1. Some Thoughts on the Hydrophobic Interaction P. Pincus Physics, Materials, Biomolecular Science & Engineering UCSB “The magnitude, range and origin of the hydrophobic interaction have been a mystery ever since the pioneering work by Kauzman and Tanford…….” J. Israelachvili, 2005

2. What is it?  Strong short range (~ 1 nm) attractive force between hydrophobic surfaces in water (Tanford, Clausson, Wennerstrøm, F. Evans …….)  Why oil is insoluble in water. ARE THESE UNRELATED?

3. OUTLINE  Interfaces – Patches E. Meyer, Q. Lin, J. Israelachvili (Israelachvili Group) A. Naydenov, P. Pincus  Molecules – H-Bonding Network Disruption D. Hone, P. Pincus

4. ISRAELACHVILI PROTOCOL Surface force apparatus with mica substrate Mica is highly Hydrophilic and Anionic – σ = 1e/nm 2 100nm<h< 0.1nm  Passivate with cationic surfactant DODAB – Langmuir Deposition  Measure forces with SFA  Look at surfaces with AFM

5. AFM IMAGES Ch. Rotsch & Manfred Radmacher--LMU Patchy surface - nearly 50-50 mixture of bilayers and bare mica Broad distribution of patches– ten’s of nanometers

6. UCSB AFM Hansma Lab Air Water

7. Forces Representative data for the normalized force vs distance curves for two DODA monolayer-coated mica surfaces ( ○ ) and for a DODA surface and a bare mica surface (●). Consistent with 1/r at short distances

8. DISJOINING PRESSURE  Negative mobile holes  Positive bilayer matrix L ~ nm-μm Bloomfield –Rouzina Attraction on L Scale Range scales with L ~ 20 nm Coulombic correlation between positive and negative patches on opposing surfaces

9. MONOLAYER INSTABILITY Gain in water/oil surface energy is sufficient to overcome screened Coulomb attraction. But why not complete segregation? Counterion Release

10. HOMOGENEOUSLY CHARGED SURFACE Gauss’ Law => φ x Electrostatic Potential φ = T(x/λ) Gouy-Chapman Length λ = (4πσℓ) -1 Bjerrum Length ℓ = e 2 /εT ≈ 0.6 nm in water All counterions bound to sheath of thickness λ !

11. ENTROPY DRIVEN PATCHES φ( X) X L T(L/λ) Patch size given by balance of counterion release against line tension of patches. Broad patch distribution L~λ ln[(ζ/T)(c s λ 2 ) -1 ] ζ is line tension, c s is salt concentration

12. PASSIVATED MICA VS BARE MICA Experimental evidence for patch mobility!

13. What is it?  Strong short range (~ 1 nm) attractive force between hydrophobic surfaces in water (Tanford, Clausson, Wennerstrøm, F. Evans …….)  Why oil is insoluble in water. ARE THESE UNRELATED?

14. H-BONDING IN WATER V U O -- U > V Polarizability of hydroxy l hydronium 10 14 ions/cm 3 => U~20 k B T t H-bond energy ~ -t 2 /(2U) ~ 5 k B T SP hybridization

15. MOLECULAR HYDROPHOBIC INTERACTTION Non-H bonding impurity --- alkyl chain Cost in H bonding energy = zt 2 /U Z is coordination number Nearest neighbor impurities gain t 2 /U ~ 5 T!!! Disruption of H-bonding network Chandler et al

16. TAKE-HOME MESSAGE  Electrostatic coupling between charged domains in proteins and other biopolymer, membrane systems  Patches stabilized by counterion release  Hydrogen bond network disruption in molecular systems  No unique “hydrophobic interaction”….