Particle size colloidal systems

Many liquid and semi-liquid pharmaceutical preparations are disperse systems. Disperse systems are defined as systems in which a substance is distributed as particles (discontinuous) into a dispersion medium (continuous). Three types of disperse systems will be discussed which are pharmaceutically relevant colloidal systems, suspensions and emulsions. In both colloidal systems and suspensions, solid particles are dispersed in a liquid. The difference is that in colloidal systems the particles do not settle, while they do in suspensions. This difference is caused by the size of the particles. In colloidal systems, the particles are so small (1 mn - 1 pm) that the Brownian motion (diffusion caused by thermal energy) is stronger than the force of gravity so that they remain suspended in the liquid and do not settle. In suspensions, the particles are larger (>1 pm) and as a consequence the force of gravity is stronger than the Brownian motion which makes them settle (if the density of the particles is larger than that of the dispersion medium). Emulsions consist of non-miscible liquids. Two types of emulsions will be discussed oil drops dispersed in water (oil-in-water emulsion or o/w emulsion) and water drops dispersed in oil (water-in-oil emulsion or w/o emulsion). There are also more complex structures such as w/o/w emulsions and bi-continuous systems. However, these systems will not be discussed. 

In practice, e.g., in nature or in fonnulated products, colloidal suspensions (also denoted sols or dispersions) tend to be complex systems, consisting of many components that are often not very well defined, in tenns of particle size for instance. Much progress has been made in the understanding of colloidal suspensions by studying well defined model systems, which allow for a quantitative modelling of their behaviour. Such systems will be discussed here. 

Although the remainder of this contribution will discuss suspensions only, much of the theory and experimental approaches are applicable to emulsions as well (see [2] for a review). Some other colloidal systems are treated elsewhere in this volume. Polymer solutions are an important class—see section C2.1. For surfactant micelles, see section C2.3. The special properties of certain particles at the lower end of the colloidal size range are discussed in section C2.17. 

A particularly interesting feature of industrial crystallization systems is the relatively wide range of particle sizes encountered. Particle sizes range over several orders of magnitude from the sub micron (nanometers) to several millimetres or more, i.e. from colloidal to coarse . Such particles comprise a large part of the world on a human scale and a great source of industrially generated wealth. 

Several colloidal systems, that are of practical importance, contain spherically symmetric particles the size of which changes continuously. Polydisperse fluid mixtures can be described by a continuous probability density of one or more particle attributes, such as particle size. Thus, they may be viewed as containing an infinite number of components. It has been several decades since the introduction of polydispersity as a model for molecular mixtures [73], but only recently has it received widespread attention [74-82]. Initially, work was concentrated on nearly monodisperse mixtures and the polydispersity was accounted for by the construction of perturbation expansions with a pure, monodispersive, component as the reference fluid [77,80]. Subsequently, Kofke and Glandt [79] have obtained the equation of state using a theory based on the distinction of particular species in a polydispersive mixture, not by their intermolecular potentials but by a specific form of the distribution of their chemical potentials. Quite recently, Lado [81,82] has generalized the usual OZ equation to the case of a polydispersive mixture. Recently, the latter theory has been also extended to the case of polydisperse quenched-annealed mixtures [83,84]. As this approach has not been reviewed previously, we shall consider it in some detail. 

Different from the metal concentration, the content of water in the reaction system exhibits an obvious effect on the particle size of the resulted metal nanoclusters. In a mixture of EG and water (10 1 in volume ratio), Pt hydroxide colloidal particles formed in the first synthesis step were 4.0 nm in average diameter, and the finally obtained Pt nanoclusters had an average particle size of... 

Colloidal systems of particles are different from molecular clusters in having a small distribution in sizes. Also, colloidal systems may be stabilized by organic molecules, resulting in chemical bonds between these and the outer monolayer of metal atoms. The Mossbauer study of such systems includes rather diverse fields ... 

Figure 8. Size and nature of colloids and particles in aqueous systems (from Buffle and VanLeewen 1992).

The defined size ranges and limits are somewhat arbitrary since there are no specific boundaries between the categories. The transition of size ranges, either from molecular dispersions to colloids or from colloids to coarse dispersions, is very gradual. For example, an emulsion may exhibit colloidal properties, and yet the average droplet size may be larger than 1 pm. This is due to the fact that most disperse systems are heterogeneous with respect to their particle size [1-2]. 

One of the most obvious properties of a disperse system is the vast interfacial area that exists between the dispersed phase and the dispersion medium [48-50]. When considering the surface and interfacial properties of the dispersed particles, two factors must be taken into account the first relates to an increase in the surface free energy as the particle size is reduced and the specific surface increased the second deals with the presence of an electrical charge on the particle surface. This section covers the basic theoretical concepts related to interfacial phenomena and the characteristics of colloids that are fundamental to an understanding of the behavior of any disperse systems having larger dispersed phases. 

A colloid is defined as a system consisting of discrete particles in the size range of 1 nm to 1 pm, distributed within a continuous phase [153], On the basis of the interaction of particles, molecules, or ions of the disperse phase with molecules of the dispersion medium-, colloidal systems can be classified as being lyophilic or lyophobic. In lyophilic systems, the disperse phase molecules are dissolved within the continuous phase and in the colloidal size range or spontaneously form aggregates in the colloidal size range (association systems). In lyophobic systems, the disperse phase is very poorly soluble or insoluble in the continuous phase. During the last several decades, the use of colloids in... 

Absorption spectra of CdS colloid indicate the formation of quantum sized CdS particles. The particle size increased upon sonication, indicated by the red shift in the onset of absorption. The particle size was highly dependent on the mercaptan used, because of the absorption of the mercaptan on the particle acting as a capping agent and the rate of H2S produced. Study of mercaptan systems revealed that there was also a thermal process responsible for CdS formation. 25% of the total CdS produced sonochemically was formed via a thermal mechanism presumably in the hot shell around the compressed bubble. CdS colloid could be dissolved quite readily by sonicating solutions under air saturated conditions [89] by the following reaction,... 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

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