11.5:
Surface Tension, Capillary Action, and Viscosity
Ever wondered why a yen coin, when carefully placed on water, floats?
Most properties of liquids are dictated by intermolecular forces. The attractive forces between similar molecules in a substance are called cohesive forces.
In water, the cohesive forces pull interior molecules equally in all directions, resulting in zero net force on average, while surface molecules experience only a downward pull, which causes the molecules to pack closely together.
The interior molecules are more energetically stable than surface molecules because they experience more cohesion, which lowers their potential energy.
Fluids, therefore, try to reduce their potential energy by minimizing the surface area, creating a surface under tension that behaves like an elastic membrane.
The energy needed to increase a liquid’s surface area by a unit amount is called its surface tension, which is often measured in joules per meter-squared.
Adhesive forces, in contrast, exist between dissimilar molecules, such as water and glass.
Upon placing a narrow capillary tube in water, the water molecules spread along the tube's surfaces via adhesion, increasing the liquid's surface area and drawing the rest of the liquid with it by cohesion. The liquid in the capillary rises until gravity overcomes the adhesive and cohesive forces. This phenomenon is called capillary action.
For water, the adhesive forces between water and glass are stronger than the cohesive forces, resulting in a concave, or inward-curving, meniscus. For mercury, the cohesive forces outweigh the adhesive forces, resulting in a convex meniscus.
Viscosity expresses a liquid's resistance to flow and is often measured in poise, or grams per centimeter-second,
Consider methanol and glycerol. Although both form hydrogen bonds, glycerol has three –OH groups instead of one and forms more hydrogen bonds per molecule. This results in a stronger attraction between glycerol molecules, making glycerol more viscous than methanol.
Viscosity is influenced by molecular shapes. Hydrocarbons of increasing molar masses and lengths interact over greater areas and entangle more frequently, resulting in stronger dispersion forces and higher viscosities.
Lastly, viscosity depends on temperature. Increased thermal energy weakens intermolecular forces, corresponding to molecules moving more freely and liquids thus flowing faster at higher temperatures.
11.5:
Surface Tension, Capillary Action, and Viscosity
Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid. However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. Because of the unbalanced molecular attractions on the surface molecules, liquids contract to form a shape that minimizes the number of molecules on the surface—that is, the shape with the minimum surface area. A small drop of liquid tends to assume a spherical shape. This is because the ratio of surface area to volume is at a minimum in a sphere. Larger drops are more greatly affected by gravity, air resistance, surface interactions, and so on, and as a result, are less spherical.
Surface tension is defined as the energy required to increase the surface area of a liquid or the force required to increase the length of a liquid surface by a given amount. This property results from the cohesive forces between molecules at the surface of a liquid, and it causes the surface of a liquid to behave like a stretched rubber membrane. Among common liquids, water exhibits a distinctly high surface tension due to strong hydrogen bonding between its molecules. As a result of this high surface tension, the surface of water represents a relatively “tough skin” that can withstand considerable force without breaking. A steel needle carefully placed on water will float. Some insects, even though they are denser than water, move on its surface because they are supported by the surface tension.
Adhesive and Cohesive Forces
The IMFs of attraction between two different molecules are called adhesive forces. Consider what happens when water comes into contact with some surface. If the adhesive forces between water molecules and the molecules of the surface are weak compared to the cohesive forces between the water molecules, the water does not “wet” the surface. For example, water does not wet waxed surfaces or many plastics such as polyethylene. Water droplets form 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.
Capillary Action
If one end of a paper towel is placed in spilled wine, 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.
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. Both consist of long molecules of cellulose that contain many −OH groups. Water molecules are attracted to these −OH groups and form hydrogen bonds with them, which draws the H2O molecules up the cellulose molecules. 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. 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.
Viscosity
When you pour a glass of water or fill a car with gasoline, the water and gasoline flow freely. But when you pour syrup on pancakes or add oil to a car engine, the syrup and motor oil do not flow as readily. The viscosity of a liquid is a measure of its resistance to flow. Water, gasoline, and other liquids that flow freely have a low viscosity. Honey, syrup, motor oil, and other liquids that do not flow freely have higher viscosities. We can measure viscosity by measuring the rate at which a metal ball falls through a liquid (the ball falls more slowly through a more viscous liquid) or by measuring the rate at which a liquid flows through a narrow tube (more viscous liquids flow more slowly).
The IMFs between the molecules of a liquid, the size and shape of the molecules, and the temperature determine how easily a liquid flows. The more structurally complex the molecule, the stronger the IMFs between them and the more difficult it is for them to move past each other. Liquids with these molecules are more viscous. As the temperature increases, the molecules move more rapidly and their kinetic energies are better able to overcome the forces that hold them together; thus, the viscosity of the liquid decreases.
This text is adapted from Openstax, Chemistry 2e, Section 10.2: Properties of Liquids.