Intrinsic near Infrared Sensor for the Determination of the Drainage Behaviour of Aqueous Detergent Solutions

Abstract

Surfactants, detergents or tensides are synonyms for substances that, even at low concentrations, can drastically alter the interfacial properties of a multiphase system. Their main application fields—among many others—are cleaning processes in households, trade and industry. A certain substance-specific concentration (c.m.c.) is generally considered to be optimal for these applications. However, the fast and inexpensive determination of tenside concentrations is still a difficult task. Our approach to this problem is based on the fact that the draining behaviour of aqueous solutions on hard surfaces depends strongly on the actual concentration of detergents. Near the critical micelle concentration a stable fluid film remains on the wetted surface. The use of optic-intrinsic sensors in the NIR allows the detection of these changes in a straightforward manner by monitoring the sensor response after retrieving the sensor from the sample liquid. A potential application of this sensor principle is expected for controlling the detergent addition mainly in large-scale cleaning devices. This would be of both economical and ecological advantage.