The surface tension at the aqueous solution/air interface has been measured for eight surface active substances at 298.15°K under atmospheric pressure. It has been found that each surface tension versus concentration curve has a distinct break at a relatively low concentration and a relatively high surface tension. This result implies that the two states of adsorbed monolayer coexist in equilibrium at the break point. The surface excess numbers of moles of surface active substances and the surface pressure versus mean area per adsorbed molecule curves confirmed that the phase transformation takes place between the gaseous and the expanded states. It has also been shown that the hydrocarbon chain length, and the ionic nature and size of polar head group of a surface active substance have a great influence on the properties inherent in the phase transition.
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