TRACKER - EXAMPLE 3

INTRODUCTION :

The competition between two surfactants can be shown through the evolution of the rheological characteristics of the interface, although the evolution of the interfacial tension does not show real differences.

PROTOCOL :

1/ Surfactants :

• BSA (10mg/l)

• C12E6 (1mg/l)
  

2/ Measurement Conditions :

• Temperature 25°C

• Duration of each measurement 1000 seconds

• Cleaning of the requisites :

- cuvette with sulfochromic acidsyringe

- capillary with chloroform then ethanol and final rinsing with distilled water

RESULTS :

1/ Measurement of the interfacial tensions,

2/ Measurement of the rheological characteristics of the interfaces,

1/ Measurement of the interfacial tensions :

The interfacial tension is measured with a rising bubble of air (about 10 µl) in a cuvette filled with 6-ml water containing either of the two surfactant of their mixture.

The following figure gives the adsorption kinetics of each surfactant and that of their mixture.

Comments :

The adsorption kinetics of each surfactant are quite similar and the final interfacial tension, which is close to the equilibrium, is about 54 mN/m. The interfacial tension of the mixture decreases faster and also gives a final value close to that reached by each surfactant.
It has to be noticed that for the BSA, the interfacial tension shows an initial plateau which is characteristic to the proteins and does not exist for the C12E6.

2/ Measurement of the rheological characteristics of the interfaces :

The viscoelastic characteristics of each interface are calculated from the response of the interfacial tension to sinusoidal variations of the volume of the bubble.

• Period of the oscillations: 10 seconds

• Deformation of the interface: +/- 5 %

The following figure shows evolution of the viscoelastic modulus of each surfactant and their mixture.

Comments :

The evolution of the interfacial viscoelastic modulus of the interface with the BSA is quite different from that with the C12E6. The former gives a high elastic interface, the latter a lower one while the evolution of their interfacial tension is quite similar (see previous figure).
Finally the evolution of the viscoelastic modulus of the interface with the mixture (black curve) shows that at the beginning of the adsorption the characteristics of the interface are close to that given by the BSA but are changing and reaching values close to that given by the C12E6.

CONCLUSION

This example shows (with the TRACKER) that the competition between two surfactants can be shown through the evolution of the rheological characteristics of the interface, although the evolution of the interfacial tension does not show real differences.