Some Thoughts on the Hydrophobic Interaction

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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
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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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OUTLINE

• 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 PROTOCOL

• Surface force apparatus with mica substrate

100nmlthlt 0.1nm
Mica is highly Hydrophilic and Anionic s
1e/nm2

• Passivate with cationic surfactant DODAB
  Langmuir Deposition
• Measure forces with SFA
• Look at surfaces with AFM

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AFM IMAGES
Ch. Rotsch Manfred Radmacher--LMU
Patchy surface - nearly 50-50 mixture of
bilayers and bare mica Broad distribution of
patches tens of nanometers
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Air
UCSB AFM Hansma Lab
Water
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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
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DISJOINING PRESSURE

• Negative mobile holes
• Positive bilayer matrix

Coulombic correlation between positive and
negative patches on opposing surfaces
L nm-µm
Bloomfield Rouzina Attraction on L Scale Range
scales with L 20 nm
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MONOLAYER 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 SURFACE
Gauss Law gt
f
Electrostatic Potential f T(x/?)
Gouy-Chapman Length ? (4psl)-1 Bjerrum Length l
e2/eT 0.6 nm in water
x
All counterions bound to sheath of thickness ? !
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ENTROPY DRIVEN PATCHES
f(X)
Patch size given by balance of counterion release
against line tension of patches.
T(L/?)
X
Broad patch distribution L?
ln(?/T)(cs?2)-1 ? is line tension, cs is salt
concentration
L
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PASSIVATED MICA VS BARE MICA
Experimental evidence for patch mobility!
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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 WATER
V
Polarizability of
O--
U gt V
SP hybridization
U
t
hydroxyl
hydronium
1014 ions/cm3 gt U20 kBT
H-bond energy -t2/(2U) 5 kBT
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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!!!
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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.