Kinds of Colloidal Particles

Colloids can be classified as hydrophilic colloids, hydrophobic colloids, or association colloids. These three classes are briefly summarized below. 

Hydrophilic colloids generally consist of macromolecules, such as proteins and synthetic polymers, that are characterized by strong interaction with water resulting in spontaneous formation of colloids when they are placed in water. In a sense, hydrophilic colloids are solutions of very large molecules or ions. Suspensions of hydrophilic colloids are less affected by the addition of salts to water than are suspensions of hydrophobic colloids. 

Association colloids consist of special aggregates of ions and molecules called micelles. To understand how these arise, consider sodium stearate, a typical soap 

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Various kinds of colloidal particles have been used in biomedical domains. In analytical chemistry, they are used as sohd supports for sample preparation. In the drug dehvery field, nanocolloids, and particularly stimuli responsive polymer-based nanogels, have been intensively explored as protein carriers for in vivo applications. The reported smdies in this direction are mainly focused on the release efficiency rather than on the driven forces involved in the loading and the release of the loaded proteins. 

Colloidal particles are of three general types, as illustrated in Figure 3.11 hydrophilic colloids tend to remain in suspension because of their strong interactions with water they are generally macromolecules such as proteins and some synthetic polymers. Exemplified by clay particles and petroleum droplets, hydrophobic colloids are generally repelled by water and remain in suspension because of their electrical charges, which tend to keep the particles away from each other. Special aggregates of ions and molecules called micelles constitute the third kind of colloidal particles. Soap ions, such as sodium stearate,... 

For tire purjDoses of tliis review, a nanocrystal is defined as a crystalline solid, witli feature sizes less tlian 50 nm, recovered as a purified powder from a chemical syntliesis and subsequently dissolved as isolated particles in an appropriate solvent. In many ways, tliis definition shares many features witli tliat of colloids , defined broadly as a particle tliat has some linear dimension between 1 and 1000 nm [1] tire study of nanocrystals may be drought of as a new kind of colloid science [2]. Much of die early work on colloidal metal and semiconductor particles stemmed from die photophysics and applications to electrochemistry. (See, for example, die excellent review by Henglein [3].) However, the definition of a colloid does not include any specification of die internal stmcture of die particle. Therein lies die cmcial distinction in nanocrystals, die interior crystalline stmcture is of overwhelming importance. Nanocrystals must tmly be little solids (figure C2.17.1), widi internal stmctures equivalent (or nearly equivalent) to drat of bulk materials. This is a necessary condition if size-dependent studies of nanometre-sized objects are to offer any insight into die behaviour of bulk solids. 

Theories or computer simulations used to calculate the potential of mean force W(r) are typically based on numerous simplifying assumptions and approximations (de Kruif, 1999 Bratko et al., 2002 Prausnitz, 2003 de Kruif and Tuinier, 2005 Home et al., 2007 Jonsson et al., 2007). Therefore they can provide only a qualitative or, at best, semi-quantitative description of the potential of mean force. Such calculations are nevertheless useful because they can serve as a guide for trends in the factors determining the interactions of both biopolymers and colloidal particles. Thus, an increase in the absolute value of the calculated negative depth of W(r) may be attributed to a predominant type of molecular feature favouring aggregation or self-association. To assist with such a theoretical analysis, expressions for some of the mean force potentials will be presented here in the discussion of specific kinds of interactions occurring between pairs of colloidal particles covered by biopolymers in food colloids. 

Polymer colloids involve dispersions containing polymer particles having sizes greater than about 1 nm. If dispersed in aqueous solution, such a polymer dispersion is called a latex. These are usually synthetic polymer particles formed by free radical polymerization [784], Many kinds of polymerization systems exist, involving almost all of the possible kinds of colloidal dispersion, including emulsion polymerization, hence the more general term heterophase polymerization is sometimes used. Several reviews are available [785-789]. Emulsion polymerization provides a convenient means of controlling the polymerization of monomers and is used to make, for example, synthetic rubber which is mostly a co-polymer of butadiene and styrene. 

In what follows, one considers for illustration purposes the case in which the charge is generated on the surface of colloidal particles or droplets by the adsorption of a surfactant, namely sodium dodecyl sulfate (SDS). We selected this case because information about the adsorption of SDS on an interface is available in the literature, and as it will become clearer later the number of parameters involved is smaller than in the case of silica. A more complex calculation about the silica and the amphoteric latex particles will be presented in a forthcoming paper. It involves several kinds of surface dipoles and equilibrium constants. 

The other kind of systems largely studied, consists of polymethylmethacrylate (PMMA) or silica spherical particles, suspended in organic solvents [23,24]. In these solvents Q 0 and uy(r) 0. The particles are coated by a layer of polymer adsorbed on their surface. This layer of polymer, usually of the order of 10-50 A, provides an entropic bumper that keeps the particles far from the van der Waals minimum, and therefore, from aggregating. Thus, for practical purposes uw(r) can be ignored. In this case the systems are said to be sterically stabilized and they are properly considered as suspensions of colloidal particles with hard-sphere interaction [the pair potential is of the form given by Eq. (5)]. 

As can be seen from Figure 2 the adsorption branch of this isotherm exhibits two distinct steps that reflect the capillary condensation inside smaller or larger mesopores at relative pressures about 0.79 and 0.9, respectively. The condensation in the relative pressure range of 0.9S-0.99S reflects condensation in secondary mesopores or small macropores, which resulted from the imprinting of agglomerates of colloidal particles. To our knowledge, this kind of isotherm has not been reported for porous carbon materials. The pore size distribution for this mesoporous carbon shown in Figure 3 exhibits two distinct peaks located about 11 nm and 24 nm, which correspond to the particle size of Bindzil 30/360 and Ludox AS-40 colloidal silicas, respectively. 

Definition and Classification of Emulsions. Colloidal droplets (or particles or bubbles), as they are usually defined, have at least one dimension between about 1 and 1000 nm. Emulsions are a special kind of colloidal dispersion one in which a liquid is dispersed in a continuous liquid phase of different composition. The dispersed phase is sometimes referred to as the internal (disperse) phase, and the continuous phase as the external phase. Emulsions also form a rather special kind of colloidal system in that the droplets often exceed the size limit of 1000 nm. In petroleum emulsions one of the liquids is aqueous, and the other is hydrocarbon and referred to as oil. Two types of emulsion are now readily distinguished in principle, depending upon which kind of liquid forms the continuous phase (Figure 2) ... 

Boiling point elevation constant, A constant that corresponds to the change (increase) in boiling point produced by a one-molal ideal solution of a nonvolatile nonelectrolyte. CoUigative properties Physical properties of solutions that depend on the number but not the kind of solute particles present. Colloid A heterogeneous mixture in which solute-like particles do not settle out also called colloidal dispersion. 

In Sections 9.1.1 and 9.2.4 some surface modifications of colloidal particles have already been discussed. Another kind of modification can be obtained by forming a sandwich between two particles of different materials such as T1O2 and CdS. Corresponding structures were formed spontaneously, when the separately prepared solutions of the colloids were mixed under certain conditions [3, 68, 69]. Such sandwich... 

In this chapter we examine some issues in mass transfer. The reader has already been introduced to some of the key aspects. In Chapter 3 (Section 7), flocculation kinetics of colloidal particles is considered. It shows the importance of diffusivity in the rate process, and in Equation 3.72, the Stokes-Einstein equation, the effect of particle size on diffusivity is observed, leading to the need to study sizes, shapes, and charges on colloidal particles, which is taken up in Chapter 3 (Section 4). Similarly some of the key studies in mass transfe in surfactant systems— dynamic surface tension, smface elasticity, contacting and solubilization kinetics—are considered in Chapter 6 (Sections 6, 7, 10, and 12 with some related issues considered in Sections 11 and 13). These emphasize the roles played by different phases, which are characterized by molecular aggregation of different kinds. In anticipation of this, the microstructures are discussed in detail in Chapter 4 (Sections 2,4, and 7). Section 2 also includes some discussion on micellization-demicellization kinetics. 

One very powerful tool for determining the size, shape, and orientation of colloidal particles in a liquid continuum is through their interaction with X-radiation. The nature of this interaction varies considerably with the colloidal system, and it can often be used to elucidate the structure of the latter when combined with the results of other experimental measurements. X-ray results alone rarely lead to unambiguous conclusions in these kinds of systems. 

The main conclusion of Paciejewska s thesis is the necessity to consider the specific kinetics of interfacial phenomena when evaluating the stability of colloidal suspensions. This applies not only to binary, but to all kinds of colloidal suspensions. A major factor is the dissolution of the dispersed phase(s)—in particular if the solubility and the intrinsic dissolution rate are relatively large. Its relevance is especially pronounced for a large total surface area, which depends on the particle concentration and the specific surface of the particles and which determines the amount of substance that can be dissolved in a given period of time. For many nanoparticle (x < 100 nm) systems (e.g. additives for paints and coatings), it will not be permissible to ignore the influence of dissolution on the interfacial properties and even on suspension stability—independent from the Gibbs-Thomson effect, which becomes relevant at particle sizes below 10 nm (cf. Sect. 3.1.4). 

Figure 3.17. Models of [A) cyclic trisilicic. (B) cubic octasilicic acids, and (C) and ( )) the corresponding theoretical colloidal particles formed by condensing monomer to form closed rings until the original species is completely surrounded by one layer of deposited silica bearing silanol groups. When formed above pH 7 the inner silica contains few silanol groups. Different kinds of incompletely condensed oligomers could form the cores of colloidal particles. There is no evidence that A and B are specifically involved. Spheres, oxygen atoms black dots, hydrogen atoms. Silicon atoms are not visible. [From Her (97b) by permission of Plenum Press.]...

Four different sol-gel routes can be used to prepare various kinds of sulfide materials. Sol-gel synthesis of sulfides usually follows colloidal chemical processing, except route A. It is very important to control the sizes of colloidal particles. As-S and Ge-S glass films could be prepared by sol-gel processing for planar waveguides, for IR optical applications. The sol-gel synthesis of multicomponent sulfide glasses needs further study, perhaps based on the Ge-S system. Route D can also be used for synthesis of multicomponent sulfides. 

In addition to ionic surfactants, nonionic surfactant molecules can also adsorb onto the particle surfaces to impart satisfactory stabihty to colloidal dispersions [20, 21]. Some very old examples include India ink and carbon black particles dispersed in the continuous aqueous phase containing a natural gum. This kind of colloidal stabilization mechanism (termed steric stabilization) was first illustrated experimentally by M. Faraday [31, 32]. Some representative polymeric materials (protective colloids) that are effective in preparing steri-cally stabilized aqueous colloidal dispersions are summarized in Table 2.7 [21]. A portion of an effective protective colloid must be hydrophobic enough to show a strong tendency to adsorb onto the hydrophobic particle surface. Furthermore, the adsorbed macromolecules must form a relatively thick hydrophilic layer surrounding the particle, which serves as a steric barrier to prevent the colloidal particles from flocculation. 

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