CHEM1612 - Pharmacy
Week 13: Surfactants
Dr. Siegbert Schmid
School of Chemistry, Rm 223
Phone: 9351 4196
E-mail: siegbert.schmid@sydney.edu.au
Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille,
Chemistry, John Wiley & Sons Australia, Ltd. 2008
ISBN: 9 78047081 0866
Surface Tension and Surfactants

Blackman, Bottle, Schmid, Mocerino & Wille: Chapters 7, 22
self-cleaning
surfaces
‘lotus effect’
goretex, teflon...
Lecture 37 - 3
Hi-tech: printing
Need to understand, and be able to control both the breakup of liquid jets
into small droplets, and the subsequent interaction of the droplets with the
target.
inkjet
Lecture 37 - 4
Surface Tension
Liquid/air interface
Any interface (liquid/liquid, liquid/solid, liquid/gas, etc.)
has an associated surface energy.

To understand, think of a liquid droplet: the molecules in
the interior of the droplet are surrounded by other
molecules of the same kind. However, those at the liquid
surface are subject to attractions only from the sides and
from below.

Bulk liquid
The effect of this uneven pull on the surface molecules is
to draw them into the body of the droplet. Therefore a
droplet assumes a spherical shape.
 The sphere is the geometry with minimum surface area.

The
liquid appears to have a skin over the surface.
BASF
Lecture 37 - 5
Surface Tension
Herminghaus, 2003.


Molecules on the surface would rather be in the bulk.
To increase the surface area, molecule within a phase must move
from the bulk to the surface, by breaking some attractive interactions,
which requires energy.
The resistance of a medium to an increase in its surface area is
called the surface tension of the medium (can be solid or liquid).
The surface tension is the energy required to increase the surface
area by a unit amount (units J/m2 = N/m).
Lecture 37 - 6
Surface Tension of Liquids


The stronger the forces between the molecules, the higher the surface
tension.
Polar solvents like water tend to have fairly high surface tension
because they interact strongly by H bonding.
hydrogen bond donor
hydrogen bond acceptor
H bond
Blackman Figure 6.32
Lecture 37 - 7
Surfactants

Surface-active agents: any molecule that is amphiphilic, i.e with
portions that are

hydrophobic (= water hating, therefore lipophilic, fat loving) and

hydrophilic (= lipophobic) portions.
Hydrophobic tail
hydrophilic head
McGraw Hill, 2006.
Figure from Silberberg, “Chemistry”,
Surfactants decrease the surface tension of water by adsorbing
at the water/air interface and disrupting the H bonds.
Soaps and detergents are surfactants, often salts of fatty acids.
Lecture 37 - 8
Demo: Adding Surfactant to Water

A beaker full of water

We
add
fine
sulphur
powder, whose weight can
be
supported
by
the
water/air interface due to
water surface tension g0.

Then we add soap, which
decreases
the
surface
tension of water, and so the
powder
cannot
be
supported anymore.
Water
Surface tension g0
Mineral oil
Water
Olive oil
Water
Lecture 37 - 9
Soap
Much of what we call dirt is non-polar. Grease for example consists of long chain
hydrocarbons.

However water, the solvent most commonly available to us is very polar and
will not dissolve ‘greasy dirt’

Soap can be viewed as an emulsifying agent, since it acts to suspend the
normally incompatible grease in the water.

Because of this ability to assist water in ‘wetting’ and suspending nonpolar
materials, soap is called a wetting agent or surfactant.
Lecture 37 - 10
Self-Assembly: Micelles



As a surfactant is added to water, the molecules adsorb at the air/liquid
interface, but otherwise are free in solution.
Above a certain conc., they spontaneously aggregate into micelles.
This occurs at the CRITICAL MICELLE CONCENTRATION (c.m.c.)
at the
c.m.c.

Reduced
Interaction
of chains
with water
Hydrophobic
interactions
between
chains
A “soap” solution contains both individual surfactants dispersed in water
and aggregates (micelles). Thus a soap-water mixture is a suspension
of micelles in water. Because the relatively large micelles scatter light
(colloidal), soapy water looks cloudy.
Lecture 37 - 11
Detergents

Artificial soaps are known as detergents

Most widely used class of detergents used are the alkylbenzene
sulfonates (with –SO3- group)

Fabric softener are often quaternary ammonium salts
Lecture 37 - 12
“Hard” and “Soft” Water

“Hard water” contains high amounts of divalent ions such as Ca2+,
Mg2+, Fe2+.
The disadvantage of soaps is that the anions form precipitates with
the cations like Ca2+ and Mg2+. This forms a scum and reduces the
soaps’ efficiency.

Ca2+ + 2 C17H35COONa(aq) → (C17H35COO)Ca (s) + 2 Na+

Lecture 37 - 13
Revision Lecture - Thursday

If you have questions send them to me by e-mail and I will
incorporate relevant material.

We will not cover any new material.

Everything we have covered in lectures (not lecture notes) is
examinable, as is the chemistry relating to your laboratory work.
Lecture 37 - 14
Lipids are broadly defined as any amphiphilic, naturallyoccurring molecules. The term is also used more
specifically to refer to fatty-acids and their derivatives, as
well as other fat-soluble sterol-containing metabolites such
as cholesterol.





fats
phospholipids (with a organophosphate group)
waxes
steroids
Hydrophobic
tail
Fats that are esters of glycerol are
called triglycerides.
Soaps are produced by saponification:
the hydrolysis of lipids to glycerol and
salts of fatty acids (carboxylate salts =
soaps) by KOH or NaOH.
Figure from Silberberg, “Chemistry”,

McGraw Hill, 2006.
Lipids
Polar-ionic
head
KOH
carboxylate salt
glycerol
Lecture 37 - 15
Self-Assembly in Lipids

In the self-assembly of surfactants and lipids, hydrophobic
interactions are important
Head
group
Tail
group
A lipid is an amphiphilic molecule,
but rarely exists as a monomer.
air
water
monolayer
Micelle
Inverse micelle
(in nonpolar
solvent)
Lipid bilayer
Lecture 37 - 16
Phospholipids
Phospatidylcholine
hydrophilic
Apolar
hydrophobic
a bilayer
Long hydrophobic tails
Polar head

Phospholipids are similar in structure to fats in that they are esters
of glycerol. However unlike fats they contain only two fatty acids.
The third ester linkage involves a phosphate group, which gives
phospholipids two distinct parts:
 long non-polar tail
 polar substituted phosphate “head”

Phospholipids tend to form bilayers in aqueous solution with the
tails in the interior and the polar heads interfacing with the polar
water molecules.
Lecture 37 - 17
Self-assembly in Phospholipids


Phospholipids prefer to form bilayer structures in aqueous solution
because their two fatty acid chains do not pack well;
Phospholipids can form either unilamellar vesicles (liposomes) or
multilamellar vesicles;
unilamellar vesicles (liposomes);
highly stable, can be used as drug
and enzyme delivery systems
multilamellar vesicles
Lecture 37 - 18




McGraw Hill, 2006.
Figure from Silberberg, “Chemistry”,
Cell Membrane
Ca. 8 nm thick
A cell membrane is a bilayer of phospholipids, embedded with
various proteins, and protects the cell from the extracellular fluid
that surrounds it.
Allows nutrients and other necessary chemicals to enter the cell
and waste products to leave, through the proteins that act as
pumps, gates, and channels.
Biological membranes are sites of biochemical reactions that
include photosynthesis, electron transfer, oxidative phosphorylation;
Facilitate cell motion; provide cell recognition and cell fusion.
Lecture 37 - 19
Fluid Mosaic Model
The most widely accepted model of this transfer of nutrients and waste
is called the fluid mosaic mode, proposed by S. J. Singer and G. L.
Nicolson, in 1972.

The phospholipid bilayer is a fluid matrix: the bilayer is a twodimensional solvent, lipids and proteins can undergo rotational and
lateral movement

Small uncharged molecules such as water,
oxygen and carbon dioxide diffuse freely
through the bilayer, while other substances
pass through “gates and passages”
provided by specific proteins embedded
in the membrane.
McGraw Hill, 2006.
Figure from Silberberg, “Chemistry”,

Lecture 37 - 20
You should now be able to

explain the molecular origin of surface tension

understand the chemical nature and action of surfactants and
detergents, and the process of self-assembly

describe the function and chemical structure of biological
membranes
Lecture 37 - 21