Adsorption of water to a phospholipid (lecithin) and a ceramide were studied by IR microspectroscopy equipped with a humidity control system and quartz crystal microbalance (QCM). The water weight ratios increase up to 12.2 wt% for lecithin and 1.2 wt% for ceramide at RH ~80%, with linear correlations with infrared OH (+NH) band areas. For lecithin, the 1230 cm-1 band (PO2-) and the 1735 cm-1 band (CO) shift to lower wavenumbers, while the 1060 cm-1 band (PO2-, POC) shift to higher wavenumber with RH. Band areas of phosphates (1230 and 1060 cm-1) increase with RH showing positive relations with the band area of bound water. Bound water molecules with shorter H bonds might be bound to these phosphate groups. Band areas of aliphatic CHs are negatively correlated with the increasing adsorption of free water. Free water molecules with longer H bonds might interact loosely with aliphatic chains of lecithin. For ceramide, only the 1045 cm-1 band (CO) shows a small red shift at higher RHs than 60%, indicating adsorption of bound water to CO bonds. Amounts of water molecules adsorbed to ceramide are very limited due to few adsorption of free water.
Keywords: Ceramide; Hydrogen bonding; Infrared spectroscopy; Lecithin; Quartz crystal microbalance; Water molecules.
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