationic surfactant with a 2-hydroxypropoxy insertion group

The rheological properties of aqueous solutions of a cationic surfactant, 3-hexadecyloxy-2-hydroxypropyl trimethyl ammonium bromide (R16HTAB), and its mixed solution system with sodium salicylate (NaSal) have been studied by using steady state and frequency sweep rheological measurements. The microstructure of the formed wormlike micelles has been ascertained by transmission electronic microscopy and 1H NMR technology. The effects of the surfactant concentration, added salt, temperature and different counterions on the viscoelastic solutions have been systematically investigated. In a salt-free state, the η0 and R16HTAB concentrations obey the relationship of the power law, i.e. η0 ∝ C2.53, in the examined concentration range. The addition of NaSal greatly promoted the micellar growth in R16HTAB aqueous solution and resulted in the generation of wormlike micelles. The causes of the formation of wormlike micelles and the strong network structure have been analyzed.

Phase behavior of new aqueous two-phase systems: 1-Butyl-3-methylimidazolium tetrafluoroborate + anionic surfactants + water

The aqueous mixtures of an anionic surfactant (sodium dodecyl benzene sulfonate, SDBS) and an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, [Bmim]BF4) within a certain range of compositions can spontaneously separate into two immiscible aqueous phases (ATPS): one surfactant-rich and the other ionic liquid-rich. The phase diagrams of the ternary system of the SDBS/[Bmim]BF4/water were drawn based on experimental data determined at 10.0, 20.0 and 30.0 °C, respectively. The borderlines of the different specific regions in the diagrams were determined using turbidity titration method. The narrow two-phase region aqueous were found to be close to the water - [Bmim]BF4 side. The formation conditions, compositions, and physicochemical properties of the aqueous two phases have been studied. The effects of temperature, surfactant and ionic liquid content on the phase separation time and phase volume ratio of the ATPS were also investigated. This phase separation phenomenon was found to be likely due to the existence of micelle aggregates with quite large size obtained by Transmission Electron Microscopy (TEM) and Steady-state Fluorescence Quenching Measurements (SFQM) methods.

Interaction Between 1-Dodecyl-3-Methylimidazolium Bromide and Sodium Carboxymethylcellulose in Aqueous Solution: Effect of Polymer Concentration

Effect of the concentration of water-soluble polyanion (sodium carboxymethylcellulose, NaCMC) on the interaction between a cationic surfactant (1-dodecyl-3-methylimidazolium bromide, C12mimBr) and NaCMC in aqueous solution has been studied by isothermal titration microcalorimetry (ITC), conductivity, surface tension, and rheological measurements. From the surfactant/polymer interacting enthalpy, it can be deduced that the electrostatic attraction between the cationic surfactant and anionic polyelectrolyte causes an endothermic process, and the C12mimBr monomers binding to the NaCMC chains to form micelle-like aggregates through hydrophobic interaction is an exothermic process. Increasing the NaCMC concentration causes the interaction between C12mimBr and NaCMC to decrease, and the characteristic surfactant concentrations, including the critical aggregation surfactant concentration (CAC), the surfactant concentration to form free micelles (Cm), and the saturation concentration of surfactant on the NaCMC chains (CS) to increase. Because of the strong electrostatic interaction between C12mimBr and NaCMC, the formation of C12mimBr/NaCMC complexes can lead to precipitation or redissolution depending on solution composition, so the critical precipitation concentration (CP) and the onset of a redissolution concentration (CR) has been determined by the electrical conductivity. The rheological results reveal a dramatic increase in solution viscosity around the CAC, attributed to interpolymer cross-linking through the formation of mixed micelles involving the carboxylic acid groups of NaCMC and the surfactant.

Formation and characteristics of aqueous two-phase systems formed by a cationic surfactant and a series of ionic liquids

Aqueous two-phase systems (ATPS) were obtained in the aqueous mixtures of a cationic surfactant and a series of ionic liquids (ILs). The effects of IL structure, temperature and additives on the phase separation were systematically investigated. The microstructures of some ATPS were observed by freeze-fracture replication technique. Lyotropic liquid crystal was found in the bottom phase besides micelles under different conditions. Remarkably, both IL structure and additives profoundly affected the formation and properties of the ATPSs. The phase separation can be attributed to the existence of different aggregates and the cation-π interactions of the cationic surfactant with the ILs, which has a significant role in the formation of ATPS. The extraction capacity of the studied ATPS was also evaluated through their application in the extraction of two biosubstances. The results indicate that the ILs with BF4(-) as anion show much better extraction efficiencies than the corresponding ILs with Br(-) as anion do under the same conditions. l-Tryptophan was mainly distributed into the NPTAB-rich phase, while methylene blue and capsochrome were mainly in the IL-rich phase.

Phase and rheological behavior of a gemini cationic surfactant aqueous system

The phase behavior of aqueous system of a gemini-type cationic surfactant, N,N′-dodecyl-2-hydroxyl-N,N,N′,N′-tetramethyl diammonium dichloride (simply written as 12-3(OH)-12(2Cl)), has been investigated by various complementary methods. A micellar solution region, a hexagonal liquid crystal phase region and two transition regions in the phase diagram are confirmed by SAXS and polarizing microscopy. Over the concentration range, 10–40 (wt%) of 12-3(OH)-12(2Cl), the viscoelastic properties of 12-3(OH)-12(2Cl) solutions at different temperatures have been determined using steady state and frequency sweep rheological measurements. The results from the FF-TEM study are in good agreement with the predictions from steady shear and dynamic rheology in which the large difference in the rheological behavior between 12-3(OH)-12(2Cl) and 12-3(OH)-12(2Br) at the same conditions might be mainly attributed to the role of the intermolecular hydrogen bonding which occurred between the hydroxyl substituted spacers and chloride counter ion and crystallization water molecule.

Interactions of two homologues of cationic surface active ionic liquids with sodium carboxymethylcellulose in aqueous solution

In the preceding paper of this series, we studied the interactions of copolymers with the ionic liquids, 1-alkyl-3-methylimidazolium bromide (CnmimBr, n = 8, 10, 12, 14, 16) and N-alkyl-N-methylpyrrolidinium bromide (CnMPB, n = 12, 14, 16). An obvious difference was detected between the interaction mechanism and the alkyl chain length of the surfactant. In the present study, we performed a systematic study on the interaction of sodium carboxymethylcellulose (NaCMC) with ionic liquids in aqueous solution by isothermal titration microcalorimetry (ITC), conductivity, turbidity, and dynamic light scattering (DLS) measurements. The existence of electrostatic attraction between NaCMC and ILs could increase the complexity of these systems. The results show that the monomers of C8mimBr can bind to the NaCMC chains and form free surfactant micelles in the solution, while no micelle-like C8mimBr/NaCMC cluster is detected. For other surfactants, the formation of surfactant/NaCMC clusters in the solution is driven by electrostatic and hydrophobic interactions, which could be divided into two types. One type is the polymer-induced surfactant/NaCMC complexes that form in the solution for the surfactant of CnmimBr (n = 10, 12, 14) or CnMPB (n = 12, 14). The other type is that the surfactant-induced surfactant/NaCMC complexes come into being for the surfactant of C16mimBr or C16MPB. Finally, the different modes of complex formation proposed have a good interpretation of the experiment results, unraveling the details of the effect of surfactant alkyl chain length and headgroup on the surfactant–NaCMC interactions.

Studies on the Physicochemical Properties of 3-Alkoxyl-2-hydroxypropyl Trimethylammonium Chloride–Sodium Dodecyl Sulfonate Binary-Surfactant Aqueous Systems

Surface tension, micelle formation, surface adsorption, and solubilization of dimethylaminoazobenzene (DMAB) are studied in aqueous solutions of 3-alkoxyl-2-hydroxypropyl trimethylammonium chloride (alkoxyl = CnH2n+1O, n = 8, 12, 14, 16), of sodium dodecyl sulfonate, and of mixtures of these cationic surfactants and the anionic surfactant at 40°C. Synergistic effects on micelle formation, surface tension reduction, and solubilization enhancement of DMAB are observed in the cationic–anionic mixed surfactant systems. The experimental results are discussed in the light of the interactions between the two kinds of surfactant ions.