X-ray diffraction and foam film investigations of PC head group interaction in water/ethanol mixtures

The influence of ethanol on single phospholipid monolayers at the water/air interface and in foam films has been investigated. Grazing incidence X-ray diffraction investigations (GIXD) of Langmuir monolayers from 1,2-distearo yl-phosphatidylcholine (DSPC) spread on water subphases with different amou nts of ethanol were performed. The thickness and free specific energy of fo rmation of foam films stabilized by 1,2-dimyristoyl-phosphatidylcholine (DM PC) at different concentrations of ethanol in the film forming dispersions were measured. The GIXD investigations show that the tilt angle of the alky l chains in the PC lipid monolayer decreases with increasing concentration of ethanol caused by a decrease of the diameter of the head groups. With in creasing ethanol content of the solution also the thickness of the aqueous core of PC lipid foam films decreases. We assume that ethanol causes a decr easing probability for the formation of hydrogen bonds of water molecules t o the PC head groups. The distinct difference between the effects of ethano l on lipid bilayers as described in the literature and on monolayers and fo am films found in this study is discussed. Whereas PC monolayers at the wat er/air interface become unstable above 25 vol.% ethanol, the PC foam films are stable up to 50 vol.% ethanol. This is related to the decrease of the s urface excess energy per lipid molecule by the interaction between the two film surfaces. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.