Common black and Newton film formation

Abstract

By model investigations of microscopic foam films (stabilized with sodium octyl sulfate, NaOS, and sodium dodecyl sulfate, NaDoS, in the presence of electrolyte) and by study of the properties of the adsorbed layers of the solutions it is shown that the adsorbed layers at the interface are essential for the formation of common black and Newton black spots and films. The adsorption isotherms are obtained using a precise spherotensiometric method. The model microscopic film with a radius of 10−2 cm that has been used makes it possible to work at very low surfactant concentrations and thus the lowest surfactant concentration at which a black spot appears has been found. The concentrations of formation of common black spots and Newton black spots are distinguished. A correlation between the probability of the formation of black spots (common and Newton) and the state of the adsorbed layer is established. For instance, the formation of common black spots and the deviation from the ideal state in the adsorbed layer are related to the formation of associates (premicelles) in the bulk and their adsorption at the interface. The formation of common black spots with a probability of one occurs at a concentration close to the saturation concentration Γ∞ of the adsorbed substance. The close packing of the molecules at the interface is a necessary condition for the formation of Newton black spots. The formation of Newton black spots with a probability of one occurs at a somewhat higher concentration (two to three times higher than the concentration corresponding to the closest packing of molecules at the interface). The dependence of the lifetime (stability) of the two types of black films (common and Newton) on the surfactant concentration is also studied. It is found that at a definite concentration (different for the common black and Newton films) the film persistence increases steeply. It is shown that the proposed surface model is not decisive for the understanding of this stability (unlike the formation of the two types of black films).