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Solutions and Water Structure. Properties of solutions Water microstructure Solute microstructure Ionic solutes Polar solutes Nonpolar solutes (the hydrophobic effect). Acid base equilibria. AH. A - + H +. BH +. B + H +. Henderson Hasselbach. AH. A - + H +. AH.

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Presentation Transcript
slide2
Properties of solutions
  • Water microstructure
  • Solute microstructure
    • Ionic solutes
    • Polar solutes
    • Nonpolar solutes (the hydrophobic effect)
acid base equilibria
Acid base equilibria

AH

A- + H+

BH+

B + H+

slide6

AH

A- + H+

monosodium glutamate
Monosodium glutamate

pK=1.88

pK=3.6

pK=9.6

Monosodium glutamate (MSG) is a flavor enhancer. It is most active over the pH range 6-8 and decreases at lower values.

water structure
Molecular structure

Supramolecular structure

Solutes

Ionic

Polar

Nonpolar (the hydrophobic effect)

Water Structure
the hydrogen bond
The Hydrogen Bond

(http://www.martin.chaplin.btinternet.co.uk/molecule.html)

tetrahedral structure of water
Tetrahedral Structure of Water
  • The lone pairs and bonding electrons repel one another
  • The OH bonds are highly polarized
  • Strong H-bonds (~10% of covalent bond)
  • Each water molecule can hydrogen bond to two neighbors allowing the formation of an extensive 3D structure
  • http://wps.prenhall.com/wps/media/objects/439/449969/Media_Portfolio/Chapter_08/FG08_13.JPG
supramolecular structure of water
Supramolecular Structure of Water
  • Water is highly hydrogen bonded (only about 15% of H-bonds break on melting ice)
  • The bonds form and break dynamically
  • Strong affinity of water for itself
  • High specific heat (to warm water must break some H-bonds)
ionic solutes

O

O

O

O

O

O

O

O

O

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Ionic Solutes
  • Ion-dipole interactions
ionic solutes15

O

O

O

O

O

O

O

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Ionic Solutes
polar solutes
Polar Solutes
  • Hydrogen bonds
  • Dipole-dipole attractions
thermodynamics of binding
Thermodynamics of Binding

DG=DH-TDS

  • Entropy cost to solvent (DS<0, -TDS>0)
  • STRONG Enthalpy gain to solvent (DH<0)
  • Net DG<0; reaction proceeds
non polar solutes

O

O

O

O

O

O

H

H

H

H

H

H

H

H

H

H

H

H

Non-polar Solutes

OIL

non polar solutes19
Non Polar Solutes
  • Weak transcient dipole (Van der Waals attraction)
  • Strong structuring of water into a hydrogen bonded clathrate cage – entropy cost
the hydrophobic effect
The Hydrophobic Effect
  • Same number of oil molecules
  • Less surface oil molecules
  • Fewer order “clathrate cage” water molecules
  • DG<0, droplets tend to merge
small molecule surfactants
Small Molecule Surfactants
  • Micelles and membranes
  • Surface activity

Hydrophilic head group (charged or polar)

Hydrophobic tail (non-polar)

amphiphilic polymers
Amphiphilic Polymers
  • Spontaneous folding
  • Surface activity

Polymer backbone

Sequence of more water soluble subunits

Sequence of less water soluble subunits