Physical Pharmacy 2 Electrokinetic properties of colloid:Electrical Double LayerKausar AhmadKulliyyah of Pharmacy
Contents Electrical double layer theories Repulsive effect of electrical double layer Potential energy of interaction Physical Pharmacy 2
Helmholtz double layer Helmholtz in 1879 introduced the concept of the electrical double layer. The charge on the particles of a lyophobic colloid was due to an unequal distribution of ions at the particle-water interface. If ions of one charge were closely bound to the particle, ions of opposite charge would line up parallel to them, forming a double layer of charges Physical Pharmacy 2
Gouy diffuse double layer Gouy proposed that the double layer is diffused, with the outer ionic layer having an electric density falling off according to an exponential law. Physical Pharmacy 2
Stern diffuse double layer Stern compromised Helmholtz and Gouy. The double layer is in two parts:1. Helmholtz layer - one layer approximately a single ion in thickness, remains essentially fixed to the interfacial surface. In this layer, there is a sharp drop of potential.2. Gouy layer – this layer extends some distance into the liquid dispersing phase and is diffuse, with a gradual fall in potential into the bulk of the liquid. Physical Pharmacy 2
Physical Pharmacy 2 Electric double layer • is a region of molecular dimension at the boundary of two substances across which an electrical field exists. • The substances must each contain electrically charged particles, such as electrons, ions, or molecules with a separation of electrical charges (polar molecules). • In the electrical double layer, oppositely charged particles attract each other and tend to collect at the surface of each substance but remain separated from one another by the finite size of each particle or by neutral molecules that surround the charged particles. • The electrostatic attraction between the two opposite and separated charges causes an electrical field to be established across the interface.
Electrical double layer Physical Pharmacy 2
Repulsive effect of double layer The repulsive effect from the double layer is responsible for stability. Verwey and Overbeek proposed that the repulsive energy is a function of: Distance between droplets The reciprocal of the effective radius of the double layer Physical Pharmacy 2
Physical Pharmacy 2 From Verwey and Overbeek VR = 4.62 x 10-6 (r/2) e-kHo VRRepulsive energy rParticle radius Valence of counter ions Ho distance between two particles = (ez/2 – 1) / (ez/2 + 1); Z = ueo/kT, o is the double layer potential Boltzmann constant
Attractive force A small attractive Van der Waals force operating between the droplets, can be given by: VA = -Ar/12H0 A is a constant depending on the polarisability of the molecules of which the droplet is composed and is known as the Hamaker constant; A ca. 10-19 J to 10-20 J. Exercise: what happen if you have big ‘r’? Physical Pharmacy 2
DLVO Theory From Deryagin, Landau, Verwey and Overbeek Describes the stability of hydrophobic suspension Combination of electrostatic repulsive energy, VR, and the attractive potential energy, VA, gives the total potential energy of interaction: Vtotal= VA + VR The forces on colloidal particles in a dispersion are due to electrostatic repulsion and London-type Van der Waals forces Physical Pharmacy 2
Physical Pharmacy 2 Interaction potentials http://griergroup.uchicago.edu/~grier/leshouches2/leshouches2.html • polystyrene sulfate spheres in deionized water at 25oC. • Curves are labelled by the spheres' radii.
Reference RJ Hunter, Foundations of Colloid Science Volume 2, Clarendon Press Oxford (1989) Physical Pharmacy 2