Lingxiang Jiang

Versatility of cyclodextrins in self-assembly systems of amphiphiles

Recently, cyclodextrins (CDs) were found to play important yet complicated (or even apparently opposite sometimes) roles in self-assembly systems of amphiphiles or surfactants. Herein, we try to review and clarify the versatility of CDs in surfactant assembly systems by 1) classifying the roles played by CDs into two groups (modulator and building unit) and four subgroups (destructive and constructive modulators, amphiphilic and unamphiphilic building units), 2) comparing these subgroups, and 3) analyzing mechanisms. As a modulator, although CDs by themselves do not participate into the final surfactant aggregates, they can greatly affect the aggregates in two ways. In most cases CDs will destroy the aggregates by depleting surfactant molecules from the aggregates (destructive), or in certain cases CDs can promote the aggregates to grow by selectively removing the less-aggregatable surfactant molecules from the aggregates (constructive). As an amphiphilic building unit, CDs can be chemically (by chemical bonds) or physically (by host-guest interaction) attached to a hydrophobic moiety, and the resultant compounds act as classic amphiphiles. As an unamphiphilic building unit, CD/surfactant complexes or even CDs on their own can assemble into aggregates in an unconventional, unamphiphilic manner driven by CD-CD H-bonds. Moreover, special emphasis is put on two recently appeared aspects: the constructive modulator and unamphiphilic building unit.

Effects of Inorganic and Organic Salts on Aggregation Behavior of Cationic Gemini Surfactants

All salts studied effectively reduce critical micelle concentration (CMC) values of the cationic gemini surfactants. The ability to promote the surfactant aggregation decreases in the order of C(6)H(5)COONa > p-C(6)H(4)(COONa)(2) > Na(2)SO(4)> NaCl. Moreover, only C(6)H(5)COONa distinctly reduces both the CMC values and the surface tension at CMC. For 12-4-12 solution, the penetration of C(6)H(5)COO(-) anions and charge neutralization induce a morphology change from micelles to vesicles, whereas the other salts only slightly increase the sizes of micelles. The 12-4(OH)(2)-12 solution changes from the micelle/vesicle coexistence to vesicles with the addition of C(6)H(5)COONa, whereas the other salts transfer the 12-4(OH)(2)-12 solution from the micelle/vesicle coexistence to micelles. As compared with 12-4-12, the two hydroxyls in the spacer of 12-4(OH)(2)-12 promote the micellization of 12-4(OH)(2)-12 and reduce the amounts of C(6)H(5)COONa required to induce the micelle-to-vesicle transition.

Bile salt-induced vesicle-to-micelle transition in catanionic surfactant systems: steric and electrostatic interactions.

The vesicle-to-micelle transition (VMT) was realized in catanionic surfactant systems by the addition of two kinds of bile salts, sodium cholate (SC) and sodium deoxycholate (SDC). It was found that steric interaction between the bile salt and catanionic surfactant plays an important role in catanionic surfactant systems that are usually thought to be dominated by electrostatic interaction. The facial amphiphilic structure and large occupied area of the bile salt are crucial to the enlargement of the average surfactant headgroup area and result in the VMT. Moreover, bile salts can also induce a macroscopic phase transition. Freeze-fracture transmission electron microscopy, dynamic light scattering, isothermal titration calorimetry, and absorbance measurements were used to follow the VMT process.

Aqueous self-assembly of SDS@2β-CD complexes: lamellae and vesicles

Cyclodextrin (CD)/surfactant complexes were usually believed to be quite soluble in water and unable to form aggregates because of the hydrophilic outer surface. However, in this work, SDS@2β-CD complex is found to be able to self-assemble into well-defined lamellar structures in concentrated aqueous solution. The lamellae possess unprecedented in-plane solid-crystalline order in addition to classical lamellar liquid-crystalline order. Such a combination in orderliness makes the lamellar phase an intermediate phase between a solid and a liquid crystal. Upon dilution, the lamellae transform to microtubes and then to vesicles. The three classes of SDS@2β-CD aggregates share a consistent building block, the channel-type crystalline bilayer membrane. Moreover, since the outer surface of SDS@2β-CD is hydrophilic, its self-assembly behavior, unlike traditional amphiphilic assembly, does not rely on the hydrophobic effect. Therefore, the present nonamphiphilic self-assembly of SDS@2β-CD is envisioned to open new possibilities for self-assembly chemistry.

“Annular Ring” microtubes formed by SDS@2β-CD complexes in aqueous solution

Traditional aqueous self-assembly of tubular structures (as well as other aggregates) usually relies on the hydrophobic effect, a relatively weak and nondirectional interaction. The resultant aggregates are inherently soft, fluid, and less-ordered. Alternatively, we report a novel kind of nonamphiphilic self-assembly of microtubes in aqueous solutions of cyclodextrin/ionic surfactant (CD/IS) complexes. This self-assembly is driven exclusively by H-bonds, relatively strong, directional interactions. The CD/IS microtubes feature an unbundling nature, ultralong persistence lengths, highly monodispersed diameters, and remarkable rigidity. Every single CD/IS microtube is constituted by a set of coaxial, equally spaced, hollow cylinders, resembling the annular rings of trees (thus termed as “annular ring” microtubes). Furthermore, bearing in mind the fundamental difference between the amphiphilic counterpart in driving forces, this H-bond-driven hydrophilic self-assembly is envisioned to complement its counterpart and expand the field of molecular self-assembly.

Self-assembly of ultralong polyion nanoladders facilitated by ionic recognition and molecular stiffness.

It is hard to obtain spatially ordered nanostructures via the polyion complexation process due to the inherent flexibility of polymers and isotropicity of ionic interactions. Here we report the formation of polyion assemblies with well-defined, periodically regular internal structure by imparting the proper stiffness to the molecular tile. A stiff bisligand TPE-C4-L2 was designed which is able to form a negatively charged supramolecular polyelectrolyte with transition metal ions. This supramolecular polyelectrolyte slowly self-assembled into polydispersed flat sheets with cocoon-like patterns. Upon the addition of an oppositely charged ordinary polyelectrolyte, the polydispersed cocoons immediately transformed into ultralong, uniform nanoladders as a result of matched ionic density recognition. The supramolecular polyelectrolytes assembled side-by-side, and the negative charges aligned in an array. This structure forced the positively charged polymers to lie along the negative charges so that the perpendicular arrangement of the oppositely charged chains was achieved. Such precise charge recognition will provide insight into the design of advanced materials with hierarchical self-assembled structures.

Helical colloidal sphere structures through thermo-reversible co-assembly with molecular microtubes.

Self-assembly is ubiquitous in nature, science, and technology and provides a general route to achieve order from disorder at various length scales.[1] Extensive effort has been exerted to molecular and colloidal self-assembly, where molecules and colloids, respectively, organize into larger-scale ordered structures. Although these two research areas have developed separately to a great extent, their combination would be very promising. Nature, for instance, utilizes hierarchical selfassembly across different length scales to construct complex, dynamic functional entities such as cells. Here we bridge the nano- and microscale by the hierarchical co-assembly between molecules and colloids, where molecular self-assembly induces the self-assembly of colloids into ordered structures.

Endowing catanionic surfactant vesicles with dual responsive abilities via a noncovalent strategy: introduction of a responser, sodium cholate†

A noncovalent strategy is proposed for endowing responseless catanionic surfactant (a mixture of cationic and anionic surfactants) aggregates with responsive abilities by addition of a responser. In this strategy, the composition of catanionic surfactant can be carefully selected to render aggregates sensitive to added responsers, and the responsers can be chosen from plenty of commercialized candidates, which bear responsive groups and will be noncovalently incorporated into the aggregates. In this paper, we report an illustrative example for the strategy: dual-responsive vesicles are realized by simply adding a responser, SC (sodium cholate), to a stimuli-inert DEAB/SDS (dodecyl triethyl ammonium bromide/sodium dodecyl sulfate) vesicular aqueous solution at a low responser/surfactant molar ratio of 0.045. The resultant DEAB/SDS/SC aggregates undergo reversible transitions between vesicles and micelles in response to temperature or pH variations. Possible mechanisms for these responsive behaviors are speculated, where the temperature-responsive hydroxyl groups and pH-responsive carboxylate group of SC are thought to be crucial. This responsive ability-endowing noncovalent strategy shows potential as a general, versatile, and economical method for fabricating stimuli-responsive self-assemblies.

General rules for the scaling behavior of linear wormlike micelles formed in catanionic surfactant systems

We report in this work on the scaling behavior of wormlike micelles formed in a series of mixed systems of oppositely charged surfactants, including sodium decanote (SD)/hexadecyltrimethylammonium bromide (CTAB), sodium laurate (SL)/hexadecyltrimethylammonium bromide, sodium didecaminocystine (SDDC)/hexadecyltrimethylammonium bromide, and sodium dilauraminocystine (SDLC)/hexadecyltrimethylammonium bromide. Steady and dynamic rheological measurements were performed to characterize these wormlike micelles. The scaling behavior for these systems at various mixing ratios was systematically investigated and was compared with that given by the Cates model. It was found that the Cates law can be applied in these systems simply by manipulating the mixing ratio or the surfactant structure. Energetic analysis demonstrates that the scaling behavior of wormlike micelles in nonequimolar mixed cationic and anionic surfactant systems can be close to that predicted by the Cates model, if the electrostatic contribution is below a threshold value.

Unveil the potential function of CD in surfactant systems

CDs may have promising functions in surfactant systems far beyond simply being disadvantageous to the formation of micelles. In this paper we review the recent literature and our work on the interesting effect of CDs on amphiphilic systems, especially on the concentrated single surfactant systems and catanionic surfactant mixed systems, both of them have been scarcely focused upon in the literature. In concentrated single surfactant systems, the 2:1 surfactant-CD inclusion complexes may form hierarchical self-assemblies such as lamellae, microtubes, and vesicles which are driven by hydrogen bonding. In nonstoichiometrically mixed catanionic surfactant systems, CDs behave as a stoichiometry booster that always selectively binds to the excess component so as to shift the mixing ratio to electro-neutral in the aggregates. In this way, CDs reduce the electrorepulsion in the aggregates and trigger their growth. Upon analysis of literature work and our own results, we expect that a new era focusing on the new function of CDs on surfactant systems will come.