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 Fig. 2, a,b for hydrophilic silica A-300, in Fig. 2, 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 [22]), the chemical shift of its protons strongly depends on association [23]. 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 [22]. Such a recorded signal of water at δН = 5–6 ppm (Fig. 2, a, c) should be attributed to strongly associated water (SAW), and a wide signal, the center of which is located at δН = 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 (Fig. 2a, b).
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 [24]. 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.
Weakly associated water in the form of a separate signal is not observed for samples taken in air (Fig. 2, a, c). In a chloroform environment, on the left shoulder of the signal of the methylene groups of the oil, one or more signals are observed that can be attributed to weakly associated water. WAW signals have greater intensity when AM1 is used as silica (Fig. 2, b, d). For AM1, three SAW signals and three WAW signals are recorded in the spectra (signals 1–3). It should be noted that for a composite created on the basis of AM1 in the spectra at δН = 0 ppm a signal is observed from tetramethylsilane (TMS) added to chloroform as a chemical shift standard (Fig. 2, d). The intensity of this signal also decreases with decreasing temperature, which may be due to the partial freezing of chloroform in the inter-hour gaps AM1.
When using acetone as a medium filling the interparticle space, which can be mixed with water and oil in any concentration ratio (Fig. 2, e), the appearance of the spectra completely changes. Several signals are recorded in the spectra: TMS signal (δН = 0 ppm), signals of methyl and methylene groups of oil (δН = 0.9 and 1.3 ppm, respectively), signals of weakly associated water WAW1, WAW2 (δН = 1–2 ppm), the signal of the H-O-H…O(CH3)2 complex (δH = 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 (δН = 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.
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 [24]. For strongly associated water, the temperature dependences of the concentration of non-freezing water in silica/(water + oil) composites based on hydrophilic and hydrophobic silicas are shown in Fig. 3, 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 Fig. 3, c,d.
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 (Fig. 3a). In the case of hydrophobic silica, a significant part of the strongly associated water becomes weakly associated (Fig. 3b).
As can be seen from the data in Fig. 3, 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’s the other way around. CDCl3 medium stabilizes water clusters of smaller radius. In this case, the distributions ΔС(R) for both types of silicas become similar. In addition, the organic medium significantly changed the ratio of signal intensities in the NMR spectra (Fig. 3, 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 non-polar hydrocarbons [26, 27].
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 Fig. 2e 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 (Fig. 4).
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 (Fig. 4). 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.