Salt-free medium

Here r/0 is the viscosity in salt-free medium and A and B are constants at high salt concentrations, the second term becomes irrelevant. The constant B, which is the second virial coefficient signifying ion-solvent interactions, is termed the Jones-Dole coefficient after the inventors (Jones, 1929). Chaotropes have a coefficient B which is less than zero, whereas kosmotropes are characterized by 1 > 0. 

Around a Charged Particle in a Salt-Free Medium... 

In this chapter, we first discuss the case of completely salt-free suspensions of spheres and cylinders. Then, we consider the Poisson-Boltzmann equation for the potential distribution around a spherical colloidal particle in a medium containing its counterions and a small amount of added salts [8]. We also deals with a soft particle in a salt-free medium [9]. 

Consider a dilute suspension of spherical colloidal particles of radius a with a surface charge density cr or the total surface charge Q = 4na a in a salt-free medium containing only counterions. We assume that each sphere is surrounded by a concentric spherical cell of radius R [5,7] (Fig. 6.1), within which counterions are distributed so that electrical neutrality as a whole is satisfied. The particle volume fraction [Pg.133]

Consider a dilute suspension of polyelectrolyte-coated spherical colloidal particles (soft particles) in a salt-free medium containing counterions only. We assume that the particle core of radius a (which is uncharged) is coated with an ion-penetrable layer of polyelectrolytes of thickness d. The polyelectrolyte-coated particle has thus an inner radius a and an outer radius b = a + d (Fig. 6.4). We also assume that ionized groups of valence Z are distributed at a uniform density N in the poly electrolyte... 

Electric behaviors of colloidal particles in a salt-free medium containing counterions only are quite different from those in electrolyte solutions, as shown in Chapter 6. In this chapter, we consider the electrostatic interaction between two ion-penetrable membranes (i.e., porous plates) in a salt-free medium [1]. 

Before considering the interaction between two ion-penetrable membranes, we here treat the interaction between two similar ion-impenetrable hard plates 1 and 2 carrying surface charge density cr at separation h in a salt-free medium containing counterions only (Fig. 18.1) [2]. We take an x-axis perpendicular to the plates with its origin on the surface of plate 1. As a result of the symmetry of the system, we need consider only the region 0 < x < h 2. Let the average number density and the valence of counterions be o and z, respectively. Then we have from electroneutrality condition that... 

Now consider two parallel identical ion-penetrable membranes 1 and 2 at separation h immersed in a salt-free medium containing only counterions. Each membrane is fixed on a planar uncharged substrates (Fig. 18.2). We obtain the electric potential distribution i/ (x). We assume that fixed charges of valence Z are distributed in the membrane of thickness d with a number density of A (m ) so that the fixed-charge density pgx within the membrane is given by... 

FIGURE 18.3 Distributions of counter-ion concentration n x) across two parallel identical ion-penetrable membranes of thickness d separated by a distance h between their surfaces in a salt-free medium. Distributions were calculated at /i = 0, 2, 6, and lOnm for Z—z— 1, d = 5nm, and the charge amount per unit area (j = ZeNd — 0.2Clm in water at 25 C (fir = 78.55). From Ref. [1]. 

It is of interest to note that this limiting expression for P h) is independent of the membrane-fixed charges ZeN and the membrane thickness d. This limiting form thus agrees with that for the interaction between two charged planar surfaces in a salt-free medium (Eq. 18.19). 

The polyketide trichoharzin (38) was only produced when Trichoderma harzianum was cultured in a salt-free medium [79]. 

So far we have discussed electrophoresis of particles and drops in electrolyte solutions, i.e., salt-containing media. For those in salt-free media containing only counterions (e.g., nonaqueous media [63]), special consideration is needed. To treat a suspension of spherical particles in a salt-free medium, one usually employs a free volume model, in which each sphere of radius a is surrounded by a spherical free volume of radius R within which counterions are distributed so that electrical neutrality as a whole is satisfied. The particle volume fraction is given by 4>= (fl// ). We treat the case of dilute suspensions, viz., < C lara/R< 1. Let the concentration (number density) and valence of the counterions be n and — z, respectively. For a spherical particle carrying surface charge density a or total surface charge Q = 4ira a, it follows from the electroneutrality condition that... 

Consider first the equilibrium potential distribution around a spherical particle in a salt-free medium. The Poisson-Boltzmann equation for electric potential i/<(r) (or its scaled form y = ze ijj/kT) at a distance r from the origin of one particle is given by... 

Ohshima, H., Surface charge density/surface potential relationship for a spherical colloidal particle in a salt-free medium, J. Colloid Interface ScL, 225, 233, 2000. 

Lazarides, H. N., Mavroudis, N. E., 1996. Kinetics of osmotic dehydration of a highly shrinking vegetable tissue in a salt free medium. /. Food Eng. 30 61-74. 

Note that Eq. (28) has a similar form as Eq. (13). However, those two equations cannot be equalized, as they are derived in different conditions, A more complicated relationship between Zeta potential and surface charge density in a non-aqueous system has been developed by Chen [28J for a suspension containing both counterions and coions and by Oshima 29] for a suspension containing only counterions (a salt-free medium). In a concentrated suspension, the Zeta potcntial/surfacc charge density relationship is also a function of the particle volume fraction [42-45]. The actual Zeta potential should be smaller than that expected from the linear relationship,... 

The same phenomenon is observed in complexes that have been compacted under the shape of circular matrices. In fact, it can be seen in Fig. 18 that water sorption rate is proportional to the viscosities reported in Table 3. Thus, the high swelling capacity exhibited by the complexes of atenolol and hdocaine in salt free medium makes them highly susceptible to the saline effect. Thus, the sorption rate decreases when a NaCl solution is used instead of water. On the other hand, the complex with metoclopramide has a very low rate of water sorption reveal a limited swelling capacity whose rate is barely affected in NaCl solution. 

H. Ohshima, /. Colloid Interface Sci., 247, 18 (2002). Surface Charge Density/Surface Potential Relationship for a Spherical Colloidal Particle in a Salt-Free Medium. 

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