Structure of adsorbed layers of nitrophenoxy-tailed quaternary ammonium surfactants at the air/water interface studied by neutron reflection

The adsorbed layers of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (ΦC10TAB) and N,N  ,N′N′,N′N′-tetramethyl-N  ,N′N′-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(ΦC10)2C6] at the air/water interface have been studied by neutron reflection. The coverage of the surfactants was obtained over the concentration range from critical micelle concentration (CMC) to CMC/100. The area per ΦC10TAB molecule changes from 50±350±3 to 390±60 Å2390±60 Å2 over this concentration range and the area per (ΦC10)2C6 molecule changes from 139±3139±3 to 288±10 Å2288±10 Å2. The overall thicknesses (single uniform layer) of the surfactant layers at CMC are about 19 and 16 Å for ΦC10TAB and (ΦC10)2C6 respectively. The distributions of the C10 chains show that the chains of both surfactants are tilted away from surface normal, with the tilt increasing in the outer part of the layer. The distribution of C10 chains in (ΦC10)2C6 is narrower than that in ΦC10TAB, indicating that the alkyl chains of (ΦC10)2C6 are more tilted. For both surfactants, the broad nitrophenoxy distribution may indicate significant positional disorder of the nitrophenoxy groups along the surface normal direction and their intermixing with alkyl chains in the adsorbed layer.

The adsorption of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (ΦC10TAB) and N,N  ,N′N′,N′N′-tetramethyl-N  ,N′N′-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(ΦC10)2C6] at the air/water interface indicates significant tilt of the hydrophobic chains from the surface normal and substantial intermixing of alkyl chains with nitrophenoxy groups.Figure optionsDownload as PowerPoint slide