Particle surface coverage, effect

Ward Jones SE, Campbell FW, Baron R, Xiao L, Compton RG (2008) Particle size and surface coverage effects in the stripping voltammetry of silver nanoparticles theory and experiment. J... 

For so-called steric stabilization to be effective, tire polymer needs to be attached to tire particles at a sufficiently high surface coverage and a good solvent for tire polymer needs to be used. Under such conditions, a fairly dense polymer bmsh witli tliickness L will be present around the particles. Wlren two particles approach, such tliat r < d + 2L, tire polymer layers may be compressed from tlieir equilibrium configuration, tluis causing a repulsive interaction. 

However, in subsequent studies [23-25,88-90] it was demonstrated that in reality the particle deposition is not a purely geometric effect, and the maximum surface coverage depends on several parameters, such as transport of particles to the surface, external forces, particle-surface and particle-particle interactions such as repulsive electrostatic forces [25], polydispersity of the particles [89], and ionic strength of the colloidal solution [23,88,90]. Using different kinds of particles and substrates, values of the maximum surface coverage varied by as much as a factor of 10 between the different studies. 

If the polymer layers increases the stability of the dispersion, it is denoted steric stabilisation. The polymer must fulfil two key criteria (i) the polymer needs to be of sufficient coverage to coat all the particle surfaces with a dense polymer layer, and (ii) the polymer layer is firmly attached to the surface. How this is engineered is beyond the scope of this article, but the consequences of not satisfying these criteria are informative in understanding the effect that polymers have on the overall interparticle interaction. Since complete or incomplete coverage of the particles results in very different properties (i.e stability or instability), this is clearly one way in which minimal change in initial conditions can lead to major differences in product. 

In a sense each monolithic column is unique, or produced as a product of a separate batch, because the columns are prepared one by one by a process including monolith formation, column fabrication, and chemical modification. Reproducibility of Chro-molith columns has been examined, and found to be similar to particle-packed-silica-based columns of different batches (Kele and Guiochon, 2002). Surface coverage of a Chromolith reversed-phase (RP) column appears to be nearly maximum, but greater silanol effects were found for basic compounds and ionized amines in buffered and nonbuffered mobile phases than advanced particle-packed columns prepared from high purity silica (McCalley, 2002). Small differences were observed between monolithic silica columns derived from TMOS and those from silane mixtures for planarity in solute structure as well as polar interactions (Kobayashi et al., 2004). 

In common with some other authors (18-20), Napper removed excess stabilizer from the dispersion medium so as to give the dispersed particles full surface coverage, leaving negligible amounts of free polymer in solution. As the solvency was worsened, no more polymer could be adsorbed, so that critical flocculation conditions do not necessarily correspond to surface saturation. In the present work, which may refer more closely with some practical applications, the stabilizer is kept at the plateau adsorption level but at the expense of complicating the system by the presence of free polymer. Clarke and Vincent (21) have reported on the effect of free polystyrene on the stability of silica with terminally-attached sytrene chains, but the very considerable differences to our studies make an assessment of the possible role played by unadsorbed polymer unproductive. 

In many practical cases, stabilisation by polymers involves a combination of steric and charge interactions. Unlike simple electrolytes, multiple adsorption effects permit polyelectrolytes to continue to adsorb well beyond the point where the adsorbed layer charge exceeds that of the particle surface. In this way, the effective charge on particles can be increased substantially at relatively low surface coverage by the polymer. 

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorbed on platinum, nickel, and palladium (1). The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil or Aerosil.f This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). The small particle size is important because it reduces the amount of radiation which is lost by scattering. A nonporous small particle form of gamma-alumina, known as Alon-C, is also available. This material is not so inert as the silica and will react with gases such as CO and CO2 at elevated temperatures. 

---