Transcript Chap.9~10 Intermolecular and interparticle Forces

10.3 Effective interaction Area of two spheres
: The Langbein Approximation
The effective area of interaction of a sphere
with a surface = the circular zone centred at a
distance–D from the surfaces( inside the sphere )
Aeff  2RD /( n  5 )
 2RD
for n=6
The interaction of a sphere and a surface
= the same as that of two planar surfaces at the
same surface separation D
10.4 Interactions of large bodies compared to
those between molecules

For two macroscopic bodies, the
interaction energy generally decays
much more slowly with distance
 the van der Waals energy between
large condensed bodies decays is
effectively of much longer range
10.4 Interactions of large bodies compared to
those between molecules

In contact b/n a small molecule and a wall
w( D )  0.74C /  6

In contact b/n a sphere of atomic dimensions
w( D )  1.6C /  6

In contact b/n two spheres
w( D )  0.8C / 

6
Increasing of the size of a sphere above atomic dimensions
w( D )  1.6( 2R /  )C /  6
10.5 Interaction Energy and Interaction
Forces : Derjagun Approximation
[ Assumption ]

Two large spheres of radii
R1 and R2

R1>>D and R2>>D

By integrating the force
between small circular
regions of area on one
surface

Surface to be locally flat
10.5 Interaction Energy and Interaction
Forces : Derjagun Approximation
 R1 R2 
W ( D )
F ( D )  2 
 R1  R2 
1.
The force b.t.n two spheres is expressed in terms of the
energy per unit area of two flat surfaces at the same
separation D
2.
The distance dependence of the force b.t.n two curved
surfaces can be quite different from that b.t.n two surfaces
even though the same type of force is operating in both.
Fig. 10. 4 Force laws betweem two curved
surfaces and two flat surfaces
16.1 Indirect access for W(D)
Thermodynamic data
on gases, liquids and
solids
Physical data
on gases, liquids and
solids
Thermodynamic data
on liquids and liquid
mixtures
PVT data, B.P
Latent heats of
vaporization
lattice energy
Viscosity, diffusion.
Compressibility, NMR
X-ray, molecular beam
scattering experiment
Phase diagrams
solubility
Partitioning, miscibility
osmotic pressure
Short-range attractive
potentials b.t.n molecules
Short-range interactions
Short-range silute-solvent
of molecules, especially
and solute-solute
repulsive forces giving
interactions
molecular size, shape and
structural role in
condensed phase
16.2 Direct access for W(D)
16.2 Direct access for W(D)
Types
Practical Applications
Adhesion
measurement
Xerography, particle adhesion.
Powder technology, ceramic
processing
Peeling
measurement
Adhesive tapes, material fracture
and crack propagation
Forcemeasuring
spring or
balance
Information
Particle
adhesion forces
and the adhesion energies
of solid surfaces in contact
( attractive short-range
forces)
The
Testing theories of intermolecular
forces
force macroscopic
surfaces as a function of
surface separation
The
full force law of an
interaction
16.2 Direct access for W(D)
Types
Practical Applications
Effects
Liquid-Liquid
or Liquid-Solid
adhesion energy
Contact Angle
Testing wettability and
stability of surface films, Information of states and
foams
adsorbed films, and of molecular
reorientation time at interfaces
A function
Equilibrium thickness of
thin free films
Equilibrium thickness
of thin absorbed films
Soap films, foams
of salt conc. or
vapour pressure
The
long-range repulsive forces
stabilizing thick wetting films
Wetting of hydrophilic surface by water, adsorption of
molecules from vapor, protective surface coatings and
lubricant layers, photographic films
16.2 Direct access for W(D)
Types
Practical Applications
Interparticle
Colloidal suspensions, paints,
spacing in liquids pharmaceutical dispersions
Sheet-like
particle spacing
in liquids
Clay and soil swelling behavior,
microstructure of soaps and
biological membranes
Coagulation
studies
Basic experimental technique for
testing the stability of colloidal
preparations
Effects
The interparticle forces
By
changing the solution
conditions and their mean
separation
By
changing the quantity of
solvents
Limits
to measure only the
repulsive parts
Information on the interplay of
repulsive and attractive forces
between particles in pure,
sulfactant and polymer solutions
10.7 Direct measurements of Surface and
Intermolecular Forces
The most unambiguous way to measure a force-law
 to position two bodies close together and
directly measure the force between them
 very straightforward
 very weak challenge coming at very small
intermolecular interaction
 surface separation controlled and measured to
within 0.1nm
The Surfaces Forces Apparatus (SFA)
1.
Measuring surface forces in
controlled vapor or
immersed in liquids is
directly measured using a
variety of interchangeable
force-measuring springs
2.
Both repulsive and
attractive forces are
measuring and a full force
law can be obtained over
any distance regimes
The Surfaces Forces Apparatus (SFA)
3.
4.
The distance resolution about 0.1nm (angstrom level)
the force sensitivity about 10-8 N
In Surfaces
a.
Two curved molecularly smooth surfaces of mica in a
crossed cylinder configuration
b.
The separation is measured by use of an optical technique
using multiple beam interference fringes
c.
The distance is controlled by use of a three-stage
mechanism of increasing sensitivity ( the coarse control
1µm – the medium control 1nm – a piezoelectric crystal
tube 0.1nm )
The Surfaces Forces Apparatus (SFA)
The force measurement
5.
a.
The force A measured by expanding or contracting the
piezoelectric crystal by a known amount
b.
The force B measured by optically how much the two
surfaces have actually moved
c.
The difference of force b.t.n two positions
= [ Force A – force B ] * the stiffness of the force-measuring
spring
The Surfaces Forces Apparatus (SFA)
The force and the interfacial energy
6.
a.
The force b.t.n two curved surfaces scale = R
b.
The adhesion or interfacial energy E per unit area two flat
surfaces E  F / 2R
by the Derjaguin approximation
c.
For given R and sensitivity F,
getting E ( an interfacial energy)
The Surfaces Forces Apparatus (SFA)
The use of SFA
7.
a.
Identifying and quantifying most of fundundamental
interactions occuring between surfaces on both aqueous
solutions and nonaqueous liquids
b.
Including the attractive van der Waaals and repulsive
electrostatic ‘double –layer’ forces, oscillatory forces,
repulsive hydration forces, attractive hydrophobic forces,
steric interactions involving polymeric systems and capillary
and adhesion
c.
The extension of measurement into dynamic interaction and
time-dependent effects and the fusion of lipid bilayers . etc
Total Internal Reflection Microscopy(TIRM)
1.
Measuring minute forces( <10-15 N) between a colloidal
particle and a surface
2.
Measuring the distance between an individual colloidal
particle of diameter ~10 µm hovering over a surface.
3.
A laser beam is directed at the particle through the surface
made of transparent glass
4.
From the intensity of reflected beam
deducing the equilibrium separation D0
5.
Providing data on interparticle interactions under conditions
closely paralleling those occurring in colloidal systems
The Atomic Force Microscope(AFM)
1.
Measuring atomic adhesion forces (10-9~10-10 N) between a
fine molecular-sized tip and a surface ( 1µm < Tip radii
< atom size )
2.
At finite distances, using very sensitive force-measuring
springs (spring stiffness=0.5 Nm-1) and very sensitive ways
for measuring the displacement (0.01nm)
3.
very short-range forces , but not longer range forces
4.
Interpreting the results is not always straightforward and
exact due to the tip geometry