Surface Tension of Surfactant Solutions with Different Concentrations

2019-09-27 08:35:39

Surface tension method: CMC is determined by the logarithmic figure of surface tension-concentration. Specific method: Measure the surface tension of a series of surfactant solutions with different concentration, make a gamma-lgc curve, and extend the linear part on both sides of the curve turning point. The concentration of intersection point is the CMC of surfactant in the system. 2) Conductivity method: When determining cmc, the square root of concentration can be plotted by conductivity to concentration or molar conductivity to concentration. The concentration of turning point is cmc. It is limited to the determination of CMC of ionic surfactants. 3) Dye method: first, a small amount of dye was added to the surfactant solution with a higher concentration (>cmc), and the dye was dissolved in micelles, showing a certain color. Then the solution was diluted with water by titration until the color changed significantly. At this time, the concentration of the solution was cmc. 4) Turbidity method: The concentration of the sudden change point of turbidity is CMC of surfactant when the turbidity of surfactant solution with appropriate amount of hydrocarbon is observed. 5) Photodispersive method: CMC can be determined by using the mutation point in the scattered light intensity-solution concentration curve.

The influence of surfactant type: under the same hydrophobic group, the CMC of ionic surfactant is larger than that of non-ionic surfactant, about two orders of magnitude difference.

2) The length of hydrocarbon chain: the critical micelle concentration of the same type of surfactants decreases with the increase of hydrophobic groups; the number of carbon atoms in the hydrocarbon chain of ionic surfactants ranges from 8 to 16, and the change of CMC with the number of carbon atoms shows a certain rule: for each additional carbon atom in the homologues, the CMC decreases by about half. Nonionic surfactants, CMC is more affected by the number of hydrophobic carbon atoms. In general, for every two additional carbon atoms, the CMC decreases to 1/10.

3) Branch of hydrocarbon chain: In surfactant molecular isomers with the same chemical composition, surface active agents with straight hydrocarbon chain have lower cmc, higher branching degree and higher cmc.

4) Location of polar groups: When the hydrocarbon chain is the same, the closer the polar group is to the middle position, the larger the CMC is.

5) The influence of other substituents in the hydrocarbon chain: when there are double bonds in the hydrocarbon chain, its CMC is higher than that of the saturated compounds. When there are phenyl in the hydrophobic chain, one phenyl is equivalent to about 3.5 CH2 groups.

6) Properties of hydrophobic chains: Surfactants containing carbon and fluorine chains have much lower CMC than hydrocarbon chain surfactants with the same number of carbon atoms, which correspondingly have much higher surface activity. The CMC of the surfactant whose hydrogen in the hydrocarbon chain is partially replaced by fluorine decreases with the increase of the degree of substitution.

7) Other factors: In addition to the chemical structure of surfactants, additives (such as inorganic salts, polar organic compounds) have an impact on the CMC of surfactants; temperature also has an impact on cmc.

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