Preamble
Proposal for the generation of osmotic pressure at aqueous interfaces
Overview of conventional colligative properties
Conventional vapor pressure lowering
Conventional freezing point depression
Conventional boiling point elevation
Conventional osmotic pressure
Much work has been done in the laboratory of Gerald Pollack (and confirmed by many other independent workers) concerning the mesoscopic properties of aqueous solutions next to surfaces [1328, 1740]. In essence, it is found that the interfacial water next to hydrophilic surfaces expels solutes to the bulk of the solution that may be several hundred microns away. These exclusion zones (named as EZ-water) can be visualized when low-molecular weight dyes, proteins, micron-sized microspheres or other solutes are used. With a laser tweezers system, the existence of force fields inside the solute-free exclusion zones have been found to reduce as a function of distance from the surface [1784]. Also the EZ-water seems to possess other physical properties such as absorption at 270 nm, greater density, greater viscosity and negative charge compared with the bulk water. It has also been experimentally verified that uncharged but highly hydrophilic nanoparticles, with high surface area, produce such a great osmotic pressure that it can be used in practical desalination processes [1768]. There is no generally-accepted explanation for these phenomena or properties. However, below I give a new explanation that is very simple, easily understood and potentially very important in a number of related fields.
Wherever water is present in solution it may be considered as being either 'bound' or 'free', although there will be a transitional water between these states. When considering the colligative properties, 'water' is considered bound to any solute when it has a very low entropy compared with pure liquid water. Such water is considered part of the solute and not part of the dissolving 'free' water. As pure liquid water consists of a mixture containing low-density water, made up of extensively hydrogen bonded structures, and higher density water, consisting of much smaller less extensive clusters, the proportions of 'bound' or 'free' water in pure liquid water can vary; the strongly-bound larger clusters behaving like 'bound' water. In bulk liquid water, the relative concentrations of the two aqueous forms is of no consequence as all the water behaves the same throughout. If volumes of the solution contain different proportions of strongly and weakly hydrogen-bonded water molecules (or even more simply that there is more extensive clustering present), then these different volumes will show a difference with respect to their water activity and chemical potential. Normally any such instantaneous differences in water activity and chemical potential between different volumes within the same mass of liquid would rapidly cause liquid movement from one to the other in order to equalize these states and so remove the potential differences. However, where there are surfaces interacting with the liquid water, the concentration of the more extensive hydrogen-bonded clusters may differ from the bulk values with the surface interactions preventing the potential equalization between bulk and surface volumes. When this occurs, the surface water has a different water activity and chemical potential to the bulk, leading to differences in osmotic pressure, and other colligative properties. The reduction in the chemical potential (μw) is -RTLn(xw) (that is, the negative energy term RTLn(xw) is added to the potential) where xw is the mole fraction of the ‘free’ water (0 < xw < 1).
As the ‘free’ water reduces as compared with its bulk value when the formation of longer-lived and more extensive hydrogen bonded clusters increases,b so the osmotic pressure increases. This increase in osmotic pressure next to the surface will displace solutes from the surface towards the bulk until its effect is equalled by the osmotic pressure of the solution. As the first effect of this solute expulsion is naturally the formation of an increased concentration band as expelled solute mixes with the prior solute concentration, the extent of the expulsion will affect the whole of the unstirred layer (~1-100 µ). It should be noted that osmotic drive does not require a membrane to separate the two solutions [1744] provided there are two phases (e.g. [1669]), here being the unstirred and stirred layers. In this context [1739], the affected aqueous layer behaves similarly to that described for exclusion zone (EZ) water by Pollack and may be a simple explanation of his experimental data [1740]. It also shows similarities with the experiments on autothixotropy [509]. The increase in density at the interface, as found in EZ water, has been explained previously by the increase in clustering causing the water to behave as though it is at a lower temperature, which also explains the ease with which this surface layer freezes. The presence of 270 nm absorption in the interfacial water, as described for EZ water [1328], has been ascribed to the delocalization of electrons within the extended clustering. As the cluster molecular-orbital LUMOs are huge, these electron delocalizations are stabilised by the addition of an electron but not by protonation, so causing the charge separation seen at these interfaces [1744].
Another effect of interfaces is the formation of evanescent waves due to the internal reflection of electromagnetic radiation. The standing electromagnetic wave produced will interact with water molecules to stabilise a standing wave of hydrogen bonded clusters that will alter the local concentration and extent of hydrogen bonded clusters so increasing the above osmotic effect, in agreement with the experimental data [1173, 1589, 1741].
It appears that a similar effect on solutes to the one described for water may occur in other polar solvents that can form hydrogen bonds [1742]; thus reinforcing the likelihood that a mechanism is acting that does not depend on the specific properties of water, such as the here-described colligative thermodynamics.
a This theory was first presented at the international conference 'Water and Nanomedicine', Academy of Sciences and Arts of Republic of Srpska, Banja Luka, Aug. 30, 2011 [1772]. [Back]
b This agrees with the earlier proposition for long-range water ordering involving the partial alignment of water molecules induced by the surface [1328]. [Back]
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This page was last updated by Martin Chaplin on 19 April, 2012
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