Some Thoughts on the Hydrophobic Interaction P. Pincus Physics, Materials, Biomolecular Science &...
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Transcript of Some Thoughts on the Hydrophobic Interaction P. Pincus Physics, Materials, Biomolecular Science &...
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
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?
OUTLINEOUTLINE
Interfaces – Patches
E. Meyer, Q. Lin, J. Israelachvili (Israelachvili Group)
A. Naydenov, P. Pincus
Molecules – H-Bonding Network Disruption
D. Hone, P. Pincus
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
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
UCSB AFM
Hansma Lab
Air
Water
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
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
MONOLAYER INSTABILITYMONOLAYER INSTABILITY
Gain in water/oil surface energy is sufficient to overcome screened Coulomb attraction.
But why not complete segregation?
Counterion Release
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 !
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
PASSIVATED MICA VS BARE MICAPASSIVATED MICA VS BARE MICA
Experimental evidence for patch mobility!
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?
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
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
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”….