Langmuir monolayers of cholesterol/ergosterol and dialkyldimethylammonium bromides (DXDABs) differing in alkyl chain length-14 (DTDAB), 16 (DFIDAB), and 18 (DODAB)-spread at the air/water interface are examined. All the systems investigated are found to be nonideal and miscible. Negative values of the total free energy of mixing, proving film stability in the whole range of compositions and surface pressures, are observed for all the studied mixtures except for DTDAB/cholesterol. The strength of interactions. quantified with Delta G(Exc) values, was found to be of the same order for mixtures of cholesterol/ergosterol and DHDAB or DODAB. Differences occurring for the mixtures of DTDAB with sterols indicate the affinity of DTDAB to ergosterol in contrast to cholesterol. (c) 2005 Elsevier Inc. All rights reserved.