Radiotracer Studies on Adsorption of Surface Active Substance at Aqueous Surface. I. Accurate Measurement of Adsorption of Tritiated Sodium Dodecylsulfate

Abstract

We carried out direct measurement of adsorbed amount at the air-solution interface to confirm the validity of the Gibbs adsorption equation by using radiochemically and surface chemically pure tritiated sodium dodecylsulfate (TSDS) and a sheet scintillation counter, which was developed for the purpose of counting β-rays of tritium in air. The adsorbed amount increased with increasing SDS concentration and showed the saturation values of 3.19×10−10 mol/cm2 in the concentration region from 3.2×10−3 to 14.4×10−3 mol/1000 g solution at 25°C. For the salt-free solution of ionic surfactant the Gibbs adsorption equation is confirmed to be valid for n=2 when activity in place of concentration is used. The equation of state for the ionic adsorbed film is further proposed. The cohesive pressure due to the attraction between hydrocarbon chains was given as a function of inverse molecular area.

Composition of mixed adsorption layers and micelles in solutions of sodium dodecyl sulfate and dodecyl acid diethanol amide

“…The presence of DOH, as an impurity in the used SDS sample, is supported also by the value for the effective surfactant adsorption, Γ TOT = 5.5×10 −6 mol m −2 , which was calculated from the slope of the isotherm just below the CMC. The latter value is considerably higher than the value determined by Tajima et al [13] for pure SDS (Γ SDS = 3.2 × 10 −6 mol m −2 ).…”

“…The isotherm of pure SDS is taken from [13], the isotherm of pure DOH is from [10]. factants, β 13 , and this isotherm was found to be very similar to the isotherm of pure DADA (see Fig. 3).…”

“…Equilibrium surface tension isotherms used to estimate the molecular interaction parameters in the micelles (β ij ) between: SDS and DOH (β 23 = −1.41); DADA and SDS (β 12 = +0.20); DADA and DOH (β 13 = +0.10). The isotherm of pure SDS is taken from [13], the isotherm of pure DOH is from [10]. factants, β 13 , and this isotherm was found to be very similar to the isotherm of pure DADA (see Fig.…”

“…3. Part of the isotherm of the mixed solution DADA + 0.0015 molar parts DOH was measured and the isotherms of pure SDS and DOH were taken from literature [10,13]. The concentration 5 × 10 −5 M was accepted as CMC of DOH, because the formation of surfactant aggregates (lenses) was observed on the solution surface above this concentration.…”

“…For the micelle interaction parameter between DADA and SDS molecules we used the value, β 12,M = 0.20, which was calculated for molar part of DADA, α = 0.05 (see Table 1). Based on the isotherms of the individual surfactants SDS [13] and DOH [10], and the isotherm of SDS + DOH, measured by us, we determined that β 23 (the molecular interaction parameter between SDS and DOH) is equal to (−1.41). From the isotherms of DADA, DOH [10] and DADA+DOH solutions we obtained β 13 = +0.1 (i.e.…”