Lyophobic systems

In colloid science, colloidal systems are commonly classified as being lyophilic or lyophobic, based on the interaction between the dispersed phase and the dispersion medium. In lyophilic dispersions, there is a considerable affinity between the two constituent phases (e.g., hydrophilic polymers in water, polystyrene in benzene). The more restrictive terms hydrophilic and oleophilic can be used when the external phase is water and a nonpolar liquid, respectively. In contrast, in lyophobic systems there is little attraction between the two phases (e.g., aqueous dispersions of sulfur). If the dispersion medium is water, the term hydrophobic can be used. Resulting from the high affinity between the dispersed phase and the dispersion medium, lyophilic systems often form spontaneously and are considered as being thermodynamically stable. On the other hand, lyophobic systems generally do not form spontaneously and are intrinsically unstable. 

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... 

It is important whether the various structural elements can be said to constitute phases or not. If not, we generally have a so-called lyophilic system, which is in principle in equilibrium. In most foods, structural elements do constitute phases, which implies that they have phase boundaries in which free energy is accumulated. This means an excess of free energy, hence a lyophobic system it costs energy to make it, and it is inherently unstable. The properties of such foods thus depend on the manufacturing and storage history and, for natural foods, on growth conditions. 

Thermodynamic description presented in Chapter IV allowed us to subdivide all colloidal systems into two large classes thermodynamically stable systems, referred to as lyophilic and those characterized by kinetic stability only, referred to as lyophobic systems. Detailed description of properties and stability of lyophobic systems is presented in chapters that follow, while in the present chapter we will focus on the properties, structure and formation conditions of the lyophilic colloidal systems. 

In free disperse systems Brownian motion, along with stabilizing action, may also reveal a destabilizing one. Such destabilizing action is typical for truly lyophobic systems, i.e. the systems that are unstable with respect to aggregation and do not belong to the class of pseudolyophilic ones. We will further show that in these systems Brownian motion is indeed the mechanism responsible for particle coagulation. 

For typically lyophobic systems (e.g. aqueous dispersions of solid hydrocarbons) with a characteristic value of A 5x 10 20 J at r 10 8 m and h0 2x1 O 10 m the interaction energy between particles in a contact, in agreement with eq. (VII. 15), is not less than... 

In this second volume colloidal systems will be treated, which differ fundamentally from the proceeding ones, in so far as the sol state — and also other states not, or rarely, met with in the case of the lyophobic systems — represent true equilibrium states. 

Colloidal systems can be divided into lyophilic and lyophobic systems. Lyophilic colloids have a strong affinity with the dispersion medium by which a solvation shell around the particle is formed. This process is called solvation and if the dispersion medium is water it is called hydration. A polysaccharide dissolved in water is an example of a lyophilic colloidal system. The solvation shell is formed by hydrogen bonds between the hydroxyl groups of the polymer molecules and the water molecules. Pharmaceutical examples are solutions of dextran, used as plasma expanders. Micelles are also lyophilic colloids. Example of such a system is the aqueous cholecalciferol oral mixture (Table 18.15). In these preparations, a lipophilic fluid is dissolved in an aqueous medium by incorporating it in micelles. Because this type of colloids falls apart on dilution to concentrations below the CMC, they are also known as association colloids. Lyophobic colloids have no affinity with the dispersion medium. Thus, in this type of colloids no solvation shell is formed around the particles. An example of lyophobic particles are colloidal gold particles (with a diameter of 1 nm - 1 pm) dispersed in water. There are no... 

Overbeek, J. Th. G. 1949b. II Phenomenology of lyophobic systems. In Colloid Science Vol. I, Irreversible Systems, ed. H. R. Kruyt. Amsterdam Elsevier. 

The lyophobic systems are also called suspensoids. They easily precipitate out forming irreversible flocculates. But if they are left undisturbed, despite their intrinsic instability they remain unchanged for long periods (see Box 3.1). Research has shown that it is the electrical potential difference between the surface and the solution far away from the particles that determines the dluturnity (stability) of the lyophobic system. This electrical potential difference arises due to the charge carried by the particles of a sol, and the charge carried by the ions in the surrounding medium. 

The optical properties of lyophobic systems are worth mentioning. These sterns are characterized by solute molecules which stay apart from each other. TTais allows Increased scattering of light as the difference in refractive indices of the solvent and solute particles is quite high. The Tyndall effect is distinct and therefore the particles of a suspensoid are more clearly visible in an ultramlcroscope. The viscosity, though, is not so affected eis the solvent flow is not hindered. 

We have a little earlier discussed the nature of lyophobic systems which are inherently unstable and the lyophilic systems which are thermodynamically stable md are made up of macromolecules having great solvation power. There exists another class of colloidal systems which, though thermodynamically stable, are made up of micelles. A micelle is a spontaneously formed aggregate of micromolecules. This aggregate achieves colloidal dimensions and is a thermodynamically stable structure. The dispersion finally formed is called the association colloidal system. 

Equation 1.44 is a reasonably smooth function, which is an indication that dispersion interactions compensate each other to a significant extent, even when there is a large difference in the nature of the contacting phases. For example, if both Aj and A2 are around 10 ° J and differ from each other by, for example, 20%, then A 10 J for a 5% difference. A 2.5 x 10 J, and for a 2% difference between A, and A2, A is lowered to 10 J. We will use these estimates later in the characterization of the contact interactions in lyophilic and lyophobic systems. 

The influence of this third factor on the trends in AF may differ significantly depending on its nature, as shown in Figure 4.33. The two limiting cases here correspond, respectively, to the complete absence and the very strong presence of this factor, namely, the dispersion down to individual molecules or ions (i.e., dissolution) on the one hand and a monotonous inCTease in the AF on the other hand. The latter represents a case of high a values, which are typical in lyophobic systems, for example, a solid body in the absence of a surface-active medium. In between these two extreme... 

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