A facile route to design pH-responsive viscoelastic wormlike micelles: Smart use of hydrotropes.

A simple and effective route to design pH-responsive viscoelastic wormlike micelles based on commercial compounds is reported. According to this route, pH-sensitive viscoelastic fluids can be easily obtained by introducing a pH-responsive hydrotrope into a surfactant solution. In this paper, the mixed system of cetyltrimethylammonium bromide (CTAB) and potassium phthalic acid (PPA) was studied in detail. This pH-sensitive fluid can be switched between a gellike state and a waterlike state within a narrow pH change. Rheology and DLS results revealed that the pH-sensitive flowing behavior was attributed to the microstructure transition between wormlike micelles and short cylindrical micelles. Combined with fluorescence anisotropy, NMR, and UV-vis, it was demonstrated that the pH response of viscoelastic fluid originated from the different binding abilities of hydrotrope to surfactant as pH varies. Furthermore, different kinds of hydrotropes can be utilized to prepare pH-responsive viscoelastic fluids in the desired pH areas.

Aqueous surfactant two-phase systems in a mixture of cationic gemini and anionic surfactants.

The phase behavior as well as the microstructures of the cationic gemini surfactant and anionic conventional surfactant aqueous two-phase system (ASTP) have been studied. The ASTP formation can be attributed to the coexistence of different kinds of aggregates in the upper and lower phases. The effects of temperature, shearing, surfactant concentration and mixing molar ratio on the phase separation of the ASTP-forming systems are systematically investigated. The ASTP can be destroyed by applying shear and increasing temperature. In this process, the lamellar structures (flat bilayers) in the ASTP are transformed into vesicles. Variation of surfactant structure also affects the phase behavior and the aggregates transformation. Appropriate molecular packing is crucial for the formation of ASTP.

Active control of surface properties and aggregation behavior in amino acid-based Gemini surfactant systems.

Two types of Gemini surfactants containing a disulfide bond in the spacer, sodium dilauroyl cystine (SDLC) and sodium didecamino cystine (SDDC), were synthesized, and their surface properties and aggregation behavior in aqueous solution were studied by means of surface tension measurements, dynamic light scattering (DLS), transmission electron microscopy (TEM), and fluorescence. During the transition of the Gemini surfactants to their corresponding monomers through the reduction of disulfide bonds, the surface tensions of their aqueous solutions, as well as their aggregation behavior, changed greatly. The reduction of SDLC and SDDC led to disruption of the vesicle, and the oxidation of corresponding monomers to Gemini surfactants led to vesicle re-formation. These results demonstrated the control of surface properties and aggregation behavior by the reversible transition between the Gemini surfactant and its monomer via reduction/oxidation reactions.

Effect of hydrotropic salt on the assembly transitions and rheological responses of cationic gemini surfactant solutions.

Cationic gemini surfactant dimethylene-1,2-bis(dodecyldiethylammonium bromide), referred to as C12C2C12(Et), was synthesized. The effect of sodium salicylate (NaSal) on the assembly formation and transition of this cationic gemini surfactant solution was studied. Addition of NaSal induced rich aggregate morphologies in the C12C2C12(Et) system. The microstructures and rheological responses resulting from the addition of NaSal were studied systematically to explore the interaction between gemini surfactants and hydrotropic salts. The rich aggregation behavior can be attributed to the special molecular structure of the gemini surfactant and the appropriate interaction between the surfactant and NaSal. The study of gemini surfactant and hydrotropic salt interaction brings promise for applications in materials synthesis as soft templates.

Temperature-sensitive aqueous surfactant two-phase system formation in cationic-anionic surfactant systems.

Temperature-induced aqueous surfactant two-phase system (T-ASTP), which was found to be of generic importance, was investigated in a series of conventional mixed cationic-anionic surfactant systems. On the basis of the investigations of turbidity, dynamic light scattering, transmission electron microscopy, and fluorescence resonance energy transfer, the formation of T-ASTP can be attributed to temperature-induced vesicle aggregation. Aggregated vesicles existed in the upper part, while the separated vesicles existed in the lower part. The phase separation temperature can be regulated by varying the surfactant composition or adding additives, such as d-sorbitol, urea, or NaBr. The hydrophobic interaction and cooperative effect between cationic and anionic surfactants played a significant role in the formation of T-ASTP.

Vesicle aggregation in aqueous mixtures of negatively charged polyelectrolyte and conventional cationic surfactant.

Vesicle aggregation induced by different environmental factors, including the addition of divalent metal ions, decrease of pH, and increase of temperature--was investigated through turbidity measurement, fluorescence measurement, and transmission electron microscope observation in aqueous solutions of hydrolyzed styrene-maleic anhydride copolymer (HSMA) mixed with dodecyltriethylammonium bromide (C(12)Et(3)). The vesicle aggregation can be explained by the dehydration of the vesicle surface through cations addition or temperature increase based on an analysis of the interaction between vesicles. Moreover, the steric repulsion was introduced to the system and the control of vesicle aggregation was achieved.

Characterizing assembly morphology changes during solubilization process of dimyristoyl phosphocholine vesicles by n-dodecyl triethylammonium bromide.

In the present work, the assembly morphology changes during the solubilization process of the sonicated unilamellar vesicles from dimyristoyl phosphocholine (DMPC) by a cationic surfactant, n-dodecyl triethylammonium bromide (DTEAB) were well characterized with DSC, FF-TEM and DLS and fluorescence probes technique. Based on an analysis on the above results, a primary multi-stage model was brought forward to sketch the assembly morphology changes during the DMPC vesicle solubilization by DTEAB. In comparison with classical models, vesicles division, tubule-like structure formation and fission to vesicle were found in the middle stages of this model. Additionally, it is the first time that the transversally-cut profiles of tubule-like structures were observed during vesicle solubilization process.

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions.

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.

Molecular packing parameter in bolaamphiphile solutions: adjustment of aggregate morphology by modifying the solution conditions.

The geometrical rule of molecular packing parameters in a bolaamphiphile solution was tested with experimental results. By modifying the solution conditions to change the molecular packing parameters, the morphology of the aggregate was successfully manipulated in a single-chain bolaamphiphile, disodium phenyl-1,4-bis (oxyhexanoate) (i.e., C6PhC6Na2 solution). Micelle-vesicle-tube transformation was observed by changing the pH and the addition of NaBr or octanol. In the mixed systems of oppositely charged bola/surfactants, the molecular packing parameter's role is related to the mixing ratio.

Vesicle formation and stability in aqueous mixtures of the hydrolyzed copolymer of styrene-maleic anhydride and conventional single-tailed cationic surfactants.

Vesicle formation in aqueous mixtures of the hydrolyzed copolymer of styrene-maleic anhydride (HSMA) and a series of single-tailed cationic surfactants (C(n)H(2n+1)N(C(m)H(2m+1))3Br, n = 8, 10, 12, 16, m = 1, 2, 3, 4) was studied by fluorescence measurement, zeta potential measurement, and transmission electron microscopy. The driving forces of vesicle formation in this kind of system are attributed to the combination of electrostatic attraction and the hydrophobic interaction. Variation of the surfactant structure had a great influence on vesicle formation. A model for the conformation of the molecular packing in the vesicle membrane was suggested on the basis of XRD measurement and Chem3D simulation. Moreover, these vesicles showed superstability to aging time, to NaBr, and to ethanol.

Transitions of organized assemblies in mixed systems of cationic bolaamphiphile and anionic conventional surfactants.

The transition from vesicles to tubelike structures has been studied in mixed systems of cationic bolaamphiphile BPHTAB [biphenyl-4,4'-bis(oxyhexamethylenetrimethylammonium bromide)] and its oppositely charged conventional surfactants with transmission electron microscopy (TEM) and dynamic light scattering (DLS). This transition can be attributed to the fact that tube-like structures are more stable aggregates than vesicles because of the special molecular packing in the aggregates of the mixed systems. The effects of temperature and salt addition on this transition have also been investigated, and the rate of the transition was found to be strongly dependent on temperature. Addition of the appropriate amount of NaBr will accelerate the transition from vesicles to tube-like structures, but the vesicles will transform into micelles at higher salt concentration. Moreover, the micelle-vesicle transition can be realized by addition of n-octanol in the mixed system of BPHTAB/sodium caprate (SC) at higher salt concentrations.

Electrochemical growth of highly oriented organic-inorganic superlattices using solid-supported multilamellar membranes as templates.

Controllable depositing of relatively thick inorganic sublayers into organic templates to fabricate organic-inorganic superlattices is of great importance. We report a novel approach to fabricating phospholipid/Ni(OH)(2) superlattices by electrochemical deposition of the inorganic component into solid-supported multilamellar templates. The well-ordered and highly oriented multilamellar templates are produced by spreading small drops of lipid solution on silicon surfaces and letting the solvent evaporate slowly. The templates which are used as working electrodes preserve the lamellar structure in the electrolyte solution. The resulting superlattices are highly oriented. The thickness of the nickel hydroxide is controlled by the concentration of nickel ions in the electrolyte bath. The electron density profiles derived from the X-ray diffraction data reveal that the thickness of the nickel hydroxide sublayers increases from 15 to 27 A as the concentration of nickel nitrate increases from 0.005 mol/L to 0.08 mol/L. We expect that the new method can be extended to depositing a variety of inorganic components including metals, oxides, and semiconductors.

Temperature-controlled vesicle aggregation in the mixed system of sodium n-dodecyl sulfate/n-dodecyltributylammonium bromide.

Temperature-controlled vesicle aggregation was investigated in a catanionic surfactant system of sodium n-dodecyl sulfate/n-dodecyltributylammonium bromide. Vesicle aggregation took place as the temperature reached the critical value (Tc). Tc can be adjusted by the variations of the total surfactant concentration and the mixed molar ratio. It was also found that the temperature variation above Tc can greatly influence the vesicle aggregation rate. The vesicle aggregation process was irreversible as long as T >/= Tc, whereas the vesicle disaggregation process occurred only below Tc.

Organized assemblies in bolaamphiphile/oppositely charged conventional surfactant mixed systems.

Five ionic bolaamphiphiles were synthesized and the aggregation behavior of bola single systems and bola/oppositely charged conventional surfactant mixed systems was studied. Small spherical vesicles were formed in all these mixed systems revealed by transmission electron microscopy (TEM). Variation of the structure of the hydrophobic chain of bolaamphiphiles has great influences on the vesicle formation ability. Vesicles were also found in the single system of a carboxylate bolaamphiphile, which was attributed to the hydrolysis of the bolaamphiphile. The results of FT-IR and X-ray diffraction (XRD) showed that bolaamphiphiles spanned through the vesicle membranes in these mixed systems. Super thermostability of the vesicles in this kind of mixed system was also investigated.