The miscibility of milk sphingomyelin and cholesterol is affected by temperature and surface pressure in mixed Langmuir monolayers

The miscibility of milk sphingomyelin (milk-SM) and cholesterol was investigated in this study. The effect of different physical states of milk-SM on its interactions with cholesterol was determined by the recording of isotherms of compression of Langmuir films for temperatures above and below the gel to Lα phase transition of milk-SM (T_m ∼ 34 °C). For T = 15 °C < T_m, the liquid expanded (LE) to liquid condensed (LC) phase transition of milk-SM monolayers was observed at surface pressures of 10–15 mN/m. For T = 43 °C > T_m, the milk-SM molecules were in a LE phase regardless of the surface pressure applied. A phase diagram pressure – milk-SM/cholesterol composition was established. This study demonstrated that both temperature and surface pressure affected the miscibility between the milk-SM and cholesterol. The strongest attractive forces (i.e. condensing effect) were identified for 30 mol% cholesterol when the milk-SM was in the LE phase state.