Researchers worldwide agree that common water, as we know it, is a complex system, which is inhomogeneous in bulk and at the surface. Water is distinct from most substances that are liquid under normal conditions because its atoms have a large number of local “orientational positions”, which is attributed to rather low density of water [4]. This results in the formation of ordered structures capable of affecting water properties. In this respect, the properties of “surface” water are of particular interest. This refers to interfacial water that interacts with different substances (for example when they dissolve in it) [1].

Water structures known from the literature can be roughly classified into two types: clusters (or clathrates) and nanosized bubbles (nanobubbles). While the former principally consist of water molecules, the second, apart from these, also contain gas molecules. There is also an intermediate hypothesis that nanobubbles can coagulate into water clusters [29].

The capacity of forming ordered structures (like the above-mentioned clusters) applies to even ordinary distilled water, i.e. water itself is the basis for the assembly process [15], [29]. Besides, other substances present in the system are also a factor contributing to the above-mentioned process. The influence of such substances is essentially determined by their solubility, and can substantially affect the water molecular organization and, therefore, properties [1], [11], [23], [24]. Formation and disintegration of supramolecular moieties occurs over time [1], [19], [29]. Thus, investigations on aqueous solutions should primarily involve the use of methods that are able to detect changes in the physicochemical or biological properties of water in the course of time.

Spontaneous process and external influences

Although, as mentioned above, structures are assembled spontaneously in water, this process can be influenced externally as well [6], [12], [13], [16], [22], [26], [29]. However, it should be noted that self-assembled structures appear to be more stable [29].

When water is treated mechanically, its hydrodynamic properties should be taken into account [9]. The generation of stable spiral waves in water significantly affects the properties of liquid. It has been reported that mechanical treatment leads to the formation of micro- and nanobubbles, which are able to generate free radicals [10], [11], [13], [21]. While the lifetime of nanobubbles is a few days [26], other nanostructured ensembles are stable during the period from a few minutes to several weeks [29].

Mechanical treatment (e.g. succussion) exerts a stabilizing effect on the structures formed in water. Thus, clustering efficacy increases due to succussion, which corresponds the similar vector to molecular vibrations, whereas without it molecules move chaotically [14]. Furthermore, secondary treatment of water (secondary mechanical treatment in particular) promotes nanobubble regeneration (i.e. the nuclei of such structures remain stable for at least several months) [11]. Similar regeneration is also induced by the addition of gas to a solution, which has initially been filtered to remove nanobubbles [12], [29].

Electromagnetic radiation can also be used to stimulate water molecules to organize into ordered layers [18], [28], [29].

Near-surface layer

The surface layer of water (the so-called “exclusion layer”) is of special interest. Particles in the surface layer behave according to laws different from those in bulk water. Since the atoms are more closely packed, supramolecular structures being formed are particularly stable [4], [7], [12], [18], [19], [28]. Thus, the presence of a hydrophobic surface facilitates the formation of ordered ensembles [26], [28] ensuring their form and symmetry [7], [11], [23]. Surfactants lower the hydrophobic properties and stability of the structures [26]. Meanwhile, there are literary data on positive effects of a hydrophilic surface on the formation of structured gel-like water layer [18], [19], [29]. Water exhibits its unique properties both in contact with hydrophobic and hydrophilic surfaces, but these properties vary depending on the surface itself [28].

The surface layer should be considered with respect to additional negative charge and the entropy factor [3], [8], [23], [28]. Structure and form generation in water is associated with the most energetically “favorable” position [3], [28].

Apart from the near-surface layer of water, that of the other interacting substance also matters considerably contributing to the formation of interfacial water structures (for example, anomalous behavior on silica surfaces) [3], [19], [28].

Dr. Gerald Pollack has discovered a new aggregate state of water, the so-called "Exclusion Zone Water" (EZ Water), which has a form of liquid water crystalls.

Alongside the near-surface layer of water, attention should be paid to microdrop clusters, the structures forming above the water surface. Such clusters are relatively stable (can persist for several weeks or even months) and are capable of catalyzing various chemical reactions [14]. Importantly, clusters should be considered over time with taking into account both charge and entropy factors [5].

Hydrogen bonds

When determining structural composition and properties of water, particular attention should be paid to research of hydrogen bonds, which are of different strength in surface water forms and bulk water. Hydrogen bonds weaken in the direction from surface to bulk [1], [7], [23]. The system of hydrogen bonds makes surface layer structures strong enough to have a long lifetime [6], [10], [23]. Competing with kinetic effects, hydrogen bonds provide stability to ordered forms [23].

The hydrogen bonds between the water molecules is indicated by black dotted lines. The yellow lines denote a covalent bond that holds oxygen (red) and hydrogen (gray) atoms together.

Interestingly, when an electric field is applied, it “opens” ring clusters and forms linear, branched structures; i.e., the number of hydrogen bonds decreases with rising field intensity, and the water structure collapses. At the same time, exposure to a magnetic field increases the strength and stability of hydrogen bonds in supramolecular clusters preventing the diffusion of water molecules [28].

The more closely the molecules are packed in the near-surface layer, the more stable and long-lived the hydrogen bonds are [7]. Hydrogen bonds are stronger in larger clusters [25], which in turn increase in size when the temperature falls, and the water structure becomes ice-like [27], [30]. Hydrogen bonds in clustered water structures can be stabilized using guest gas molecules [24]. Different authors have reported varying effects of temperature on hydrogen bonds. In alcoholic solutions, the strength of hydrogen bonds is proportional to the alkyl chain length – the greater the length is, the higher the dissociation energy becomes (the stronger the bond is) [16].

Effects of solvent

When mixed together, the molecules of water and of a solvent (e.g. ethanol) can form new ordered structures, which are easy to detect by various techniques [6], [11], [12], [17], [20], [22]. Water-alcohol aggregates are larger than water ones [22]. Yet it should be noted that water-alcohol mixtures produce several types of clusters with varying characteristics [20]. Some experiments have demonstrated that this kind of clusters is most stable in solutions containing 60% (v/v) of ethanol [17].

Small organic molecules (including those of ethanol) play an equally important role in formation of nanobubbles and ensuring their stability. Due to the presence of such components in the system, the pressure inside the nanobubbles decreases and their stability rises [12]. However, other authors have concluded that the structures formed in more polar solvents prove to be smaller and less stable [8], [11].

Similarly to the solvent factor, the effect of heating has been described in different ways: some peer-reviewed sources, for example, suggest that heating promotes structure formation in water-alcohol mixtures [6], while others report that such structures (particularly nanobubbles) destroy when heated [11], [23].

Conclusion

Ordered structures can be formed in water or aqueous solutions. Such structures should be considered within the systems where they are generated, i.e. water clusters, solute molecules, gas and impurity components, and the surface involved in the formation of ordered structures. Self-assembly of the entire system is mediated by more favorable energy and spatial factors. The process can also be influenced by a number of manipulations. It should be emphasized that the presence of dissolved gas facilitates the formation of nanostructures. The structures themselves can be stabilized by the formation of a large number of weak hydrogen bonds.

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