CHEMISTRY 2000 Topic #2: Intermolecular Forces – What Attracts Molecules to Each Other? Spring 2010 Dr. Susan Lait

2 Properties of Liquids: Surface Tension Several properties of liquids can be explained by looking at intermolecular forces. For example, we know that small samples of liquid tend to be round (e.g. water droplets). Why? Molecules within a liquid are attracted to all of the molecules around them via intermolecular forces. Molecules on the surface of a liquid can only interact with the molecules immediately below them and beside them. As such, the surface molecules feel a net attraction toward the interior of the liquid and act as a “skin” for the liquid. The energy required to break through this surface “skin” is referred to as surface tension. Some insects take advantage of surface tension to literally walk on water!

3 Properties of Liquids: Surface Tension A sphere is the shape with the smallest surface area-to-volume ratio – giving the smallest ratio of surface molecules to bulk molecules (i.e. non-surface molecules). This maximizes the intermolecular forces: This effect can also be seen when looking at the convex (“inverted”) meniscus of mercury (Hg) in a glass test tube: Note that water behaves differently because water molecules also experience intermolecular forces attracting them to the glass!

4 Properties of Liquids: Surface Tension What happens if we add a solute to the liquid (now a solvent)? If the intermolecular forces between the solute and the solvent are stronger than the intermolecular forces between solvent molecules, the solute will be pulled into the solution away from the surface. If the intermolecular forces between the solute and the solvent are weaker than the intermolecular forces between solvent molecules, the solute will tend to stay at the surface and is referred to as a surfactant (“surface active agent”). Surfactants reduce surface tension. Detergents are important surfactants (in water). They consist of a long nonpolar “tail” and a small polar “head” (often ionic). e.g. dodecylsulfate ion

5 Properties of Liquids: Capillary Action When a liquid is in contact with a solid surface, we must also consider intermolecular forces between the liquid molecules and the molecules of the solid. If the liquid molecules are more strongly attracted to the surface than to each other, wetting will be observed as the liquid sample spreads over the surface to maximize those attractions. e.g. water in a glass test tube If the liquid molecules are more strongly attracted to each other than to the surface, non-wetting will be observed as the liquid sample appears to ‘bead’ on the surface. e.g. mercury in a glass test tube

6 Properties of Liquids: Capillary Action The familiar concave meniscus of water on glass is an example of wetting leading to capillary action – specifically, capillary rise. Glass consists of polar Si-O bonds so, if a narrow glass tube is placed in water, the water molecules are attracted to the glass. Other water molecules are attracted to the initially “stuck” molecules, and are pulled up by surface tension. This continues until the force of gravity pulling water molecules down is equal to the force of surface tension pulling water molecules up. Because the force of attraction between the water and the glass has to support the weight of the column of water (i.e. counteract the force of gravity), narrow capillaries allow the liquid to rise higher (compared to wide capillaries). A nonwetting liquid-surface combination will give capillary depression. How do you think that works?

7 Properties of Liquids: Viscosity Viscosity is a liquid’s resistance to fluid flow. Liquids with high viscosities tend to be thick and difficult to pour. Viscosity increases as intermolecular forces increase. As a result, viscous liquids tend to be either large or polar (or both). e.g. gasoline vs. mineral oil (nonpolar, ~8 carbons) (nonpolar, ~15-40 carbons)