The delightful iridescence and evanescence of soap bubbles arises from the fact that their walls are nothing but a film of water sandwiched between two layers of surfactant molecules. This unusual construction also endows soap bubbles with a uniquely large ratio of surface area to contained liquid. Tinakorn Kanyanee et al. (Anal. Chem. doi:10.1021/ac052198h; 2006) set out to exploit this feature — by ‘wiring up’ soap bubbles and using them as the functional heart of a fully automated detection system for trace gases.
The authors' bubble factory uses a sealed, transparent plastic box with water at the bottom to keep the environment moist and so prolong bubble life. Soap solution is first supplied to the box through the inner of two concentric tubes. As the solution spreads from this tube's slightly recessed tip, it coats the tip of the surrounding outer tube, through which a controlled blast of air is delivered, inflating the soap film and creating a bubble. Precise metering of the air pulse ensures that the bubble is inflated just enough to touch two opposing stainless-steel electrode rods that jut into the box. The bubble-box reproducibly generates long-lived bubbles of uniform size and wall-thickness, so that the conductance measured between the electrodes for a given bubble geometry depends on the bubble's chemical composition.
To use their system for trace-gas analysis, the authors added the oxidizing agent hydrogen peroxide to the soap solution and exposed the resulting bubble to air containing traces of sulphur dioxide. The sulphur dioxide readily diffused into the bubble, where it was oxidized and sulphuric acid was produced. The presence of acid increased the bubble's electrical conductance.
This effect enabled quantitative determination of part-per-billion levels of sulphur dioxide within minutes. The approach should also be applicable to a range of other soluble and reactive gases. But as Kanyanee and colleagues point out, the wider message is that soap bubbles are an attractive, yet largely unexplored, tool for concentrating trace gases to enable their detection. Whether these species are then detected through conductance measurements, or through bursting the bubble and analysing its liquid by other means, there is, in that sense, more to this story than mostly froth and bubble.