The existence of a local minimum of surface tension near the critical micelle concentration has been reported for various surfactant solutions. In some cases, this is interpreted as an indication for the presence of impurities. Recently, this phenomenon has been interpreted as an inhibition of the aggregation for POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) in HPN (3-hydroxypropionitrile). This notion is supported by the present experiments on the phospholipid DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) at the surface of HPN. Comparison of the results of both systems delivers improved understanding of the aggregation of surfactants in solution.