Intramolecular forces do, however, play a role in determining the types of intermolecular forces that can form. Intermolecular forces come in a range of varieties, but the overall idea is the same for all of them: A charge within one molecule interacts with a charge in another molecule.
Depending on which intramolecular forces, such as polar covalent bonds or nonpolar covalent bonds , are present, the charges can have varying permanence and strengths, allowing for different types of intermolecular forces. So, where do these charges come from? In some cases, molecules are held together by polar covalent bonds — which means that the electrons are not evenly distributed between the bonded atoms.
This type of bonding is described in more detail in the Chemical Bonding module. This uneven distribution results in a partial charge: The atom with more electron affinity, that is, the more electronegative atom, has a partial negative charge, and the atom with less electron affinity, the less electronegative atom, has a partial positive charge. This uneven electron sharing is called a dipole. When two molecules with polar covalent bonds are near each other, they can form favorable interactions if the partial charges align appropriately, as shown in Figure 3, forming a dipole-dipole interaction.
Hydrogen bonds are a particularly strong type of dipole-dipole interaction. Hydrogen bonds occur when a hydrogen atom is covalently bonded to one of a few non-metals with high electronegativity , including oxygen, nitrogen, and fluorine, creating a strong dipole. The hydrogen bond is the interaction of the hydrogen from one of these molecules and the more electronegative atom in another molecule. Hydrogen bonds are present, and very important, in water, and are described in more detail in our Water: Properties and Behavior module.
Hydrogen bonds and dipole-dipole interactions require polar bonds, but another type of intermolecular force , called London dispersion forces , can form between any molecules , polar or not. The basic idea is that the electrons in any molecule are constantly moving around and sometimes, just by chance, the electrons can end up distributed unequally, creating a temporary partial negative charge on the part of the molecule with more electrons.
This partial negative charge is balanced by a partial positive charge of equal magnitude on the part of the molecule with fewer electrons, with the positive charge coming from the protons in the nucleus Figure 4. These temporary partial charges in neighboring molecules can interact in much the same way that permanent dipoles interact. The overall strength of London dispersion forces depends on the size of the molecules: larger molecules can have larger temporary dipoles, leading to stronger London dispersion forces.
Now, you might ask, if molecules can develop temporary partial charges that interact with each other, these temporary charges should also be able to interact with permanent dipoles , right?
And you would be correct. These interactions are called, very creatively, dipole-induced dipole interactions. As you might have guessed, London dispersion forces and dipole-induced dipole interactions are generally weaker than dipole-dipole interactions. These forces , as well as hydrogen bonds , are all van der Waals forces , which is a general term for attractive forces between uncharged molecules.
The primary intermolecular forces present in most oils and many other organic liquids — liquids made predominantly of carbon and hydrogen atoms , also referred to as non-polar liquids — are London dispersion forces , which for small molecules are the weakest types of intermolecular forces.
These weak forces lead to low cohesion. On the other end of the cohesion spectrum , consider a dewdrop on a leaf in the early morning Figure 6. How can such a thing exist if, as explained earlier, liquids flow and take the shape of the container holding them?
As described above and in the Water module, water molecules are held together by strong hydrogen bonds. These strong forces lead to high cohesion: The water molecules interact with each other more strongly than they interact with the air or the leaf itself. This high cohesion also creates surface tension. Surface tension results from the strong cohesive forces of some liquids.
These forces are strong enough to be maintained even when they experience external forces like the gravity of an insect walking across its surface. Adhesion is the tendency of a compound to interact with another compound.
Remember that, in contrast, cohesion is the tendency of a compound to interact with itself. Adhesion helps explain how liquids interact with their containers and with other liquids. One example of an interaction with high adhesion is that between water and glass. Both water and glass are held together by polar bonds. This is because the forces between the mercury particles are very strong, so the particles clump together.
This force between particles of the same type is called cohesion. Water particles do not have such strong cohesion, so they wet surfaces. A measure of how fast or slowly a liquid can flow is its viscosity. Crude oil, for example, is a liquid that does not flow very easily.
It is said to have high viscosity. Heating crude oil lowers its viscosity and enables it to flow more freely through pipes. Other liquids, such as water, flow easily without being heated. A steel needle carefully placed on water will float. Some insects, like the one shown in Figure 3, even though they are denser than water, move on its surface because they are supported by the surface tension.
Figure 3. The IMFs of attraction between two different molecules are called adhesive forces. Consider what happens when water comes into contact with some surface. For example, water does not wet waxed surfaces or many plastics such as polyethylene. Water forms drops on these surfaces because the cohesive forces within the drops are greater than the adhesive forces between the water and the plastic.
Water spreads out on glass because the adhesive force between water and glass is greater than the cohesive forces within the water. When water is confined in a glass tube, its meniscus surface has a concave shape because the water wets the glass and creeps up the side of the tube. On the other hand, the cohesive forces between mercury atoms are much greater than the adhesive forces between mercury and glass.
Mercury therefore does not wet glass, and it forms a convex meniscus when confined in a tube because the cohesive forces within the mercury tend to draw it into a drop Figure 4. Figure 4. Differences in the relative strengths of cohesive and adhesive forces result in different meniscus shapes for mercury left and water right in glass tubes.
If you place one end of a paper towel in spilled wine, as shown in Figure 5, the liquid wicks up the paper towel. A similar process occurs in a cloth towel when you use it to dry off after a shower. These are examples of capillary action —when a liquid flows within a porous material due to the attraction of the liquid molecules to the surface of the material and to other liquid molecules.
The adhesive forces between the liquid and the porous material, combined with the cohesive forces within the liquid, may be strong enough to move the liquid upward against gravity. Figure 5. Towels soak up liquids like water because the fibers of a towel are made of molecules that are attracted to water molecules. Most cloth towels are made of cotton, and paper towels are generally made from paper pulp. The water molecules are also attracted to each other, so large amounts of water are drawn up the cellulose fibers.
Capillary action can also occur when one end of a small diameter tube is immersed in a liquid, as illustrated in Figure 6. If the liquid molecules are strongly attracted to the tube molecules, the liquid creeps up the inside of the tube until the weight of the liquid and the adhesive forces are in balance.
The smaller the diameter of the tube is, the higher the liquid climbs. Thus on increasing stirrer speed to create inversion, a lower stirrer speed on lower volume fraction of the specified phase is required for reversion of the inversion process. In addition, the inversion band is not the same when starting from an oil-dispersed system or a water-dispersed one.
It should be noted that if mass transfer is taking place, the conditions for phase inversion will most certainly change due to the presence of solute at the interface. Figure 2. Inversion curves for binary immiscible liquid-liquid systems. Source : McClarey and Mansouri There is considerable controversy about the effect of wettability of the containing vessel and also of the agitator on phase inversion, but there are no general conclusions on this respect, although it has been suggested that wettability may only be significant at low stirring rates.
In pipe systems, the mechanisms of mixing leading to inversion are much more complex and, from the point of view of the experimentalist, uncontrolled.
There is little information in the literature about phase inversion in pipes. Unlike the case of stirring speed, there seems to be no effect of mixture velocity or droplet size on phase inversion. Phase fraction and temperature, on the other hand, seem to be the key parameters. Arirachakaran, S. Bird, R. John Wiley and Sons.
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