Particulate systems dimensions

A particle is a single unit of material having discrete physical boundaries which define its size, usually in micrometers, )Am (1 /tm (lO" A) = 1 X cm = 1 x 10 m). The size of a particle is usually expressed by the dimension of its diameter. Typically, particle science is Limited to particulate systems within a size range from 10 to 10 lm. 

The special needs of size characterization methods for the biological and pharmaceutical systems that are now so important in the medical sciences today are reviewed. Submicrometre and subcolloidal particulate systems are more frequently encountered that do not have the sharply defined interfaces familiar to the analyst of 25 years ago. Philosophically it is pointed out that many particles with indeterminate interfaces will move under the application of external forces as if they were inside a sphere of influence and it is, in fact, the dimensions of this sphere that are measured. Under these conditions it is valid to measure the particle size characteristics of particles down to the molecular dimensions, at least, of some of the larger protein molecules, and some methods will certain reach down into these size regions without excessive difficulty. 

Most physical properties of a particulate system are ensembles or statistical values of the properties from their individual constituents. Commonly evaluated particle geometrical properties are counts, dimension (size and distribution), shape (or conformation), and surface features (specific area, charge and distribution, porosity and distribution). Of these properties, characterization of particle size and surface features is of key interest. The behavior of a particulate system and many of its physical parameters are highly size-dependent. For example, the viscosity, flow characteristics, filterability of suspensions, reaction rate and chemical activity of a particulate system, the stability of emulsions and suspensions, abrasiveness of dry powders, color and finish of colloidal paints and paper coatings, strength of ceramics, are all dependent on particle size distribution. Out of necessity, there are many... 

The disc centrifuge photosedimentometer (DCP) is an instrument used to determine the particle size and size distribution of latices, pigments and other particulates. Separation is based on Stokes Law for the sedimentation of particles in a centrifugal force field. Particles are assumed to be spherical. In the most common mode of operation the separation takes place in a cylindrical cavity of known dimensions and containing a known volume of fluid. A dilute suspension of the particles of Interest is injected onto the surface of the fluid while the disc is spinning and the particles sediment in the centrifugal force field radially outward past a detector system. The appearance time of the particle at the detector, t, in minutes is given by... 

For a successful incorporation of a pigment into the latex particles, both type and amount of surfactant systems have to be adjusted to yield monomer particles, which have the appropriate size and chemistry to incorporate the pigment by its lateral dimension and surface chemistry. For the preparation of the miniemulsions, two steps have to be controlled (see Fig. 14). First, the already hydrophobic or hydrophobized particulate pigment with a size up to 100 nm has to be dispersed in the monomer phase. Hydrophilic pigments require a hydro-phobic surface to be dispersed into the hydrophobic monomer phase, which is usually promoted by a surfactant system 1 with low HLB value. Then, this common mixture is miniemulsified in the water phase employing a surfactant system 2 with high HLB, which has a higher tendency to stabilize the monomer (polymer)/water interface. 

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