COMPARISON OF NEUTRON REFLECTION AND SURFACE-TENSION MEASUREMENTS OF THE SURFACE EXCESS OF TETRADECYLTRIMETHYLAMMONIUM BROMIDE LAYERS AT THE AIR-WATER-INTERFACE

Neutron reflection and surface tension (ring and plate methods) have been used to study the adsorption of four deuterated isotopic species of tetradecyltrimethylammonium bromide (C-14TAB) at the air/water interface at 298 K. The critical micelle concentration (cmc) was found to be (3.7 +/- 0.1) x 10(-3) M for three of the four isotopic species, in good agreement with earlier determinations. The fourth species showed a slight surface tension minimum but was otherwise similar. The close similarity of the surface tension curves indicated that the surface properties of C-14TAB are independent of deuteration, a result of importance for the interpretation of neutron reflectivity. Neutron reflection has been used to measure the surface concentrations of the fully deuterated and chain-deuterated isotopes of C-14TAB from 3 x 10(-4) to 4.5 x 10(-3) M. The area per molecule at the cmc (A) was found to be 46 +/- 2 angstrom 2 and continues to decrease above the cmc. The gamma-In c plots gave a limiting area per molecule at the cmc, which depended on both the method used to determine gamma and the method of analysis of the data. The simplest approach using a Pt-Ir ring and a straight line plot gave a value of 60 angstrom 2, in agreement with some earlier determinations, but, using a plate and a polynomial fit to the data, the value was found to be in close agreement with the neutron measurement, i.e., 45 +/- 2 angstrom 2. The implications of the difficulty in determining A from surface tension measurements are discussed in detail.