Micellar and surface behavior of sodium deoxycholate characterized by surface tension and ellipsometric methods

Sodium deoxycholate was used as a model compound to test a mass-action model of micelle formation developed in conjunction with a surface tension method for estimating micellar properties including aggregation number, micellar association constants, and the molecular area of the surfactant anion. The model simultaneously combines the Gibbs adsorption equation, the Langmuir adsorption isotherm, and a mass balance equation to yield the micellar properties from surface tension versus concentration data. The data are fit to the model across the whole range of concentrations studied (10−6 to 10−1 M). Ellipsometric measurements of the thickness of the surface layer of deoxycholate anions at the air—water interface were used to confirm that only a monolayer formed. The micelle properties were measured in a base electrolyte solution of 0.01 M NaOH and sodium chloride concentrations up to 0.50 M. The micelle aggregation number ranges from N = 12.4in0.01 MNaOH toN = 42 in 0.01 M NaOH and 0.50 M NaCl. The surface concentration at the CMC and beyond is Λm = 2.0 ± 0.19 × 10−10 molelcm2. The average area occupied by a deoxycholate anion at the air-water interface is 84A˚2 measured by ellipsometry or calculated from Λm. The average deoxycholate film thickness measured by ellipsometry was 6.3A˚in good agreement with the dimensions of the deoxycholate ion. The critical micelle concentrations determined from the surface tension data are reported and vary from 0.0064 M in 0.01 M NaOH to 0.0009 M in 0.1 M NaOH and 0.50 M NaCl.