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

Journal article

Neutron reflection and surface tension (ring and plate methods) have been used to study the adsorption of four deuterated isotopic species of tetradecyltrimethylammonium bromide (C14TAB) at the air/water interface at 298 K. The critical micelle concentration (cmc) was found to be (3.7 ± 0.1) × 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 C14TAB 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 C14TAB from 3 × 10-4 to 4.5 × 10-3 M. The area per molecule at the cmc (A) was found to be 46 ± 2 Å2 and continues to decrease above the cmc. The γ-ln c plots gave a limiting area per molecule at the cmc, which depended on both the method used to determine γ 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 Å2, 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 Å2. The implications of the difficulty in determining A from surface tension measurements are discussed in detail. © 1992 American Chemical Society.

Authors: Simister, E; Thomas, R; Penfold, J; Aveyard, R; Binks, B

• Publication status: Published
• Journal: JOURNAL OF PHYSICAL CHEMISTRY
• Volume: 96
• Issue: 3
• Pages: 1383-1388
• Publication date: 1992-02-06
• DOI: 10.1021/j100182a066
• EISSN: 1541-5740
• ISSN: 0022-3654