Electrical Properties of Interfaces PDF

ELECTRICAL PROPERTIES OF INTERFACES Charges on the dispersed particles in liquid medium may come from: Selective adsorption of a particular ions from solution. The majority of particles dispersed in water acquire -ve charge due to preferential adsorption of OH⁻ ions. The ions from electrolyte...

ELECTRICAL PROPERTIES OF INTERFACES Charges on the dispersed particles in liquid medium may come from: Selective adsorption of a particular ions from solution. The majority of particles dispersed in water acquire -ve charge due to preferential adsorption of OH⁻ ions. The ions from electrolyte or in the case of pure water (H⁺ or OH⁻ ions). Example: 1- Addition of KI dilute solution to an equimolar solution of AgNO₃. → Positively charged colloidal ppt of AgI are produced due to adsorption of Ag ⁺ ions (present in excess). 2- Addition of AgNO₃ dilute solution to an equimolar solution of KI. → Negatively charged colloidal ppt of AgI are produced due to adsorption of I ⁻ ions (present in excess).  Ionization of groups: The total charge is pH dependant. Example: The charge on colloid protein molecules will depend on the pH of the dispersion medium: (a) In alkaline solution: The carboxylic acid groups of the protein molecules → carboxylate anions. NH₂ – R – COO⁻ (b) In acidic solution: → The amino groups of the molecules protonated. NH3 – R+ – COOH Thus, proteins are negatively charged in alkaline solution and positively charged in acid solutions. At certain pH (Iso-electric point IEP) the protein exists as zwitterion, which is electrically neutral, although both groups are ionized. The solubility of the protein is at a minimum at its IEP and therefore precipitation is facilitated at this pH NH3 –+R- – COO  The charge due to the difference in dielectric constant DEC between the particle and dispersion medium. It is less common mechanism. When a particle possesses a higher DEC than its dispersion medium → it acquires positive charge and vice versa. Due to a transfer of electrons from the substances of high DEC to those of lower one. DEC is a physicochemical property of a solvent relating to the amount of energy required to separate two opposite charged regions in the solvent as compared to separate the same in the vacuum. It is a measure of its efficiency to induce a dipole in another molecule. Applications of DEC: 1. Measure of polarity of solvent. ↑ DEC →↑ polarity of solvent. 2. Solubilization of drug.↑ DEC →↑ solubilization of solvent. 3. Selection of solvents o solvent mixtures for drugs. The Electrical Double Layer Consider a solid surface acquired +ve charges (due to adsorption of cations) in contact with a solution containing ions. The rest of the cations and the total number of anions are remained in solution. The anions are attracted to the +vely charged surface by electrical forces which also serve to repel the approach of any further cations once the initial adsorption is complete. At equilibrium, some of the excess anions approach the surface, while the remainder anions are distributed in a decreasing amount with distance from the charged surface. The concentration of anions and cations are equal → electrically neutral system. The diffuse double layer therefore aa₁cc₁ composed of: → 1. First layer (aa₁bb₁) tightly bound layer 2. Second layer (bb₁cc₁) which is more diffuse. As shown in the figure: the line aa₁ having +ve ions (potential determining ions). A region of tightly bound solvent molecules and some - ve ions (counter ions) limited by the line bb1 (shear plane) is present. The degree of attraction of solvent molecules and counter- ions is such that if the surface is moved relative to the liquid→ the shear plane is bb₁ rather than aa₁ the true surface. Three situations other than that represented by the previous figure are possible: 1. If the counter ions in the tightly bound layer (solvated layer) < the +ve ions (potential determining ions) on the solid surface → the net charge at bb₁ will be +ve. 2. If the counter ions in the tightly bound layer (solvated layer) = the +ve ions (potential determining ions) on the solid surface → then electric neutrality occurs at the plain bb₁ (not cc₁). 3. If the total charge of the counter ions in the region aa₁bb₁ > the ions (potential determining ions) → the net charge at bb₁ will be -ve. The electro thermodynamic potential = (Nernst potential) (E): The potential at the solid surface aa₁ (due to the potential determining ion) It is the difference in potential between the actual surface and the electro-neutral region of the solution. The electro kinetic potential = (zeta potential) (d): The potential at the shear plane bb₁ is known as the zeta potential It is the difference in potential between the surface of the tightly bound layer (shear plane) and the electro- neutral region of the solution. The zeta potential rather than the Nernst potential, governs the degree of repulsion between adjacent, similarly charged, dispersed particles. Flocculation: If the Zeta potential is reduced below a certain value, the attractive forces exceed the repulsive forces and the particles come together.