|
Soap bubbles may be notoriously delicate and short-lived, but Purnendu Dasgupta of Texas Tech University, Lubbock, thinks that they could make highly useful tools for analytical chemistry. He has already developed a device that can detect sulphur dioxide by measuring the electrical conductivity of a captured soap bubble, and is now in the process of using bubbles as the basis for an entirely novel separation system. Soap bubbles consist of a layer of water sandwiched between two layers of soap molecules, which, like all surfactants, possess a hydrophilic (water-loving) end and a hydrophobic (water-avoiding) end. The hydrophilic ends point towards the water layer and the hydrophobic ends point away from it. Initially generated as a thin film, these layers naturally form into spheres to minimise their surface energy. As these spherical bubbles are hollow, they have a large surface area compared to their liquid volume. It is this property that initially brought them to the attention of Dasgupta, because they possess a large area for absorbing gaseous compounds. After spending the past decade exploring the analytical potential of liquid drops and films, Dasgupta and his colleagues decided to turn their attention to bubbles. They have now built a device that can generate bubbles of specific sizes, keep them stable for over 30 minutes and measure their electrical conductivity. This device consists of a clear polystyrene box, with holes on either side where electrodes with spherical caps can be inserted. A solution of the surfactant Triton X-100 and glycerol (to promote bubble stability) is combined with a stream of air at a nozzle that pokes through the top of the box. This produces a bubble that can be captured between the two moveable electrodes, allowing its conductivity to be measured. The bottom of the box contains water to produce a humid environment, which is also important for maintaining bubble stability. Dasgupta and his team found that they could control the size of the bubble by combining constant amounts of liquid solution and gas, with the same amounts always producing a bubble of the same size. The size of the bubble is important, because the only factors that can affect a bubble's electrical conductivity are the bubble membrane's thickness and chemical make-up. For the same amount of liquid mixture, larger bubbles have thinner membranes and Dasgupta showed that they are less able to conduct electricity. If the bubble size is held constant, however, any change in the bubble's conductivity must be due to changes in the chemical make-up of the bubble membrane. To show that this was the case, Dasgupta and his team added different concentrations of sulphuric acid to the bubble mixture and showed that the conductivity increased in line with the sulphuric acid concentrations. They then exposed bubbles doped with hydrogen peroxide to a gas stream containing tiny amounts (parts per billion) of sulphur dioxide, which was absorbed by the bubbles to form sulphuric acid. They found that bubbles exposed to higher levels of sulphur dioxide had much greater electrical conductivity than those exposed to lower levels. For Dasgupta, perhaps the most interesting aspect of this research was the finding that he and his team could create bubbles that were stable for 30 minutes. They have now extended this to over an hour and are hoping to develop versions of the device that can maintain a bubble for more than a day. "At this point, you can start doing multi-stage separations," says Dasgupta. For instance, by incorporating chiral compounds into the bubble membrane, bubbles could theoretically be used to conduct enantiometric separations. Then they really would become an important tool for analytical chemists. Related links:
|
 |
|
|
|
|
