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 dodecy lbenzenesulfonate solutions. A straightforward correlation was found betwee n the foam stability and the so-called "entry barrier", which prevents the emergence of pre-emulsified oil drops on the solution surface. In the prese nt article we perform a systematic experimental study of the entry barriers for several oils by means of the recently developed film trapping techniqu e. The latter consists of trapping oil drops in wetting films on a solid su bstrate, followed by a controlled increase of the capillary pressure of the meniscus that compresses the drops against the substrate. At a certain cri tical capillary pressure, P-C(CR), the asymmetric oil-water-air films ruptu re and the drops enter the water-air interface. This event is observed micr oscopically, and P-C(CR) is determined as a function of various parameters (type of oil, surfactant concentration, drop size, and others). The entry b arrier increases with the surfactant concentration, especially in the range where the surfactant micelles are expected to stabilize the asymmetric fil ms. The results obtained with a series of alkanes (from octane to hexadecan e) show that the entry barrier increases with the alkane chain length. Furt hermore, it is shown that the presence of a spread oil (even as an ultrathi n, molecular layer) on the surface of the foam film might lead to a signifi cant 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 te rms of the disjoining pressure, we estimate from the experimental data the critical disjoining pressure, IIASCR, at which the asymmetric oil-water-air film ruptures and the drop entry occurs. The estimates show that the curva ture of the asymmetric film is very important in the overall consideration of the mechanical equilibrium in the system and there is a big difference b etween the numerical values of P-C(CR) and IIASCR, unlike the case of plana r films where P-C(CR) = IIASCR. Additionally, we find that P-C(CR) is a wea k function of the oil drop size and of the asymmetric film radius, while II ASCR scales as (film radius)(-1) for all of the studied systems. These resu lts are discussed with respect to the possible mechanisms of film rupture. Concerning the foam stability, P-C(CR) is a more convenient quantity for de scription of the entry barriers, because its magnitude correlates with the foam height, whereas the magnitude of IIASCR does not.