Colloidal dispersions electrolyte concentration, effect

Among the various branches in colloid and interface science, polymer adsorption and its effect on the colloid stability is one of the most crucial problems. Polymer molecules are increasingly used as stabilizers in many industrial preparations, where stability is needed at a high dispersed phase volume fraction, at a high electrolyte concentration, as well as under extreme temperature and flow velocity conditions. 

When the particle volume fraction / was increased to 0.015, the oscillations of the effective interaction between identical charged particles became larger than those for / = 0.005 at an electrolyte concentration of 10 5 M (Fig. 3). The effective interaction between identical charged particles versus the distance between particles is plotted in Fig. 3 for various values of Z (Z = 300, Z = 600, Z = 1200). As shown in Fig. 4, the colloidal dispersion has a disordered liquidlike structure for Z = 600, but a more ordered structure for Z = 1200. When the electrolyte concentration was increased to 10-4 M, the interaction between identical charged particles became completely screened. As shown in Fig. 5, no oscillations of the effective interaction potential were present for Z = 300,600, and 1200. 

When the electrolyte concentration, c, is low, while the concentration of colloidal particles, n, and their effective charge are high, i.e. when c q nleNA, the value of x is close to the initial electrolyte concentration. In the other words, under these conditions essentially all of the electrolyte should transfer into pure dispersion medium. This means that in the case of highly developed diffuse layers of ions and rather compact arrangement of particles, when the ionic atmospheres come into contact, the co-ions (Na+ in the present... 

For a limited number of metal surfaces, adsorption of a molecular species in a thin (monomolecular layer) film results in a huge increase in the effective vibrational Raman scattering cross-section (again, as with RR scattering, up to ca. 106 times) of the adsorbate species. The SERS effect was discovered more than ten years ago for pyridine adsorbed at a silver electrode surface in contact with an aqueous electrolyte [1, 2]. In the intervening period, many hundreds of papers devoted to SERS phenomena have been published, extending the studies to other metals than silver, to non-aqueous as well as aqueous electrolytes, to colloidal dispersions of metals as well as metal electrodes, and even to vacuum-deposited thin film systems under UHV conditions. This review will concentrate on studies of metal-electrolyte interfaces. 

Electrically stabihzed colloidal dispersions are very sensitive to the addition of electrolytes. If the concentration of ions in the solution increases, decreases as a result of both entropic and electrical screening effects, leading to a reduction in the repulsive potential. On the other hand, colloid particles dispersed in organic media (low dielectric constant) cannot be effectively stabilized by charges because is extremely short. In these cases, steric stabilization is recommended. Steric stabihzation is imparted by nonionic amphiphilic molecules (usually polymeric molecules). The lyophobic moiety will adsorb onto the surface of the colloidal particles, while its lyophilic moiety will be extended in the continuous phase. When two sterically stabihzed particles approach each other, the concentration of the lyophilic segments in the portion of the continuous phase between the particles is increased. This higher local concentration results in an osmotic pressure that... 

A consequence of the small size and large surface area in colloids is that quite stable dispersions of these species can be made. That is, suspended particles may not settle out rapidly and droplets in an emulsion or bubbles in a foam may not coalesce quickly. Charged species, when sedimenting, present a challenge to Stokes law because the smaller counterions sediment at a slower rate than the larger colloidal particles. This creates an electrical potential that tends to speed up the counterions and slow down the particles. At high enough electrolyte concentrations the electric potentials are quickly dissipated and this effect vanishes. 

Steric stabilization of a colloidal dispersion is achieved by attaching long-chain molecules to colloidal particles (Fig. 3.6). Then when colloidal particles approach one another (for example due to Brownian motion), the limited interpenetration of the polymer chains leads to an effective repnlsion which stabilizes the dispersion against flocculation. Steric stabilization has several advantages compared to charge stabilization. First, the interparticle repulsion does not depend on electrolyte concentration, in contrast to charge-stabilized colloids where the electric double-layer thickness is very sensitive to ionic strength. Second, steric stabilization is effective in both... 

The above terminology ( inert vs. specific ) was adopted for studies of the surface charging of colloids. Different experimental methods are used and different quantities are measurable for colloids than for the Hg electrode, but the model of an electrical double layer is analogous. Studies of pH-dependent surface charging of colloids are usually carried out in the presence of an inert electrolyte and an acid or base (used to adjust the pH) with an anion or cation in common with the inert electrolyte. Products of dissolution of the solid are also present in solution at low concentration (we are only interested in sparingly soluble solids), but are ignored in most studies. Sometimes, the concentration of dissolution products is measured, and very occasionally the concentration of dissolution products (which are water-soluble salts) is controlled by addition of these salts to the dispersion. The effect of addition of Al(iii) salt on the potential of alumina was studied in [35]. At the lEP, the solubility of Al species is low thus, the lEP was not very different from that in a 1-1 electrolyte. The solubility problem is discussed in more detail in Section 1.6. 

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