Water near an extended hydrophilic surface

In nature, water is often found to occur in contact with interacting surfaces which are larger than a water molecule in size. Examples include protein and DNA molecules, silica and mica surfaces, and water in zeolite pores, to name a few. Recently studies have been initiated to understand the nature of such water from a microscopic perspective. We have already discussed proteins and DNA in detail in previous chapters. In this section, we shall cover a few interesting non-biological susbstances. 

The effects of the difference in hydrophobicity/hydrophilicity on water structure was captured in an elegant thought, or rather computer, experiment where the partial charges of surface atoms were tuned from zero to the normal charge distribution. When the partial charge is zero q = 0), then the surface behaves like an apolar/ hydrophobic surface. On the other hand, when the charge distribution is normal iq = 1), the surface is hydrophilic. In both scenarios, the water dynamics in the adjacent layer slows down but for quite different reasons [5]. We will discuss this important result in more detail. 

In the case of the hydrophilic sihca surface, it forms strong HBs with water molecules and the local density of water becomes high. 

It is important to note the difference that two confining surfaces makes to the structural arrangement of water molecules, even pretty far into the liquid, such as 6-8 layers. Such a long-range effect of the surface is not seen when there is only one hydrophilic surface. Clearly frustration is reinforced when the liquid is confined between the two interacting layers. 

Like silica, a mica surface also acts as a hydrophilic surface. Mica is a potassium salt of alumino-silicate. If one replaces some of the Si atoms from silica by Al, then because of the charge mis-match, the structure becomes negatively charged and the extra negative charge is compensated by ions. Like the silica surface, here also one finds that water molecules form hydrogen-bonding with the surface polar atoms and some of the ions fill the interstitial position in the water structure. The density of water also increases near the surface. 

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