From what has been seen so far it is easy to see the polar associations of water molecules will hardly be maintained at temperatures above 100°C. In reality, the dimeric form also coexists in the vapor state at temperatures well above 100°C(1). But we are interested in what happens at temperatures below 100°C where, as the temperature decreases, increasingly more complex oligomeric forms begin to appear. In particular, the molecules can associate in single file to form progressively longer linear chains (aliphatic forms), or assume closed chain forms (associations of the aromatic type).
Without going into too much detail on the physics underlying this matter, the so-called “continuous model” (also called polymeric model) corresponds to knowing the structure of water under certain conditions of temperature and pressure and is equivalent to being able to answer the following points:
– Know the number of molecules involved respectively by zero, one, two, three or four hydrogen bonds
– Knowing how many of the molecules involved in the hydrogen bonds form open-chain or closed-chain oligomers
– identify the typological form according to the number of cycles or open chains
If we add the fact that everything is played in domains ranging in size from 3 to 20 Angstrons, it is easy to understand why we are far from being able to aspire to experimental confirmations while there are several valid numerical simulations.
The simulation of a few thousand molecules already requires significant computing resources; the simulation of an entire drop of water (consisting of about 10E21 molecules) would be much more expensive. The polymeric model considers liquid water as a network of more or less regular hydrogen bonds and responds very well to many purely chemical questions. valid numerical simulations.
This theory is preferred by chemists; physicists prefer the “discrete model” where water is made up of a myriad of microscopic icicles, each made up of no more than a hundred molecules and having a very short average life (the time between formation of the icicle and its dissolution is equal to about one pico-second). The reason for this preference is that this model, unlike the polymeric one, can be found experimentally through the use of infrared or Raman spectroscopy. In choosing between the two approaches we are therefore faced with the dilemma of a model – the continuous one – which cannot be tested but which manages to explain almost all the so-called “anomalous” behaviors of water, compared to the discrete model which can be found experimentally but which cannot explain many of the anomalous behaviors of liquid water.
However, let’s see the discontinuous model in a little more detail and check how it differs from the continuous model. Let’s start by trying to understand what ice exactly is and why it forms. As the temperature drops, the thermal energy is less and less able to contrast the aggregating effect of the hydrogen bonds which favor the formation of linear and cyclic polymers with a percentage increase of the cyclic form compared to the linear one as the temperature drops. In particular, in cold water the concentration of aggregates with five molecules (pentamers) and six molecules (hexamers) are predominant compared to aggregates with lower or higher numbers of molecules(2). Analyzing the ice by X-ray (or even better neutron) diffraction we find the typical hexagonal lattice; for obvious electrostatic reasons, the various layers could never be pentagonal, under penalty of instability of the crystalline structure. Therefore, below 10°C, the hexagonal cycles begin to aggregate with each other, expanding and forcing the pentagonal cycles to occupy the remaining interstices. Below 0°C the hexamers become predominant and tend to occupy all the spaces, forcing the few remaining pentamers to disintegrate, to “escape” from the crystalline matrix in formation and return to monomeric molecules. At this point it is also easy to understand why ice, being formed solely of hexagonal layers regularly superimposed on each other, is less dense than cold liquid water (i.e. at a temperature between 0°C and 4°C) where the gaps still incomplete hexagonal matrices are filled by pentamers(3).
The “microscopic icicles” model predicts a part of completely disordered molecules in which parts each formed by only a hundred completely ordered molecules in hexagonal form are included. It is difficult to explain how so few ordered molecules have the energy to tend to break up or, even worse, to aggregate new unrelated water molecules. Furthermore, since we are not dealing with a homogeneous medium, the energy would be exclusively of the interfacial type. However, these problems do not arise for the polymeric model as it is all homogeneously made up of polymers of various sizes, many of which are branched, both linear and cyclic.
Based on these considerations we tend to prefer the continuous model and in the next in-depth articles we will completely abandon the discrete model.
Then in a study of further detail we will be forced to abandon also the polymeric model to arrive at the interfacial water model(4).
Turin january, 24 2022
Gianfranco Pellegrini
Notes
(1) See note (11) of the previous article “Simple chemistry applied to water”.
(2) The graph below indicates the concentration of the various polymers in cold water. As can be seen,
pentamers stand out with a concentration close to 40% and hexamers with a concentration of around
25%.
3) Anyone who lives in cold places knows that to prevent the water pipes from breaking, it is necessary to prevent freezing. In reality, it is not so much the freezing that breaks the pipes, but the increase in specific volume (i.e. the reduction in density). Given that the maximum is obtained at 4°C when the water is still liquid, it is incorrect to say that freezing must be avoided. In reality it is expected to keep the temperature of the water in the pipes above 4°C but most people think that this is just a safe temperature above the risk zone. Well, if you want to maintain a safety zone you must still keep it above 4°C (e.g. 7°C or, even better 9°C) which is what you do when you set the anti-ice function in the heating systems in the mountains.
4) The so-called “polywater” model has not brought much success to the in-depth studies of this precious substance. In fact, the so-called “polywater disaster” was one of the two incidents (the other being the famous “Benveniste affair”), which prevented scientists from progressing in water studies. In the 1960s, the Russian chemist Nikolai Fedyakin discovered that water, under certain conditions, became difficult to freeze and/or vaporize. He turned to the greatest Russian chemist of the time (Boris Derjaguin) who was fascinated by the discovery and dedicated an entire research team to deepening this phenomenon. The desire for easy sensationalism on the part of an incautious journalist led him to write that a drop of polymerized water dropped into the ocean would lead to the polymerization of all the water in the globe, transforming it into a useless jelly. The Cold War did the rest: let’s not forget that it was the time when the USA and the USSR were competing to excel and the USSR had already scored some important coups in the aerospace field. The “polywater” became so ridiculous that Derjaguin himself was forced to give in.
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