Analysis of light scattering data on the sodium caseinate assembly as a response to the interactions with likely charged anionic surfactant

We report on the modification of the parameters of sodium caseinate nanoparticles, measured basically by static and dynamic light scattering (the weight-average molar mass Mw; the radius of gyration RG, the hydrodynamic radius Rh, their ratio (ρ=RG/Rh), reflecting the shape of the scattering particles; and the thermodynamic affinity for an aqueous medium (the second virial coefficient A2)), as a response to the interactions with the likely charged anionic surfactant–CITREM (the equimass mixtures of the esters of the stearic and palmitic acids with a citric acid) over a wide range of the surfactant low concentrations at pH 7.2. We found that the protein association, induced by the interactions with the likely charged anionic surfactant, was most pronounced at the medium value of the ionic strength under the ionic strength variation from 0.005 to 0.1 M, namely, at 0.05 M. The detailed dependence of the parameters of the surfactant-modified sodium caseinate on the CITREM concentration allowed recognizing the three concentration regions with the specific parameters of the complex (protein+surfactant) nanoparticles at the ionic strength that is peculiar to the most marked protein association (0.05 M). The first concentration region is at CCITREM<1.75 mg L−1, which is rather far from the original cmc region for CITREM (cmcCITREM≈15 mg L−1), where the formation of the most large and linear complex (protein+surfactant) nanoparticles, possessing the highest thermodynamic affinity for the aqueous medium, is revealed. The second concentration range is from CCITREM=1.75 mg L−1 up to the original cmc point for the surfactant, where the less rigid linear complex (protein+surfactant) nanoparticles, possessing both the lower molar mass and the lower thermodynamic affinity for the aqueous medium, than those in the region (I), were generally formed. The third concentration region, which is defined in the close vicinity to the original surfactant cmc (cmcCITREM⩾15 mg L−1), is characterized, by a slight decrease in the molar masses of the modified protein nanoparticles and by their quite close thermodynamic affinity for the aqueous medium as compared with the same parameters observed in the second region. The sharp collapse of the complex (protein+surfactant) nanoparticles is found to be in the third concentration region. This collapse apparently suggests the cluster formation of the surfactant molecules in the interior of the protein associates in the close vicinity to the surfactant cmc region. The further measurements of the enthalpy of the protein–surfactant interactions along with the intrinsic viscosity and the apparent net charge of the surfactant-modified protein nanoparticles support the light scattering data obtained.