Responsive wormlike micelle with pH-induced transition of hydrotrope based on dynamic covalent bond

Interactions Between Cationic Micellar Solution and Aromatic Hydrotropes with Subtle Structural Variations

Wormlike micelles (WLMs) with tunable viscoelastic characteristics have emerged as indispensable smart materials with a wide range of applications, which have garnered intense interest over the past few decades. However, quantitatively predicting the effect of various hydrotropes on the rheological behaviors of WLMs remains a challenge. In this article, micelles were formed in a mixture of 3-hexadecyloxy-2-hydroxypropyltrimethylammonium bromide (R16HTAB) and aromatic hydrotropes (e.g., sodium benzoate, sodium cinnamate and their derivatives, respectively) in an aqueous solution. The phase behavior, viscoelasticity and thickening mechanism were systematically studied by macroscopic observation, rheological measurements, electrostatic potential analysis and cryogenic transmission electron microscopy (Cryo-TEM). Rheological measurements were used to probe the remarkable viscoelastic properties of micelles stemming from their lengthening and entanglement under the interaction between R16HTAB and hydrotropes with structural variations. For an equimolar system of R16HTAB and cosolute (40 mM), the relaxation time decreases in the following order: SpMB > SoHB > S4MS > SmMB > S5MS > SB > SmHB > SoMB > SpHB. These results allow us to predict the possible rules for the self-assembly of R16HTAB and aromatic hydrotropes, which is conductive to directionally designing and synthesizing smart wormlike micelles.

Stimuli-Responsive Vesicles and Hydrogels Formed by a Single-Tailed Dynamic Covalent Surfactant in Aqueous Solutions

Controlling the hierarchical self-assembly of surfactants in aqueous solutions has drawn much attention due to their broad range of applications, from targeted drug release, preparation of smart material, to biocatalysis. However, the synthetic procedures for surfactants with stimuli-responsive hydrophobic chains are complicated, which restricts the development of surfactants. Herein, a novel single-tailed responsive surfactant, 1-methyl-3-(2-(4-((tetradecylimino) methyl) phenoxy) ethyl)-3-imidazolium bromides (C14PMimBr), was facilely fabricated in situ by simply mixing an aldehyde-functionalized imidazolium cation (3-(2-(4-formylphenoxy) ethyl)-1-methyl imidazolium bromide, BAMimBr) and aliphatic amine (tetradecylamine, TDA) through dynamic imine bonding. With increasing concentration, micelles, vesicles, and hydrogels were spontaneously formed by the hierarchical self-assembly of C14PMimBr in aqueous solutions without any additives. The morphologies of vesicles and hydrogels were characterized by cryogenic transmission electron microscopy and scanning electron microscopy. The mechanical properties and microstructure information of hydrogels were demonstrated by rheological measurement, X-ray diffraction, and density functional theory calculation. In addition, the vesicles could be disassembled and reassembled with the breakage and reformation of imine bonds by adding acid/bubbling CO2 and adding alkali. This work provides a simple method for constructing stimuli-responsive surfactant systems and shows great potential application in targeted drug release, drug delivery, and intelligent materials.

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-Induced reversible conversion between non-Pickering and Pickering high internal phase emulsion

Solid-stabilized high internal phase emulsions have received extensive attention. Many previous studies have confirmed that solid emulsifiers in high internal phase Pickering emulsions (HIPPEs) provide a great interface mechanical barrier. With the development of research, novel solid-stabilized emulsions have emerged. These emulsions are stabilized by the electrostatic repulsion between the surfactants and hydrophilic solid particles. They are distinct from Pickering emulsions in that the solid particles do not exist at the oil-water interface, but are dispersed in the continuous phase, so it is called a non-Pickering emulsion. However, high internal phase non-Pickering emulsions (HIPNPEs) are rarely reported. Herein, HIPNPEs that are synergistically stabilized by anionic surfactants with dynamic covalent bonds and negatively charged nano-SiO₂ particles were prepared. In the presence of dodecylamine, the acidity causes the dynamic covalent bonds to break and the surfactant to be inactivated. Additionally, the long-chain amine is protonated and adsorbed on nano-SiO₂ particles to form a new surfactant for stabilizing HIPPEs. However, alkalinity causes the HIPNPEs to form again. In addition, rheological tests confirmed that the HIPNPEs and HIPPEs had similar rheological behaviors, which were typical gel-like fluids. The emulsion can quickly respond to realize the conversion between the different types of high internal phase emulsion by simple stimulation, which provides a new direction for stimulus-responsive high internal phase emulsions.

Advances of supramolecular interaction systems for improved oil recovery (IOR)

Improved oil recovery (IOR) includes enhanced oil recovery (EOR) and other technologies (i.e. fracturing, water injection optimization, etc.), have become important methods to increase the oil/gas production in petroleum industry. However, conventional flooding systems always encounter the problems of low efficiency, high cost and complicated synthetic procedures for harsh reservoirs conditions. In recent decades, the supramolecular interactions are introduced into IOR processes to simplify the synthetic procedures, alter their structures and properties with bespoke functionalities and responsiveness suitable for different conditions. Herein, we primarily review the fundamentals of several supramolecular interactions, including hydrophobic association, hydrogen bond, electrostatic interaction, host-guest recognition, metal-ligand coordination and dynamic covalent bond from intrinsic principles and extrinsic functions. Then, the descriptions of supramolecular interactions in IOR processes from categories and advances are focused on the following variables: polymer, surfactant, surfactant/polymer (SP) complex for EOR and viscoelasticity surfactant (VES) for clean hydraulic fracturing aspects. Finally, the field applications, challenges and prospects for supramolecular interactions in IOR processes are involved and systematically addressed. The development of supramolecular interactions can open the way toward adaptive and evolutive IOR technology, a further step towards the cost-effective production of petroleum industry.

On the effects of organic-acids isomers on temperature-responsiveness in wormlike micelles (WLMs) systems

Responsive wormlike micelles (WLMs) consisted of cationic surfactants and organic-acids are fascinating due to their reversible molecular recognition properties. However, it is unknown how the structure of organic-acids alters the stimuli-responsiveness of WLMs systems. Herein, the peculiar nature of temperature-responsive behaviors in three WLMs systems were systematically investigated. These were manufactured by combining N-erucamidopropyl-N,N-dimethylamine (UC₂₂AMPM) with isomers of organic-acids: o-phthalic acid (o-PA), m-phthalic acid (m-PA) and p-phthalic acid (p-PA) at molar ratio of 2:1 (named as o-EAPA, m-EAPA and p-EAPA respectively). The phase behaviors, macro- and micro-rheology, as well as the mechanism of temperature-responsiveness were explored by visual inspection, rheological and optical methods. The results showed that the three systems exhibited different responsiveness with increase of temperature. Among them, the viscosity and viscoelasticity of o-EAPA were gradually decreased with temperature increase from 30 °C to 90 °C. On the other hand, those of p-EAPA were firstly increased and subsequently decreased, exhibiting the highest viscosity during the heating process. This peculiar phenomenon was attributed to the hydrophilic difference of organic-acids isomers, leading to variations of micelle transitions upon temperature increase. This study is the first report of aromatic-acids isomers inducing different on temperature-responsiveness, and finding beneficial for the development of responsive WLMs for different applications.

Photoresponsive Viscoelastic Solutions Based on Chiral Wormlike Micelles in Mixed Solutions Containing an Amphiphile Derived from Rosin

A novel rosin-based photoresponsive anionic amphiphile, sodium N-azophenyl maleopimaric acid imide carboxylate (AzoMPCOONa), has been successfully synthesized. Its molecular structure was characterized by ¹H and ¹³C NMR and mass spectrometry (MS). The photoisomerization of AzoMPCOONa was evaluated by ultraviolet (UV)-visible spectrometry and ¹H NMR. The structure of AzoMPCOONa could be converted between the trans and cis isomers by irradiation with UV/visible light. Importantly, a fascinating photoresponsive viscoelastic solution was prepared by mixing AzoMPCOONa and cetyltrimethylammonium bromide (CTAB). The properties of the photoresponsive viscoelastic solution were further investigated by rheology, circular dichroism (CD), and cryogenic transmission electron microscopy (cryo-TEM). Initially, the AzoMPCOONa/CTAB system was a gel-like solution composed of entangled wormlike micelles possessing the right-handed chiral structure. After UV irradiation for 10 min, the gel-like solution transformed into a slightly viscous solution, its zero-shear viscosity dramatically reduced by 2 orders of magnitude, and the aggregates were converted into rod-like micelles and spherical micelles. In addition, the right-handed chiral structure of the aggregates disappeared. These dramatic changes in the viscosity and the aggregate structure can be attributed to the photoisomerization of the azobenzene group in AzoMPCOONa, which led to changes in the molecular geometry and the packing parameter of the AzoMPCOONa/CTAB system. Interestingly, the right-handed chiral structure of wormlike micelles also is photoresponsive. The results reveal the superiority of forest resources for preparing viscoelastic solutions.

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.

Fabrication of self-healing pectin/chitosan hybrid hydrogel via Diels-Alder reactions for drug delivery with high swelling property, pH-responsiveness, and cytocompatibility

Self-healing hydrogels with pH-responsiveness could protect loaded drugs from being destroyed till it arrives to the target. The pectin-based hydrogel is a candidate due to the health benefit, anti-inflammation, antineoplastic activity, nontoxicity, and biospecific degradation, et al. However, the abundant existence of water-soluble branched heteropolysaccharide chains influenced its performance resulting in limitation of the potential. In the present study, we prepared a series of self-healing pectin/chitosan hydrogels via the Diels-Alder reaction. Moreover, pectin/chitosan composite hydrogel was prepared as a contrast. By comparison, it can be seen that the Diels-Alder reaction greatly improved the cross-linking density of hydrogels. The self-healing experiments showed excellent self-healing performance. In different swelling mediums, significant transformation in the swelling ratio was shown, indicating well-swelling property, pH- and thermo-responsiveness. The drug loading and release studies presented high loading efficiency and sustained release performance. The cytotoxicity assay that showed a high cell proliferation ratio manifested great cytocompatibility.

Facile synthesis of ultralong hydroxyapatite nanowires using wormlike micelles as soft templates

Ultralong hydroxyapatite nanowires were synthesized by formation of an entangled long wormlike micelle structure in a three-phase reaction system.

pH-Responsive Non-Pickering Emulsion Stabilized by Dynamic Covalent Bond Surfactants and Nano-SiO2 Particles

A novel stimulus-responsive non-Pickering emulsion stabilized by nano-SiO₂ particles was prepared in our recent study. 4-formylbenzoic acid and hexylamine through a dynamic covalent bond form a surface-active substance, which was confirmed by Fourier transform infrared (FTIR) and ¹H NMR. Through optimization experiments, it was proved that a stable emulsion can be formed by low surfactant concentration (below cmc) and low nano-SiO₂ particle concentration (0.5 wt %). In this emulsion, nano-SiO₂ particles are not located at the interface of oil-water but dispersed in the continuous phase of the emulsion, which is different from the Pickering emulsion. The negatively charged nano-SiO₂ particles and anionic surfactants repel each other, thereby synergistically stabilizing the emulsion so that the concentrations of surfactants and nanoparticles required to stabilize the emulsion are reduced. In addition, the system can also control the formation and fracture of dynamic covalent bonds by changing pH, thereby controlling the stability and demulsification of the emulsion. At the same time, this non-Pickering emulsion could be used as a microreactor for chemical synthesis and still had a high yield after three cycles. This study provides a new application direction for this environmentally friendly emulsion.

Light-Triggered Rheological Changes in a System of Cationic Wormlike Micelles Formulated with a Photoacid Generator

"Smart" fluids displaying large changes in their rheological properties in response to external stimuli have been of great interest in recent years. For example, "smart" wormlike micelles (WLMs) that respond to pH can be readily formulated by combining a cationic surfactant such as cetyltrimethylammonium bromide (CTAB) with an aromatic compound such as 1,2-dihydroxybenzene (DHB). Here, we show that a pH-responsive aqueous formulation as mentioned above can be simultaneously made responsive to ultraviolet (UV) light by incorporating a photoacid generator (PAG) into the system. A commercially available PAG, diphenyliodonium-2-carboxylate, is used here. Upon exposure to UV light, this PAG irreversibly photolyzes into iodobenzene (IB) and benzoic acid (BA), with the formation of BA, leading to a drop in pH. WLMs formed by mixtures of CTAB, DHB, and the PAG are systematically characterized before and after UV irradiation. As the PAG photolyzes, an increase in the viscosity of WLMs occurs by a factor of 1000. We show that the ratio of the zero-shear viscosity η₀ (after UV/before UV) depends on the initial pH of the sample. The UV-induced increase in η₀ can be attributed to the growth of WLMs in solution, which in turn is influenced by both the ionization state of DHB and the presence of IB and BA.

pH-Responsive aggregates transition from spherical micelles to WLMs induced by hydrotropes based on the dynamic imine bond

In recent years, the use of dynamic chemical bonds to construct stimulus-responsive micelle systems has received increasing attention. However, current reports focus on the construction of dynamic covalent bond surfactants using dynamic chemical bonds, and the method of applying dynamic covalent bonds to hydrotropes has not been reported yet. In this study, a novel pH-responsive worm-like micelle system was constructed by mixing cetyltrimethylammonium bromide (CTAB), 4-hydroxybenzaldehyde (HB) and p-toluidine (MB) at the molar ratio of 60 mM : 40 mM : 40 mM. The formation mechanism of the dynamic covalent bond hydrotropes and the rheological behavior of the micelles were investigated via rheology, 1H-NMR spectroscopy and Cryo-TEM. The results show that as the pH increases, the viscosity of the solution first decreases and then increases rapidly. The microscopic aggregates in the solution transition from spherical micelles to worm-like micelles (WLMs), and the solution changes from a water-like fluid without viscosity to a gel system that can withstand its own weight. The transformation of the aggregates and their rheology can be attributed to the formation of MB-HB-, which is a type of hydrotrope with dynamic covalent bonds. Moreover, the transition from spherical micelles to worm-like micelles in this system is reversible.

pH-Responsive Wormlike Micelles Formed by an Anionic Surfactant Derived from Rosin

A novel pH-responsive wormlike micellar viscoelastic solution was constructed by a rosin-based anionic surfactant (Na-MPA-AZO-Na) in the presence of cetyltrimethylammonium bromide (CTAB). The viscoelasticity, aggregate morphology, and pH-responsiveness of the pH-responsive wormlike micelles have been investigated through the method of rheology and cryogenic-transmission electron microscopy. Its corresponding mechanism has been studied using ¹H NMR and ¹H-¹H 2D NOESY HNMR. The zero-shear viscosity (η₀) of the wormlike micellar solution rapidly decreases by 3 orders of magnitude as the pH increases from 5.21 to 9.56. The viscoelastic fluids and water-like solutions can be converted by tuning the pH between 3.62 and 12.00, and the corresponding aggregates also transform between wormlike micelles and spherical micelles. In addition, the wormlike micellar cross-sectional diameter is approximately 10 nm, which is remarkably larger than that of the common wormlike micelles. The phenomenon can be attributed to the large steric volume of the rosin rigid skeleton. When the pH is 12.00, a "pseudo" Gemini surfactant is constructed by Na-MPA-AZO-Na and CTAB through the electrostatic interactions. Wormlike micelles also can be formed with the increasing concentrations. The η₀ of the wormlike micellar system shows strong dependence on concentration with an exponent of 9.6 (η₀ ∝ C^9.6). This work further promotes new applications of forest resources.