Water and the Energy of Interaction of Solutes

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Water and the Energy of Interaction of Solutes
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Water is an unusual low molecular weight compound

• M 18. cf with ammonia (17), methane (16),
  hydrogen sulfide (34)
• Water has the highest boiling point by over 100?
  C (H2S -61?)
• Water has the highest dielectric constant ? (80)
• Water has a solid density lower than the liquid
  (9).
• Water has a high viscosity
• Water has a high surface tension

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Most of waters properties stem from

• Its polarity (Exemplified by ?)
• High ? reduces the attraction force between
  oppositely charged particles Fk(q1?q2)/ ?r2
• Its hydrogen bonding ability
• Directionality ?? Structure (even in liquid
  state)
• Acts as both donor and acceptor

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Hydrogen bonding

• Partially covalent and partially electrostatic
• Interatomic approach of H and acceptor is LESS
  than van der Waals distance but much more than
  covalent distance
• O????H 0.19 nm, not 0.26 (vdW), but O H is
  0.10 nm
• Interaction energy is much less than that of
  covalent bonds.

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Energy of interactions
Bond Energy
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Noncovalent Interactions

• charge-charge
• longest range force, non-directional
• ?? 1/r
• charge-(permanent)dipole
• directional dependence
• ?? 1/r2
• Dipole-dipole
• directional
• ?? 1/r3

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Noncovalent Interactions

• Charge-induced dipole
• depends on polarizibility of inducee
• ?? 1/r4
• Dipole-induced dipole
• depends on polarizibility of inducee
• ?? 1/r5
• Dispersion
• involves synchronization of fluctuating polarity
• ?? 1/r6

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Noncovalent Interactions

• van der Waals repulsion
• Overlap of outer electron clouds
• ?? 1/r12

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The Solvent Water

• Hydration (polarity effect)
• Waters charged ends associate with charged
  solutes
• Exchange of intramolecular/intrasolute H-bonds
  for intermolecular H-bonds with water

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Exchange of H-bonds
H
H
?G lt 0
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The organization of aqueous compartments by
hydrophobic entities

• (It is said that) Water molecules are organized
  by hydrophobic surfaces, producing a decrease in
  entropy in the system (and an increase, by
  release of heat, in the universe).
• The association of hydrophobic molecules/parts of
  molecules is energetically advantageous, because
  more water-water interaction is possible than if
  the molecules are separated in space

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Amphipathic structures (micelles)
2-D Micelle
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Amphipathic structures figure 12-4
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Chemical Reactions in Water