Colloids, lyophobic

Lyophobic colloids Lyotropic liquid crystals Lyotropic mesophases Lyotropic polymers Lyral [31906-04-4]... 

Lyophobic colloids, ordinarily, have an electric charge of definite sign, which can be changed only by special methods. 

The stability of lyophobic colloids is intimately associated with the electrical charge on the particles. Thus in the formation of an arsenic(III) sulphide sol... 

Liquid junction potential 63, 549 Literature of analytical chemistry 6, 122, 156, 251, 253, 498, 499, 640, 641, 813, 815 Lithium, D. of as aluminate, (g) 459 Litmus 265 Litre xxix, 78 Littrow mounting 661 Logarithms four figure, 843 Lovibond comparator 655 Low voltage d.c. arc 763, 771 Lubricants for glass stopcocks 85 Lyophilic colloids 419 Lyophobic colloids 419 stability of, 419... 

EJV Verwey and J Th G Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. 

REFERENCE VERWEY AND OVERBEEK THEORY OF THE STABILITY OF LYOPHOBIC COLLOIDS... 

Varanasi, P. P., Ryan, M. E., and Stroeve, P., Experimental study on the breakup of model viscoelastic drops in uniform shear flow. I EC Research 33, 1858-1866 (1994). Verwey. E. J., and Overbeek, J. T. G., Theory of the Stability of Lyophobic Colloids. ... 

Verwey, E.J.W. Overbeek, J.Th.G. (1948) Theory of stability of lyophobic colloids. Elsevier, Amsterdam... 

It is also important to emphasize that conventional consciousness of colloid or fine particle technology, like better dispersion and control of rheological properties of dipping solution, are not to be overlooked. The growth of nanoparticles in liquid phase is almost exactly regulated by the nuclei-growth theory as well as stability of lyophobic colloids suggested half a century ago. 

Lyophobic colloids are known by a variety of terms, depending on the nature of the phases involved. Some of these are listed in Table 1.4. Some of the terms (e.g., aerosol, gel) are somewhat ambiguous, so the reader is warned to make certain that the system is fully understood, particularly when the original literature is consulted. Remember that a common feature of all systems we consider is that some characteristic linear dimension of the dispersed particles falls in the range defined in Section 1.1a. When we deal with two-phase colloids in this book, we are primarily concerned with systems in which the dispersed phase is solid and the continuous phase is liquid. 

Similarly, charged solid particles (such as latex spheres) —kinetically stable lyophobic colloids —may exist in colloidal crystalline phases (with body-centered or face-centered cubic structures) as a consequence of thermodynamically favored reduction in free energies (see Chapter 13). Even neutrally charged spherical particles ( hard spheres ) undergo a phase transition from a liquidlike isotropic structure to face-centered cubic crystalline structures due to entropic reasons. In this sense, the stability or instability is of thermodynamic origin. 

Because of the particle sizes involved, classically the optical microscope has been the instrument of choice especially for lyophobic colloids. Excellent books and manuals are available (Bradbury 1991 Cherry 1991 Schaeffer 1953) on the numerous variations of optical microscopy, and we do not go into all the details. Our purpose here is merely to point out some very elementary principles that make this method ideally suited for direct examination of colloids. We also use this introduction as a first step in pointing out modern techniques that fall under the class of microscopy but use principles (e.g., electron tunneling see Vignette 1.8) and radiation (e.g., electron or x-ray) other than those used in optical microscopy. 

What are lyophilic colloids What are lyophobic colloids Give some examples. 

Equation (45) shows that as long as balances, volumetric flasks, and viscometers are available, [17] can be determined. All that is required is to measure viscosity at a series of concentrations, work up the data as (l/c)[(ij/i70) — 1], and extrapolate to c = 0. If the experimental value of [17] turns out to be 2.5 (V2/M2), then the particles are shown to be unsolvated spheres. If [17] differs from this value, the dispersed units deviate from the requirements of the Einstein model. In the next section we examine how such deviations can be interpreted for lyophobic colloids. 

For simplicity, consider a lyophobic colloidal particle with a surface that carries a charge. Electroneutrality requires that an equal amount of opposite charges (counterions) be present... 

Since van der Waals forces are responsible for the coagulation of lyophobic colloids, the mitigating influence of the continuous phase on the attraction between dispersed particles imparts a measure of stability to the system. This result was anticipated in our remarks about... 

The sensitivity of aqueous lyophobic colloids to electrolyte content is due to the dependence of interparticle repulsion on this concentration. 

In the quantitative sections of this chapter the primary emphasis has been on establishing the relationship between the electrophoretic properties of the system and the zeta potential. We saw in Chapter 11 that potential is a particularly useful quantity for the characterization of lyophobic colloids. In this context, then, the f potential is a valuable property to measure for a lyophobic colloid. For lyophilic colloids such as proteins, on the other hand, the charge of the particle is a more useful way to describe the molecule. In this section we consider briefly what information may be obtained about the charge of a particle from electrophoresis measurements. 

The discussions above highlight the following (a) the structure of a dispersion is a complicated function of interaction forces (b) equilibrium thermodynamics dictates what is possible and what is not, but (c) for lyophobic colloids, ultimately it is kinetics that determines whether the structures predicted by thermodynamics can be realized in practice. One of the major objectives of thermodynamic and kinetic studies of colloidal systems is to be able to predict and... 

FIG. 13.8 Plot of ne, versus d, the minimum separation between surfaces, for two spheres of equal radius (100 nm). Curves are drawn for different values of k with constant values of A (2 10 19 J) and p0 (25.7 mV). (Redrawn with permission from E. J. W. Verwey and J. Th. G. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, Netherlands, 1948.)... 

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