Colloidal suspensions, preparation techniques

Some preparation methods specific to the formation of nanoparticle suspensions are provided in References [20,62,63]. Many such methods are simply conventional colloidal suspension preparation methods that have been extended to produce smaller particle sizes, but others involve novel approaches. Some ofthese involve making nanoemulsions as a first step. For example, membrane, microfluidic and nanofluidic devices have been used to make nanoscale emulsions of all kinds, as already noted earlier, and the emulsion droplets so generated can be used in turn to make sohd microparticles and nanoparticles. If the nanoparticles are intended to encapsulate other materials, then a double emulsification technique can be used, at elevated temperature, to prepare a multiple emulsion (i.e. 

After preparation, colloidal suspensions usually need to undergo purification procedures before detailed studies can be carried out. A common technique for charged particles (typically in aqueous suspension) is dialysis, to deal witli ionic impurities and small solutes. More extensive deionization can be achieved using ion exchange resins. 

This chapter describes the basic principles involved in the development of disperse systems. Emphasis is laid on systems that are of particular pharmaceutical interest, namely, suspensions, emulsions, and colloids. Theoretical concepts, preparation techniques, and methods used to characterize and stabilize disperse systems are presented. The term particle is used in its broadest sense, including gases, liquids, solids, molecules, and aggregates. The reader may find it useful to read this chapter in conjuction with Chapters 8, 12, and 14, since they include some of the most important applications of disperse systems as pharmaceutical dosage forms [1]. 

Colloidal suspensions of silver or gold particles in water can be prepared typically by the reduction of a silver salt (e.g., silver nitrate) using a reducing agent such as D-glucose or a citrate. A novel technique [32] involves the laser ablation of silver foil in water using a 355-nm laser with a pulse energy of about 50 mJ and a 10-Hz repetition rate. 

A related technique used to produce very thin, virtually continuous tapes (0.1 mm) of ceramic materials is tape casting. This involves the preparation of a colloidal suspension which is transported on a carrier... 

Bissessur and coworkers explored the inclusion of poly(2-ethylaniline) (PEA) and poly(2-propylaniline) (PPA) into GO, in addition to polyaniline [90]. The technique of intercalation differed from previously reported methods. They showed that polyaniUnes can be directly inserted into GO without the preparation of precursor phases. The polymers were first prepared from the monomers by oxidation with ammonium peroxydisulfate in acidic medium. GO, synthesized by using the Hummers method, was dispersed in deionized water with the aid of sonication. Colloidal suspensions of the polymers in NMF were then added to the aqueous GO suspensions. The reaction mixtures were then acidified and heated at 60 °C for 90 min. The intercalated phases were isolated via freezedrying. A similar process was used to intercalate polypyrrole into GO [91]. 

Clearly, the availability of the four-stepwise hierarchy in the structure of the nanosilica powder could cause some dependencies of the structural features and other characteristics of the colloidal dispersions on the pretreatment techniques and other conditions, which can influence the particles differently from different hierarchic levels. Therefore, several types of the sample preparation technique were applied. There were (a) ultrasonic (US) treatment of the freshly prepared suspensions of nanosilica (at different concentrations) at =1-12 h (Figures 1.89,1.90, and 1.95) (b) pretreatment of a dry silica powder in the ball mill for t cA = 1-24 h, then preparation of the aqueous suspensions sonicated for 1-9 h (c) MCA of the aqueous suspensions of silica in the ball mill for 0.1-24 h (d) MCA of the mixtures of the aqueous suspensions of silica and other compounds for 5-7 h and (e) addition of some compounds (polymers, biomacromolecules, surfactants, drugs) to the ball-milled suspensions of silica (vide infra). 

A similar technique was used for the preparation of polystyrene (PS)-Si02 nanohybrids, where colloidal silica solutions were mixed with PS solutions by means of ultrasonic homogenization [60]. Also, latex-silica nanohybrid films were synthesized upon mixing aqueous colloidal suspensions of silica and nanolatex polymer beads [61-63], Other silica-based nanohybrid systems with poly(ethylene oxide) (PEO) [64, 65], polyfvinyl alcohol) (PVA) [66], PS [67], polybutylacrylate [68], or PMMA [69] can be prepared by using the same suspension blending method. 

Traditional electron microscopy is conducted in high vacuum, which imposes specific efforts to sample preparation. Particles from colloidal suspensions have to be deposited onto an appropriate substrate (e.g. on carbon or silica films) and dried. Alternatively, the suspensions can be shock-freezed and particles are subsequently excavated from the continuous phase by special cryo-preparation techniques (Schmidt et al. 1994, pp. 694—705). The sample preparation can be considerably reduced with environmental scanning electron microscopes (ESEM), which are operated up to 1000 Pa and, thus, even facilitate the analysis of wet surfaces. However, the ease in operation is at the expense of resolution (Danilatos 1993). 

Static and dynamic scattering techniques are spectroscopic characterisation methods in the sense of Sect. 2.2. These techniques evaluate the functional dependency of measurement signals on a spectral parameter, i.e. on time, space, or classically on wavelength or frequency. The major advantage of spectroscopic methods is the reduced sample preparation (no fractionation), but they involve the inversion problem. That is, the spectrum is a—most frequently incomplete and discrete— nonlinear projection of the size distribution. Beside the scattering techniques, there are further spectroscopic methods which are based on the extinction of radiation or on any other response of the particle system to an external field. This section describes optical, acoustic, and electroacoustic methods that have gained relevance for the characterisation of colloidal suspensions. 

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