Water-alcohol solutions have been studied for quite a long time. For example, we owe the availability of accurate data on their density at different temperatures to the great Russian scientist Dmitry Ivanovich Mendeleev. His findings formed the basis of alcoholometric tables in many countries of the world (1). More than one hundred and fifty years have passed. Nevertheless, despite its simple composition (only two low-molecular components: water and alcohol), the nature of intermolecular bonds and interactions in water-alcohol solutions (WASs) has not been fully researched.
It is well known that various anomalies are inherent in the physical and chemical properties of WASs (1, 4, 5, 8, 9, 13). For example, when ethyl alcohol is mixed with water, a decrease in the volume of the solution is observed (50ml water and 50ml ethanol produce about 97ml of solution). At the same time, when alcohol concentration is increased, the intensity of such a compression first increases (alcohol concentration peaks at about 54%), and then decreases (1). When water and alcohol are mixed, heat is released, and the maximum mixing heat is observed at an alcohol concentration of 36% (1). The boiling point also sharply deviates from the norm when mixing alcohol with water (1). This behaviour of alcohols (particularly ethyl alcohol) when mixed with water has attracted the attention of researchers. Using the latest analytical approaches, physicists and chemists from different countries are studying the general and internal structure of various WASs under various conditions.
The formation of water-alcohol solutions is the process of mixing water and alcohol (we will hereinafter assume alcohol to mean ethanol). Many experimental studies have discovered that alcohol and water do not mix entirely, and the properties of such mixtures are different from ideal solutions (1, 4, 5, 8). Intermolecular bonds and interactions in WASs are complex. Ethyl alcohol and water are capable of forming hydrogen bonds (1, 5). Water molecules are polar: each molecule can form four hydrogen bonds (two uncompensated positive charges on the hydrogen atoms and two negative charges on the oxygen atom) (1, 5). Due to the presence of hydrogen bonds, water molecules are oriented relative to each other and form a three-dimensional network consisting of associates (1, 4). Since the energy of such bonds is small, associates can decay and re-form in various combinations. It has been demonstrated that under different conditions, a corresponding dynamic equilibrium is established in liquid water between ordered and disordered structures (associated and non-associated molecules) (1, 17).
Ethanol molecules resemble the two-faced god Janus. Like water molecules, they can form hydrogen bonds (although only two). They have a polar hydrophilic (hydroxyl residue, -OH) part on the one hand, and a non-polar hydrophobic (alkyl, CH3-CH2-) group on the other (4, 8, 9, 11). Due to these features, associates can also form in ethyl alcohol (1, 8). What is more, when alcohol and water are blended, mixed associates also arise, namely water-water, ethanol-ethanol, and ethanol-water pairs of molecules connected by hydrogen bonds. Since alcohol molecules are larger than those of water, the incorporation of such molecules disrupts the structure of water associates. Moreover, the incorporation of water molecules into the structure of alcohol does not cause it to change significantly. When a small amount of alcohol is dissolved in water its structure changes slightly. However, when the concentration of alcohol is increased, it is disturbed. In the medium alcohol concentration range, a dynamic equilibrium is established between associates of identical molecules and aggregates of different molecules. A stable, more ordered system is formed. When the concentration of alcohol in the solution is high, the structure of alcohol with water molecules included in it prevails (1, 4, 5, 8, 11, 12).
Change in the structure of a water-alcohol solution depending on alcohol concentration. At a concentration of 20%, the energy of hydrogen bonds is at its highest, and the greatest structuredness of the solution is observed, as is the formation of clathrate structures.
A more detailed explanation is in the text. Source: (Dolenko et al., 2015)
Thus, various associative processes occur in WASs, depending on the composition and external conditions. It has been established that various structures are formed in these systems based on the components of the WAS. However, some mechanisms of how these structures form, as well as the nature of their interactions, remain unclear.
Anomalies of the 'water-ethanol' binary system at different concentrations have long been a subject of interest to scientists. There is an abundance of experimental and theoretical research into the nature of such anomalies. Let us consider some of the studies devoted to aqueous solutions of ethyl alcohol that have been published over the past few years.
Studying the change in the strength of bonds between homogeneous and heterogeneous components of WASs with a change in the concentration of alcohol in the solution can help determine the internal structure of the solution, and consequently its properties. Using Raman spectroscopy, Russian scientists from Lomonosov Moscow State University performed a detailed analysis of the strength of hydrogen bonds in WASs of different concentrations (20%, 70% and 96%) at various temperatures (-10 to 70°C) (4). Their calculations showed that the energy of hydrogen bonds was highest in a 20% ethanol solution. A strengthening of hydrogen bonds was observed both between water molecules and between water and ethanol molecules. It is well known that when the concentration of ethanol in water changes, the nature of intermolecular interactions in the components of the solution also changes. The authors adhere to the hypothesis that due to the hydrogen bonds and hydrophobic interaction caused by alkyl groups of ethanol molecules, at a certain concentration clathrate structures (networks of water molecules surrounding small non-polar groups of alcohol) can form in WASs. The authors believe that the enhancement of hydrogen bonds which they found at an alcohol volume concentration of 20% confirms the hypothesis that clathrate-like structures with a strong hydrogen bond exist in aqueous-ethanol solutions of a certain concentration (4). Also using Raman spectroscopy, a group of Chinese scientists found that ethanol enhances hydrogen bonds in water. Similar processes occur when water is frozen. In other words, in a 20% alcohol WAS, its structural organisation changes, and the system undergoes a phase transition (11).
Scientists from Japan recently received slightly different data. They carried out a detailed analysis of molecular interactions in WASs at a variety of concentrations (from 0 to 100%) and found that the strongest hydrogen bonds are observed in solutions with a concentration of 40-60%. At an ethanol concentration below 5%, the main form of intermolecular interaction in the WAS is hydrophobic hydration, and at a concentration above 5% - hydrogen bonds. At a concentration range of 10-40%, ethanol mainly interacts with water molecules, while with a further increase in concentration, interaction with other ethanol molecules begins to prevail. At concentrations above 60%, water-ethanol clusters decompose (5).
Two groups of scientists from Denmark (3) and Germany (13) studied the hydrogen bonding of water and ethanol clusters using various spectroscopic methods. Both groups showed in their work that ethanol is a much stronger acceptor of a hydrogen bond compared to water (water is a hydrogen bond donor not because it is superior to ethanol in this quality, but because it is a worst acceptor). In addition, water forces ethanol to transform into a less stable ‘skewed’ or gauche conformation by virtue of a complication of the structure – the dimerisation of alcohol molecules (3, 13).
Another important topic in research into WASs is the study of formation and interaction of homogeneous and heterogeneous associates by methods of molecular dynamics. For example, a group of scientists from Italy attempted to study the mechanism of ethanol and water cluster formation in WASs with a concentration of between 5% and 40% using time-of-flight mass spectrometry (12). The clusters they found were larger than those which had been discovered previously. The scientists demonstrated that with an increase in the concentration of ethanol, the cluster structure of water formed by a network of hydrogen bonds is disturbed, since ethanol molecules are capable of replacing interaction with water aggregates. However, up to a certain concentration, the association of ethanol molecules with each other prevails over this process. An increase in ethanol concentration promotes the formation of hydrated ethanol clusters (12). The complex mechanism of ethanol and water clustering in WASs was also confirmed by Canadian scientists using a method of IR spectroscopy of WAS microvolumes they developed using photo-thermal microfluidic cantilever deflection spectroscopy (7).
Molecular dynamics methods were used to investigate the heterogeneity of ethanol and water cluster distribution, the effect of temperature on the formation of associates, and the degree of hydrophobic interaction in WASs. Similarly, scientists from Hungary conducted an analysis of WAS lacunarity in concentrations from 0% to 100%, studying two-component and one-component clusters based on hydrogen bonds. The findings showed that the distribution of one-component clusters (especially ethanol) at a low ethanol concentration can be characterised by a multifractal distribution and, in most cases, these clusters are not isolated, but form 'islands' in a space consisting of two-component clusters (6).
Scientists from India have demonstrated how the degree of structural heterogeneity for low-concentration (up to 35%) WASs depends on temperature (8). Using computer simulations, they established that lowering the temperature leads to an aggregation of ethanol clusters even in the lowest concentration WASs. Ethanol clusters are short-lived, but a decrease in temperature causes them to stabilise. The authors suggest that this can be mediated by multiple hydrophobic interactions between ethyl groups (8). Another team of Indian scientists also used molecular dynamics to prove that the degree of hydrophobic interaction within WASs actually increases with an increase in ethanol concentration, as far as 25% (9).
Japanese scientists extensively studied the dependence of WAS surface structure on bulk structure (18). Analysing extensive data in the literature, they tried to explain the anomalous changes in the surface tension of WASs with a change in alcohol concentration by means of changes in the internal structure of the solution. It is known that the addition of even a small amount of ethanol to water reduces its surface tension, and when more ethanol is added it continues to decrease before increasing again. The authors suggest that this phenomenon is closely related to a change in the bulk structure of WASs: first, hydrogen bonds between water molecules are locally enhanced due to hydrophobic hydration, the surface concentration of ethanol increases, and it forms a monolayer (surface tension decreases). A further increase in ethanol concentration leads to an increase in the size of ethanol and water associates and the destruction of the three-dimensional network of hydrogen bonds in the solution, causing a subsequent decrease in the surface concentration of ethanol and an increase in surface tension (18).
Another area of WAS research is the analysis of nanostructure formation in such systems (2, 10, 16). Some scientists believe that when alcohol and water are mixed, stable bulk nanobubbles (nano-objects filled with gas) are formed, since the solubility of air in WASs is lower than in either water or ethanol on its own (10, 16). However, other scientists who also register the appearance of nanostructures when mixing water and ethanol question whether these structures are nanobubbles (2).
It is noteworthy that the strength of hydrogen bonds has not only been studied in WASs prepared in the laboratory with ethanol and water. Japanese scientists conducted research and aggregated data for popular alcoholic beverages, such as whisky, sake, shōchū (a Japanese spirit stronger than sake) and alcoholic fruit-based cocktails (14, 15). The scientists establish that hydrogen bonds are enhanced in alcoholic beverages due to original ingredients or those added during their preparation (for example, aging whisky in oak barrels). Moreover, the strengthening of hydrogen bonds depends on the amounts of these components (both acidic and basic). It is higher, for example, in whisky aged in oak barrels than in shōchū. This, in turn, improves the organoleptic properties of the drink, reducing the characteristic odour and 'taste' of ethanol (14,15).
WASs are therefore incredibly complex systems. Depending on composition and external conditions, various associative processes take place, due to which bonds between identical molecules are established, complexes of heterogeneous molecules are formed, and interaction between these complexes and individual water and alcohol molecules occurs. When the composition of the solution and/or external conditions change, the associates rearrange, and the strength of the various bonds changes, leading to a change in the physical properties of the system as a whole. Similar principles underlie the structure of many supramolecular systems, including living systems. It would seem that the phrase in vino veritas is taking on a whole new meaning.
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