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Determination of CMC of
Surfactant by Conductivity
Methods
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Contents
Purposes and Demands
 Principle
 Apparatus and Reagent
 Procedure
Data Records and Processing
Questions
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Attentions
 Demonstrated
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Purposes and Demands
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1. Master the technique and principle
that is used to determine the CMC of
surfactant solution.
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2.Understand more deeply about the
properties of surfactant solution.
Principle

Provided that the solution is rather dilute,
if concentration of surfactant solution
increases to a certain value, its ions or
molecules will come to association reaction,
and start to be micelles, and this certain
value of concentration is called CMC
(shorted for critical micelle
concentration).

Since most properties of physical
chemistry would make a sudden change
along with the formation of micelle,
CMC is considered as the measurement
for surfactant.

So, it is vital for doing further
researches on physical chemistry
properties of surfactant to determine
the CMC and master the condition what
correlative to micelle.

There are many ways to determine CMC
by which make physical chemistry
properties of the solution have a
sudden change along with the
concentration changing of surfactant.
Generally, there are four methods to
measure CMC showed as follows:

1. Conductivity methods
By the relationship between
conductivity of ionic surfactant
solution and concentration, draw
curves of conductivity versus
concentration or molar conductivity
on which turning points is the CMC.
The method is likely to the ionic
surfactant except for surfactant of
which CMC is quite big and surface
active is bad.
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2. Surface tension method.
Determine surface tension of solution
towards several different
concentrations, and then draw a graph
of surface tension against lgc the
logarithm of concentration, and the
turning points is the CMC.
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3. The means of dyestuff
The color of the micelle would change if
adsorbs colored organic ion of some
dyestuff. The dyestuff method is concerned
with the maximal absorption spectrometer
change for determination with dyestuff for
indicator.
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4. Solubilization method.
The solubilization that surfactant
act on organics has an obvious change
when the concentration is equal to
CMC.

The experiment we shall be concerned
with the conductivity method to
determine the value of CMC.

As to general electrolyte solution, the
capacity of electric conductivity is due to
conductance that is defined as the
reciprocal of resistance and the
conductance of a homogeneous body of
uniform cross section is proportional to
the cross-sectional areas A and inversely
proportional to the length L:
and conductance is expressed in units
of Ω－１.
кis the conductance determined when A
is 1m2 and L is m. L/A is the cell
constant.
There is an relationship between conductivity к
and molar conductivity Λ
Where Λis the conductive ability of 1mol
electrolyte solution and C is molar concentration.
Λ changes along with electrolyte’s concentration.
As to thin solution of strong electrolyte, Λ is
expressed in the equation

Where Λ0 stands for the molar
conductivity when concentration is
limitless thin and A is constant.

For ion-surfactant solution, when the
concentration of solution is equal to
CMC, micelle come to being,
conductivity and molar conductivity
have obvious change, thus, it can be
used as an important method to
determined CMC.
Apparatus and Reagent
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DDS—ⅡＡ conductivity detector
Thermostatic bath
Volumetric flask
Pipette
KCl
Sodium lauryl sulfate (repeat crystal
process using ethanol as solvent for three
times)
Conductance water
Procedure

1. Regulate the temperature of
thermostatic bath at 25±0.1℃.
2. Make up 8-10 solution of
concentration between３×10－3 to３×10
－2mol·-l in 25ml volumetric flasks
with conductance water.
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3. Determine conductivity of solution
from high concentration to low one.
Electrodes should be washed up with
solution before determination.
Data Records and Processing
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1. Calculate conductivity and molar
conductivity of terms of sodium lauryl
sulfate solution.
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2. List the data. Draw a graph of K to c,
and figure out the value of CMC from the
turning point in the curve.
Questions
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1. Why temperature of liquid should
be kept constant?
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2. Whether this method can be used
to determine non-ion surfactant’s CMC?
If not, which method can be used to?
Attentions
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1. Surfactant should be dissolved
absolutely in water.
2. The value of CMC is limited.
Outline in the use of
DDS-11A conductivity detector
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1. Switch on the electrical source
and allow it warm-up for several min
(until pointer turns stable).
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2. Turn the constant knob to be electrode
constant (it’s been checked and determined
by the factory).
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3. Emendation. Determine conductivity of
H2O on low round and HAc is on high round.
Turn the knob, driving the pointer point to
full scale namely ↗1.0.
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4.Determination.
Choose
proper
measuring range from large range to
small one and try to control the
pointer pointing to the full scale in
order to get accurate data.
That
just
104→103→102……10……1……0.1
is:
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5. Reading and record. The data
marker is in the form of ×10x×106(s/cm). X stands for 4 , 3 or 2 and 1
in the measuring range.