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
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?
OUTLINE
 Interfaces – Patches
E. Meyer, Q. Lin, J. Israelachvili (Israelachvili Group)
A. Naydenov, P. Pincus
 Molecules – H-Bonding Network Disruption
D. Hone, P. Pincus
ISRAELACHVILI PROTOCOL
•
Surface force apparatus with
mica substrate
100nm<h< 0.1nm
Mica is highly
Hydrophilic and
Anionic – σ = 1e/nm2
Passivate with cationic surfactant DODAB – Langmuir Deposition
Measure forces with SFA
Look at surfaces with AFM
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
UCSB AFM
Air
Hansma Lab
Water
Forces
0
4
0.01
0.1
-F/R (mN/m)
-Force/Radius, -F/R (mN/m)
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 (●).
8
1
10
12
100
0
200
400
600
800
1000
D (Å)
0
Consistent with 1/r at short distances
200
400
600
800
Separation Distance, D (Å)
1000
DISJOINING PRESSURE
Negative mobile holes
Positive bilayer matrix
L ~ nm-μm
Coulombic correlation between positive
and
negative patches on opposing surfaces
Bloomfield –Rouzina Attraction on L Scale
Range scales with L~ 20 nm
MONOLAYER INSTABILITY
Gain in water/oil surface
energy is sufficient to
overcome screened Coulomb
attraction.
But why not complete segregation?
Counterion Release
HOMOGENEOUSLY CHARGED SURFACE
Gauss’ Law
φ
=>
E  4e / 
Electrostatic Potential φ = T(x/λ)
Gouy-Chapman Length λ = (4πσℓ)-1
Bjerrum Length ℓ = e2/εT ≈ 0.6 nm in
water
x
All counterions bound to sheath of thickness λ !
ENTROPY DRIVEN PATCHES
φ(X)
Patch size given by
balance of counterion
release against line
tension of patches.
T(L/λ)
L
Broad patch distribution
L~λ ln[(ζ/T)(csλ2)-1]
ζ is line tension, cs is salt concentration
X
PASSIVATED MICA VS BARE MICA
Experimental evidence
for patch mobility!
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 WATER
V
Polarizability of
O--
t
U > V
SP hybridization
U
2 H 2O  OH   H 3O 
hydroxyl
1014 ions/cm3 => U~20 kBT
H-bond energy ~ -t2/(2U) ~ 5 kBT
hydronium
MOLECULAR HYDROPHOBIC INTERACTTION
Disruption of H-bonding network
Chandler et al
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!!!
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”….