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. 2025 Jun 17;5(5):501–507. doi: 10.1021/acsphyschemau.5c00029

Liquid−Solid Phase Transitions in Nanoscale Mixtures of Water and Organic Substances by the Data of NMR Spectroscopy

Tetiana Krupska †,‡,*, Myroslav Lenov ‡, Qiliang Wei †, Jinju Zheng †, Weiyou Yang †, Volodymyr Turov †,‡,*
PMCID: PMC12464749  PMID: 41019628

Abstract

The process of water clustering in the interparticle gaps of hydrophilic (A-300) and hydrophobic (AM1) silicas in different media was studied using 1H NMR spectroscopy. It has been established that when equal amounts (100 mg/g) of water and oil are introduced into the interparticle gaps of compacted hydrophilic or hydrophobic silica by grinding under the influence of mechanical load, the water transforms into a nanosized state with cluster radii in the range of 1−50 nm. In air, the main part of water is in a strongly associated state with a network of hydrogen bonds similar to liquid water. Replacing air with a chloroform medium leads to the stabilization of weakly associated forms of water, which are observed in the NMR spectra in the form of one or several signals with chemical shifts δH = 1−2 ppm. A comparison of the intensities of the NMR signals of water and oil allows us to conclude that the oil is partially frozen not only in air, but also in chloroform, which has unlimited solubility in relation to oil. In the medium of acetone, which is capable of dissolving both water and oil, in the interparticle gaps of hydrophobic silica, the formation of several types of clusters of strongly and weakly associated water is observed, existing as spatially separated nanodroplets, slowly (on the NMR time scale) exchanging protons or molecules with each other. It has been shown that the hydrophobic walls of silica particles have such an ordering effect on clusters of water and acetone, oil or TMS located in the interparticle gaps that a significant part of it turns into a solid state at temperatures (up to 287 K), which is several tens of degrees higher than the bulk freezing temperature.

Keywords: clusterization, hydrophilic silicas, hydrophobic silicas, 1H NMR spectroscopy, interparticle gaps, nanosized state

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The problem of water interaction with nonpolar hydrocarbons and silica materials is extremely important for increasing the efficiency of the oil production industry. − Modern methods of oil production involve its displacement from silica formations in the presence of active additives such as surfactants, salts and clay minerals. Therefore, the ability to control liquid−solid phase transitions at the silica−hydrocarbon−water phase boundaries can contribute to increasing the yield of formations and reducing the cost of oil extraction processes.

The nanosized state of matter is characterized by the fact that the number of atoms (or molecules) in the surface layer of particles and in the volume becomes comparable. Since the number of bonds in which the molecules of the surface layer participate is less than in the volume, these molecules (or atoms) become more reactive. Accordingly, in the nanosized state, the solubility of substances increases, the temperature of chemical reactions decreases, and the transfer of energy of chemical interactions accelerates. − In recent years, significant efforts have been made to use nanoparticles for medical purposes. Due to their high reactivity, even short-term effects on cells can significantly affect cellular metabolism. − However, nanoparticles must be used with great caution, since in some cases they can penetrate into cells and disrupt their normal functioning. ,

One of the simplest ways to study the properties of a substance in a nanoscale state is to use mesoporous materials as a solid matrix. Then, by filling the pores or interparticle gaps with a certain amount of the substance, it can be transferred to a nanoscale state. Pyrogenic silicas, which consist of nonporous particles, can be used as a mineral matrix. This makes it possible to study the effect of the silica surface on the phase state of substances in interparticle gaps under conditions of their complete or partial filling with the substance being studied or a mixture of different substances (solution) in contact with the silica surface or (with partial filling of the pores) also with air. To register the solid−liquid phase transition, an original method of low-temperature 1H NMR spectroscopy can be used, − which is based on the use of liquid NMR in a wide temperature range in which phase transitions (freezing-thawing) of the studied substances occur. The method uses a large (several orders of magnitude) difference in the values of transverse relaxation of nuclear spins for solids and liquids. When water or organic substances freeze, they cease to be registered in the liquid NMR spectra.

Fumed silicas, which are synthesized by burning chlorosilanes in the flame of a hydrogen burner, − consist of structurally ordered primary nanosized particles united by interparticle interactions into a system of micronsized aggregates and agglomerates, in which the interparticle gaps form a nonrigid mesoporous structure. , The surface of silica particles contains a significant amount of silanol groups (0.8−1.5/nm2), which serve as centers for the primary adsorption of water − from the gas phase. The spaces between the hydroxyl groups of the surface are formed by siloxane bonds, which serve as hydrophobic centers and can sorb nonpolar substances (saturated hydrocarbons). Thus, fumed silicas are biphilic in nature. The amount of water or nonpolar organic substances absorbed by fumed silicas from a gaseous medium (for example, at p/p 0 = 0.5) is determined by the adsorption potential of the surface − and usually for materials with different specific surface areas is 1−5 mmol/g, i.e., several weight percent of the mass of the adsorbent. However, if you prepare a mixture of silica with water or liquid nonpolar substances, then due to the lability of the structure of silica aggregates and agglomerates, under the influence of mechanical load, an arbitrary amount of liquids can be introduced into the interparticle gaps of silica. Since, the interparticle space in fumed silicas is a system of nanosized cavities, under conditions when most of the pore space remains unoccupied, the embedded substances turn into a system of surface clusters (or domains) that interact with the surface by forming hydrogen bonds, or by the van der Waals interactions. The properties of substances in a clustered state can differ greatly from those in bulk. −

The aim of the work was to study the possibility of the formation of cluster structures of water introduced into the interparticle gaps of hydrophilic and hydrophobic silicas in the presence of high-boiling hydrocarbons (vegetable oil) and the influence of a liquid organic medium on this process.

Experimental Section

We used hydrophilic and hydrophobic silicas produced by the Kalush Experimental Plant of the Chuiko Institute of Surface Chemistry of NAS of Ukraine. Methyl silica was obtained by chemical modification of fumed silica grade A-200 with a specific surface area according to BET S BET = 185 m2/g with dimethyldichlorosilane. The initial bulk density of A-300 (S BET = 295 m2/g) and AM1 was 50 mg/cm3. To obtain a more compact form, methyl silica powder was moistened with an organic solvent (methanol) and dried at 373 K for 12 h, after which it was additionally kept at 300 K for 18 days. When grinding dry substances, 100 mg/g of vegetable oil was added to the silica powders, and then 100 mg/g of distilled water and additionally ground for several minutes to move water and oil into the interparticle gaps A-300 or AM1 with the corresponding removal of a certain amount air. − Hydrophilic silica was used as wetting-drying nanosilica A-300 with a bulk density of C d = 300 mg/cm3. Measurements were carried out on one sample in air and an organic solvent.

The method of converting liquid substances such as water or oil into a nanosized state in the interparticle gaps of silicas (both hydrophobic and hydrophilic) by grinding them together in a porcelain mortar is quite universal and can be used to prepare nanostructured mixtures (composites) if their concentration does not exceed 30% by weight. Complete reproducibility of the results is achieved when the mixture homogenization process is carried out for more than 5 min. If it is necessary to prepare large volumes of composite systems, ball mills of various designs can be used. However, since the pressure of the balls on the walls is significantly less than with manual grinding, the homogenization time should be 20−30 min. The bulk density of the composite can be used as a criterion for the reproducibility of the process.

NMR spectra were recorded on a high-resolution NMR spectrometer (Varian ″Mercury″) with an operating frequency of 400 MHz. Eight 60° probe pulses with a duration of 1 μs and a bandwidth of 20 kHz were used. The temperature in the sensor was regulated with an accuracy of ±1 degree. Signal intensities were determined by measuring peak areas using a signal decomposition procedure assuming a Gaussian waveform and optimizing the zero line and phase with an accuracy of ±10%. To prevent overcooling of water in the objects under study, the concentration of nonfreezing water was measured by heating samples precooled to a temperature of 210 K. Temperature dependences of the intensity of NMR signals were carried out in an automated cycle, when the sample was kept at a constant temperature for 3 min. To prevent the appearance of additional intense signals in the spectra, CDCl3 and (CD3)­CO were used as an organic medium, in which the proportion of the main isotope (deuterium) was 99 wt %.

The process of freezing (melting) of interfacial water localized in interparticle gaps or in the pores of the adsorbent occurs in accordance with changes in the Gibbs free energy caused by the action of the surface. The smaller the distance from the surface of the studied water layer (the larger the size of the clusters of adsorbed water), the smaller it is. At T = 273 K, water freezes, the properties of which do not differ from bulk water, and as the temperature decreases (without taking into account the effect of supercooling), water that is part of increasingly smaller clusters freezes, and for interphase water the following relation is ΔG ice = K 1ΔT m , where K 1 = −0.036 kJ/(mol·Deg). By determining the temperature dependence of the concentration of nonfreezing water C uw (T) from the signal intensity in accordance with the method described in detail in − , the amounts of strongly and weakly bound water (SBW and WBW, respectively) can be calculated.

Typically, weakly bound water can be considered the part of water that melts at a temperature T > 265 K. For highly hydrated systems, part of the WBW may not differ in its properties from bulk water. Water that melts at lower temperatures is classified as strongly bound water. To determine the geometric dimensions limited by the solid surface of nanosized liquid droplets (domains), the Gibbs−Thomson equation , can be used, relating the radius of spherical or cylindrical nanodroplets (R) with the magnitude of the freezing temperature depression ΔT m = K 2/R, where K 2 = 50 (Deg/nm).

Results and Discussion

1H NMR spectra of samples containing compacted silica with equal mass quantities of water and oil immobilized on its surface, taken at different temperatures, recorded in air (filling the empty space in the interparticle gaps) and CDCl3 environment are shown in Figure a,b for hydrophilic silica A-300, in Figure c,d − for hydrophobic AM1. The spectra contain two main signals related to the protons of water and oil. Since water is a strongly associated liquid (each molecule can participate in the formation of four hydrogen bonds, the chemical shift of its protons strongly depends on association. − In this case, the chemical shift of water protons varies from δH = 1−1.5 ppm for molecules that do not participate in the formation of hydrogen molecules (weakly associated water, WAW) up to δH = 7 ppm in ice. In liquid water, the chemical shift is δH = 5 ppm, which corresponds to the participation of each molecule in 2.5−3 hydrogen bonds. Such a recorded signal of water at δH = 5−6 ppm (Figure a,c) should be attributed to strongly associated water (SAW), and a wide signal, the center of which is located at δH = 1 ppm − the signal of oil protons (CH2 and CH3 groups), as well as (possibly) the signal of weakly associated water, which has similar chemical shift values (Figure a,b).

1.

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1H NMR spectra of water and oil immobilized in the interparticle gaps of hydrophilic silica A-300 in air medium (a) and CDCl3 medium (b); hydrophobic silica AM1 in air (c), in CDCl3 (d) and acetone (e), containing 100 mg/g H2O and 100 mg/g of oil.

With decreasing temperature, the intensities of both signals decrease due to the partial freezing of substances adsorbed on the surface of silica particles. Water is characterized by a significant decrease in freezing temperature (compared to bulk water), due to a decrease in the free energy of interfacial water caused by adsorption interactions. The broad, asymmetric signal of vegetable oil protons is a signal of a mixture of fatty acid triglycerides, which contains a fused signal of the CH3 and CH2 protons of fatty acid protons (the main signal), as well as the CH and CH2 groups of glycerol. Since the signal of protons in strong magnetic fields cannot be attributed only to the signal of CH2 and CH3 groups of oil, to determine the concentration of strongly associated water, it can be assumed that the total intensity of the signals of all protons of substances present in the colloidal system relates to the total amount of substance in the liquid phase (water and oil), which is 200 mg/g (the signal intensity of hydrocarbons, the amount of which is expressed in grams, is close to the signal intensity of the same amount of water).

Weakly associated water in the form of a separate signal is not observed for samples taken in air (Figure a,c). In a chloroform environment, on the left shoulder of the oil methylene group signal, one or more signals are observed that can be attributed to the signals of the methylene groups of glycerol and weakly associated water (WAW). Oil signals are more intense when AM1 is used as silica (Figure b,d). For AM1, three SAW signals and three signals are recorded in the spectra, which can be attributed to the signals of the CH2 groups of glycerol and WAW (signals 1−3). It should be noted that for a composite created on the basis of AM1 in the spectra at δH = 0 ppm, a signal is observed from tetramethylsilane (TMS) added to chloroform as a chemical shift standard (Figure d). The intensity of this signal also decreases with decreasing temperature, which may be due to partial freezing at a relatively high temperature of both TMS and chloroform, localized in the interhour gaps AM1.

When using acetone as a medium filling the interparticle space, which can be mixed with water and oil in any concentration ratio (Figure e), the appearance of the spectra completely changes. Several signals are recorded in the spectra: TMS signal (δH = 0 ppm), signals of methyl and methylene groups of oil (δH = 0.9−1.5 ppm), signals of weakly associated water WAW (δH = 2 ppm), signals of weakly associated water WAW1, WAW2 (δH = 1−2 ppm), the signal of the H−O−H···O­(CH3)2 complex (δ H = 3 ppm), which are in a state of rapid exchange with water molecules that are part of strongly associated water clusters (δH = 3−4 ppm) and two signals that can be attributed to strongly associated water, which does not take part in the exchange of protons (or molecules) with other forms of water SAW1 and SAW2 (δH = 4.3 and 5.5 ppm). All these signals are observed separately, which makes it possible to determine the intensities of most signals using the integration procedure. For water impurity in acetone (d6), a signal with a chemical shift of δH = 2.8 ppm is characteristic. In the spectra shown in Figure e, such a chemical shift value for water dissolved in acetone is recorded only at T = 287 K. With decreasing temperature, the intensity of this signal decreases, and its chemical shift increases. Considering that at the same time there is also a decrease in the signal intensity of the methyl groups of acetones (δH = 2.0 ppm), it can be assumed that with partial freezing of acetone, the contribution to the signal of the protons of water included in the complexes H−O−H···O­(CH3)2 increases from the protons of strongly associated water that quickly exchange with it, having a greater chemical shift.

As the temperature decreases, the intensities of the water and oil signals decrease due to the partial freezing of substances in the interparticle gaps of silica. Water is characterized by a significant decrease in freezing temperature (compared to bulk water), due to a decrease in the free energy of interfacial water caused by adsorption interactions. For strongly associated water, the temperature dependences of the concentration of nonfreezing water in silica/(water+oil) composites based on hydrophilic and hydrophobic silicas are shown in Figure a,b, and the distributions along the radii of clusters of adsorbed water in the air and CDCl3 medium, calculated in accordance with eq 2in Figure c,d.

2.

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Temperature dependences of the concentration of water and oil immobilized in interparticle gaps (a, b) and the distribution along the radii of clusters of adsorbed water (c, d) for silicas A-300 (a, c) and AM1 (b, d) in air and CDCl3 medium.

For hydrophilic silica A-300 in an air environment, almost all sodium-associated water is strongly bound both in an air environment and in a CDCl3 environment (Figure a). In the case of hydrophobic silica, a significant part of the strongly associated water becomes weakly associated (Figure b).

As can be seen from the data in Figure c,d, water adsorbed on the surface of A-300 and AM1 is in the form of a system of clusters, the radius of which is 1−50 nm. In the case of hydrophobic silica, water clusters with radii R > 10 nm and R < 20 nm are predominantly formed in the air. For A-300, replacing the air environment with a chloroform environment stabilizes larger clusters of strongly associated water (with a radius of 10 nm or more). For AM1 it is the other way around. CDCl3 medium stabilizes water clusters of smaller radius. In this case, the distributions ΔC(R) for both types of silicas become similar. In addition, the organic medium significantly changed the ratio of signal intensities in the NMR spectra (Figure a,b). These changes may be associated with various processes occurring at the interface between the solid and liquid phases under conditions of clustering of the latter. Thus, on the one hand, a decrease in the concentration of SAW in the air can occur due to a corresponding increase in the contribution from WAW, which is poorly distinguishable in liquid NMR spectra against the background of an intense signal from the aliphatic CH2 and CH3 groups of the oil. On the other hand, this process may be influenced by a decrease in the oil signal, due to stabilization under the influence of the surface of its solid state at temperatures higher than the bulk freezing temperature. A similar effect was previously observed in many systems containing highly dispersed silicas, water and nonpolar hydrocarbons.

Since the freezing point of acetone is 178 K, with its absolute excess one could expect that both oil and water would go into a dissolved state, which they would remain in throughout the entire temperature range accessible to measurement. However, it turned out that this was not the case. From the data in Figure e it follows that with a decrease in temperature due to partial freezing, the intensities of all signals decrease, and this occurs over almost the entire range of temperature changes. Moreover, with a decrease in temperature at relatively high temperatures (up to room temperature), even substances such as acetone and tetramethylsilane freeze in the interparticle gaps of methyl silica (Figure ).

3.

3

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Temperature dependences of signal intensities related to different substances in the interparticle gaps AM1.

If we analyze the course of the temperature dependences of signal intensities related to different substances localized in the interparticle gaps AM1, we can do the conclusion, that simultaneous freezing of all substances present in the adsorption layer takes place (Figure ). Consequently, the silica surface promotes the synchronous transition to the solid state of both a solution of water and oil in acetone and clusters of different types of water. The fact that acetone is the dominant substance allows us to conclude that the influence of the surface on the liquid/solid phase transition relates primarily to the formation of molecular crystals, which in a limited pore space can occur at a temperature several tens of degrees above the bulk melting temperature. It should be noted that during the experiments, the temperature in the NMR spectrometer sensor did not drop below the bulk freezing temperature of acetone. Therefore, we should talk specifically about the freezing of acetone in the limited space of the interparticle gaps AM1, and not about the slow process of its melting with increasing temperature.

As follows from the data in Figure b, in the case of a hydrophobic surface of methyl silica, which borders on water clusters in interparticle gaps, there is a temperature range (up to T = 290 K) in which water is in the form of metastable ice. Thus, in nanosized cavities filled with liquid (water or organic substances), the surface stabilizes the metastable state of frozen substances at temperatures tens of degrees higher than the bulk temperature of their freezing.

Conclusions

When equal amounts (100 mg/g) of water and oil are introduced into the interparticle gaps of compacted hydrophilic (A-300) or hydrophobic (AM1) silica by grinding under the influence of mechanical load, the water transforms into a nanosized state with cluster radii at the range of 1−50 nm. In the air, the main part of water is in a strongly associated state with a network of hydrogen bonds similar to liquid water.

Replacing air with a chloroform medium leads to the stabilization of weakly associated forms of water, which are observed in the NMR spectra in the form of one or several signals with chemical shifts δ H = 1−2 ppm. A comparison of the intensities of the NMR signals of water and oil allows to conclude that the oil is partially frozen not only in air, but also in chloroform, which has unlimited solubility in relation to oil.

In the medium of acetone, which is capable of dissolving both water and oil, in the interparticle gaps of hydrophobic silica, the formation of several types of clusters of strongly and weakly associated water is observed, existing as spatially separated nanodroplets, slowly (on the NMR time scale) exchanging protons or molecules with each other.

In the case of a hydrophobic surface of methyl silica, which borders on water clusters in interparticle gaps, there is a temperature range (up to T = 287 K) in which water is in the form of metastable ice. Thus, in nanosized cavities filled with liquid (water or organic substances), the surface stabilizes the metastable state of frozen substances at temperatures tens of degrees higher than the bulk temperature of their freezing.

Acknowledgments

The authors are grateful to the Ningbo University of Technology for financial support of this study.

Glossary

Abbreviations

NMR

nuclear magnetic resonance

WAW

weakly associated water

SAW

strongly associated water

WBW

weakly bound water

SBW

strongly bound water

TMS

tetramethylsilane

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. All authors T.K., M.L., Q.W., J.Z., W.Y., and V.T. consent to participate and give permission for publication of this article. T.K. conceptualization, data curation, investigation, methodology, resources, and draft; M.L. data curation, investigation; Q.W. data curation, investigation; J.Z. data curation, investigation; W.Y. data curation, investigation, resources; V.T. conceptualization, data curation, investigation, methodology, project administration, resources, supervision, validation, roles/writingoriginal; writingreview and editing

This work was supported by the National Natural Science Foundation of China (Grant No. 52372063) for the financial support of this study.

The authors declare no competing financial interest.

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