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Solutions and Water Structure

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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Solutions and Water Structure

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  1. Solutions and Water Structure

  2. Properties of solutions • Water microstructure • Solute microstructure • Ionic solutes • Polar solutes • Nonpolar solutes (the hydrophobic effect)

  3. Acid base equilibria AH A- + H+ BH+ B + H+

  4. Henderson Hasselbach AH A- + H+

  5. AH A- + H+

  6. 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.

  7. Soy protein solubility

  8. Molecular structure Supramolecular structure Solutes Ionic Polar Nonpolar (the hydrophobic effect) Water Structure

  9. Water Molecular Structure O ~1 A H H 104°

  10. The Hydrogen Bond (http://www.martin.chaplin.btinternet.co.uk/molecule.html)

  11. 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

  12. 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)

  13. 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

  14. O O O O O O O H H H H H H H H H H H H H H Ionic Solutes

  15. Polar Solutes • Hydrogen bonds • Dipole-dipole attractions

  16. 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

  17. O O O O O O H H H H H H H H H H H H Non-polar Solutes OIL

  18. Non Polar Solutes • Weak transcient dipole (Van der Waals attraction) • Strong structuring of water into a hydrogen bonded clathrate cage – entropy cost

  19. The Hydrophobic Effect • Same number of oil molecules • Less surface oil molecules • Fewer order “clathrate cage” water molecules • DG<0, droplets tend to merge

  20. Small Molecule Surfactants • Micelles and membranes • Surface activity Hydrophilic head group (charged or polar) Hydrophobic tail (non-polar)

  21. Amphiphilic Polymers • Spontaneous folding • Surface activity Polymer backbone Sequence of more water soluble subunits Sequence of less water soluble subunits

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