Difference between revisions of "Intermolecular and interparticle forces"
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Revision as of 19:12, 29 September 2008
- 1 Intermolecular energies
- 2 An Example: Milk
- 3 Another Example: Water, the most "complex" fluid
- 4 Flow properties from molecular energies
- 5 Impact of Bond Type on Physical Properties of a Solid
- 6 Forces near surfaces
- 7 (Modern) forces near sufaces
- 8 Interactions from molecular attraction
- 9 Derjaguin Force Approximation
- 10 Casimir Effect
- 11 Repulsive Van Der Waals Forces
I've always thought that phase diagrams are rich with information that is often hard to glean without a basic introduction to what all of those lines and points mean. I found this tutorial very helpful when I was working at Corning: Phase Diagrams Explained --BPappas 19:34, 28 September 2008 (UTC)
An Example: Milk
The gastroscience of milk provides many insights into the interparticle forces in a colloid. 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.
Another Example: Water, the most "complex" fluid
In contrast to the simple example presented in many introductory physics textbooks, water can actually be considered one of the most "complex" fluids. A leader in the field of understanding the intricacies of phase transitions in water is Prof. Gene Stanley at Boston University. In a recent presentation (September 24, 2008) in the "Squishy Physics" lecture series at Harvard, he explained that there are 63 recorded anomalies in the physical properties of water. Some of these may be due to the asymmetrical structure of the molecule, which allows two different packing structures. At very low temperatures (well below zero celsius), the water forms regions in which these different types of packing occur. As the water freezes, the correlation length between similar packing structures increases, until the entire material forms a solid.
Flow properties from molecular energies
Impact of Bond Type on Physical Properties of a Solid
Melting point: The melting point of solids has an almost monotonically increasing relation with the cohesive energy - e.g., the following substances are arranged in order of increasing melting temperature : Ne (VdW bond), Na (simple metal), Fe (transition metal), KCl (ionic bond), Si (covalent bond).
Electrical and Thermal conductivity: For conducting current, some of the valence electrons must be free to move in response to an electric field - only in metallic bonds are the valence electrons delocalised sufficiently for this to be possible. Hence metals are good electrical conductors. Thermal conductivity is related to electrical conductivity (Wiedemann-Franz relation, first observed in 1853) - hence metals are also good thermal conductors.
Optical properties: Metals reflect light, non-metals are transparent. (http://www.imsc.res.in/~sitabhra/teaching/cmp03/class3.html)
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.
Interactions from molecular attraction
- (a) A molecule near a flat surface.
- (b) A sphere near a flat surface.
- (c) Two flat surfaces.
Derjaguin Force Approximation
Where W(D)is the energy of interaction of two flat plates.
Derjaguin Force Approximation has been deemed fairly accurate from a number of experiments. One experiment measuring interaction forces between colloidal particles of different sizes were conducted to investigate the validity of the approximation. Forces between silica particles of 2.0, 4.8, and 6.8mm in diameter were measured by an atomic force microscope. In this investigtion, the Derjaguin approximation is confirmed at all distances investigated. This approximation holds even at small distances, which are comparable to the surface roughness or the characteristic distance of a heterogeneously charged substrate. To read more about the investigation, the research by Samuel Rentsch, et al. is uploaded to the wiki. Research pdf
Most forces that are usually taken into account in soft matter physics are electro- or magneto-static in nature. That means that the forces are specifically due to the electric or magnetic attraction or repulsion between charges. As distances become sufficiently smaller (on the order of 10's of nm) quantum effects begin to become apparent.
From another perspective, since radiation can be modeled by waves, then each object generates a boundary condition for the radiation in the surrounding space. Thus two objects, when close enough, create strict boundary conditions on the electromagnetic field that is contained between them. When changing the distance between the objects these boundary conditions change. Boundary conditions impose a quantization of the frequencies of radiation than can exist in the space between the two objects. According to quantum mechanics, all radiation can be modeled as a simple harmonic oscillator and each radiation state has a non-zero energy even when no photons occupy that state. Thus even the vacuum has a non-zero energy that is determined by summing this fundamental energy over all modes of the field. However, this fundamental energy changes with the boundary conditions. Thus as two objects are moved with the respect to eachother, the "vacuum-energy" contained changes. A change of energy with respect to position manifests itself as a force!
Thus when two objects get close enough that the vacuum energy between them starts to change appreciably with distance, once can observer a non-negligible force between the two objects called the "Casamir Effect"
For example, at around 10nm, the force can be as high as 1atm of pressure.
Repulsive Van Der Waals Forces
While we usually think of Van der Waals forces as being attractive, Hamaker predicted repulsive Van der Waals forces in 1937 and they have since been seen experimentally. Repulsive van der Waals forces have been used to explain the properties of liquid helium, and have been seen on thin liquid hydrocarbon films on alumina and quartz. They are of great interest to researchers, despite their rarity, because they can be measured more easily by AFM techniques than attractive Van der Waals forces. This paper describes the use of AFM to study repulsive forces between Teflon thin films and silica and alumina.