Structure of Water and

In a water molecule the two H atoms form an almost tetrahedral angle with a strong dipole moment. A shell of about four water molecules exists at a distant of 28 nm, followed by a second at approximately

45 nm, and beyond 80 nm no shell can be identified. In addition to this short range order, a network of hydrogen bonds exits with a very short lifetime of 10~12 to 10~13 s.

In sub-cooled, very pure water the nucleation of ice crystals (homogeneous nucleation) starts around

— 41°C. Normally water contains about 106 particles which act as nuclei for crystallization (heterogeneous crystallization). They become more effective as their structure approaches that of ice. Ice crystals exist in nine forms, of which the cubic and hexagonal at under atmospheric pressure. The growth rate of crystals depends on the diffusion of molecules to the nuclei, on finding a suitable place, and on the transportation of the freed energy to the heat sink. With extreme cooling rates (of the order of 105°C min-1), crystallization can be avoided and water solidifies into an amorphous, glass-like phase. Figure 1 shows that the amorphous phase of ice is only stable below

— 160°C; in the range — 160°C to — 130°C amorphous and cubic phases can exist, and between — 130°C and — 65°C all the forms can be present, depending on the speed of warming. It is technically impossible to cool pure water quickly enough, to produce glassy ice; even small droplets (1 mm) injected into liquid nitrogen may freeze at a rate less than 103 °C min-1. The freezing behaviour of water changes completely if other substances are present in the water, e.g. cryoprotective agents (CPAs). The most widely used CPAs are:

• protein: human serum albumin, gelatin

• amino acids: glycine, arginine, alanine

• alcohols: mannitol, polyethylene glycol (PEG)

• carbohydrates

- monosaccharides: glucose, fructose

- disaccharides: lactose, maltose, sucrose, trehalose

— polysaccharides: dextran, cyclodextrins

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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