Suspensions of Iron Oxide Nanoparticles Stabilized by Anionic Surfactants

Abstract

Two common anionic surfactants, sodium oleate (SO) and sodium dodecyl benzene sulfonate (SDBS) were used to re-suspend iron oxide nanoparticles in aqueous solutions. At certain SO concentrations, the SO formulations produced highly stable suspensions. In contrast, SDBS-stabilized nanoparticles exhibited poor stability at all concentrations. The adsorption isotherm of SO on iron oxide nanoparticles revealed that stable suspensions were obtained when the equilibrium SO concentration (after adsorption) reached its critical micelle concentration (CMC). At this ''optimal'' condition, the maximum SO adsorption was reached, and the zeta-potential of the particles was highly negative (* -50 mV). According to the SO isotherm, this optimal formulation coincided with the formation of a highly compact SO bilayer. The SDBS isotherm, on the other hand, revealed that SDBS is not strongly adsorbed on the surface of iron oxide nanoparticles and that is likely that a patchy, loosely packed bilayer, is formed on the surface of the iron oxide nanoparticles when the equilibrium SDBS concentration reaches its CMC. The DLVO theory confirmed the connection between formulation conditions and the corresponding stability. This works confirmed that the formation of a surfactant bilayer is an important element in producing stable nanoparticle suspensions with anionic surfactants. It was also confirmed that for anionic surfactants, electrostatic repulsions are an important factor in establishing an energy barrier against flocculation. This work also introduced two more elements into the design of nanoparticle suspensions. The first element is that, in order to ensure the best possible dispersion, the surfactant concentration in solution at equilibrium with the adsorbed surfactant should be close or slightly above its CMC. The second element is that the molecular structure of the surfactant should facilitate the formation of closely packed bilayers.

Heat of adsorption of surfactants and its role on nanoparticle stabilization

“…An increase in the extent of adsorption produced highly packed surfactant structures with high charge density, leading to more effective electrostatic repulsions, as shown in a previous study with SO-stabilized iron oxide nanoparticles [5]. That study does not discount the possibility of steric and head group hydration arguments, but the measured zeta potential and DLVO calculations were enough to explain the differences in colloidal stability.…”

“…Surfactant adsorption was calculated as (initial or added surfactant concentration -final surfactant concentration)*volume of surfactant solution/mass of iron oxide nanoparticles [5].…”

“…where M is the molar mass of SO (304.4 g·mol -1 ), q is the amount of surfactant adsorbed on iron oxide nanoparticles (mass of adsorbed surfactant divided by mass of nanoparticles), N A is Avogadro's number, and S s is the specific surface area of iron oxide (S s = 47 m 2 ·g -1 ), determined using the BET technique [5]. One can also estimate the maximum number of available adsorption sites (x) for SO directly adsorbed (first layer) on iron oxide nanoparticles:…”

“…Previous studies on the interaction between surfactant and iron nanoparticles have involved the use of various techniques such as surface tension [4,5], thermogravimetric analysis [6][7][8], and Fourier transform infrared spectroscopy (FTIR) [7,8]. The results from those studies suggest that a bilayer is formed on the particles at high surfactant concentrations [5][6][7]. Stable suspensions are obtained when the surface of the particles are coated with a closely-packed surfactant layer [5].…”

“…The results from those studies suggest that a bilayer is formed on the particles at high surfactant concentrations [5][6][7]. Stable suspensions are obtained when the surface of the particles are coated with a closely-packed surfactant layer [5]. Finding surfactants with the ability to adsorb on metal oxide surfaces and formed highly packed layers is, still, an empirical endeavour.…”