Module 4.4: Liquids

4.4.1 Order

I Liquids are held together by intermolecular forces (like solids) but have enough energy to move past one another (like gases).

1. A liquid is short-range order; it arranges orderly over short distances

   1. the tetrahedral arrangement of water molecules is short-range order

2. there is also long-range order; its orderly arranagement is repeated over long distances.

   1. crystalline ice (a solid) possess long-range order in a hexagonal lattice

4.4.2 Viscosity, Surface Tension, and Capillary Action

II viscosity: A liquid's resistance to flow

1. the viscosity of a liquid is dependent on its intermolecular forces.

   1. high viscosity has sluggish flow and is hard to move
   2. low viscosity has natural flow and is easy to move

2. viscosity is hard to predict for a liquid as:

   1. the ease of which positions the molecules can take as the liquid flows is significant

   2. entanglement (esp. for long chain molecules) affects viscosity

   3. viscosity decreases as temperature increases due to higher kinetic energy

III surface tension: The net inward pull towards the center of the liquid

1. molecules with strong intermolecular forces will have high surface tension

2. surface tension can be used to explain the everyday phenomena of liquids

   1. water drops are spherical in a waxy surface due to surface tension

3. surface tension decreases as temperature increases

   1. higher kinetic energy results in more motion to overcome intermolecular forces

4. when the liquid has strong interactions with the surface, it tends to maximize its contact by spreading

   1. water and paper interaction (known as “wetting”)
   2. chromatography

IV capillary action: The rise of liquids in narrow tubes

1. occurs when there are favorable interactions between the tube and the liquid

   1. adhesion is the forces that bind the molecules of a liquid to a surface

   2. cohesion is the forces that bind the molecules of a liquid together

2. the shape of a liquid's meniscus is a relative indicator of cohesion vs. adhesion

4.4.3 Liquid Crystals

V Liquid crystals: substances that flow like viscous liquids but lie in a moderate orderly array

1. liquid crystals are an example of mesophase (intermediate of long-range & short-range)

VI liquids can be anisotropic or isotropic

1. anisotropic: materials with properties that depend on the direction of measurement

   1. all liquid crystals are anisotropic; its viscosity is different with different direction

2. isotropic: materials with properties that do not depend on the direction of measurement

   1. water is isotropic; their viscosity is the same with different direction

3. liquid crystals can be classified into three classes:

   1. nematic phase: molecules that lie in the same direction but staggered

   2. smectic phase: molecules that lie in the same direction but line up as well

   3. cholesteric phase: ordered layers but each layer is at a different angle

VII liquid crystals can also be classified by how it is prepared

1. thermotropic: liquid crystals that are prepared by melting the solid phase

   1. these compounds act as liquid crystals at a certain range of temperature

   2. these compounds become isotropic liquids when heated above a certain temperature

   3. thermotropic liquid crystals are used in digital watches, computer screen, and thermometers

2. lyotropic: a material that forms liquid crystal phases upon the addition of a solvent

4.4.4 Ionic Liquids

VIII ionic liquids: liquids that are ionic compounds that have low vapor pressures and can dissolve organic compounds

1. typically has a relatively small anion (like $\textrm{BF}_{4}^{2-}$) and a huge cation to prevent crystallization

2. in general, cations should not be symmetrical and contain a large nonpolar region to prevent crystallization

3. Ionic liquids are a big part of green chemistry, as normal liquid solvents tend to be harmful to the environment and have a health risk due to high toxic vapors