Spreading a spoonful of oil on a pond of water can create a thin layer called a Langmuir-Blodgett (LB) film, which can be used in sensor devices, electronic switches and non-linear optical materials. Such applications have until now been limited by the presence of a large amount of defects in the films.
In order to improve this, three teams of chemists and chemical engineers have made a nearly defect-free LB film, which could increase the application possibilities of LB films.
An LB film consists of a single layer or many layers of organic material, each layer being one-molecule thick, deposited on a solid or liquid substrate. The film created by these scientists is made of one layer of steric acid molecules.
‘Each molecule has the shape of an upside down lollipop,’ said Paul S. Cremer, a Texas A&M chemist and leader of one of the teams. ‘The head represents the carboxylic acid and the long chain represents the tail.’
The most striking feature of these molecules is that they are amphiphilic: the head is water-soluble whereas the tail is water-insoluble.
‘These molecules love water on one side and hate water on the other side,’ Cremer said, ‘but the two sides are bound, they cannot come apart.’
Ideally, the molecules sit in a perfect lattice next to each other throughout the whole layer. Until the recent discovery the molecules tended to cluster randomly, making one or more layers, and leaving holes in between the clusters.
Over the past two years, the three groups from different US institutions have been trying to make a highly regular layer of steric acid molecules with the least number of detects as possible.
Joseph A. Zasadzinski, professor of chemical engineering at the University of California came up with the idea of using more basic (non-acid) solutions.
‘If you try to make these films in an acid solution, you get many more detects,’ Cremer said. ‘But if you use a basic solution, the carboxylic acid of every molecule loses a proton. That leaves a negative charge at the base of each molecule. Then if you bring in a cadmium ion, which is doubly positively charged, in between the molecules, it sticks there, locking the molecules in place.’
Acting like glue, cadmium ions stick the bases of each molecule with its neighbour. The molecules cannot escape from each other nor can they jump onto each other to form more layers.
‘We are locking the lollipop heads together,’ said Cremer, ‘so we can have only one layer all the way along the surface without big holes or the wrong number of layers in some places on the surface.’
Cremer notes that scientists could have predicted such result many years ago, but they would not have had the techniques to prove it.
They would also had to have the right basic (non-acid) solution, to consider an LB film made of steric acids, and to choose cadmium as the ‘stiffening’ agent.
Cremer added that though this achievement is unprecedented, it is only one way to make a nearly-defect free LB layer.
‘Our LB film is the best ordered among all other known LB films,’ he said, ‘but this is one way of doing it. We have not solved all the problems yet.’