Surface chemistry, topography, and spectroscopy of a mixed monolayer of 10,12-pentacosadiynoic acid and its mannoside derivative at the air-water interface

N-(11-O-α-D-Mannopyranosyl-3,6,9-trioxa)undecyl 10,12-pentacosa-diynamide (PDTM) was synthesized from 10,12-pentacosadiynoic acid (PDA). The pure and mixed PDA/PDTM monolayers at the air - water interface were characterized by recording surface pressure-area (π-A) isotherms, surface potential - area (ΔV-A) isotherms, Brewster angle microscopy, and surface UV-visible spectroscopy on a subphase with different pH values and ionic strengths. A first-order phase transition was observed in the mixture but not in any of the pure compound. An overshot at the beginning of the phase transition plateau of the π-A isotherms was identified as the result of hydrogen bonding between the headgroups of PDA and PDTM. 2-D phase diagrams of the binary mixed monolayer of PDA/PDTM were obtained through the analysis of these results. Subphase ion and pH can shift the phase boundary of the phase diagram. The PDA/PDTM mixed monolayers have good miscibility in a wide range of molar ratio. The absorption spectra showed that high ratio of PDTM can reduce the effective conjugation length of the polydiacetylene backbone and prevent the appearance of the blue film phase.