Determination of adsorption of an ionic surfactant on latex from surface tension measurements

The adsorption isotherm of sodium dodecyl sulfate (SDS) on polystyrene (PSt) latex is evaluated from surface tension (γ) measurements of aqueous SDS solutions and PSt latex solutions. A video-enhanced pendant bubble/drop tensiometer is applied for the equilibrium and dynamic surface tension measurement of SDS aqueous solutions and PSt latex solutions. The bulk concentrations of un-absorbed SDS molecules (CSDS, SDS in water bulk phase) in PSt latex solutions are evaluated from equilibrium surface tension data, which is uniquely related between the equilibrium γ and bulk concentration CSDS for SDS aqueous solutions. From the mass balance calculation, mtotal=CSDS×Vwater+ΓSL×A (here A is the surface area of pendant drop of latex solution), the surface concentration of SDS (ΓSL) on PSt latex is then determined for aqueous latex solutions with different SDS concentrations. A unique relationship between ΓSL and CSDS is resulted and the Langmuir adsorption isotherm, with maximum surface concentration of SDS at PSt latex (Γ∞=5.75×10−10 mol cm−2) and equilibrium constant a=6.49×10−6 mol cm−3, describes this dependence well.