Liquid-Expanded to Liquid-Condensed Transitions in Lipid Monolayers at the Air/Water Interface

The so-called liquid-expanded to liquid-condensed phase transitions in monolayers of n-pentadecanoic and n-hexadecanoic acids at the air/ aqueous HC1 interface and of dihexadecanoylphosphatidylcholine at the air/aqueous NaCl interface have been studied in detail. The experiments followed critical methods for monolayer spreading, surface pressure measurement, and humidity control and used ultrahigh purity lipids. It has been demonstrated unambiguously from the pressure isotherms, and confirmed by surface potential measurements, that the transition is simply first order. The frequently reported “higher order” nature of this transition is the result of inadequate monolayer techniques, poor humidity control, and the use of insufficiently pure monolayer substances or substrates. Some results on temperature effects are also reported. It is proposed that the use of the notation “liquid expanded” and “liquid condensed” be abandoned. Analysis of the published results on phase transitions observed in phospholipid monolayers at the oil/water interface confirms that they are not first order.