Temperature and subphase effect on alkanols monolayers at the air/water interface

The regular behavior of spread films consists in the shift of the pi/A isotherm toward larger molecular area values as temperature increases. Spread monolayers of long-chain aliphatic alcohols (1-hexadecanol, 1-octadecanol, and 1-eicosanol) are studied as a function of temperature and subphase composition. In the case of 1-hexadecanol we observe an anomalous trend of the pi/A isotherm at different temperatures, consisting in the shift of the curve toward lower area values. The same behavior has already been reported in a previous paper in the case of 1-tetradecanol. The values of the limit area (A(lim)), modulus of compressibility (C(S)-1), and surface entropy (S(S)) evaluated from the pi/A isotherms confirm that this anomalous temperature-induced lowering of the film area is related to the squeezing-out of water molecules from the hydration shells around the polar head groups of the amphiphile at the air-liquid interface.