Abstract
Hydrophilic silica nanoparticles alone are not surface active. They, however, develop a strong electrostatic interaction with ionic surfactants and consequently affect their surface behavior. We report the interfacial behavior of n-heptane/anionic-surfactant-solutions in the presence of hydrophilic silica nanoparticles. The surfactants are sodium dodecyl sulfate (SDS) and dodecyl benzene sulfonic acid (DBSA), and the diameters of the used particles are 9 and 30 nm. Using experimental tensiometry, we show that nanoparticles retain their non-surface-active nature in the presence of surfactants and the surface activity of surfactant directly increases with the concentration of nanoparticles. This fact was attributed to the electrostatic repulsive interaction between the negatively charged nanoparticles and the anionic surfactant molecules. The role of electrostatic repulsion on increasing surface activity of the surfactant has been discussed. Further investigations have been performed for screening the double layer charge of the nanoparticles in the presence of salt. Moreover, the hydrolysis of SDS molecules in the presence of silica nanoparticles and the interaction of nanoparticles with SDS inherent impurities have been studied. According to our experimental observations, silica nanoparticles alleviate the effects of dodecanol, formed by SDS hydrolysis, on the interfacial properties of SDS solution.
Highlights
Surfactants are extremely versatile chemical products of the chemical industry
This is probably the reason why much less attention has been paid to studies of the surface/interfacial behavior of systems comprised of surfactants and nanoparticles with the same electric charge
The results indicated that the dynamics of adsorption and the response of the interfacial layer to the surface perturbations were faster for systems containing nanoparticles
Summary
The relatively fast adsorption dynamics of both fresh (empty points) and aged solutions (filled points) indicates that the formed complexes of nanoparticle-SDS impurity (dodecanol) are not surface active and do not adsorb at the interface This can be inferred from the overall trends of interfacial elasticity (Fig. S3), the surface pressure, and the standard deviation of fitting the Laplace equation during the drop surface compression (Fig. 3). It can be stated that the observed SDS interfacial behavior in the presence of nanoparticles can be attributed to two different effects of the nanoparticles: 1- the electrostatic repulsive interaction, leading to the increased surfactant surface activity and the lower IFT values, 2- the adsorption of the dodecanol on the surfaces of particles, affecting the adsorption dynamic and resulting in faster interfacial relaxation. The dynamic and equilibrium values of IFT of DBSA solutions with nanoparticles behaves quite similar to the pure surfactant system at respective higher concentrations
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