Surfactant adsorption and aggregate structure at silica nanoparticles: Effects of particle size and surface modification

Abstract

The influence of particle size and a surface modifier on the self-assembly of the nonionic surfactant C 12 E 5 at silica nanoparticles was studied by adsorption measurements and small-angle neutron scattering (SANS). Silica nanoparticles of diameter 13 to 43 nm were synthesized involving the basic amino acid lysine. A strong decrease of the limiting adsorption of C 12 E 5 with decreasing particle diameter was found. To unveil the role of lysine as a surface modifier for the observed size dependence of surfactant adsorption, the morphology of the surfactant aggregates assembled on pure siliceous nanoparticles (Ludox-TMA, 27 nm) and their evolution with increasing lysine concentration at a fixed surfactant-to-silica ratio was studied by SANS. In the absence of lysine, the surfactant forms surface micelles at silica particles. As the concentration of lysine is increased, a gradual transition from the surface micelles to detached wormlike micelles in the bulk solution is observed. The changes in surfactant aggregate morphology cause pronounced changes of the system properties, as is demonstrated by turbidity measurements as a function of temperature. These findings are discussed in terms of particle surface curvature and surfactant binding strength, which present new insight into the delicate balance between the two properties.

Aggregate Formation of Surface-Modified Nanoparticles in Solvents and Polymer Nanocomposites

“…As opposed to the monofunctional grafts, it is also possible that polycondensed groups form on the NP surface in particular in presence of water. 75,76 Such hydrophobic patches may induce additional attractive interactions between NPs in the still hydrophilic solvent, in analogy with surfactant micelles or proteins which may adsorb on NP surfaces [77][78][79][80][81][82][83] and destabilize the colloidal suspension [65][66][67][84][85][86] . As with the ethanol-water mixture, the combination of remaining electrostatic repulsion with the solvophilic character of the surface modification needs to be invoked to provide an explanation for the observed order in the scattered intensities.…”

“…Moreover, the formation of "patches", which may be surface micelles or adsorbed proteins was found to induce strong interactions. [65][66][67] Depending on the solvophilicity of such patches, the net interaction may be sterical repulsion, or attractive bridging. In the present article, the grafting of silane molecules having different reactive groups is used to modify interactions between NPs in precursor suspensions, and between NPs and a polymer matrix in nanocomposites.…”

The effects of surface curvature on the adsorption of surfactants at the solid–liquid interface

“…Comparing eqn. (9) and (12) shows that (13) This ratio is equal to unity for a planar surface, and decreases with decreasing particle radius R.…”

“…Probably the best-studied situation is the adsorption of ionic and non-ionic surfactants (and their mixtures) on to hydrophilic silica surfaces. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] In many of these studies, the silica is colloidal, with radii on the order of 100 Å. The surfactants may either form uniform bilayers, defective bilayers, or surface micelles, and the presence of these adsorbed layers has been found to play a significant role in the effective interactions between the colloidal particles.…”

“…Central to these phenomena is the interplay between the radius of curvature of the solid surface and the spontaneous curvature of the micelles, which is controlled by the molecular architecture of the surfactant. [3,7,[9][10][11]13] Despite its industrial relevance to the lubricant industry, the adsorption of amphiphilic molecules on to inorganic surfaces from oil-based solutions has received less detailed attention. One important example is an overbased detergent, this being a combination of inorganic particles (such as calcium carbonate) solubilised with ionic surfactants.…”