Honglan Fu

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.

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.

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.

Vesicle formation of 1:1 cationic and anionic surfactant mixtures in N,N-dimethylformamide and tetrahydrofuran solutions

In the last two decades, many works on vesicle formation from natural amphiphiles (mainly phospholipids) and synthetic surfactants were reported [1,2]. However, up to 10 years ago amphiphiles used to form vesicles are mainly the double chained compounds including natural and synthesized amphiphiles. In 1989, Kaler et al. [3] revealed the vesicle formation from mixed cationic and anionic surfactants using cetyltrimethyl-ammonium tosylate and sodium dodecylbezene sulfonate. Similar work was done in our laboratory, using mixed surfactants of carboxylate and alkyltrimethylammonium compounds [4,5]. However, most works on vesicle formation have been involved in aqueous systems and those in non aqueous systems are less. Some works about vesicle formation in aprotic solvent systems were reported [6,7], which were concentrated on the systems of double-chained fluorocarbon surfactants [8–11]. On the other hand, the studies about vesicle formation in non aqueous polar solvent and mixed polar solvents’ systems are mainly on the liposome of phospholipids [12– 14]. Comparing with phospholipids and other double-chained amphiphiles, the cationic and anionic surfactants used as precursors for vesicle formation have the predominances of simple structure, excellent stability, and convenience in production. Therefore, investigation on vesicle formation by cationic and anionic surfactants is of great significance in both theoretics and applications. It is well known that many drugs are insoluble in water but soluble in organic solvents, and many useful reactions can be carried out in nonaqueous systems. However, the investigations about vesicle formation of mixed cationic and anionic surfactants in non-aqueous polar solvents are scanty. Obviously, it is important to study the vesicle formation in non aqueous or mixed solvents. In a previous paper [15], we reported the * Corresponding author.