Monolayers formed by glycine and taurine conjugates of chenodeoxycholic acid (GCDC and TCDC respectively) and ursodeoxycholic acid (GUDC and TUDC respectively) and cholesterol (Ch), and, in addition, those formed by different binary mixed systems of these bile acids (BAs) with Ch (GCDC/Ch, GUDC/Ch, TCDC/Ch and TUDC/Ch) were investigated in terms of surface pressure (n), surface potential (d V) and surface dipole moment (pI) as a function of average surface (occupation) area (A). On the substrate of 5 M aqueous NaCl solution at pH 1.2 and 25"C, the n-A curves demonstrated that Ch forms a typical condensed monolayer film whereas each conjugated BA forms a nontypical expanded film which is clearly different from a typical one formed by free BAs such as deoxycholic acid (DC) and chenodeoxycholic acid (CDC).
Every mixed system showed that its z--A, Al/-A and pi-A curves are inserted inbetween those of the respective pure systems in a regular sequence of mole fraction of one component in the binary mixture. All the mixed systems of BA with Ch were found to be completely immiscible with each other and to form a "patched monolayer" on the substrate, and this led us to conclude that compliance with the additivity rule as for A, AV or pI is a necessary, but not wholly sufficient, condition for ideal mixing in the two-dimensional phase.