Surface Densities of Adsorbed Layers of Aqueous Sodium Myristate Inferred from Surface Tension and Infrared Reflection Absorption Spectroscopy
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 carboxylate bands, after eliminating the H2O vapor noise in the polar group region (1800-1300 cm-1), and for having larger reflectance-absorbance intensities due to the smaller complex refractive index of D2O than that of H2O in the hydrocarbon stretching region (2950-2850cm-1). The surface densities of adsorbed sodium myristate layers at pH = 12 as determined from tension data by using the Gibbs adsorption isotherm agree to better than 10% to those determined from IRRAS data by using the model of either an isotropic film or an anisotropic film on the surface. The surface densities at pH = 12 range from 1 x 10-6 to 4 x 10-6 mol/m2 as the concentration increases from 0.05 to 4 mM. At pH 8-9, the surface density is 8 x 10-6 mol/m2 at 4mM,as explained by the lower tension. The frequencies of both antisymmetric and symmetric methylene stretching vibration bands are lower at natural pH, indicating more ordered and almost all-trans conformations at the higher surface densities.