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”….