ABSTRACT
The lifetime of the vertical film (Trup) and the aqueous core thickness of rupture (Drup), which characterizes the drainage curve of aqueous nonionic solutions, are summarized in
Table 3 [79]. Although it is difficult to determine the position of foam film rupture in a vertical foam film, we would like to discuss about foam film stability assuming that the film drainage parameters (Trup, Drup) monitored at the center of the frame reflect the film properties at the rupture. There was a difference in the Trup values, which depends on the hydrocarbon chain length. It was found that the Drup increased with temperature in the case of C10E8 and C12E8 solutions and the Drup value remarkably decreased with increasing hydrocarbon chain length from C12E8 to C16E8. The foam films were ruptured on the order of 100 nm for both the C10E8 and C12E8 solutions in the higher temperature region, whereas the black films were formed for both the C14E8 and C16E8 solutions under the same conditions. These results clearly show that the intermolecular hydrophobic interaction in the adsorption layer on the surfaces of foam films
Trup/sec Drup/nm
Surfactants 2°C 15°C 40°C 2°C 15°C 40°C
was a significant factor in the stability of the vertical thick foam films during drainage. The two surfactants (C10E8, C12E8), which occupy an area of 0.655 and 0.589 nm2, respectively, form unstable foam films with a Drup of 315 and 84 nm at 40°C, whereas the other two surfactants (C14E8, C16E8), which occupy an area of 0.496 and 0.381 nm2, respectively, form stable foam films with a Drup of 2 and 16 nm, respectively. However, the foam film of the C14E8 solution was ruptured at a silver film with a Drup of 34 nm at 2°C, whereas the Newton black film was observed at 15°C and 40°C. These results may reflect the relative differences in the contribution of the two factors: liberation of restricted water around the hydrophobic moiety of nonionics with increasing temperature and the strengthening of the hydrophobic interaction which prevents fluctuation in the vertical thick foam films during drainage. Thus the black film was not maintained at 2°C because the hydrophobic interaction could be reduced by an increase in the HLB of the surfactant with decreasing temperature. The CMC of the POE alkyl ether solution decreases with increasing temperature [178]. It can be assumed that the black film formation of the systems, which act as an effective hydrophobic interaction, was performed with a decrease in the CMC around the high temperature region. Furthermore, a Newton black film was observed in the C16E8 solution even when the temperature was lowered to 2°C. This is in line with the decrease in the CMC with an increase in hydrocarbon chain length. With the increase in the hydrophobic interaction, the lifetime of the vertical films was prolonged and a sufficient drainage of the films resulted.
