Abstract
Infrared reflection absorption spectroscopy (IRRAS) has been used for probing the surface densities and compositions of adsorbed layers of aqueous sodium tetradecanoate, or myristate, at 25 °C. Aqueous sodium myristate normally becomes protonated (myristic acid) to an extent of about 0.5−1%, yielding a natural pH from 8 to 9, depending on concentration. The myristic acid, and possibly an acid−soap complex, are quite surface active compared to myristate, making impractical the application of the Gibbs adsorption isotherm for determining surface densities from tension data. At pH = 12 (in 10 mM NaOH), only myristate is expected in the bulk, and the tension is higher; for 2 mM total surfactant concentration, the tension is ∼ 43 mN/m vs 23 mN/m. IRRAS spectra confirm that only myristate is present in the monolayer at pH = 12. At natural pH (8−9), in addition to the band due to the myristate group, a significant band due to myristic acid is observed. Solutions in D2O were used for observing the carbonyl and ca...
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