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THE HYDROPHOBIC EFFECT

THE HYDROPHOBIC EFFECT. S is a non-polar solute. D G o > 0. S np → S w. At equilibrium. The “free energy of transfer”. Henry’s law SS. With Henry’s law SS as the choice, the free energy of transfer is the difference in solvation energy of S in the two solvents. OH. (CH 2 ) n. CH 3.

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THE HYDROPHOBIC EFFECT

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  1. THE HYDROPHOBIC EFFECT S is a non-polar solute DGo > 0 Snp → Sw At equilibrium The “free energy of transfer” Henry’s law SS With Henry’s law SS as the choice, the free energy of transfer is the difference in solvation energy of S in the two solvents

  2. OH (CH2)n CH3 CH3 (CH2)n COOH CH3 (CH2)n CH3 EXPERIMENTAL DATA FOR THE FREE ENERGY OF TRANSFER Alkanes Alkanols Fatty acids

  3. Calories/mole CH3(CH2)nCH3 2436 = excess free energy of 2 -CH3 groups over –CH2- group 2436/2 = 1218 Cal/mole = excess free energy of –CH3 The contribution of a –CH3 group must be 884 +1218 = 2102 Cal/mole Alkanes CH3-(CH2)n-OH -833 = excess free energy of 1 -CH3 group and 1 –OH group taking 1218 Cal/mole = excess free energy of –CH3 (from the alkanes) Alkanols

  4. Fatty Acids For n  8, the fatty acids are solids, and heptane was employed as a non-polar solvent

  5. Fatty Acids CH3-(CH2)-COOH Cal/mole (Nc does not include the COOH carbon)

  6. Fatty Acids Cal/mole CH3-(CH2)-COOH -3500 is the excess due to one –CH3 and the –COOH contribution Taking again the excess contribution of a –CH3 group to be 1218 Cal/mole Group contributions to the free energy of transfer

  7. The temperature dependence of the Free energy of transfer C2H6 (ethane) C6H6 (benzene) CCl4 (carbon tetrachloride) C6H6 or CCl4 C2H6 C2H6

  8. Solubility equilibrium of ethane between water and C6H6 or CCl4 1. 2. Similar for both solvents 3. DHo<0 4. DHo is temperature dependent; DCp for the transfer to water > 0

  9. TABLE 1 Thermodynamic Parameters for the Transfer of Ethane from Organic Solvents to Water • The thermodynamic “signature” of the hydrophobic effect • For the process Snp → Sw: • DGo > 0 • ΔHo is relatively small and can be either positive or negative • ΔSo < 0 and dominates the free energy change • ΔCp > 0

  10. Molecular origin of the hydrophobic effect A methane clathrate Structure of ice

  11. GENERALITY OF THE HYDROPHOBIC EFFECT Leucine(H2O) + Isoleucine(H2O) ‑‑--> (Leucine‑Isoleucine) + (H2O) DGo < 0 (about 25 Cal/A2 of contact area)

  12. SINGLE-CHAIN AMPHIPHILES Polar “head group” Hydrophobic “tail”

  13. Is the interior liquid or solid? How stable is the structure? What determines the shape of the micelle?

  14. SOLUBILIZATION OF NON-POLAR SOLUTES BY THE MICELLE INTERIOR Non-polar solute

  15. HOW STABLE IS THE MICELLE STRUCTURE? DGo

  16. FORMATION OF MICELLES: THE PHASE SEPARATION MODEL Amphiphile as micelle Amphiphile as monomer

  17. THERMODYNAMICS OF THE PHASE SEPARATION MODEL OF MICELLE FORMATION

  18. DETERMINATION OF THE CMC Insoluble dye Concentration of dye in solution CMC Total concentration of amphiphile

  19. CHAIN LENGTH DEPENDENCE OF THE CMC betaines glucosides Trimethylammonium bromides Nc = total carbons in the chain

  20. The equations of the best-fit lines are Alkyl betaines Alkyl glucosides Alkyl Trimethylammonium bromides Substituting in Alkyl betaines Alkyl glucosides Alkyl Trimethylammonium bromides

  21. (CH2)n CH3 Alkyl betaines Alkyl glucosides Alkyl Trimethylammonium bromides Intercept = (excess of –CH3) + (polar head group contribution) = 1218 cal/mol + (polar head group contribution) Alkyl betaines Alkyl glucosides Alkyl Trimethylammonium bromides

  22. HOW STABLE IS THE MICELLE STRUCTURE? Alkyl betaines Alkyl glucosides Alkyl Trimethylammonium bromides DGo

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