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Some Thoughts on the Hydrophobic Interaction

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'The magnitude, range and origin of the hydrophobic interaction have been a ... Passivate with cationic surfactant DODAB Langmuir Deposition. Measure forces with SFA ... – PowerPoint PPT presentation

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Title: Some Thoughts on the Hydrophobic Interaction


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

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

5
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
6
Air
UCSB AFM Hansma Lab
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

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
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 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 ? !
11
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
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
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
15
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!!!
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.
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