Polystyrene latex adsorbed layer thickness

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Cosgrove et al. (1990) used small-angle neutron scattering on a polystyrene latex in water to obtain the adsorbed layer thickness of polyethylene oxide at three different temperatures. At higher temperatures a greater adsorbed amount was observed, which was consistent with water becoming a poorer solvent for the polyethylene oxide, but in all cases the layer thickness was about 20-30 A, an order of magnitude less than the hydrodynamic layer thickness. Neutron... 

The effect of droplet size and its distribution on the adsorbed layer thickness may be inferred from a comparison of the results obtained with the o/w emulsions with those recently obtained using polystyrene latex dispersions containing grafted PEO chains of (molecular weight 2000) (49). As discussed earlier, the viscoelastic behavior of the system (which reflects the steric interaction) is determined by the ratio of the adsorbed layer thickness to the particle radius (8/R). The larger this ratio, the lower the volume fraction at which the system changes from predominantly viscous to predominantly elastic response. With relatively polydisperse systems, ( )cr shifts to higher values when compared to monodisperse systems with the same mean size. 

The effect of droplet size and its distribution and the adsorbed layer thickness may be inferred from a comparison of the results obtained with the O/W emulsions with those obtained using polystyrene latex dispersions containing grafted... 

A very similar effect of the surface concentration on the conformation of adsorbed macromolecules was observed by Cohen Stuart et al. [25] who studied the diffusion of the polystyrene latex particles in aqueous solutions of PEO by photon-correlation spectroscopy. The thickness of the hydrodynamic layer 8 (nm) calculated from the loss of the particle diffusivity was low at low coverage but showed a steep increase as the adsorbed amount exceeded a certain threshold. Concretely, 8 increased from 40 to 170 nm when the surface concentration of PEO rose from 1.0 to 1.5 mg/m2. This character of the dependence is consistent with the calculations made by the authors [25] according to the theory developed by Scheutjens and Fleer [10,12] which predicts a similar variation of the hydrodynamic layer thickness of adsorbed polymer with coverage. The dominant contribution to this thickness comes from long tails which extend far into the solution. 

Several experimental parameters have been used to describe the conformation of a polymer adsorbed at the solid-solution interface these include the thickness of the adsorbed layer (photon correlation spectroscopy(J ) (p.c.s.), small angle neutron scattering (2) (s.a.n.s.), ellipsometry (3) and force-distance measurements between adsorbed layers (A), and the surface bound fraction (e.s.r. (5), n.m.r. ( 6), calorimetry (7) and i.r. (8)). However, it is very difficult to describe the adsorbed layer with a single parameter and ideally the segment density profile of the adsorbed chain is required. Recently s.a.n.s. (9) has been used to obtain segment density profiles for polyethylene oxide (PEO) and partially hydrolysed polyvinyl alcohol adsorbed on polystyrene latex. For PEO, two types of system were examined one where the chains were terminally-anchored and the other where the polymer was physically adsorbed from solution. The profiles for these two... 

In this paper we present results for a series of PEO fractions physically adsorbed on per-deutero polystyrene latex (PSL) in the plateau region of the adsorption isotherm. Hydro-dynamic and adsorption measurements have also been made on this system. Using a porous layer theory developed recently by Cohen Stuart (10) we have calculated the hydrodynamic thickness of these adsorbed polymers directly from the experimental density profiles. The results are then compared with model calculations based on density profiles obtained from the Scheutjens and Fleer (SF) layer model of polymer adsorption (11). 

Garvey et al.85) made a similar sedimentation study on poly(vinyl alcohol) adsorbed on polystyrene latex particles. Adsorbance of the polymer was also measured. Both the thickness of the adsorbed layer and the adsorbance increased linearly with the square root of the molecular weight. The volume occupied by a polymer molecule in the adsorbed layer was approximately equal to that of the effective hydrodynamic sphere in bulk solution. However, the measured values of LH were greater than the hydrodynamic diameters of the polymer coils in solution. Thus, it may be concluded that adsorbed poly(vinyl alcohol) assumes a conformation elongated in the direction normal to the surface. 

In order to test the model used here, calculated values of the limiting free polymer concentration 0 at which phase separation occurs are compared with the experimental data [6] on the aqueous dispersions of polystyrene latex particles with adsorbed polyethylene oxide and with polyethylene oxide as the free polymer. Since no information is available regarding the thickness of the adsorbed layer, the values used by Vincent et al. [6] in their theoretical calculations are adopted. Table 1 compares the experimental values of the limiting volume fraction of the free polymer with our calculated values for two different molecular weights of the free polymer. The simple model used here gives reasonably good agreement with the experimental values. 

The data of Woods and Krieger [47] illustrate this effect for polystyrene latexes of 200, 660 and 1100 nm diameter (Figure 13.5). Woods and-Krieger corrected their data for the thickness. A, of an adsorbed layer of nonionic surfactant with a fully-extended length of 4,5 nm, using the following equation for the effective volume fraction... 

The influenee of the adsorption layer thiekness on doublet lifetime is shown in Fig. 5 for one value of the Hamaker eonstant. There is high specificity in the thiekness of a polymer adsorption layer.p-Casein adsorbed on to polystyrene latex causes an increase in the radius of the particle of 10-15 nm (79). Alayer of P-Mactoglo-bulin appears to be in the order of 1-2 nm thick, as compared to 10 nm for the caseins (80). 

Information about the adsorbed layer strueture of P-casein at the hydrophobic surface can be obtained by employing neutron refleetivity, small-angle X-ray scattering (SAXS), and dynamic light scattering. It was found that the layer of P-casein adsorbed to a hydrocarbon oil/water interface or an air/water interface (70, 71) consisted of a dense inner part, 2 nm thick, and a protein volume fraction of 0.96, immediately adjacent to the interface. Beyond that a second dilute region with a protein volume fraction of 0.15 extended into the aqueous phase. A similar structure of P-casein adsorbed on to polystyrene latex particles was observed with SAXS (65). The electron-density profile cal-... 

For systems stabilised by nonionic surfactants or macromolecules, the energy-distance curve also shows a minimum whose depth depends on particle size, the Hamaker constant and the thickness of the adsorbed layer [94, 95]. This is illustrated in Figure 14.15, which shows the energy-distance curves for polystyrene latex particles containing poly(vinyl alcohol) (PVA) layers of various molecular... 

Figure 13 demonstrates the segment density profile of sodium poly(styrene sulfonate) adsorbed on polystyrene latex in NaCl solution [48]. The segment density profiles of Na poly(styrene sulfonate) do not practically depend on the surface charge. The shape of curve depends on the NaCl concentration. An increase of salt concenliation leads to an increase of the thickness of the adsorption layer, as predicted by theory. 

The simultaneous adsorption of poly(vinylpyrrolidone) (PVP) and an anionic hydrocarbon or fluorocarbon surfactant from their binary mixtures on polystyrene latex was studied by Otsuka et al [59]. The bare particles and the particles coated with PVP/surfactant were sized by sedimentation field flow fractionation (SFFF). The adsorption of PVP was enhanced by LiDS or LiFOS at low surfactant concentrations, but decreased at high surfactant concentrations. The conformation of adsorbed PVP changed from loops and trails to trains with the increasing surfactant concentration. However, in the PVP-LiFOS system, the fraction of train segments increased steeply at a lower surfactant concentration and was greater than that in the PVP-LiDS system. The thickness of the adsorbed layer was also determined by photon correlation spectroscopy after prefractionation by SFFF. The thickness of the adsorbed layer decreased with increasing surfactant concentration. (For the characterization of adsorbed surfactant layers, see Ref. 56, pp. 205-216). 

The preparation of rough silver films by vapor deposition results in reproducible and stable surfaces for SERS. For example, deposition of 20-nm Ag films onto Teflon, polystyrene, or latex spheres [29,30] has been performed. These substrates produced strong SERS intensities for various organic adsorbates and good reproducibility between multiple rims. However, vapor deposition can be slow and needs access to a vacuum system. There are also some variables that need to be controlled, such as the film thickness, deposition temperature, and use of annealing procedures. Moreover, unless the experiment is performed under vacuum, the film is exposed to the atmosphere after deposition. Even a brief exposure to the atmosphere results in contamination of the surface and the formation of an inactive oxide layer. 

---