The use of methyl orange for the characterization of micelles in aqueous nonionic surfactant solutions

Abstract

As has been shown for the system poly(ethylene glycol) (PEG-600) — H₂O, a gradual change in the number of water molecules per oxyethylene (EO) unit leads to a corresponding change of the wavelength of the absorption maximum of methyl orange (MO). This effect has been used to investigate the alterations of the hydration state of oligoether chains in nonionic surfactans solutions. The measurements have been carried out with polydisperse preparations of p-(1,1,3,3-tetramethylbutyl)phenyoxypoly(oxyethylene glycol) with an average EO-chain length varying from 3.1 (OPE-3.1) to 90 (OPE-90). Special attention has been focused on the influence of lower alkanols on solutions of OPE-9. The shift of the MO peak sometimes parallels the micellization of nonionic surfactants, but not necessarily. The main information obtained by the MO method is about the hydration state of the EO chains, i.e., their aggregation state. By varying the EO degree of the OPEs from low (3.1, 5.4) to medium (9, 10, 13, 18) to high values (70, 90), distinct alterations in the aggregation behavior have been observed. It has also been shown solubilization and the structure-breaking action of alkanols affect the hydration state of the oligoether chains.