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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.
:O:
Soaps are a good example:
non-polar tail
hydrophobic
So soaps & other amphipathic compounds are able to clean grease
from objects by making the grease soluble in water.
K +1
:O:
..
-1
polar
head group
hydrophilic
• 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:
:O:
:O:
non-polar tail
hydrophobic
S
Na +1
:O:
..
polar
head group
hydrophilic
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
Effect of Chain Length on Boiling Point
220
170
Effect of H-Bonding on Physical
Properties
70
alkanes
alcohol
20
-30
2
1
3
4
5
9
8
7
6
10
1500
bp (C)/viscosity (cp)
bp (C)
120
-80
-130
-180
Number of Carbons
Effect of Chain Length on Viscosity
12
1000
bp (C )
viscosity (cp)
500
Viscosity @ 20C (cp)
0
10
1-propanol 2-propanol
8
alkanes
6
alcohols
4
2
0
1
2
3
4
5
6
7
Number of Carbons
8
9
10
propylene
glycol
glycerol
Density of organic compounds
The density of organic compounds depends on their ratio of “heavy” and
light atoms. For example:
C8H18
C8H14Cl4
 more dense, sinks
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
cyclohexane
C6H14
C6H12
0.66
0.78
diethyl ether
1-butanol
C4H10O
C4H9OH
0.71
0.81
Top: H2O, red food coloring;
Bottom: CH2Cl2
D&D p.197-4
McKee p70