NONIONIC SURFACTANT MONOLAYERS

The foam stability of aqueous solutions of pentaethylene glycol n-decy l ether and pentaethylene glycol n-dodecyl ether surfactants was relat ed to experimental studies carried out on thin horizontal microscopic single films of the surfactant solutions using the microinterferometri c film apparatus developed by Scheludko [1]. It was observed that an i nitial decrease in film thickness and reduction in potential at the ai r/solution interface of the film, occurred at a critical surfactant co ncentration where ''black spots'' appeared in the film when it was obs erved in reflected light. The black spots appeared to expand and coale sce, until the entire sheet was thinned down to a metastable common bl ack film. In addition, a sharp increase in foam stability occured abov e this surfactant concentration, which was about 0.1 c. m. c., and cor responded to a high surface adsorption density of surfactant. We belie ve that the cause of the phenomenon was a change in state of the surfa ctant monolayer as originally proposed by Scheludko and Exerowa [2,3]. Such a change (condensation) from gaseous to liquid expanded monolaye rs, has already been established for ionic surfactants and may change the rheological properties of the adsorbed film, enhancing the foam st ability Finally the foam stability was determined at a range of electr olyte (KCI) concentrations from which it could be suggested that two t ypes of foam stabilization methods were operating. In the low electrol yte concentration, the stability appeared to be controlled by the meta stable common first black film, but at high electrolyte concentrations the stability seemed to be dependent on secondary ''Newton black'' fi lms.