Difference between revisions of "Intermolecular and interparticle forces"

Revision as of 21:43, 27 September 2008

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Intermolecular energies

3D Pressure-volume isotherms	2D Spreading pressure-area isotherms
Hisrshfelder, Fig. 4.1.1	Gaines, Fig. 4.7
The figure at left shows a sample pressure-volume isotherm. Note that the horizontal lines between the liquid and gas phases are an unstable state. The fluid discontinuously transforms from the intersection at one side of the dashed curve to the other (e.g. boiling water undergoes a sudden change from liquid to vapor).	The figure on the right shows the spreading of a thin layer of myristic acid on the surface of a liquid. Since the system is two-dimensional, the pressure is replaced by a force per volume (dyne/cm). As the layer is compressed or the temperature is raised, it exerts more pressure along its boundary.

An Example: Milk

Milk is an example of a colloidal dispersion, which illustrates several key features common to other such colloids. On its own, fresh whole milk segregates into a cream layer floating on top of a fat-depleted liquid. However homogenization was developed in France around 1900 to overcome this problem. By forcing hot milk through a surface of small nozzles, turbulence in the fluid tears the 4-micron fat globules into smaller particles closer to a micron in size. The original membrane surrounding the globules is insufficient to cover the greatly increased surface area of the globules. Since they are hydrophobic, they attract casein proteins from the surrounding liquid, which weight them down. The combination of smaller particle size and greater density allows Brownian motion to keep the particles in suspension.

Aggregation is another phenomenon that can lead to phase separation in a colloid. In the case of milk, additional ingredients or a change in acidity can cause the globules to stick together and separate from the liquid. This can happen with the addition of an acid, such as lemon juice. The astringent tannins in beverages like tea and coffee make this process more likely (which could be why one rarely adds both milk and lemon juice to tea).

To read more about the gastroscience of milk, see On Food and Cooking by Harold McGee (in the section "Unfermented Dairy Products") or Chapter 4 ("Colloidal dispersions") of Soft Condensed Matter by Richard A. L. Jones.

Flow properties from molecular energies

	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.
	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.
	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.

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