Surface pressure-molecular area (Pi-A) isotherms and fluorescence microscopy were used to investigate the interactions between N-stearoyl-glutamic acid (L- and D-) and L-arginine at the air/water interface. N-stearoyl-glutamic acids (C-18-Glu) with different chirality (L- and D-) were spread at the air-water interface onto subphases containing varied concentrations of L-arginine at 5. The apparent binding affinity of C-18-Glu to L-arginine was obtained by fitting the plots of the change in mean molecular area of C-18-Glu vs. L-arginine concentration. The thermodynamic properties of the binding events such as binding constant and Gibbs free energy were estimated from the binding curves. We found that N-stearoyl-L-glutamic acid (C-18-L-Glu) had a stronger binding affinity to L-arginine as compared to N-stearoyl-D-glutamic acid (C-18-D-Glu) at low to moderate surface pressures (below similar to 22 mN/m). The C-18-D-Glu had stronger binding to L-arginine at higher surface pressure. Domains with different shapes in C-18-L-Glu and C-18-D-Glu monolayers were observed under a fluorescence microscope in situ at the air/water interface. Herein, we present a mechanism for C-18-L-Glu and C-18-D-Glu interacting with L-arginine at the air/water interface. (C) 2013 Elsevier Inc. All rights reserved.