Solvation and hydrophobic forces

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Molecular ordering at surfaces

Israelachvili, Fig. 13.1

The liquid density at:

  • (a) The vapor/liquid interface.
  • (b) The solid/liquid interface.
  • (c) The solid/liquid/solid interface.

Solvation force - oscillatory

Israelachvili, Fig. 13.2

Note that the number of spheres in contact with the surface also varies with the maxima and minima.

The corresponding oscillatory solvation forces are shown in the lower graph.

Measured oscillatory forces

Israelachvili, Fig. 13.5

Forces between mica sheets separated by a liquid. The doted line is the theoretical calculation.

Not so good!!

Molecular “ordering” at a surface is a significant factor.

The hydrophobic efffect

Hydrophobicity refers to the physical property of a molecule that is repelled from a mass of water. Hydrophobic molecules tend to be non-polar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules in water often cluster together forming micelles. Water on hydrophobic surfaces will exhibit a high contact angle. Examples of hydrophobic molecules include the alkanes, oils, fats, and greasy substances in general. Hydrophobic materials are used for oil removal from water, the management of oil spills, and chemical separation processes to remove non-polar from polar compounds.

According to thermodynamics, matter seeks to be in a low-energy state, and bonding reduces chemical energy. Water is electrically polarized, and is able to form hydrogen bonds internally, which gives it many of its unique physical properties. But, since hydrophobes are not electrically polarized, and because they are unable to form hydrogen bonds, water repels hydrophobes, in favour of bonding with itself. It is this effect that causes the hydrophobic interaction — which in itself is incorrectly named as the energetic force comes from the hydrophilic molecules. Thus the two immiscible phases (hydrophilic vs. hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called phase separation.

Israelchvili, Fig. 13.12

Water near a “hydrophobic” surface re-arranges to maintain hydrogen bonding. This decreases the entropy and hence raises the free energy.

Most significant in molecular interactions:

  • (a) Solubility
  • (b) Micellization
  • (c) Association
  • (d) Protein folding

Less so in

  • (e) Adhesion
  • (f) Wetting
  • (g) Flocculation
  • (h) Flotation.

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