some thoughts on the hydrophobic interaction p. pincus physics, materials, biomolecular science...
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Some Thoughts on the Hydrophobic Interaction Some Thoughts on the
Hydrophobic Interaction
P. PincusPhysics, Materials, Biomolecular Science &
EngineeringUCSB
“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
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What is it?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?
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OUTLINEOUTLINE
Interfaces – Patches
E. Meyer, Q. Lin, J. Israelachvili (Israelachvili Group)
A. Naydenov, P. Pincus
Molecules – H-Bonding Network Disruption
D. Hone, P. Pincus
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ISRAELACHVILI PROTOCOLISRAELACHVILI PROTOCOL
• Surface force apparatus with mica substrate
Mica is highlyHydrophilic and
Anionic – σ = 1e/nm2
100nm<h< 0.1nm
Passivate with cationic surfactant DODAB – Langmuir Deposition
Measure forces with SFA
Look at surfaces with AFM
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AFM IMAGESAFM 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
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UCSB AFM
Hansma Lab
Air
Water
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ForcesForces
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
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DISJOINING PRESSUREDISJOINING 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
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MONOLAYER INSTABILITYMONOLAYER INSTABILITY
Gain in water/oil surface energy is sufficient to overcome screened Coulomb attraction.
But why not complete segregation?
Counterion Release
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HOMOGENEOUSLY CHARGED SURFACEHOMOGENEOUSLY CHARGED SURFACE
Gauss’ Law => /4 eE
φ
x
Electrostatic Potential φ = T(x/λ)
Gouy-Chapman Length λ = (4πσℓ)-1
Bjerrum Length ℓ = e2/εT ≈ 0.6 nm in water
All counterions bound to sheath of thickness λ !
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ENTROPY DRIVEN PATCHESENTROPY DRIVEN PATCHES
φ(X)
XL
T(L/λ)Patch size given by balance of counterion release against line tension of patches.
Broad patch distribution L~λ ln[(ζ/T)(csλ2)-1]
ζ is line tension, cs is salt concentration
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PASSIVATED MICA VS BARE MICAPASSIVATED MICA VS BARE MICA
Experimental evidence for patch mobility!
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What is it?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?
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H-BONDING IN WATERH-BONDING IN WATER
V
U
O-- U > V
Polarizability of
OHOHOH 322
hydroxyl
hydronium
1014 ions/cm3 => U~20 kBT
t
H-bond energy ~ -t2/(2U) ~ 5 kBT
SP hybridization
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MOLECULAR HYDROPHOBIC INTERACTTION
MOLECULAR HYDROPHOBIC INTERACTTION
Non-H bonding impurity --- alkyl chain
Cost in H bonding energy = zt2/U Z is coordination number
Nearest neighbor impurities gain t2/U ~ 5 T!!!
Disruption of H-bonding network
Chandler et al
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TAKE-HOME MESSAGETAKE-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”….