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Determination of CMC of Surfactant by Conductivity Methods
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Contents
Purposes and Demands
1. Master the technique and principle that is used to determine the CMC of surfactant solution.
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.
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.
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.
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 1m 2 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 Λ 0 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
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 －2 mol · -l in 25ml volumetric flasks with conductance water.
3. Determine conductivity of solution from high concentration to low one. Electrodes should be washed up with solution before determination.
Data Records and Processing
1.
Calculate conductivity and molar conductivity of terms of sodium lauryl sulfate solution.
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
1.
Why temperature of liquid should be kept constant?
2.
Whether this method can be used to determine non-ion surfactant ’ s CMC? If not, which method can be used to?
Attentions
1.
Surfactant should be dissolved absolutely in water.
2. The value of CMC is limited.
Outline in the use of DDS-11A conductivity detector
1. Switch on the electrical source and allow it warm-up for several min (until pointer turns stable).
2. Turn the constant knob to be electrode constant (it ’ s been checked and determined by the factory).
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.
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 10 4 →10 3 →10 2 …… 10 …… 1 …… 0.1
is:
5. Reading and record. The data marker is in the form of ×10 x ×10 6 (s/cm). X stands for 4 , 3 or 2 and 1 in the measuring range.