imk209 fantasia
TRANSCRIPT
Electrostatic
Forcesby, FaNtasi’athe forces between particles that are caused
by their electric charges.
• Van der Waals attractive force• Electrostatic repulsive force
• Electrical double layer
COLLOID
• Colloidal particles often carry an electrical charge and therefore attract or repel each other.
• The charge of both the continuous and the dispersed phase, as well as the mobility of the phases are factors affecting this interaction.
Why Lemon Makes Milk Curdle??
Electrostatic coating
• Painting industry• principle :charge
powder particles to similar polarity, charged particles repel each other, forming a cloud across the coating area.
DLVO THEORY• Boris Derjaguin • Lev Landau• Evert Verwey• Theo Overbeek
VT = VA + VR + VS• Where,• VT = Total interaction force• VA = van der Waals attraction• VR = electrical double layer repulsive
• Two types of force in a stable colloid:
van der Waals' force-attractive and binds particles together
electrostatic repulsion-The total interaction potential can be calculated as a function of distance, with colloid stability being attained when two forces balance each other.
Figure 1: Types of colloidal stabilization
• There are two fundamental mechanisms that affect dispersion stability (figure 1):
Steric repulsionElectrostatic or charge stabilization
Application of DLVO Theory
• Since 1940s, the DLVO theory has been used to explain phenomena found in colloidal science, adsorption and many other fields. Due to the appearance of nanoparticles, DLVO theory becomes even more popular. Because it can be used to explain both general nanoparticles such as fullerenes particles and microorganisms.
ELECTROSTATIC DOUBLE LAYER: DEFINITION A structure that appears on the surface
of an object when it is placed into a liquid
Consists of two parallel layers of charge surrounding the object
The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body
Important in determining the physical properties of the system as a whole
FIRST LAYER SECOND LAYER The surface charge
(either positive or negative)
Comprises ions adsorbed directly onto the object due to a host of chemical interactions
Is firmly adsorbed on the surface of the object
Also called the diffuse layer
Composed of free ions attracted to the surface charge via the Coulomb force
Electrically screens the first layer
Is loosely associated with the object
ELECTROSTATIC DOUBLE LAYER:
APPLICATION: ICE-CREAM
Zeta potential• electrical potential that exists at the "shear
plane" of a particle, which is some small distance from its surface.
• electric potential in the interfacial double layer (DL) at the location of the slipping plane versus a point in the bulk fluid away from the interface
• charge that develops at the interface between a solid surface and its liquid medium
• Zeta potential is considered to be the electric potential of this inner area including this conceptual sliding surface.
Zeta Potential (Smoluchowski’s Formula)
The Smoluchowski (for high ionic strengths) or Huckel (for low ionic strengths) equations are used to obtain the Zeta potential from the measured mobilities.
Zeta Potential vs. Surface Potential
The relationship between zeta potential and surface potential depends on the level of ions in the solution.
Zeta potential [mV] Stability behavior of the colloid
from 0 to ±5 Rapid coagulation or flocculation
from ±10 to ±30 Incipient instability
from ±30 to ±40 Moderate stability
from ±40 to ±60 Good stability
more than ±61 Excellent stability
• BIOMEDICAL• EMULSIONS• CLAY TECHNOLOGY: (a) SOIL MECHANICS (b) OIL WELL TECHNOLOGY (c) CERAMICS• WATER PURIFICATION AND INDUSTRIAL
WASTE• MINERALS AND ORE FLOTATION
Zeta Potential Applications
Effect of ionic strength and pH
Ionic stabilization…• charged groups on the surface of the emulsion
droplets will increase the repulsive forces- Ionic emulsifier will form an electricity charge
double layer in the aqueous solution surrounding each oil droplet.
• Ionic strength effect???- the thickness of the electrical double
layer • Ionic strength
- electrical repulsion is greater than Van der Waals attraction
Ionic stabilization…
• almost all food formulation will contain some proteins –proteins may be neutralpositively charged, or negatively charged depending of the pH of the system compared to their isoelectric point, pI
(pH < pI, positive charge; pH > pI, net negative charge).• The charge on a droplet is important because it
determines the nature of its interactions with other charged species.
Two droplets charges of opposite sign = attracted toward each other two droplets charges of similar sign = repelled to each other
• properties of emulsions stabilized by;-ionized emulsifiers
(pH and ionic strength of the aqueous phase)• pH is adjusted
1. emulsifier loses its charge, or 2. if salt is added to “screen” the electrostatic interactions between the droplets (repulsive forces may no longer be strong enough to prevent the droplets from aggregating)
Remember!!!-the energy barrier modified by changing the pH and
pI-effect directly the forces involved
• Increase pH above the isoelectric point (pI)
• the electrostatic interaction energy
• Decreasing ionic strength-Increase energy barrier-stabilizing the suspension-increase turbidity
For example from our previous slide,
• The situation whereby ‘Why Lemon Makes Milk Curdle??’
• decreases the pH of the emulsion toward the isoelectric point of the proteins,
• thereby reducing the electrostatic repulsion between the droplets and causing droplet aggregation.
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