Intermolecular and interparticle forces
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Revision as of 18:02, 24 September 2008 by Nsinha (Talk | contribs) (→Flow properties from molecular energies)
Contents
Intermolecular energies
| 3D Pressure-volume isotherms | 2D Spreading pressure-area isotherms |
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| Hisrshfelder, Fig. 4.1.1 | Gaines, Fig. 4.7 |
Flow properties from molecular energies
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For short time scales and simple liquids, the viscosity η can be approximated by the product of the instantaneous modulus G0 and the relaxation time τ. |
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Erying model: When the strain is generated molecules are "trapped" inside an energy barrier of size ε and "jump" to a relaxed state with the characteristic time τ. While inside the barrier, the molecule vibrates with the characteristic frequency ν of the solid. 
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Combining these equations yields the Arrhenius behavior. In this case, ε is the heat of vaporization of the liquid, which is the upper bound of the energy barrier. This behavior can be seen experimentally by plotting the logarithm of viscosity as a function of the reciprocal of the temperature. |
Forces near surfaces
- Bulk phases are characterized by density, free energy and entropy – not by forces.
- Molecular forces average out.
- Not so at surfaces.
(Modern) forces near sufaces
- (a) This potential is typical of vacuum interactions but is also common in liquids. Both molecules and particles attract each other.
- (b) Molecules attract each other; particles effectively repel each other.
- (c) Weak minimum. Molecules repel, particles attract.
- (d) Molecules attract strongly, particles attract weakly.
- (e) Molecules attract weakly, particles attract strongly.
- (f) Molecules repel, particles repel.
Israelachivili Fig.10.1
Interactions from molecular attraction
Israelachivili Fig.10.2
- (a) A molecule near a flat surface.
- (b) A sphere near a flat surface.
- (c) Two flat surfaces.
Derjaguin Force Approximation
Israelachivili Fig.10.3
Where W(D)is the energy of interaction of two flat plates.
