AK Lecture properties of water and hydrophobic effect

Polarity (of water)

Uneven charge distribution caused by bent shape and oxygen's high electronegativity; gives water its dipole moment.

Hydrogen bond

Weak electrostatic attraction between a slightly positive H (on O or N) and a slightly negative O or N; gives water its cohesive properties.

Cohesion

Attraction between water molecules due to hydrogen bonding; responsible for surface tension and transpiration pull.

Adhesion

Attraction between water molecules and polar surfaces (e.g., xylem walls); assists capillary action in plants.

Specific heat

Amount of heat required to raise 1g of a substance by 1°C; water's high value buffers temperature changes.

Heat of vaporization

Energy needed to convert liquid to gas; water's high value makes evaporative cooling (sweating) effective.

Density anomaly of water

Ice is less dense than liquid water due to an open H-bond crystal lattice; ice floats, insulating aquatic ecosystems.

Hydration shell

A layer of water molecules surrounding a solute ion or polar molecule, oriented by partial charges to dissolve the solute.

Hydrophilic

Water-loving"; describes polar or charged molecules that readily dissolve in water.

Hydrophobic

Water-fearing"; describes nonpolar molecules that are poorly soluble and tend to aggregate in water.

Hydrophobic effect

Entropy-driven aggregation of nonpolar molecules in water; caused by the release of low-entropy clathrate cage water

Clathrate cage

Ordered, ice-like water structure encasing a nonpolar molecule; its low entropy makes nonpolar dissolution unfavorable.

Entropy (ΔS)

Measure of disorder; the hydrophobic effect is driven by a large increase in water's entropy upon cage release.

Amphipathic

A molecule containing both hydrophilic (polar) and hydrophobic (nonpolar) regions; e.g., phospholipids, bile salts.

Phospholipid bilayer

Spontaneously formed double layer of amphipathic phospholipids in water; hydrophobic tails inward, heads outward. Basis of cell membranes.

Micelle

Spherical amphipathic aggregate with hydrophobic core and hydrophilic surface; formed by detergents and fatty acid salts via the hydrophobic effect.

Protein folding (hydrophobic driving force)

Spherical amphipathic aggregate with hydrophobic core and hydrophilic surface; formed by detergents and fatty acid salts via the hydrophobic effect.

Gibbs free energy equation

ΔG = ΔH – TΔS; a reaction is spontaneous when ΔG < 0. Hydrophobic aggregation is driven by a large positive ΔS term.