The influence of spreading solvent traces in the atmosphere on surface tension measurements by using a micro-film balance and the captive bubble method

Two different methods for investigating monolayer properties are presented, the micro-film balance and the captive bubble method. Both require the elimination of traces of the spreading solvent in the atmosphere, as in the micro-film balance experiment a drop is kept in a small hermetically closed chamber and in the captive bubble method the monolayer is spread inside a bubble. The influence of different organic spreading solvents on the surface tension at the air/water interface was checked for the two methods. The surface tension of water was measured by the micro-film balance when exchanging the air in the chamber after spreading. Using a small amount of methanol as solvent, similar results in the captive bubble experiments are obtained. In the presence of a monolayer, traces of solvents interact with the monolayer molecules and can be removed only immediately after spreading. In such cases, a good agreement with surface pressure isotherms determined by a Langmuir trough is achieved. When different amounts of chloroform, heptane and methanol are evaporated in the cell atmosphere, the π/A isotherms of 1-monoglycerid monolayers change dramatically up to atmosphere saturation by the hydrocarbons. These changes could not be removed by air exchange in the cell. Therefore, the micro-film balance is applicable in investigating interactions between monolayer and gas phase. Taking into account these results, dipalmitoyl phosphatidylcholine monolayers were investigated by both the captive bubble and the micro-film balance. The results of both methods are in good agreement when the methodological requirements stated above are met.