Effect of Oily Additives on Foamability and Foam Stability. 2. Entry Barriers

In the preceding paper of this series we studied the effect of several oils of different chemical structure on the foaming properties of sodium dodecylbenzenesulfonate solutions. A straightforward correlation was found between the foam stability and the so-called "entry barrier", which prevents the emergence of pre-emulsified oil drops on the solution surface. In the present article we perform a systematic experimental study of the entry barriers for several oils by means of the recently developed film trapping technique. The latter consists of trapping oil drops in wetting films on a solid substrate, followed by a controlled increase of the capillary pressure of the meniscus that compresses the drops against the substrate. At a certain critical capillary pressure, PCCR, the asymmetric oil−water−air films rupture and the drops enter the water−air interface. This event is observed microscopically, and PCCR is determined as a function of various parameters (type of oil, surfactant concentration, drop size, and others). The entry barrier increases with the surfactant concentration, especially in the range where the surfactant micelles are expected to stabilize the asymmetric films. The results obtained with a series of alkanes (from octane to hexadecane) show that the entry barrier increases with the alkane chain length. Furthermore, it is shown that the presence of a spread oil (even as an ultrathin, molecular layer) on the surface of the foam film might lead to a significant change of the magnitude of the entry barrier. For decane and dodecane, the layer of spread oil reduces the entry barrier, whereas for hexadecane the effect is the opposite. As far as we know, such a role of oil spreading in the antifoaming action of oils has not been reported so far. Since the stability of thin liquid films is usually discussed in the literature in terms of the disjoining pressure, we estimate from the experimental data the critical disjoining pressure, ΠASCR, at which the asymmetric oil−water−air film ruptures and the drop entry occurs. The estimates show that the curvature of the asymmetric film is very important in the overall consideration of the mechanical equilibrium in the system and there is a big difference between the numerical values of PCCR and ΠASCR, unlike the case of planar films where PCCR = ΠASCR. Additionally, we find that PCCR is a weak function of the oil drop size and of the asymmetric film radius, while ΠASCR scales as (film radius)-1 for all of the studied systems. These results are discussed with respect to the possible mechanisms of film rupture. Concerning the foam stability, PCCR is a more convenient quantity for description of the entry barriers, because its magnitude correlates with the foam height, whereas the magnitude of ΠASCR does not.