Imparting pH and temperature dual-responsiveness in a micellar solution of cationic surfactants by introducing a hydrotrope.

Over the recent years, intelligent, multi-responsive micelles have received considerable attention due to their promising application in a variety of fields, including biomedical technology, drug delivery, separation, and catalysis. However, the design of such systems with controlled self-assembly is challenging both experimentally and theoretically and is still in the nascent stage. In this study, a novel dual-stimuli triggered wormlike micellar solution is prepared by mixing cationic surfactants 3-hexadecyloxy-2-hydroxypropyltrimethylammonium bromide (R16HTAB) and sodium hydrogen phthalate (SHP). The viscoelasticity, aggregate morphology, and pH- and thermo-responsive behavior of the micellar solution are examined by rheological measurements, cryogenic-transmission electron microscopy (cryo-TEM), nuclear magnetic resonance (1H NMR) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The dual-sensitive fluid can be switched between a water-like state and a gel-like state by adjusting the pH and temperature. The variations in the flowing behavior are ascribed to the microstructural transition between wormlike micelles, short cylindrical micelles, and spherical micelles. Furthermore, based on the experimental results, dual-responsive behavior of the mixed solution is attributed to the different binding modes between SHP and the surfactant with the variation in the pH and temperature. We hope that the proposed system provides a new route for developing multi-stimuli-responsive materials that are capable of adapting to local environmental variations.

A pH, thermal and light triple-stimuli responsive micellar solution formed by a cationic surfactant and trans-o-hydroxycinnamic acid.

Controlling the viscoelastic characteristics of wormlike micelles is of great significance to both basic theory and practical applications. In this article, a novel multi-stimuli responsive wormlike micellar solution was prepared by mixing cationic surfactant 3-hexadecyloxy-2-hydroxypropyltrimethylammonium bromide (R16HTAB) with trans-o-hydroxycinnamic acid (OHCA). Rheological studies, nuclear magnetic resonance (1H-NMR) spectroscopy, UV-vis spectroscopy, and cryogenic-transmission electron microscopy (cryo-TEM) were utilized to investigate the wormlike micellar system's multi-responsive activity. The results showed that the self-assembled structure and viscoelasticity of the mixed system could be regulated by pH, temperature, and light irradiation. With the increasing trans-OHCA concentration, η0 of the mixed solution increases first and then decreases and the turning point is presented at 30 mM trans-OHCA, indicating the transformation of spherical micelles to wormlike micelles, and then to short micelles. The microstructure of the mixed systems could be reversibly altered by adjusting the pH between 6.41 and 3.90, which was ascertained by cryogenic-transmission electron microscopy (cryo-TEM). The relationship of temperature and η0 obeys the Arrhenius law, attributed to the decreasing micellar contour length. η0 of a 40 mM R16HTAB/15 mM OHCA solution sharply increases after UV irradiation, mainly because the cis-isomer could insert into the micelle more easily, and the transition mechanism was studied by UV-vis and 1H NMR. The multi-responsive self-assembled system may open a new vista for building multi-functional aggregates to adapt to various environmental changes.

Temperature/pH-Responsive Hexagonal Liquid Crystal for Curcumin Release.

The temperature/pH-responsive hexagonal liquid crystal constructed in a polyoxyethylene (20) sorbitan monooleate (Tween 80)/water-soluble chitosan (WCS)/ethyl oleate (EtOL)/H2O system was used to encapsulate curcumin. We discuss the effect of temperature and pH on the in vitro release of curcumin and the rheological properties of a curcumin-encapsulated liquid crystal. The drug-encapsulated hexagonal phase containing relatively high WCS content (9.9 wt %) exhibited good temperature response, as indicated by three phase transformation temperature obtained by temperature scanning. The viscosity of the drug-encapsulated hexagonal phase decreased as the temperature rose, which exhibited diverse constancy and temperature sensitivity. Also, the in vitro release rate of curcumin increased as the temperature increased. After introducing WCS, the release rate of curcumin decreased obviously the pH dependence, where the release of curcumin was slower as the pH was higher. All of the results indicated that the temperature/pH-responsive hexagonal liquid crystals were proposed to be potential slow release carriers to deliver curcumin.

Molecular Dynamics Simulation of the pH-Induced Structural Transitions in CTAB/NaSal Solution.

We performed molecular dynamics simulations to study the pH-induced structural transitions for aqueous mixtures of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) and a hydrotrope (sodium salicylate, NaSal). We obtained rigid cylindrical, spherical, and flexible cylindrical micelles at pH 7, 2, and 0, respectively, which agrees well with the experimental results of Umeasiegbu et al. (Langmuir 2016, 32, 655). By analyzing the different micellar structural properties, including distribution and molecular orientation of CTA+ and Sal- inside the micelle, we found that the binding form of the protonated salicylate molecules with CTA+ is different from that of Sal- ions. Because of the protonation of salicylate molecules with reduction in pH, their hydrogen bonding interactions with water molecules strengthened and the electrostatic interactions with CTA+ headgroups weakened. Thus, the repulsion of the CTA+ headgroups led to the breakage of the cylindrical micelle into spherical ones. At pH 0, the H-bond-strengthened cation-π interactions between salicylate and CTA+ were verified. We concluded that the penetration of salicylate molecules inside the micelle and the strong association of Cl- ions on the micellar surface play a key role in the formation of a flexible cylindrical micelle. This work provides an atomic-level insight into the mechanism of pH-induced shape transitions in the CTAB/NaSal systems, which is expected to be helpful to understand the aggregate behavior of cationic surfactant-hydrotrope solution.

Effects of additives on the viscoelastic responses of cationic gemini surfactant solutions.

Several additives, including inorganic (NaCl) and organic salts (derivatives of benzoate), were added into aqueous solutions of a gemini cationic surfactant, 2-hydroxypropyl-1,3-bis (myristyldimethylammonium chloride) (abbreviated as 14-3(OH)-14(2Cl)). The mixed systems were investigated using rheological measurement, cryo-TEM and 1H NMR analysis. The results showed that addition of salts induced rich aggregate morphologies in the 14-3(OH)-14(2Cl)/salt systems. The influence of an inorganic salt on the viscoelasticity of 14-3(OH)-14(2Cl) solutions is much weaker than that of organic salts. Furthermore, the ability of three organic salts in enhancing the viscoelasticity of 14-3(OH)-14(2Cl) solutions is in the order sodium m-hydroxybenzoate > sodium o-hydroxybenzoate > sodium p-hydroxybenzoate. The different roles of these organic salt isomers arise from the different types of hydrogen bonding formed between 14-3(OH)-14(2Cl) and the organic counter ions.

Phase behaviour and temperature-responsive properties of a gemini surfactant/Brij-30/water system.

The phase behaviour of a ternary system composed of cationic gemini surfactant 2-hydroxypropyl-1,3-bis(myristyldimethylammonium chloride) (14-3(OH)-14(2Cl)), nonionic surfactant polyoxyethylene laurel ether (C12H25(OCH2CH2)10OH, Brij-30) and H2O was studied and its phase diagram was determined. In several different phase regions identified in the phase diagram, the viscoelastic worm-like micellar solution region was investigated. The focus of the investigation was the increase in the viscosity of the micellar solution with the temperature and the Brij-30 content, as suggested by a partial region of the triangular phase diagram. With an increase in the Brij-30 content or temperature, one-dimensional micellar growth occurred, and a maximum appeared in the zero-shear viscosity, η0, versus Brij-30 content or temperature curves; this was true for mixed solutions containing 14-3(OH)-14(2Cl) and Brij-30 in different mass ratios (WBrij-30/W14-3(OH)-14(2Cl)). After the maximum point, the decrease in viscosity is the result of the micellar shortening in the size with Brij-30 content and of the micellar branching in the network structure with temperature. This rheological analysis was performed to investigate the structural changes in different solutions as well as the effects of various factors on micellar growth. Various techniques were used to study the phase-transition processes occurring in this system, including cryogenic transmission electron microscopy, small-angle X-ray scattering measurements, and differential scanning calorimetry. The transition mechanisms of the aggregates were analysed on the basis of the obtained results.

Thermo-responsive properties driven by hydrogen bonding in aqueous cationic gemini surfactant systems.

A series of unexpected thermo-responsive phenomena were discovered in an aqueous solution of the cationic gemini surfactant, 2-hydroxypropyl-1,3-bis(alkyldimethylammonium chloride) (n-3(OH)-n(2Cl), n = 14, 16), in the presence of an inorganic salt. The viscosity change trend for the 14-3(OH)-14(2Cl) system was investigated in the 20-40 °C temperature range. As the temperature increased, the viscosity of the solution first decreased to a minimum point corresponding to 27 °C, and then increased until a maximum was reached, after which the viscosity decreased again. In the 16-3(OH)-16(2Cl) system, the gelling temperature (T(gel)) and viscosity changes upon heating were similar to those in the 14-3(OH)-14(2Cl) system above 27 °C. The reversible conversion of elastic hydrogel to wormlike micelles in the aqueous solution of the 16-3(OH)-16(2Cl) system in the presence of an inorganic salt was observed at relatively low temperatures. Various techniques were used to study and verify the phase-transition processes in these systems, including rheological measurements, cryogenic transmission electron microscopy (cryo-TEM), electric conductivity, and differential scanning calorimetry. The abovementioned phenomena were explained by the formation and destruction of intermolecular hydrogen bonds, and the transition mechanisms of the aggregates were analyzed accordingly.

An independent 1D single-walled metal-organic nanotube transformed from a 2D layer exhibits highly selective and reversible sensing of nitroaromatic compounds.

The syntheses and structures of two new Zn(II) complexes, a 2D graphite-like layer {[Zn(PIA)H2 O]⋅H2 O}n (1) and an independent 1D single-walled metal-organic nanotube (SWMONT) {[Zn2 (PIA)2 (bpy)2 ]⋅2.5 H2 O⋅DMA}n (2), have been reported based on a "Y"-shaped 5-(pyridine-4-yl)isophthalic acid ligand (H2 PIA). Interestingly, the 2D graphite-like layer in 1 can transform into the independent 1D SWMONT in 2 with addition of 2,2'-bipyridine (bpy), which represents the first successfully experimental example of an independent 1D metal-organic nanotube generated from a 2D layer by a "rolling-up" mechanism.

2-Hy-droxy-N,N,N-trimethyl-3-tetra-decyl-oxypropan-1-aminium bromide.

In the crystal structure of the title compound, C(20)H(44)NO(2) (+)·Br(-), the cation and anion are connected via an O-H⋯Br hydrogen bond, forming an ionic pair. The cation is disordered over two conformations related by a mirror plane, and the anion is situated on a mirror plane so that the asymmetric unit contains half of the ionic pair. The long alkyl chain in the cation adopts an all-trans conformation. The crystal packing exhibits weak inter-molecular C-H⋯O inter-actions.

Poly[[µ(2)-1,3-bis-(imidazol-1-ylmeth-yl)benzene][µ(2)-2,2'-dihy-droxy-1,1'-methyl-enebis(naphthalene-3-carboxyl-ato)]zinc].

In the title compound, [Zn(C(23)H(14)O(6))(C(14)H(14)N(4))](n), the Zn(II) ion is four-coordinated in a distorted tetra-hedral geometry. The 1,3-bis-(imidazol-1-ylmeth-yl)benzene and 2,2'-dihy-droxy-1,1'-methyl-enebis(naphthalene-3-carboxy-l-ate) ligands con-nect the Zn(II) ions alternately in different directions, forming a layered structure parallel to the ac plane. Topological analysis reveals that the whole structure is a (4,4) network. The layers are further assembled into a three-dimensional supra-molecular structure via C-H⋯O and C-H⋯π inter-actions.

2-Hy-droxy-3-oct-yloxy-N,N,N-trimethyl-propan-1-aminium bromide.

In the title compound, C(14)H(32)NO(2) (+)·Br(-), organic cationsstacked parallel to the a axis andbromide anions placed between the head groups of the cations form ionic pairs via weak inter-molecular O-H⋯Br hydrogen bonds. The octyl chain in the cation adopts an all-trans conformation. The O-CH(2)-CH(-OH)-CH(2) portion of the molecule is disordered over two sets of sites with occupancy factors of 0.57 (3) and 0.47 (3).

3-Dodec-yloxy-2-hydr-oxy-N,N,N-trimethyl-propan-1-aminium bromide.

In the title compound, C(18)H(40)NO(2) (+)·Br(-), the ion pairs formed by the hydrogen-bonded bromide anions and organic cations are arranged into thick layers with the alkyl groups directed to the inside and the trimethyl-aminium groups and the bromide anions situated on the layer surface. The long alkyl chain in the cation adopts an all-trans conformation. In the crystal structure, molecules are connected by intermolecular O-H⋯Br hydrogen bonds, forming ionic pairs that are further connected into an extended chain structure via C-H⋯O hydrogen-bonding interactions. The crystal is chiral but nearly 90% of atoms in the unit cell are related by a pseudo-inversion center. The crystal shows racemic twinning with a 0.33:0.67 domain ratio.

3,3'-Dimethyl-1,1'-methyl-ene-diimidazolium dibromide.

In the crystal structure of the title compound, C(9)H(14)N(4) (2+)·2Br(-), the cation and anions have crystallographic mirror symmetry, with the mirror plane running through the central CH(2) group for the cation. The latter are stacked along the a axis, forming channels hosting the bromide anions. The crystal packing is stabilized by C-H⋯Br hydrogen-bonding inter-actions, generating a two-dimensional network.

1-Hexadecyl-3-methyl-imidazolium bromide monohydrate.

In the crystal structure of the title compound, C(20)H(39)N(2) (+)·Br(-)·H(2)O, the 1-hexa-decyl-3-methyl-imidazolium cations are stacked along the b axis, forming channels parallel to [100] which are occupied by the bromide anions and water mol-ecules. The crystal is stabilized by O-H⋯Br, C-H⋯O and C-H⋯Br hydrogen-bonding inter-actions, generating a two-dimensional network.

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.

Wormlike micelles formed by mixed cationic and anionic gemini surfactants in aqueous solution.

The formation and the properties of wormlike micelles in aqueous solutions of mixed cationic and anionic gemini surfactants, 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3(OH)-12) and O,O'-bis(sodium 2-dodecylcarboxylate)-p-benzenediol (C(12)ϕC(12)), have been studied by steady-state and dynamic rheological measurements at 25°C. With the addition of a small amount of C(12)ϕC(12) into the solution of 12-3(OH)-12, the total surfactant concentration of which was always kept at 80 mmol L(-1), the solution viscosity was strongly enhanced and its maximum was much larger than that of the mixed system of propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3-12) and C(12)ϕC(12). The results of dynamic rheology measurements showed that 12-3(OH)-12/C(12)ϕC(12) formed longer wormlike micelles in comparison with 12-3-12/C(12)ϕC(12). This was attributed to the effect of hydrogen bonding occurring between 12-3(OH)-12 molecules, which was an effective driving force promoting micellar growth. As few C(12)ϕC(12) participated in the micelles, the electrostatic attraction between the oppositely charged head groups of 12-3(OH)-12 and C(12)ϕC(12) made the molecules in the aggregates pack more tightly. This reinforced the hydrogen-bonding interactions and greatly promoted the micellar growth.

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.

How far can hydroxyl radicals travel? An electrochemical study based on a DNA mediated electron transfer process.

Using photocatalytic reactive nanoparticles and DNA composite modified gold electrodes, exploiting the sensitivity of DNA-mediated electron transfer to base pair stacking, we examined the effects of DNA oxidation damage by photocatalytically generated hydroxyl radicals (˙OH) on charge transfer efficiency and proposed an electrochemical technique to read the effective diffusing distance of ˙OH.