Asymmetric and symmetric bolaform supra-amphiphiles: formation of imine bond influenced by aggregation

Dynamic covalent surfactants and their uses in the development of smart materials

Dynamic covalent chemistry, which leverages the dynamic nature of reversible covalent bonds controlled by the conditions of reaction equilibrium, has demonstrated great potential in diverse applications related to both the stability of covalent bonds and the possibility of exchanging building blocks, imparting to the systems the possibility of "error checking" and "proof-reading". By incorporating dynamic covalent bonds into surfactant molecular architectures, combinatorial libraries of surfactants with bespoke functionalities can be readily fabricated through a facile strategy, with minimum effort in organic synthesis. Consequently, a multidisciplinary field of research involving the creation and application of dynamic covalent surfactants has recently emerged, which has aroused great attention in surfactant and colloid science, supramolecular chemistry, self-assembly, smart materials, drug delivery, and nanotechnology. This review reports results in this field published over recent years, discusses the possibilities presented by dynamic covalent surfactants and their applications in developing smart self-assembled materials, and outlines some future perspectives.

pH and Magnetism Dual-Responsive Pickering Emulsion Stabilized by Dynamic Covalent Fe3O4 Nanoparticles

Herein, we describe pH and magnetism dual-responsive liquid paraffin-in-water Pickering emulsion stabilized by dynamic covalent Fe₃O₄ (DC-Fe₃O₄) nanoparticles. On one hand, the Pickerinfigureg emulsions are sensitive to pH variations, and efficient demulsification can be achieved by regulating the pH between 10 and 2 within 30 min. The dynamic imine bond in DC-Fe₃O₄ can be reversibly formed and decomposed, resulting in a pH-controlled amphiphilicity. The Pickering emulsion can be reversibly switched between stable and unstable states by pH at least three times. On the other hand, the magnetic Fe₃O₄ core of DC-Fe₃O₄ allowed rapid separation of the oil droplets from Pickering emulsions under an external magnetic field within 40 s, which was a good extraction system for purifying the aqueous solution contaminated by rhodamine B. The dual responsiveness enables Pickering emulsions to have better control of their stability and to be applied more broadly.

Rational design of dynamic imine surfactants for oil-water emulsions: Learning from oil-induced reversible dynamic imine bond formation

HYPOTHESIS

Dynamic imine surfactants (DIS) can be constructed by the formation of dynamic imine bonds (Dibs) between aromatic aldehydes and aliphatic amines in water. Because of the nature of Dibs in water, a thermodynamic equilibrium state was achieved between the DIS and aldehyde and amine precursors to form a dynamic combinatorial library (DCL). When the DIS served as sole emulsifier to form oil-H₂O emulsions, the precursors migrated between the H₂O phase and the oil phase, which altered the DCL equilibrium. The DIS concentration and emulsion stability also changed.

EXPERIMENTS

By mixing 4-(2-sulfobetaine-ethoxy)-benzaldehyde (SBBA) and aliphatic amines of C_nH_2n+1NH₂ (n = 4, BA; n = 6, HA; n = 8, OA; n = 10, DA) in water, four amphoteric DIS (SBBA-BA/HA/OA/DA) were prepared. Dib formation was characterized using ¹H NMR. The DIS surface activity was studied by surface tension and fluorescence probe methods. The reversible switching of DIS and its wormlike micelles were explored.

FINDINGS

SBBA-OA (or SBBA-DA) DIS was not a suitable emulsifier for stable hydrocarbon (HC)-H₂O emulsions. OA and DA were more soluble in the HC phase than the H₂O phase. The precursors of OA and DA migrated from the H₂O to the HC phase, and the thermodynamic equilibrium state of DCL shifted towards Dib dissociation. The Dib could be regenerated by HC phase removal. A novel strategy where volatile HC (such as pentane) was used as a trigger was developed to switch the DIS reversibly and its self-assemblies (such as wormlike micelles) in water without inorganic salt accumulation. The SBBA-HA (or SBBA-BA) DIS was a suitable emulsifier for stable emulsions because HA and BA were more soluble in the H₂O phase.

pH-responsive pickering foam created from self-aggregate polymer using dynamic covalent bond

HYPOTHESIS

Responsive surfactant systems based on dynamic covalent bond exhibit an unsatisfactory foamability and foam stability, despite their documented functionality in emulsions. As such we anticipate that the foaming performance should be improved by introducing Pickering effect, which is possible when the responsiveness of the dynamic covenant bonds controls not only the hydrophobicity of polymers but also their aggregation behavior (to form nanoparticles).

EXPERIMENTS

Here we created surface active nanoparticles made from self-aggregated polymers consisting of PAH (polyallylamine hydrochloride)-BA (benzaldehyde). The covalent imine bonds between originally hydrophilic PAH and hydrophobic BA are dynamic in that their formation and breakage is a function of solution pH, confirmed by ¹H NMR and dynamic interfacial tension measurement.

FINDINGS

At pH 7.4, a stable foam is achieved in the PAH-BA (amino to aldehyde ratio at 1:0.2) solution; while at pH 2.5, it defoams due to breakage of dynamic bonds corresponding to the measured diminishing surface activity. The reversibility of foaming-defoaming has been demonstrated by alternatively changing pH for multiple cycles, with the foaming performance persistent. The foam stability can be improved by more hydrophobic compounds e.g. at a lower amino to aldehyde ratio or using PAH-cinnamaldehyde (CA). The reversible and responsive foaming demonstrated in a Pickering system provides a new method to create novel foaming systems with properties desirable to many applications.

Responsive morphology transition from micelles to vesicles based on dynamic covalent surfactants

A dynamic covalent bond is widely used to fabricate stimuli responsive systems due to its reversible molecular recognition properties. In this study, we developed a pH-responsive morphology transition system based on a mixture of a cationic surfactant CTAB and two nonamphiphilic precursors, 4-hydroxybenzaldehyde (HB) and octylamine (OA), at a molar ratio of 100 : 60 : 60 (CTAB/HB/OA). The morphology transition of CTAB/HB/OA was characterized by 1H NMR spectroscopy, Fourier transform infrared spectroscopy, macroscopic appearance observation, dynamic light scattering, and rheological and cryo-TEM measurements. The phase behavior of CTAB/HB/OA solutions underwent transition from a water-like fluid to a transparent gel-like solution and then converted into a turbid low-viscosity solution upon increasing the pH. Upon increasing the pH from 4.93 to 7.99, the morphology was transformed from spherical micelles to wormlike micelles. Upon further increasing the pH to 12.02, the wormlike micelles gradually disappeared with the formation of vesicles. Thus, a morphology transition from micelles to vesicles can be triggered by varying the pH of CTAB/HB/OA solutions. This drastic variation in morphology behavior was attributed to the pH dependent ionization and formation of the anionic surfactant HB-OA-. Besides, over 3 cycles of morphological alternation among spherical micelles, wormlike micelles and vesicles of the CTAB/HB/OA solutions can be obtained by adjusting the pH.

Asymmetric maltose neopentyl glycol amphiphiles for a membrane protein study: effect of detergent asymmetricity on protein stability

An asymmetric MNG, MNG-8,12, provided enhanced stability to human G protein-coupled receptors (GPCRs) compared to the symmetric MNG, MNG-3.

Maintaining protein stability in an aqueous solution is a prerequisite for protein structural and functional studies, but conventional detergents have increasingly showed limited ability to maintain protein integrity. A representative novel agent, maltose neopentyl glycol-3 (MNG-3), has recently substantially contributed to membrane protein structural studies. Motivated by the popular use of this novel agent, we prepared asymmetric versions of MNG-3 and evaluated these agents with several membrane proteins including two G protein-coupled receptors in this study. We found that some new MNGs were significantly more effective than MNG-3 at preserving protein integrity in the long term, suggesting that these asymmetric MNGs will find a wide use in membrane protein studies. In addition, this is the first study addressing the favorable effect of detergent asymmetric nature on membrane protein stability.

Construction of Various Supramolecular Assemblies from Rod-Coil Molecules Containing Biphenyl and Anthracene Groups Driven by Donor-Acceptor Interactions

Rod-coil amphiphilic functional molecules, comprising a rigid aromatic building block and hydrophilic oligoether dendrons as the coil segments, were synthesized. These compounds exhibit a powerful self-organizing ability to form supramolecular nanoparticles and long nanofibers in tetrahydrofuran/water solution, by controlling the intermolecular interaction of the rigid blocks. These molecules are able to form supramolecular polymers and, subsequently, to form sheetlike nanoaggregates, through charge-transfer interactions by the addition of a guest molecule, tetracyanoquinodimethane. Notably, upon addition of water-soluble 2,4,6-trinitrophenol, the self-assembly of these molecules exhibits the antagonistic effect owing to donor-acceptor and hydrophobic-hydrophilic interactions among the molecules. The experimental results reveal that various morphologies of rod-coil molecular assemblies can be obtained by tuning the molecular interaction and the hydrophilicity of guest electron-acceptor molecules. Interestingly, the cross-coupling reaction between phenylboronic acid and chlorobenzene occurs within the charge complexes of these molecular aggregates. This occurs in the nanoenvironment that affords an extremely concentrated reaction zone and reduces the activation energy barrier required for the cross-coupling reaction.

Dynamic Covalent Silica Nanoparticles for pH-Switchable Pickering Emulsions

Dynamic covalent surfactants have been recently reported for preparation of pH-switchable emulsions [ Sun , D. Langmuir , 2017 , 33 , 3040 ]. In this study, dynamic covalent silica (SiO₂-B) nanoparticles of switchable wettability were fabricated by a pH-responsive dynamic (covalent) imine bond between hydrophilic amino silica (SiO₂-NH₂) nanoparticles and hydrophobic benzaldehyde molecules. The properties of SiO₂-B were characterized by Fourier transform infrared spectroscopy, elemental analysis, contact angle measurement, and ζ potential measurement. The hydrophilicity and hydrophobicity of SiO₂-B were shown to be readily switchable by adjusting pH between 7.8 and 3.5. At pH 7.8, SiO₂-B was partially hydrophobic and adsorbed at oil-water interface to stabilize O/W Pickering emulsions, which were characterized by electrical conductivity, optical microscopy, and confocal laser scanning microscopy. Upon lowering the pH to 3.5, the dynamic covalent bond is dissociated to convert partially hydrophobic SiO₂-B into highly hydrophilic SiO₂-NH₂ and surface-inactive benzaldehyde. Both of them desorb from oil-water interface, resulting in a rapid oil-water separation of the Pickering emulsions. Alternating stabilization and phase separation of the Pickering emulsions over 3 cycles were demonstrated by adjusting the pH. The pH-switchable Pickering emulsions show great potential in application to effective oil-water separation of emulsions.

Cooperative Effects of Zwitterionic-Ionic Surfactant Mixtures on the Interfacial Water Structure Revealed by Sum Frequency Generation Vibrational Spectroscopy

Cooperative effects of a series of equimolar binary zwitterionic-ionic surfactant mixtures on the interfacial water structure at the air-water interfaces have been studied by sum frequency generation vibrational spectroscopy (SFG-VS). For zwitterionic surfactant palmityl sulfobetaine (SNC₁₆), anionic surfactant sodium hexadecyl sulfate (SHS), and cationic surfactant cetyltrimethylammonium bromide (CTAB) with the same length of alkyl chain, significantly enhanced ordering of interfacial water molecules was observed for the zwitterionic-anionic surfactant mixtures SNC₁₆-SHS, indicating that SNC₁₆ interacts more strongly with SHS than with CTAB because of the strong headgroup-headgroup electrostatic attraction for SNC₁₆-SHS. Meanwhile, the SFG amplitude ratio of methyl and methylene symmetric stretching modes was used to verify the stronger interaction between SNC₁₆ and SHS. The conformational order indicator increased from 0.64 for SNC₁₆ to 7.17 for SNC₁₆-SHS but only 0.94 for SNC₁₆-CTAB. In addition, another anionic surfactant sodium dodecyl sulfate (SDS) was introduced to study the influence of chain-chain interaction. Decreased SFG amplitude of interfacial water molecules for SNC₁₆-SDS was observed. Therefore, both the headgroup-headgroup electrostatic interaction and chain-chain van der Waals attractive interaction of the surfactants play an important role in enhancing the ordering of interfacial water molecules. The results provided experimental and theoretical bases for practical applications of the surfactants.

pH Switchable Emulsions Based on Dynamic Covalent Surfactants

Dynamic covalent surfactants were designed to prepare pH switchable emulsions. A dynamic covalent bond between nonamphiphilic building blocks (polyethylenimine (PEI) and benzaldehyde (B)) was introduced to form the dynamic covalent surfactant PEI-B. The dynamic nature of covalent bond in PEI-B was confirmed by ¹H NMR and fluorescence probe analysis. Stable emulsions were successfully prepared with interfacial active PEI-B at pH 7.8 with various water/paraffin oil ratios under sonication. When lowering the pH to 3.5, a complete phase separation was observed as a result of breaking dynamic covalent bond in the interfacial active PEI-B. After tuning the pH back to 7.8, stable emulsion was obtained again due to the reformation of the dynamic covalent bond and hence interfacial active PEI-B. The emulsification and demulsification were dependent on the formation and breaking of dynamic covalent bond in PEI-B. Such pH-triggered emulsification and demulsification can be switched at least three times. Application of dynamic covalent surfactants will open up a novel route for preparing responsive emulsions.

Controlling amphiphilic copolymer self-assembly morphologies based on macrocycle/anion recognition and nucleotide-induced payload release

We report here a new approach to creating diversiform copolymer-derived self-assembly morphologies that relies on macrocycle/anion recognition in aqueous media. This approach exploits the anion binding features of a water-soluble form of the so-called 'Texas-sized' molecular box. When this tetracationic receptor is added to an aqueous solution of an amphiphilic copolymer bearing tethered carboxylate anion substituents, binding occurs to form a macrocycle/polymer complex. As the concentration of the box-like receptor increases, the relative hydrophilic fraction of the copolymer complex likewise increases. This leads to changes in the overall morphology of the self-assembled ensemble. The net result is an environmentally controllable system that mimics on a proof-of-concept level the structural evolution of organelles seen in living cells. The macrocycle/anion interactions respond in differing degrees to three key biological species, namely ATP, ADP, and AMP, which may be used as "inputs" to induce disassembly of these vehicles. As a result of this triggering and the nature of the morphological changes induced, the present copolymer system is capable of capturing and releasing in controlled manner various test payloads, including hydrophobic and hydrophilic fluorophores. The copolymer displays low inherent cytotoxicity as inferred from cell proliferation assays involving the HUVEC and HepG2 cell lines.

The fabrication of a supra-amphiphile for dissipative self-assembly

Dissipative self-assembly is a challenging but attractive field of supramolecular science, because it generally concerns complex systems but is more close to the self-assembly of living bodies. In this article, we realized dissipative self-assembly by coupling a supra-amphiphile with a chemical oscillator. The supra-amphiphile was fabricated with iodine and a double hydrophilic block copolymer containing PEG segments, as the non-covalent interaction between PEG and iodine could turn PEG hydrophobic, leading to the formation of the supra-amphiphile. The self-assembly and disassembly of the supra-amphiphile could be controlled by varying the concentration of iodine. Therefore, the dissipative self-assembly of the supra-amphiphile was realized when it was coupled with the IO3--NH3OH+-OH- chemical oscillator, which was able to produce iodine periodically. Meanwhile, the kinetic data of the self-assembly and disassembly of the supra-amphiphile could be estimated by the theoretical simulation of the chemical oscillations. This line of research promotes the self-assembly of supra-amphiphiles one step forward from thermodynamic statics to a dissipative system, and also suggests a new strategy to investigate the kinetics of stimuli-responsive molecular self-assembly.