Structural Biochemistry/Unique Properties/Versatility as a Solvent

Overview
Water is a polar solvent that tends to not react with the solute. Since many biomolecules are either polar or charged, water readily dissolves these hydrophilic compounds. Water is an inert solvent that solvates salts by separating the cations and anions, therefore reducing their electrostatic interactions and abilities to form crystals. This may occur because the water-water hydrogen bonds are replaced with water-solute bonds. On the other hand, water is a poor solvent for hydrophobic molecules such as lipids. The non-polar molecules in water undergo hydrophobic interactions; the water changes its hydrogen bonding patterns around the hydrophobic molecules. The bonding patterns that form constrains the water molecule which decreases entropy.

The need for water to have high entropy helps to promote protein folding. When the hydrophobic portions of the protein are exposed to the water, more of the water needs to be organized around it, decreasing entropy. When the protein is folded (a decrease in entropy that is overcome by favorable non-covalent bond formation and increased water entropy), the hydrophobic parts have less surface area in contact with water, freeing up the water to favorably make and break hydrogen bonds.

•Molecules such as butanol have a polar hydroxyl group and thus can from intermolecular hydrogen. Alcohols, aldehydes, ketones and compounds containing N-H bonds all form hydrogen bonds with water molecules and tend to be soluble in water. •Water is effective in screening the electrostatic interactions between dissolved ions because it has a high dielectric constant, a physical property that reflects the number of dipoles in a solvent. •The strength of the ironic force F= (Q1 Q2)/ er ², for water at 25 ºC, e is 78.5. Ionic interactions between dissolved ions are much stronger in less polar environments. •Oxygen and nitrogen and carbon dioxide gases are all nonpolar. they are not very soluble in water. •Non polar compounds force energetically unfavorable changes in the structure of water.

Hydration Process
Water involves the hydration process. The “positive ends” of H2O are attracted to the negatively charged anion and that the “negative ends” are attracted to the positively charged cations. Salt dissolves in water by the hydration process. The strong water-ion interaction takes over the force of the positive and negative ions of the solid. When ionic substances (salts) dissolve in water, they break up into individual cations and anions. For example, when sodium chloride (NaCl) dissolves in water, the resulting solution contains Na+ and Cl- ions moving freely.

Reference
Zumdahl/Zumdahl, ''Chemistry, 7ed. Houghton Mifflin Company, Boston New York''