Colloids BTF3823 PDF

COLLOIDS (BTF3823) 1  TOPIC OUTCOMES Distinguish types of colloids based on the properties. Analyze the properties of colloids 2  CONTENTS 1. Definition of colloids 2. Types of colloids 3. Properties of colloids...

COLLOIDS (BTF3823) 1  TOPIC OUTCOMES Distinguish types of colloids based on the properties. Analyze the properties of colloids 2  CONTENTS 1. Definition of colloids 2. Types of colloids 3. Properties of colloids 3  INTRODUCTION TO COLLOIDS A colloid is defined as a substance microscopically dispersed throughout another substance. The word colloid comes from a Greek word kolla, which means glue thus colloidal particles are glue like substances. Colloids can be made settle by the process of centrifugation. 4  INTRODUCTION TO COLLOIDS Dispersed Systems Dispersed systems consist of particulate matter (dispersed phase), distributed throughout a continuous phase (dispersion medium). They are classified to:  Colloidal dispersion E.g. colloidal silver sols, natural & synthetic polymers  Coarse dispersions (> 0.5 um) E.g. emulsions, suspensions 5  Property True solution Suspension Colloidal solution Nature Homogeneous Heterogeneous Appears to be homogenous but actually heterogeneous Particle size < 1 nm > 100 nm Between 1 nm to 100 nm Sedimentation Do not settle Settle on standing Do not settle Diffusion Diffuse quickly Unable to diffuse Diffuse slowly Visibility Particles invisible Particles visible by Particles scatter light and naked eye or under can be observed under microscope ultramicroscope Filterability Pass easily through Unable to pass Pass through filter paper animal membrane through animal but not through animal and filter paper membrane or filter membrane Appearance Clear and paper Opaque Translucent 6 transparent 7 Solutions, Colloids, and Suspensions 8  COLLOIDS o Colloidal dispersions consist of two distinct phases – a dispersed phase and a dispersion phase or medium. o The dispersed phase is also called as internal or discontinuous phase and the dispersion medium, as external or continuous phase. o Size of particles lies between that of true solution and suspension (between 1 nm to 100 nm). o Examples: o Sols (solid in liquid) o Gels (liquids in solids) o Emulsions (liquid in liquid) 9  Colloids Characteristics of Colloids have medium size particles (between 1 nm to 100 nm). cannot be filtered. can be separated by semipermeable membranes. scatter light (Tyndall effect – in which the path of a beam of light through the colloid is visible due to scatter light). 10  Examples of OF EXAMPLES Colloids COLLOIDS Milk Cheese Whipped Cream Fog 11  SIZE AND SHAPE OF COLLOIDS Particles lying in the colloidal size have large surface area when compared with the surface area of an equal volume of larger particles. Specific surface: surface area / unit weight or volume of material The shape of colloidal particles in dispersion is important:  The more extended the particle the greater its specific surface the greater the attractive force between particles of dispersed phase and dispersion medium.  Flow, sedimentation and osmotic pressure of the colloidal system affected by the shape of colloidal particles.  Particle shape may influence the pharmacological action. 12 SHAPES OF COLLOIDS 13 Classification of colloids Classification is based on: 1. Physical state of dispersed phase and dispersion medium. 2. Types of particles of the dispersed phase. 14 Classification based on physical state of dispersed phase and dispersion medium Dispersed Dispersion Type of Example phase medium colloid Solid Solid Solid sol gem stones Solid Liquid Sol Paints, cell fluids Solid Gas Aerosol Smoke, dust Liquid Solid Gel Cheese butter, jellies Liquid Liquid Emulsion Milk, hair cream Liquid Gas Aerosol Fog, mist, cloud, insecticide sprays Gas Solid Solid sol Pumice stone, foam rubber Gas Liquid Foam Froth, whipped cream, soap-lather 15  TYPES OF COLLOIDS Lyophilic colloids (solvent loving)  When there is great interaction (or affinity) between the dispersed phase and the dispersion medium, a lyophilic colloid is formed. Colloid particles are solvated and they are mostly solids in liquids.  It may be hydrophilic or lipophilic.  If water is the dispersion medium, it is hydrophilic.  Hydrophilic colloids may be : a) True solutions acacia or povidone in water b) Gelled solutions polymer with high concentrations like gelatin and starch c) Particulate dispersion bentonite forms hydrosol in water 16 Lyophobic colloids (solvent hating colloids) In lyophobic the colloidal particles exhibit little interaction or affinity with the dispersion medium. Hence lyophobic colloidal dispersions are not solvated. Lyophobic colloids may be: 1. Hydrophobic 2. Lipophobic Special technique needed for their preparation. Unstable: require stabilizing agents. Irreversible: once precipitated, don’t return. 17 Hydrophobic colloids have water as dispersion medium and the particles are not hydrated. Eg. Polystyrene steroids form hydrophobic colloids. Gold, silver and sulphur (inorganic particles) can also be formed as hydrophobic. Lipophobic dispersions are water-in-oil emulsions. When metals and their sulphides simply mixed with dispersion medium, they don’t form colloids. need stabilizing to preserve them. irreversible. Example colloidal solutions of gold, silver, Fe(OH)3, As2S3, etc. 18  Association colloids  Association/amphipiles colloids are molecules characterized by having a hydrophilic head and a lipophilic tail.  Association colloids result from the formation of micelles formed by surfactant.  At low concentrations, behave as normal, strong electrolytes At higher concentrations exhibit colloidal state properties due to the formation of aggregated particles (micelles).  Ex: anionic, cationic, non-ionic 19 Parameters Lyophilic colloid Lyophobic colloid Association colloid Disperse Consists of large Consists of Consists of micelles of phase organic molecules of inorganic particles small organic colloidal dimensions or (gold or silver) molecules particulate substances Solvation Solvated No solvation Hydrophilic portions solvated Spontaneity Spontaneously formed Does not form Spontaneously formed once disperse phase spontaneous. Needs above CMC comes in contact with special methods dispersion medium Stability Thermodynamically Thermodynamically Thermodynamically stable and reversible unstable and stable and reversible irreversible Viscosity Increased as the conc Not increased Increases when increase. Due to micelles increases solvation Effect of Stable in presence of Sensitive even in Electrolytes reduce electrolytes low conc of electrolytes. low conc of CMC and salting out Desolvation and salting electrolytes 20 out in high conc PROPERTIES OF COLLOIDS Optical Property Kinetic Property Electrical Property 21 Optical property It can be discussed by the following: 1. The Faraday Tyndall effect 2. Electron microscope 3. Light scattering 22 OPTICAL PROPERTIES Faraday Tyndall Effect When a beam of light falls at right angles to the line of view through a solution, the solution appears to be luminescent and due to scattering of light the path becomes visible. Quite strong in lyophobic colloids while in lyophilic colloids it is quite weak. 23 Colloids scatter light, making a beam visible. Solutions do not scatter light. 24  When a strong beam of light is passed through a colloidal sol, the path of light is illuminated (a visible cone formed).  This phenomenon resulting from the scattering of light by the colloidal particles.  The same effect is noticed when a beam of sunlight enters a dark room through a slit when the beam of light becomes visible through the room.  This happens due to the scattering of light by particles of dust in the air. 25 26 Electron microscope  Ordinary/light microscope cannot reveal the structure of colloids separated by smaller distance.  The electron microscope has a higher resolving power of about 5 Aº. The resolving power is directly correlated to the wavelength of radiation. The shorter the wavelength, the more efficient is the resolving power. So it can reveal the structure of colloids separated by smaller distance.  So, electron microscope is capable of yielding pictures of actual particles size, shape and structure of colloidal particles. 27 Electron Microscope Transmission electron microscopy (TEM) 28 Scanning electron microscopy (SEM) 29 Light Scattering This property depends on the Faraday-Tyndall effect It is widely used for determining the molecular weight of colloids. Can be used to obtain information on the shape and size of these particles. Used to study proteins, association colloids and lyophobic sols. Light scattering and intensity depends on: 1. Wavelength of the incident beam 2. Intensity of incident beam 3. Difference in the refractive index between the particles and the medium. 30 The intensity of light scattered may vary at different angles and can be used to obtain some indication as to the shape of the macromolecules (colloidal particles). Hc / T = (1/M) + 2Bc T: turbidity C: conc of solute in gm /litre of solution M: molecular weight B: interaction constant H: constant for a particular system 31 Dynamic Light Scattering (DLS) 32 Kinetic property Brownian movement Diffusion Osmotic pressure Sedimentation Viscosity 33 KINETIC PROPERTIES a) Brownian movement  DEFINITION: colloidal particles are subjected to random collision with molecules of the dispersion medium (solvent) so each particle move in irregular and complicated zigzag pathway.  The motion of the molecules cannot be observed because the molecules are too small to see.  The velocity of particles increases with decreasing particle size and viscosity.  Increasing the viscosity of dispersion medium (by glycerin) decrease then stop Brownian motion. 34 Brownian movement: Zig - zag movement of colloidal particles in a colloidal sol This Brownian motion arises due to the uneven distribution of the collisions between colloid particle and the solvent molecules. 35 Diffusion  Diffusion is a process where the solute molecules move from a region of higher concentration to one of lower concentration until the concentration of the system attains equilibrium.  Diffusion is a direct result of Brownian movement. 36  Solution (H2O + Pure Solutes) H2O Semipermeable membrane Only water passes through osmotic membranes and faster from the side on which water is more concentrated. 37  Solution (H2O + Pure Solutes) H2O Semipermeable membrane Diffusion rates tend to equalize as flow continues. 38 Osmotic pressure Using osmotic pressure, it is possible to estimate the molecular weight and this is based on van 't hoff equation: Osmotic pressure, π = cRT Replacing c by c/M (where c = the grams of solute / liter of solution, M = molecular weight), R- gas constant, T – absolute temperature π = (c/M)RT or π/C = RT/M The osmotic pressure depends on molar concentration of the solute and on absolute T. The osmotic pressure is inversely proportional to molecular weight. 39  Osmotic Pressure If applied pressure is P too low, H2O flows into the region of higher solute concentration... “Down the P concentration gradient” for H2O. Membrane H2O + Pure Solutes H2O 40 P Osmotic Pressure If applied pressure is too high, H2O flows into the region of lower solute concentration... Against the natural concentration gradient P for H2O. --Reverse Osmosis Membrane H2O + Pure Solutes H2O 41 P Osmotic Pressure Minimum pressure required to maintain equal flow rates (to prevent infusion of H2O). Proportional to solute P concentration differences across membrane. Membrane H2O + Pure Solutes H2O 42 Sedimentation o Brownian movement keeps the dissolved molecules of colloidal particles in continuous random motion. Hence it offsets sedimentation due to gravity. o Therefore stronger force must be employed to bring about sedimentation of colloids particles. o It can be achieved by ultracentrifuge which provide stronger force so promote sedimentation in measurable manner. 43 Viscosity: o Definition: the resistance to flow of a system under an applied pressure. o Viscosity provides information on molecular weight and shape of particles in colloids. 44 o The more viscous a liquid, the greater the applied force required to make it flow at a particular rate. o The viscosity of colloidal dispersion is affected by the shape of particles of the disperse phase: o Spherocolloids dispersions are of relatively low viscosity. o Linear particles dispersion are of high viscosity. If linear colloidal particles coil up into sphere, the viscosity of the system falls due to the changing the shape. 45 Electrical property  Electric double layer  Electrophoresis  Electroosmosis 46  ELECTRIC PROPERTIES OF COLLOIDS The particles of a colloidal solution are electrically charged and carry the same type of charge, either negative (eg. Kaolin, sulphur colloids) or positive (ferric hydroxide colloid). The colloidal particles therefore repel each other and do not cluster together to settle down. 47  The charge on colloidal particles arises because of the dissociation of the molecular electrolyte on the surface. E.g. As2S3 has a negative charge During preparation of colloidal As2S3, H2S is absorbed on the surface and dissociate to H+ (lost to the medium) and S-2 remain on the surface of colloid. 48 Electric double layer The presence of electric charges on the dispersed particles influences the distribution of positive and negative ions in the solution that surround each particles. 49  Thermal motion also has some influence over the distribution of charges. The resultant effect is that each particles is surrounded by an electric double layer. It has a combination of charged surface. Unequal distribution of co-ions and counter-ions near the surface. A negatively charged particle attracts the positive counter-ions surrounding the particle. Electric Double Layer (EDL) is the layer surrounding a particle of the dispersed phase and including the ions adsorbed on the particle surface and a film of the counter-charged dispersion medium. 50 The Electric Double Layer is electrically neutral. It consists of three parts: Surface charge - charged ions (commonly negative) adsorbed on the particle surface Stern layer – counter-ions (charged opposite to the surface charge) attracted to the particle surface and closely attached to it by the electrostatic force. Diffuse layer - a film of the dispersion medium (solvent) adjacent to the particle. Diffuse layer contains free ions with a higher concentration of the counter-ions. The ions of the diffuse layer are affected by the electrostatic force of the charged particle. 51  ZETA POTENTIAL Zeta potential is a scientific term for electrokinetic potential in colloidal dispersions. It is usually denoted using the Greek letter zeta (ζ), hence ζ- potential. The electric potential at the boundary of the double layer is known as the Zeta potential of the particles and has values that typically range from +100 mV to -100 mV. 52  DEFINITION ZETA POTENTIAL Zeta potential or electrokinetic potential is defined as the difference in the potential between shear plane and electroneutral region of the solution. Zeta potential: It is the potential observed at the shear plane. Zeta potential is more important than nernst potential because the electrical double layer also moves, when the particle is under motion. 53  Factors affecting zeta potential 1. pH : In aqueous media, the pH of the sample is one of the most important factors that affects its zeta potential. Zeta potential versus pH curve will be positive at low pH and negative at high pH. There may be a point where the plot passes through zero zeta potential. This point is called the isoelectric point. Least stable of colloidal system 54  Factors affecting zeta potential 2.Thickness of double layer: The thickness of the double layer depends upon the concentration of ions in solution and can be calculated from the ionic strength of the medium. The higher the ionic strength, the more compressed the double layer becomes. The valency of the ions will also influence double layer thickness. 55  Factors affecting zeta potential 3. Concentration of a formulation component: The effect of the concentration of a formulation component on the zeta potential can give information to assist in formulating a product to give maximum stability. 56  Rheology + Quiz – 6 Jan 2024 TEST 2 - 13 Jan 2025 57 58