Comparison of neutron reflection and surface tension measurements of the surface excess of tetradecyltrimethylammonium bromide layers at the air/water interface
- Physical Chemistry Lab., Oxford (United Kingdom)
- Rutherford-Appleton Lab., Oxon (United Kingdom); and others
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{sub 14}TAB) at the air/water interface at 298 K. The critical micelle concentration (cmc) was found to be (3.7 {+-} 0.1) x 10{sup -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{sub 14}TAB 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{sub 14}TAB from 3 x 10{sup -4} to 4.5 x 10{sup -3}M. The area per molecule at the cmc (A) was found to be 46 {+-} 2 {angstrom}{sup 2} and continues to decrease above the cmc. The {gamma}-ln 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}{sup 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}{sup 2}. The implications of the difficulty in determining A from surface tension measurements are discussed in detail. 20 refs., 6 figs., 3 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 379476
- Journal Information:
- Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 3 Vol. 96; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
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