Critical Micelle Concentration Experiments
Theory of Critical Micelle Concentration Experiments
Definition
Certain molecules can be classified according to their
affinity towards polar and non-polar solutes. An affinity towards polar
substances such as water is said to be hydrophilic, conversely an affinity
towards non-polar substances such as hydrocarbons is said to be hydrophobic. A
molecule may be either completely hydrophobic or hydrophilic, or it could have
portions that are hydrophilic and portions that are hydrophobic. Such a
molecule is said to be amphiphilic.
Amphiphilic molecules are very interesting, especially in
the way they interact with water. The behavior can be considered to be similar
to that of a magnet- the two ends of the molecule have opposite polarity and
the water has the same polarity as one of the ends. The end with the same
polarity as water is repelled away from the water (hydrophobic) and the end
with the opposite polarity is attracted towards the water (hydrophilic). Two
optional configurations exist, surfactants and micelles.
This type of arrangement occurs at the boundary between any
polar or non-polar liquid and air, if a non-polar liquid is used then the
hydrophobic portion points downwards and the hydrophilic portion points
upwards. The presence of these molecules at the surface disrupts the cohesive
energy of the surface and thus lowers the surface tension. Such molecules are
called surfactants, short for surface active molecules.
The other possible arrangement is with a cluster of
molecules forming a ball within the liquid, known as a micelle. In the case of
a polar liquid such as water the hydrophilic portions make up the exterior of
the ball and the hydrophobic portions make up the interior.
In a non-polar liquid the arrangement would be
such that the hydrophilic portions are pointed towards the center of the
molecule and the hydrophobic portions are pointed away from the center. The
formation of micelles is less favored than surfactants, which allows the
investigation of amphiphiles through comparing surface tension with the
concentration
