Liquid Condensed Matter Mediated Assembly and Functionality of Dispersoid

Bao Li; Lixin Wu

doi:10.7536/PC220304

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (7) : 1600-1609 DOI: 10.7536/PC220304

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Liquid Condensed Matter Mediated Assembly and Functionality of Dispersoid

Bao Li
, Lixin Wu ^*

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Abstract

Condensed matter chemistry is a new research field that studies the multi-level structures of condensed matter constructed by intermolecular interactions for realizing functionalities and chemical reactions. Compared with solid-state condensed matter chemistry, the study of liquid condensed matter chemistry involves multiphase states, such as how liquid condensed matter affects the state and functional properties of dispersoids. It is important to understand the aggregation behaviors of dispersoids from the perspective of condensed matter chemistry, which is not only beneficial to the preparation of the expected structures, but also can deepen the understanding of the formation process of the assembled structure. In this paper, based on a brief overview of the physical and chemical properties of liquid condensed matter, especially those related to dispersion and dissolution, typical examples are selected to illustrate the assembly process, assembly and disassembly, and structural transformation of dispersoids in the liquid condensed matter. In terms of the influence of the liquid condensed state on properties of dispersoids, the UV-vis absorption, electron transfer, chirality regulation and catalysis are discussed. In these processes, as the continuous phase, the properties of the liquid condensed state, such as dielectric constant, polarity and viscosity, play key roles in the existing states and properties of the dispersoids. However, due to the limitation of the detection range of the present instruments, it is difficult to accurately measure the fast, variable and subtle forces between liquid condensed matter and dispersoids in time and space. Therefore, it is an important and effective strategy to perform mutual fitting from both experimental and theoretical aspects to illustrate the role of liquid condensed matter.

Contents

1 Introduction

2 Properties of liquid condensed matter and its relationship with a dispersoid

3 Influence of liquid condensed state on aggregation behaviors of dispersoid

3.1 Regulation of dispersoid assembly processes by liquid condensed matter

3.2 Regulation of aggregation states of dispersoid by liquid condensed matte

3.3 Regulation of assembly structures of dispersoid by liquid condensed matter

4 Influence of liquid condensed state on the properties of dispersoid

4.1 Liquid condensed state reduced solvatochromism

4.2 Photoinduced electron transfer controlled by liquid condensed matter

4.3 Modulation of dispersoid chirality by liquid condensed matter

4.4 Effects of liquid condensed matter on catalytic reactions of dispersoid

5 Conclusion

Graphical abstract

Keywords

liquid condensed matter / dispersoid / assembly / functionality

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Bao Li, Lixin Wu. Liquid Condensed Matter Mediated Assembly and Functionality of Dispersoid. Progress in Chemistry, 2022, 34(7): 1600-1609 DOI:10.7536/PC220304

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1 Introduction

Since the concept of "condensed matter chemistry" was put forward by Academician Xu Ruren of Jilin University, the connotation and extension of solid-state condensed matter chemistry and its applications in different fields have been developed and elaborated in detail^[1,2]^[3,4]. Relatively speaking, there is little discussion on the chemistry of liquid condensed matter. A liquid condensed state is a system of particles held together by intermolecular forces and having no particular shape, but having certain volume and flow characteristics^[5]. Although there is a strong intermolecular interaction between the particles in the liquid condensed state, the combination is relatively firm, but it is not a rigid connection. The particles have a certain degree of freedom and fluidity, and can be used as a continuous phase to disperse the dispersoid (generally in the solid condensed state) to form a discontinuous phase^[6]. Influenced by the particle structure and properties of the liquid continuous phase, dispersoids can exist as monodisperse or multiple aggregates with different structures^[7,8]. Further changes in the environment of the dispersion can induce changes in the aggregate structure of the dispersoid and acquire new functional properties^[9⇓⇓~12].

The relationship between the liquid condensed state and the assembly structure of the dispersoid is very complex. According to the different structures and properties of the dispersoid particles, there are various types of supramolecular interactions with different strengths between the liquid condensed state and the dispersoids. It is difficult to summarize and sort out all the contents in one article^[13,14]. Based on the basic concept of liquid condensed state, this paper discusses the physical and chemical properties of liquid condensed state and its relationship with dispersoid, and preliminarily explains the influence of liquid condensed States on the assembly structure and properties of dispersoid through the analysis of typical examples. It should be noted that this paper mainly focuses on the assembly behavior, assembly structure and properties of dispersoids in the liquid condensate. Although the liquid condensate also has an important impact on the activation sites and reaction mechanism of organic reactions carried out in it, this paper focuses on the impact of the liquid condensate on the catalytic reaction process, especially on the catalyst^[15]. In addition, ionic liquids have unique behavior characteristics and are not within the scope of this article^[16]. In this paper, the effects of common liquid properties such as water and organic reagents on the structure and properties of dispersoid assembly are discussed, and the existing scientific problems and further development directions in this field are expected to be extracted, so as to deepen the understanding of liquid condensed matter systems.

2 Properties of liquid condensed matter and its relation to dispersoid

The difference of particle structure in liquid condensed state leads to the different interaction between particles, which makes the liquid different in density, viscosity, saturated vapor pressure, acidity and alkalinity, polarity and dielectric constant^[17]. These properties of liquids also play different roles for various dispersoid systems^[18]. The most direct effect of these effects on liquid and dispersoid systems is the difference in solubility, which directly affects the state of the dispersoid in the liquid.

The process of dispersoid entering the liquid condensed state and forming supramolecular assemblies can be divided into three steps: dissolution, particle recognition, and assembly (Fig. 1)^[19]. In the process of dissolution, the dispersoid particles overcome the supramolecular force between the liquid particles, "squeeze" into the interior, and interact with the liquid particles, so that the liquid particles are polarized, and the polarization in turn affects the charge distribution of the dispersoid particles. The result of the interaction between the two is that the liquid molecules as the continuous phase surround the incoming dispersoid particles and reduce the energy of the system. In this process, the matching of liquid particles and dispersoid particles, that is, the strength of the force between them, determines the difficulty of the latter entering the former^[19]. With the increase of the number of dispersoid particles, the collision probability between them increases, and two or more dispersoid particles combine under a specific force through particle recognition, which further reduces the energy of the system. On the basis of the subunits containing two or more dispersoid particles generated by the above process, the subunits with relatively weaker interaction force continue to combine with each other to further reduce the interfacial energy and reach an equilibrium state to obtain an assembly structure. If the environment of the equilibrium system, such as the polarity of the liquid, is changed so that the force between the subunits is destroyed without destroying the force of particle recognition, the arrangement of the subunits is re-dominated by the new force, resulting in a new assembly structure. For example, in some cases, the assembly morphology shown by electron microscopy has changed, but the powder X-ray diffraction results show that the subunit has not changed in this process^[20]. If the environment of the dispersion system changes greatly, it may also affect the recognition of the primary particles, thus causing fundamental changes in the assembly structure.

The assembly structure of dispersoids is the result of the cooperation of various supramolecular forces. Compared with covalent bonds, most of the supramolecular forces have lower bond energy and are easily affected by the surrounding environment, especially the continuous phase^[21,22]. The formation of supramolecular assemblies with different structures can be induced by properly considering the properties of the liquid and making use of them^[23,24]. According to the types of forces commonly used in supramolecular assembly, the regulation of liquid on the assembly structure of dispersoids can be considered from the following aspects. When the dispersoid particles are mainly bound by electrostatic interaction, the dielectric constant of the liquid will play an important role in the assembly process^[25,26]. The low dielectric constant reduces the electrostatic interaction potential between the dispersoid particles, resulting in their easier aggregation. When the dielectric constant of the liquid is increased, its ability to disperse charged particles is stronger, so that the dispersoid tends to be less likely to aggregate. By using this feature, the strength of electrostatic interaction in the system can be changed by selecting liquids with different dielectric constants, so as to effectively regulate the assembly structure of charged particles.

Another factor that has an important influence on the assembly structure of the dispersoid is the polarity of the liquid. The polarity of the liquid has a broader effect on the assembly of the dispersoid than the dielectric constant^[27,28]. Although in most cases, liquids with high dielectric constant have greater polarity, there are other measures of polarity according to different criteria, so there is no one-to-one correspondence between them. For example, Gutmann acceptor number for measuring Lewis acidity and Gutmann donor number for measuring Lewis basicity of liquid^[29]; The π^* scale for measuring dipole-dipole and dipole-induced dipole interactions in protic liquids, the α scale for hydrogen bond donor ability and the β scale for hydrogen bond acceptor ability^[30,31]. When the particles of the dispersoid are nonionic compounds, the polarity of the liquid plays an important role in its assembly and assembly process. Liquids with similar polarities and dispersoids generally have good compatibility, so the aggregation and deaggregation of the dispersed phase can be controlled by adjusting the polarity of the condensed state.

For protic liquids, they can act as hydrogen bond donors to form hydrogen bonds with dispersoids, thus affecting the assembly of dispersoids with hydrogen bond acceptors^[32]. When there is no strong hydrogen bond acceptor in the system, the protic liquid will form a hydrogen bond network with itself. Therefore, when studying dispersoid assembly structures based on hydrogen bonding, it is necessary to consider the effect of removing the liquid on the assembly process, that is, the liquid does not act as a hydrogen bond donor or acceptor. Based on this, the liquid particles are required not to contain atoms with large electronegativity such as N or O, to reduce the number of C-H as much as possible, and to have good solubility in the dispersoid. Based on the above considerations, chloroform is the more suitable liquid condensed phase in such systems, but if the hydrogen bond of the dispersoid is weak, carbon tetrachloride can be used to eliminate the influence of the weak ability of chloroform as a hydrogen bond donor on the assembly.

In the supramolecular assembly system formed by dispersoids based on coordination, liquids with high electronegative groups, especially those containing atoms such as N, O and S, which are easy to occupy the coordination sites of metal ions, have the ability to participate in the formation of supramolecular assemblies as important components and affect the assembly structure^[33]. In general, liquid molecules occupy the coordination sites of metal ions in a monodentate coordination manner. Compared with other multidentate ligands, the combination of liquid molecules and metal ions is relatively weak, and they can be replaced or lost, resulting in structural and property changes, which can be used to construct dynamic supramolecular assembly structures^[34].

In a liquid system in which the dispersoid energy is relatively stable, there may be a variety of forces. For example, if there are not only strong coordination bonds, but also relatively strong hydrogen bonds and weak intermolecular forces in the system, the coordination bonds are generally used for the first level of recognition and assembly to form stable binding motifs. The building units are assembled secondarily under the action of hydrogen bonds to form assemblies with specific structures. The assembly exists as an aggregate of a certain size under the combined influence of weak van der Waals forces between each other and the surrounding liquid environment. According to the principle of minimum energy, the lower the surface energy of the aggregate, the more stable its structure in solution. Therefore, with the change of the external liquid environment, the structure of the assembly will change accordingly to reduce the surface energy. The factors affecting the stabilization of aggregates formed by different types of building motifs are different, and it is difficult to express them quantitatively by a unified formula. According to different system characteristics and parameters, appropriate theoretical methods can be selected to simulate. For example, the state of particles is simulated by Monte Carlo method, and then the equilibrium thermodynamics and structural characteristics of the system are calculated. The molecular dynamics method is used to simulate the motion of particles in a very short time, so as to understand the diffusion and dynamic flow of particles.

The above is a general discussion on the influence of liquid condensation on the assembly of dispersoids and the stability of the formed aggregates. The following is a detailed explanation of the influence of liquid condensation on the assembly and properties of dispersoids through specific examples.

3 Effect of liquid condensed state on aggregation behavior of dispersoid

3.1 Regulation of Liquid Condensate on Dispersoid Assembly Process

The aggregation of dispersoids in the liquid condensed state to form ordered assemblies is the result of the continuous dynamic adjustment of various building units under the action of supramolecules, and finally forms a relatively stable assembly structure^[35]. By regulating the driving force of assembly, trapping metastable intermediates is an important means to deepen the understanding of the assembly process. Through this method, the role of each driving force in the assembly process can be understood, thus providing a guarantee for the further controlled assembly of specific structures. Due to the physical and chemical properties of different liquid condensates, the intermolecular forces of the same type of molecules will be different. By adjusting the composition of liquid condensates to change the size of supramolecular forces, the assembly process of dispersoids can be regulated, and then the assembly mechanism can be understood^[36].

In the aqueous phase assembly of an amphiphilic polypeptide PSar₃₀-(L-Leu-Aib)₆(S30L12) (fig. 2a) composed of hydrophilic polysarcosine (PSar), chiral leucine (L-Leu) with a hydrophobic α-helix as a repeating unit, and aminoisobutyric acid (Aib), the polypeptide first assembles to form a layered structure with a certain arc^[37]. After heating, the hydrophobic interaction in the system is enhanced due to dehydration, which further rapidly curls the curved layered structure and forms thermodynamically stable nanotubes (Figure 2B). In order to enhance the understanding of the formation and transformation of the assembly structure, the strategy of adding cosolvent to the system at room temperature was used to slow down the assembly process and obtain the intermediate state of the assembly. When ethanol (EtOH) was added to the system, ethanol could insert into the hydrophobic part of the α-helix of the assembled structure, enhance the hydrogen bonding in the liquid, and reduce the mutual contact between water and polypeptide, thus playing a heating effect and inducing the transformation from polypeptide oligomers to nanotubes. When the proportion of ethanol in water was low (1% and 10% by volume), the assembly morphology of the polypeptide was similar to that in pure water, forming a curved layered structure first, then twisting to form a short spiral ribbon, gradually lengthening, and finally forming a nanotube. When the proportion of ethanol in the aqueous solution is high (20% and 30% by volume), the twisted ribbons are directly formed, then the spiral ribbons are formed, and finally the nanotubes are assembled. The results of electron microscopy showed that the above process was reversible when the volume content of ethanol was 30%. Further analysis showed that compared with pure water solution, the hydration layer around Leu and Aib decreased in water and ethanol solution, which induced the hydrophobic interaction between the edges of polypeptide assemblies and led to the formation of nanotubes. In this process, ethanol molecules are inserted into the polypeptide to relax its molecular packing, and when the proportion of ethanol is higher, the bending of the layer structure is greatly reduced, resulting in different assembly morphologies at lower and higher concentrations, respectively (Fig. 2C).

In this example, by introducing ethanol into water to weaken the force between the liquid condensed state and the dispersoid, a thermodynamically stable intermediate assembly process was obtained, and the assembly process was precisely regulated. Using a similar strategy, the assembly process of dispersoids can be regulated step by step by selecting appropriate mixed liquid condensates, and then a profound understanding of the assembly mechanism can be obtained by analyzing the morphology and structure of the intermediates.

3.2 Regulation of liquid condensed state on the aggregation morphology of dispersoid

In the liquid condensed state, the dispersoid changes the surface polarity and reduces the interfacial energy by fine-tuning its molecular conformation to adapt to its existing environment. For a certain dispersoid, its surface polarity can be adjusted in a narrow range, but the dispersoid will form aggregates through its own molecular interaction, reduce the contact with the continuous phase, and thus increase the dispersion in the continuous phase. Simply put, the polar dispersoid is dispersed in the non-polar liquid in the form of aggregation and the less polar part of the dispersoid faces the outside of the aggregate, while the non-polar dispersoid is also dispersed in the polar liquid in the form of aggregation and the more polar part of the dispersoid faces the outside of the assembly. Using this strategy, the dispersion or aggregation state of the dispersoid can be regulated by adjusting the polarity of the liquid, which is particularly effective for the assembly structure regulation of amphiphilic dispersoids^[38].

Polyoxometalate anions were coated with cationic surfactants with azobenzene groups at the end to form organic-inorganic hybrid supramolecular complexes based on electrostatic interaction (Fig. 3)^[39]. Due to the highly symmetrical structure and charge delocalization of polyoxometalate anions, cationic surfactants are uniformly distributed on the surface of anions in solution, forming spherical-like complexes. In this case, the cationic head group of the surfactant is close to the polyoxometalate anion through electrostatic interaction between the organic and inorganic components, while the terminal azobenzene group is located outside the complex. When the structure of azobenzene is trans, the surface polarity of the complex is weak, so the complex is monodisperse in the mixed solvent of toluene (PhMe) and tetrahydrofuran (THF) with weak polarity. On the contrary, in the mixed solvent of methanol (MeOH) and tetrahydrofuran, its strong polarity does not match the weak polarity of the dispersoid, which leads to the aggregation of the dispersoid and reduces the interfacial energy. Under the irradiation of ultraviolet light, the azobenzene changes from trans to cis structure, which increases the polarity of the supramolecular complex. In the above mixed solvent, its existing state is just opposite to that of azobenzene in trans structure, that is, it is in aggregation state in toluene and tetrahydrofuran and in dispersion state in methanol and tetrahydrofuran. The above process is reversible, and the aggregation state of the dispersoid is changed when the azobenzene is transformed from cis to trans under visible light irradiation.

The above results indicate that the physicochemical properties of the liquid condensate, especially the polarity, not only affect the supramolecular interactions between the surface groups of the dispersoid, but also change the existing state of the dispersion or aggregation of the supramolecular complex. This characteristic can be used to achieve specific functionality, such as phase transfer catalysis, by controlling the aggregation and deaggregation processes of dispersoids^[40].

3.3 Regulation of Liquid Condensation on the Assembly Structure of Dispersion

When the polarity of the continuous phase changes, the corresponding supramolecular forces will also change, which will induce the structural transformation of the dispersoid assembly. For example, the strength of π-π stacking can be used to control the packing tightness between dispersoid molecules, thus realizing the reversible transition from folding to unfolding.

In the phenylethynyl/perylene bisimide oligomer (PBI) (Figure 4A), the strength of intermolecular π-π stacking has a significant effect on the configuration of the assembly due to the presence of a large π conjugated system within the molecule^[41]. When chloroform (CHCl₃) is used as solvent, the π-π stacking interaction between molecules is relatively weak, so that the oligomer exists in an unfolded form; In the less polar methylcyclohexane (MCH), the strong π-π stacking interaction greatly reduces the distance of the π conjugated groups, thus forming a folded structure (Fig. 4B). The fluorescence spectrum of PBI in chloroform solution shows an obvious monomer emission peak at 520 nm (Fig. 4C, solid line), which also indicates that the oligomer exists in an unfolded form. In MCH (Figure 4 C, dotted line), the monomer emission band at 520 nm almost completely disappears, and the excimer-like broad emission band from 550 to 850 nm has a maximum intensity at about 640 nm, indicating that PBI has a strong π-π interaction in the folded state.

For a single dispersoid particle, its relatively definite polarity results in a smaller number of liquid condensate species to match. In a larger range, the polarity of dispersoids can be effectively controlled by obtaining a certain aggregation structure through the assembly of particles, so as to match the liquid condensed state. Based on this idea, the assembly structure of dispersoids can be effectively regulated by selecting the appropriate polarity of liquid condensates, and the use of mixed liquid condensates is a simple and effective strategy^[42,43].

In Anderson-type polyoxometalates (TTAs) (Figure 5A), which are covalently modified by triazine derivatives on both sides and have good solubility in methanol when the countercation is sodium ion, they assemble to form randomly distributed spherical structures at a concentration of 0.1 mg·mL^-1, and in some places, the spherical structures tend to be arranged in chains (Figure 5B)^[44]. When the volume content of water in the solvent is increased to 10%, obvious nanowire structures can be observed, and the length can reach tens of microns (Figure 5C). If the volume content of water is further increased to 20%, lamellar structures with different sizes can be produced (Figure 5d). The above morphological changes are derived from the polarity-induced increase in packing force driven by hydrophobic interactions of the organic moieties. When sodium ion is replaced by other cations, such as tetrabutylammonium cation, TTA can form assemblies with regular morphology in the liquid condensed state with weaker polarity.

The above examples show that the liquid condensate plays an important role in the assembly process, aggregation morphology and assembly structure regulation of dispersoids. Since the liquid condensate and the dispersoid interact with each other through supramolecular forces, a weak change in the polarity of the liquid condensate may lead to a change in the aggregation behavior of the dispersoid^[45]. By making full use of this feature, supramolecular assemblies with different morphologies can be obtained by changing the liquid condensed state.

4 Effect of liquid condensed state on the properties of dispersoid

4.1 Solvatochromism Induced by Liquid Condensation

As a typical example of the important influence of the liquid condensed state on the dispersoid properties, the dye molecules with asymmetric structure show different UV-Vis absorption signals in the liquid condensed state with different polarities, and the maximum absorption wavelength of the absorption spectrum shifts with the change of the polarity of the solid condensed state^[46,47]. Conversely, the maximum absorption wavelength of the UV-visible absorption spectrum of a certain dye molecule in different liquid condensed States can also be used to evaluate its polarity, especially in evaluating the polarity of mixed liquid condensed States.

When 4- (nitrostyryl) phenol tetrabutylammonium salt (NPT) was used as a probe molecule (Figure 6a), it showed light brown, purple, royal blue, and sky blue colors in ethane-1,2-diol, toluene, ethyl acetate, and N, N-dimethylacetamide (DMA), respectively (Figure 6B)^[47]. Correspondingly, the maximum absorption wavelengths of the UV-Vis absorption spectra are 484, 516, 556, and 620 nm, respectively (Fig. 6C), and all come from the intramolecular charge transfer process, that is, the π-π^* electron transition from the electron-donor phenolate moiety to the electron-acceptor 4-nitrophenyl moiety. In addition to the maximum absorption wavelength, the solvent also has an important effect on the absorption intensity and peak shape of the UV-Vis absorption spectrum of the dispersoid. Solvent-induced discoloration of a dispersoid results from differential solvation of the ground and first excited States of a light-absorbing molecule (or its chromophore)^[46]. If the ground state molecule obtains a better stable state than the excited state molecule by solvation with the increase of solvent polarity, it will lead to a blue shift of the spectrum. On the contrary, with the increase of solvent polarity, the molecules in the first excited state are more stable than those in the ground state, which will lead to the red shift of the spectrum. In this process, an important criterion for the selection of the solvent is to ensure the solubility of the dispersoid in it to eliminate the effect of aggregation on the spectrum due to poor solubility. It should be pointed out that the polarity of the solvent can be evaluated by this method to obtain multiple parameters, and these parameters can be weighted by different weights to produce a composite figure of merit.

4.2 Photoinduced Electron Transfer Regulated by Liquid Condensate

In liquid condensed state, most of the intermolecular and intramolecular electron transfer of dispersoids need to be carried out by means of the continuous phase, so the efficiency of electron transfer is closely related to the physical and chemical properties of liquid condensed state, especially the dielectric constant and the ability to donate and accept protons related to electron transfer^[48].

In a compound 10⁵ (Figure 7 A) composed of an acetylenic (cc) -linked photosensitizer [Ru(bpy)₃]²⁺(bpy 2,2 ′ -bipyridine) and an electron acceptor anthraquinone (AQ), the electron transfer rate from the photosensitizer to the acceptor can be changed by several orders of magnitude from 10⁵ to 10¹¹s^-1 under illumination by selecting a suitable solvent as the continuous phase^[49]. The nanosecond transient absorption spectrum shows that electron transfer from the metal ion Ru²⁺ to the ligand bpy occurs first upon illumination. In high dielectric constant solvents, the formation of a charge-separated state and the rapid (on the order of a few picoseconds) establishment of a pseudo-equilibrium excited state can be demonstrated by ultrafast transient absorption spectroscopy and time-resolved infrared spectroscopy, and electrons can be transferred from the donor to the acceptor; However, in low dielectric constant solvents, this process cannot occur. The plot of fluorescence lifetime versus the dielectric constant of the solvent (Fig. 7B) shows that in acetonitrile, which has a high dielectric constant, the fluorescence lifetime is only 6.7 ns, while in butyronitrile and acetone, which have relatively low dielectric constants, the fluorescence lifetime is only 6.7 ns.The fluorescence lifetime increases to several hundred nanoseconds, and in dichloromethane and 1,2-dichloroethane with lower dielectric constant, the fluorescence lifetime increases to more than 3000 ns, indicating that the dielectric constant greatly affects the electron transfer process.

4.3 Dispersoid chirality controlled by liquid condensed state

One of the important strategies to achieve chirality amplification is to arrange single chiral molecules in a specific environment to form chiral supramolecular assemblies by self-assembly^[50]. The method of regulating the liquid condensed state to achieve this process has the characteristics of simple operation and wide range of regulation, so this method has attracted wide attention^[51]. By choosing a suitable liquid phase as the continuous phase, not only the chiral control and helical structure of the assembly can be effectively realized, but also the electron transfer process of the assembly can be affected^[52].

L-PyG and L-PyPhG, two amphiphilic chiral molecules modified by pyridine and cyanostilbene at the end, were dissolved in ethanol and dimethyl sulfoxide (DMSO), respectively, and both formed left-handed chiral nanotubes in ethanol and right-handed chiral nano-twisted helices in DMSO (fig. 8)^[53]. The reason for the formation of the two opposite helical structures is that the protic solvent ethanol can form hydrogen bonds with the terminal pyridine, while the aprotic solvent DMSO cannot form similar hydrogen bonds. Although the molecular chirality in the compound is preserved in both solvents, the nanoassembly shows opposite circular dichroism and circularly polarized luminescence signals at the nanoscale. For l-PyPhG, both signals are inverted when the solvent is changed from EtOH to DMSO, which is consistent with the chiral inversion of the chiral nanostructure. However, for l-PyG, the nanotube and nanotwist exhibit the same sign on the chiral spectrum, while the handedness of the nanostructure is opposite. When the dye molecule acridine orange is added to the system, the nanotube is easy to combine with the dye molecule through π-π stacking due to the large planarity of the nanotube, so that effective energy transfer (ET) is realized; Nano-twisted helices can not stably bind to dye molecules, so they can not transfer energy effectively.

4.4 Effect of Liquid Condensation on Dispersoid Catalytic Reaction

Liquid condensed matter is the reaction medium of most catalytic reactions, including homogeneous catalysis and heterogeneous catalysis, which has an impact on the reaction rate, yield and selectivity of catalytic reactions^[13]. Generally speaking, the better the solubility of the substrate in the solvent, the better its dispersion, and the higher the probability of collision, which is conducive to the reaction. On the other hand, if the solubility of the product of the catalytic reaction in the solvent is relatively poor, the product will continue to precipitate from the continuous phase as the reaction proceeds, which can reduce the concentration of the product in the system and promote the reaction to proceed in the positive direction. The mass transfer ability of the solvent is also an important factor affecting the catalytic reaction rate^[54]. When the solvent viscosity is high and the mass transfer ability is weak, the reaction rate will generally decrease significantly, and the reduced reaction rate will also affect the reaction selectivity.

In addition to the above general effects of liquid condensates on catalytic reactions, liquid condensates may also affect catalytic reactions in other ways. For example, when there is a strong supramolecular force between the solvent and the substrate molecule, such as hydrogen bonding, it will have a strong impact on the reactivity of the substrate^[55]. The solvent may also be directly used as the substrate of the catalytic reaction, which can effectively improve the contact probability between the reactants to obtain a high conversion rate. In addition to the interaction between the solvent and the substrate, the supramolecular interaction between the solvent and the catalyst may also affect the activity of the catalyst.

The polarity of the liquid condensate is still an important factor in catalytic reactions. In the reaction of furan-2-ylmethanol catalyzed by zeolite catalyst H-ZSM-5 with microporous structure, when different liquid condensed States are used, although the same catalytic products are obtained, the catalytic reaction will occur at different reaction sites^[56]. When the less polar 1,4-dioxane is used as the dispersed phase, the reaction takes place in the curved pores; When more polar water is used as the reaction medium, the reaction occurs in straight macropores (fig. 9). The difference of reaction sites is due to the polarity change caused by the different degree of silanol defects in the two types of pores. The kinetic difference caused by the co-adsorption of the dispersoid and the liquid condensed state on the catalyst makes the component with larger polarity in the system enter the small pores, while the component with smaller polarity enters the large pores, thus realizing the selection of the reaction site for the polarity control of the dispersed phase.

In liquid phase catalytic system, the contact probability between substrate and catalyst is one of the important factors affecting the efficiency of catalytic reaction. Weak aggregation and transient recognition between different molecules can enhance the capture ability of the catalyst to the substrate, reduce the distance between the substrate and the catalyst, and then effectively promote the smooth progress of the catalytic reaction. As shown in Fig. 10, in an aqueous catalytic system {Mo₁₅₄}@CDC composed of cationized β-cyclodextrin (CDC) and ring-shaped polyoxometalate anions { Mo₁₅₄ } through electrostatic interaction, the hydrophobic catalytic substrate cyclohexene and the cavity of cyclodextrin have good size and hydrophilic-hydrophobic matching, which can realize the host-guest binding between them^[57]. This combination greatly shortens the distance between the substrate cyclohexene and the active catalytic center polyoxometalate cluster, so that cyclohexene can be oxidized to cyclohexenol and so on. The hydrophilicity of the oxidized product is enhanced, the host-guest interaction between the product and the hydrophobic cavity of the cyclodextrin is weakened, and the increased volume of the product promotes the product to be separated from the cavity of the cyclodextrin and return to the aqueous solution, so that the cavity can be reused to absorb new cyclohexene molecules. In this process, the distance between the substrate and the catalyst is shortened and the catalytic efficiency is improved by using the transient host-guest binding between the substrate and the cyclodextrin.

5 Conclusion

In a system composed of a liquid condensate and a dispersoid, the liquid condensate as a continuous phase has an important influence on the existence and properties of the discontinuous phase dispersed in the liquid condensate. The physical and chemical properties of liquid condensates, such as polarity and dielectric constant, play a key role in the dissolution, molecular recognition and assembly of dispersoids. The physical and chemical properties of the continuous phase can be precisely controlled by selecting appropriate solvents or using solvents mixed in different proportions, so as to realize the whole process control of the dispersoid from dissolution to controlled assembly. In addition to the regulation of the assembly structure of dispersoids, the liquid condensed state also has an important impact on the properties of dispersoids. The same asymmetric dye molecule exhibits different UV-Vis absorption spectra in solvents with different polarities. The polarity of the solvent also has a profound impact on the assembly structure and circular dichroism spectrum of chiral dispersoids. In addition, the solvent also plays a key role in the electron transfer and catalytic reaction of the dispersoid.

The influence of liquid condensed state on the structure and properties of dispersoid is multifaceted and multi-level, and only some of them are summarized and elaborated in this paper. Although considerable progress has been made in this field, more efforts are still needed to deepen the understanding of the system in the following aspects: Most of the current research work is based on the elaboration and simple explanation of experimental phenomena.There is still a lack of deeper research and discussion, and it is an important and effective means to predict the influence of liquid condensate on the assembly and properties of dispersoid by theoretical calculation. The enthalpy change, entropy change and free energy change in the process of dissolution and assembly of dispersoid in liquid condensed state are calculated by using physical and chemical principles, so as to quantitatively describe the above process. Sometimes, a single parameter of the liquid condensate can not explain the assembly law and thermodynamic process of the dispersoid in it, while a joint quality factor constructed by multiple reasonably weighted parameters may explain the experimental phenomena and physical processes more reasonably. In this process, how to select the appropriate parameters and the weighted proportion is the focus of the study. In most cases, the influence of liquid condensates on the assembly and properties of dispersoids is dynamic, so the development of high-precision characterization methods, such as ultrafast spectroscopy for monitoring the assembly process and electron microscopy with atomic resolution, is of great significance.

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