Mixed monolayers of fatty acids with distearoylglycerophosphocholine

Mixed monolayer states of long normal-chain fatty acids with distearoylglycerophosphocholine (DSPC) have been studied by a previously described thermodynamic treatment. The surface pressures of mixed monolayers of tetradecanoic, pentadecanoic and octadecanoic acids with DSPC were measured at various compositions and temperatures. The two-dimensional phase diagrams of both the tetradecanoic acid-DSPC and pentadecanoic acid-DSPC systems were determined. They both exhibited eutectic type phase diagrams. In the octadecanoic acid-DSPC system the two components were immiscible both in the liquid-expanded and liquid-condensed states. The apparent molar entropy and energy changes for the tetradecanoic acid-DSPC system were calculated. The values for tetradecanoic acid were negative, as is the usual case. However, the values of DSPC were positive, and their absolute values wer about $\text{1}\text{4}$ of that of tetradecanoic acid.