Latex spheres

Figure C2.6.11. SEM of AB2 stmcture, fonned in aqueous mixtures of PS latex spheres witli <3 = 68 nm and a = 264 nm (courtesy of Prof R H Ottewill).

The term electrophoresis refers to the movement of a soHd particle through a stationary fluid under the influence of an electric field. The study of electrophoresis has included the movement of large molecules, coUoids (qv), fibers (qv), clay particles (see Clays), latex spheres (see Latex technology), basically anything that can be said to be distinct from the fluid in which the substance is suspended. This diversity in particle size makes electrophoresis theory very general. 

Lest I leave the erroneous impression here that colloid science, in spite of the impossibility of defining it, is not a vigorous branch of research, I shall conclude by explaining that in the last few years, an entire subspeciality has sprung up around the topic of colloidal (pseudo-) crystals. These are regular arrays that are formed when a suspension (sol) of polymeric (e.g., latex) spheres around half a micrometre in diameter is allowed to settle out under gravity. The suspension can include spheres of one size only, or there may be two populations of different sizes, and the radius ratio as well as the quantity proportions of the two sizes are both controllable variables. Crystals such as AB2, AB4 and AB13 can form (Bartlett et al. 1992, Bartlett and van... 

New methods of emulsion polymerization, particularly the use of swelhng agents, are needed to produce monodisperse latexes with a desired size and surface chemistiy. Samples of latex spheres with uniform diameters up to 100 pm are now commercially available. These spheres and other mono-sized particles of various shapes can be used as model colloids to study two- and three-dimensional many-body systems of very high complexity. 

Radko, SP Chrambach, A, Electrophoretic Migration of Submicron Polystyrene Latex Spheres in Solutions of Linear Polyacrylamide, Macromolecules 32, 2617, 1999. 

Monodisperse DOP and latex spheres mixed with 220Rn... 

Our experiments are typically carried out at DNA concentrations of 20-50 /ig/ml with 1 ethidium per 300 bp, so that depolarization by excitation transfer is negligible.(18) The sample is excited with 575-nm light, and the fluorescence is detected at 630, 640, or 645 nm. Less than one fluorescent photon is detected for every 100 laser shots. The instrument response function e(t) is determined using 575-nm incident light scattered from a suspension of polystyrene latex spheres. 

When selecting a particle for analysis, it is important to consider the effects of particle size on fluorescence histogram coefficient of variation (CV). Small particles of uniform size, such as 1 pm diameter latex spheres, exhibit a sharp, narrow histogram with a relatively small CV (Fig. lA). Spherical bacteria. 

Calibration of these single-particle counters is usually carried out using monodisperse polystyrene latex or polyvinyl latex spheres, which are available in sizes from 0.1 to 3 /im and have a refractive index of 1.6 alternatively, aerosols with lower refractive indices may be generated from liquids such as dioctyl phthalate (m = 1.49). Whitby and Willeke (1979) discuss the... 

Similarly, charged solid particles (such as latex spheres) —kinetically stable lyophobic colloids —may exist in colloidal crystalline phases (with body-centered or face-centered cubic structures) as a consequence of thermodynamically favored reduction in free energies (see Chapter 13). Even neutrally charged spherical particles ( hard spheres ) undergo a phase transition from a liquidlike isotropic structure to face-centered cubic crystalline structures due to entropic reasons. In this sense, the stability or instability is of thermodynamic origin. 

FIG. 2.1 Sedimentation field flow fractionation (SdFFF) (a) an illustration of the concentration profile and elutant velocity profile in an FFF chamber and (b) a schematic representation of an SdFFF apparatus and of the separation of particles in the flow channel. A typical fractionation obtained through SdFFF using a polydispersed suspension of polystyrene latex spheres is also shown. (Adapted from Giddings 1991.)... 

The type of crystalline structure that is formed depends on the concentration of the particles as well as the magnitude of the Debye-Hiickel thickness. For large Debye-Hiickel thicknesses a body-centered cubic crystal is formed, whereas for smaller values a face-centered cubic crystal is preferred. An example of the latter observed experimentally in a dispersion of latex spheres is shown in Figure 13.3. Note that this crystallization phenomenon is analogous to crystallization of simple atomic fluids, as is evident from Figure 13.3a, which shows the coexistence of a crystal with a liquidlike structure. 

In the optical microscope, measurements were made with a Crookes image-splitting eyepiece that was calibrated with a measuring slide. The electron microscope (Hitachi HU-11) was calibrated with latex spheres of known size. 

Scattering intensity measured by the pulse height analyzer is related to particle size by calibration with monodisperse latex spheres or nearly mono-disperse NaCl particles. Calibration uncertainties have been studied and discussed (86-91). These studies show that the smallest particles that can be sensed by the ASASP probe are somewhat larger than the 0.12 xm stated by the manufacturer. Similarly, it is reported that detection of particles larger than about 2 xm is unreliable because of attenuation of the laser power. 

We have to ask, however, how large an error is introduced when the excluded volume effect is neglected. Before considering this question, a recent result of cluster size dis-tritution for antigen coated latex spheres which were cross-linked by antibodies, may be discussed. Schulthess et al.178) measured this distribution as a function of the mean number of bonds per latex particle, b = af, where f is the number of antigens bound per latex particle and a the extent of reaction. 

At low monomer conversion, the polymerization leads to fairly small molecules, and the branching process can proceed largely unimpeded by the finite volume. Thus, the common behavior of randomly branched polymers is observed. At larger conversion of monomer, the polymer has grown in size, such that the largest species have already reached dimensions of the latex sphere. On further branching, the molecular dimensions... 

FIGURE 5.16 Latex spheres produced on one of the space shuttle missions. Each one is a perfect sphere of diameter 0.01 mm, formed by surface tension in the absence of gravity. 

This natural process by which dissolved and/or particulate surface-active materials end up in the atmosphere has been modeled and studied in the laboratory. As summarized by Detwiler and Blanchard (ref. 46), tests in suspensions of bacteria (ref. 76,96,97), latex spheres (ref. 98), dyes (ref. 99), and in sea water and river water (ref. 96,100,101) have demonstrated successful transfer of all manner of surface-active material from the bulk fluid, or the bulk interface, to the droplets ejected when bubbles burst. (This situation can be pictured as an extension of the common industrial adsorptive-bubble-separation process (ref. 102) into a third dimension or phase — the atmosphere.) Further laboratory tests with various tap waters, distilled waters, and salt solutions have shown that no water sample was ever encountered that did not contain at least traces of surface-active material (ref. 46). 

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