Tensammetry with accumulation on the hanging mercury drop electrode : Part 5. The behaviour of mixtures of oxyethylated alcohols

Abstract

The behaviour of the following mixtures of polydispersed oxyethylated alcohols having a well-defined n-alkyl radical was investigated: 18-14/6-14, 18-14/10-14, 18-14/18-10, 18-14/18-6, 18-6/18-10, 6-14/10-14 and 18-6/18-10/18-14 (the first number denotes the number of carbon atoms in the n-alkyl chain, and the second is the number of oxyethylene subunits). The influence of preconcentration potential in the range more negative than - 1.0 V vs. SCE is described in detail. The results are compared with the behaviour of model mixtures of poly(ethylene glycols). The surfactant mixtures 18-14/6-14 and 18-14/10-14 behave similarly to a model mixture of components having sufficiently different properties. Separate peaks for both components appear on the tensammetric curves of the mixtures, but only at certain preconcentration potentials are the peak heights similar to the peak heights for the individual components alone. The behaviour of the 18-14, 18-10 and 18-6 mixtures, in alt combinations, depends strongly on whether premicellar (associated) forms are present in the solution. These premicellar forms are indicated by a common very narrow peak on the tensammetric curves. If the monomeric forms predominate (i.e., at low-total surfactant concentrations) the peak heights for the mixtures are approximately additive after preconcentration at a suitable potential. If premicellar forms predominate (as at total concentrations > 0.1 mg l −1), the behaviour of the mixtures is a result of competition between the monomeric and premicellar forms in the adsorption process. This behaviour approximates the behaviour of a mixture consisting of sufficiently different components; monomeric forms behave like the first component, and premicellar forms like the second component of the model mixture. For the surfactant mixture 18-14/18-10, which behaves additively under certain conditions, it is possible to determine the total concentration within the range 10-35μgl −1. The 6-14/10-14 mixture forms a single nonadditive peak.