CHE2060 4: Physical properties & interactions 4.1 Physical properties of organic molecules Solids, liquids & gases Melting point Boiling point 4.2 Types of intermolecular interactions van der Waals interactions Dipolar interactions Hydrogen bonding 4.3 Solubility 4.4 Surfactants Micelles & emulsions Labs Melting point determination Viscosity of organic compounds Distillation of wine Daley & Daley Chapter 4: Physical Properties

Surfactants Amphipathic lipids Micelles vs. emulsions

Surfactants Surfactants are a class of molecules with ‘dual’ or ‘two-faced’ physical natures. One end is polar The other end is non-polar So these amphipathic molecules are able to interact with (or be soluble in) both polar and non-polar molecules & solvents. Soaps are a good example: :O: K +1 :O: .. non-polar tail hydrophobic -1 polar head group hydrophilic So soaps & other amphipathic compounds are able to clean grease from objects by making the grease soluble in water. Normally grease & water don’t mix… …but soaps create a molecular bridge between grease & water… surfactants greatly increase solubility! Bile salts are a surfactant that dissolve lipids in body fluid. D&D p.194-6

Soap scum & ‘advanced’ soaps So, what’s soap scum? It’s a precipitate formed when metal ions in hard water interact with the carbonate head group of soap & form a precipitate that builds up on surfaces. Hard water ions: calcium, magnesium, iron Like most precipitates, soap scum is dissolved by acids.. Typical cleaners use HCl. Some surfactants are less likely to form “scum” precipitates because of their chemical structures & properties. Sodium dodecanyl sulfate (aka sodium lauryl sulfate): :O: polar head group hydrophilic :O: S :O: non-polar tail hydrophobic Na +1 :O: .. SDS doesn’t precipitate with as many metals…. So less soap scum. D&D p.194-6

Surfactants & lung development Surfactants can lower surface tension & this helps allow oxygen to gain access to the surface of lung alveoli. The inner surface of lungs is coated with a solution of lipid in water. DPPC (dipalmitoylphophatidyl choline) is the lipid of choice. polar head non-polar tails Hyaline membrane disease? Animals born prematurely haven’t yet developed the ability to produce DPPC, & without the surfactant their lungs aren’t able to inflate & stay inflated. They can be saved by being given the missing surfactants until they develop their own. D&D p.194-6

Soap bubbles Soap bubbles are two lipid (soap) monolayers with a layer of water in between them. The non-polar fatty carbon tails point outward into the air, also non-polar. Polar head groups face into the layer of very polar water.

Lipid structures in water When mixed with water (or an aqueous solution), amphipathic lipids spontaneously form 3D structures to increase lipid solubility. Liposomes are formed by closing a bilayer to create a hollow sphere filled with aqueous solution. Micelles are spherical structures one lipid ‘thick’ with no filling. Emulsions are micelles that hold non-polar, hydrophobic molecules in their cores where they are solubilized by the surfactant’s tails. Note that the polar heads are hydrophilic & interact with the water, while the hydrophobic non-polar tails interact only with each other or other hydrophobic molecules (hiding from water). non-polar molecules Salad dressing, hollandase sauce, mayonnaise Emulsions

Lipids, emulsions and dairy science Milk fat droplets have very weird and elaborate structures. Triacylglcyerol core surrounded by a monolayer of phospholipids. All enclosed by a phospholipid bilayer studded with proteins. Processing, like pasteurization and homogenization, changes the structure of milk.

Effect of chain length on physical properties

Density of organic compounds The density of organic compounds depends on their ratio of “heavy” and light atoms. For example: C8H18 C8H14Cl4 The ability of molecules to pack together well (efficiently) also affects density, but is less important than MW and ratio of heavy to light atoms. density (g/mL) n-pentane C5H12 0.62 1-bromobutane C4H9Br 1.27 hexane C6H14 0.66 cyclohexane C6H12 0.78 diethyl ether C4H10O 0.71 1-butanol C4H9OH 0.81  more dense, sinks Top: H2O, red food coloring; Bottom: CH2Cl2 D&D p.197-4 McKee p70