Neutron reflectivity studies of Aerosol-OT monolayers adsorbed at the oil/water, air/water and hydrophobic solid/water interfaces

The structures of monolayers of Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate) adsorbed at the oil/water, air/water and hydrophobic solid/water interfaces have been determined using neutron specular reflection in conjunction with isotopic substitution. The areas per molecule were found to be 78 +/- 2 Angstrom 2 and 80 +/- 5 Angstrom(2) at the critical micelle concentration (CMC = 2.5 x 10(-3) M), increasing to 132 +/- 5 Angstrom(2) at 8.0 x 10(-6) M and 213 +/- 20 Angstrom 2 at 2.5 x 10(-6) M at the air/water and hydrophobic solid/water interfaces respectively. The thicknesses of the AOT layer at the air/water and solid/water interfaces were found to be 18 +/- 2 Angstrom and 15 +/- 2 Angstrom with no significant change with concentration. At the oil/water interface the thickness is much larger than the fully extended molecule and is 26 +/- 2 Angstrom at an area per molecule of 68 +/- 2 Angstrom 2. This is probably caused by roughening of the oil/water interface by thermal fluctuations. (C) 1998 Elsevier Science B.V.