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

•The interactions between milk sphingomyelin and cholesterol are involved in the formation of membrane domains.•The melting temperature of milk-SM is Tm = 34 °C.•Below Tm, cholesterol has a weak condensing effect on milk-SM.•Above Tm, cholesterol and milk-SM are miscible with strongest area condensation for 30 mol% chol.•The physical state of milk-sphingomyelin controls its assembly with cholesterol.

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 (Tm ∼ 34 °C). For T = 15 °C < Tm, 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 > Tm, 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.

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