Association and solubility studies of mixed polyglycol ether surfactants in water

The effect of varying the molecular structure and the type of counter ion of polyglycol ether based anionic surface-active components on the processes of phase separation, adsorption to the air-water interface and molecular association in aqueous model metalworking formulations containing a polypropylene glycol was studied. Phase separation and micellization were studied via static surface tension measurements and by using an instrument, which enables the automatic scanning of temperature and concentration dependent pH, turbidity and conductivity data. The anionic surfactants investigated were highly water-soluble and the surface tension isotherms of all formulations were found to display a common anomalous trend characterized by an initial maximum in the Gibbs surface excess function followed by a plateau region before a second maximum occurs just before the concentration taken to indicate micellization. The aqueous mixtures in most cases displayed concurrent phase separation and micelle formation, which has to be taken into account when comparing critical micelle concentrations (CMCs). The present availability of parallel data, turbidity data in particular, allowed incipient phase separation to be detected. This is a circumstance, which would otherwise pass unnoticed, since it is not visually detectable. Results are considered industrially useful since the control of the isotropic temperature–concentration space is a key aspect in the design and fine-tuning of environmentally benevolent aqueous functional fluids for specific applications.