Colloids 1& 2

• nanosized particles dispersed in a continous phase

• sizes between molecular and coarse dispersions

• one nanometer to 1 micron size

• if light is shined, will see a cone of light - reflects light towards you

colloids def

• high surface to volume ratio

• many preparations are colloids

• understands behaviour of dispersions, emulsions and suspensions

colloid properties

• too small to sediment

• forced to sediment in ultracentrifuge

• too small to be seen with optical microscope

• seen by electron microscope

• cant pass through semi-permeable cellophane membrane - use to separate colloids and electrolytes

• can be filtered out

• slower diffusion than molecular particles due to larger size

colloid size characteristics

• different shapes

• depends on medium

• if particle dispersed in a hostile environment (environment that it is not attracted to) - particles ball up

• if particle dispersed in a comfortable medium, it stretches out and unwinds, spreading out to maximize contact with the surrounding liquid (increasing its surface area).

colloid shape

• mechanical disintegration - irregular shapes

• condensation - regular shapes

• emulsification - spherical droplets

factors that can influence the shape - method of preparation

• hydrophilic polymers in water - high interaction - open structure (increase SA)

• hydrophilic polymer in non-solvent - balls up

factors that can influence the shape - interaction with dispersion medium for hydrophilic polymers

• natural feature born into the material that you can't easily change.

• Because of how its molecules lock together chemically and physically, the material naturally crystallizes or forms into a specific shape when turned into a powder

• kaolin - plate-like

factors that can influence the shape - Inherent physical and chemical nature of the substance

• according to their interaction with the medium

classifcation of colloids

• solvent loving

• disperse phase has affinity for particles

• form dispersons easily - spontaneous

• e.g. proteins in water

• can be hydrophilic or lipophilic

lyophilic colloid

• solvent hating colloids

lyophobic

• amphiphilic - cationic, anionic, non-ionic, amphylotic

• surfactants when they form micelles

• hydrophobic and hydrophilic

• dont exist below CMC

• 5nm size

• 50 or more monomers

• lyophobic - on inside, lyophilic on inside

• spontaneous if above CMC

• tween solution

assosication colloids

• most interested

• affinity for water allows for solvation - solvent molecules form around solute

• e.g. water soluble polymers - methylcellulose

• energetically favourable - spontaneous dispersion - increase in entropy and negative delta G

hydrophilic colloids

• disperse phase as little attraction for dispersion medium

• inorganic particles dispersed in water

• Au, Ag or sulfur

• o/w or w/o

• not spontaneous

• method - colloid mills - to shear particles into colloid size range

• Ultrasonic treatment: Using sound waves to create microscopic bubbles that violently collapse, using that shockwave to shatter and break down the particles.

• Condensation Method (Building small things up) - Starting from a molecular level, building up and precipitating/depositing particles out of a supersaturated solution until they reach the correct colloidal size.

lyophobic colloids

• exhibit hydrophilic and hydrophobic properties

• particulate colloids - swells or breaks apart into tiny colloidal-sized pieces. The particles become hydrated (soak up water), but they do not dissolve into a true solution.

• referred to as hydrophobic

• bentonite

• kaolin

clays

• viscosity - lyophilic(increase by prescence of disperse phase) form a gel at high conc, assosication - increases as micelles increase, no change in lyophobic

• stability with electrolytes - lyophilic (stable, very high conc - salting out), assosication (CMC reduced, salting out at higher concs), lyophobic - unstable

• solvation - lyophilic (disperse phase solvated), assosication (lyophilic portion is solvated), lyophobic (little solvation)

comparision of different types of colloids

• able to scatter light

• shows turbidity - cloudy appearance - known as Faraday-Tyndall effect

• scattered light - goes into, incident light (intensity of beam)

• more scattering by smaller particles, lower wavelength of light scattered more

• slight blue appearance

optical properties of colloids

• measure molecular weight of colloids - using debye equation

• shape and size of colloidal particles

• measured at 90 degrees

use of light scattering properties of colloids

• An optical constant measures the molecular weight (mass) of large particles, such as polymers or colloidal particles, by looking at how they scatter light.

debye equation

• uses dark field- microscopy - observable contrast

• particles viewed against a dark background at right angle to the beam of incident light

• scattering cause right spots - indicate particles

• electron microscope to see particles

visualising colloids

• zig-zag motion of colloids

• due to random collisions of colloidal particles with molecules in dispersion medium

• increase viscosity - decreases brownian motion

• smaller molecular weight particles - have higher brownian motion

brownian motion of colloids

• particles migrate from the region of high to low conc spontaneously

• passive

• diffusion coefficent - how fast the particle moves

• high molecular weight particles - move at a lower rate than small particles

• inversely proportional to molecular weight

diffusion of colloids

• equation used to find the radius of a particle by utilizing its diffusion coefficient

• for albumin or haemoglobin

• used to identify and understand colliodal particles

diffusion for spherical partciles

• necessary to balance osmotic flow

• semi permeable membrane - only solvent molecule can pass, to balance solute conc, the solvent molecules move more to the solute high conc

• more solvent in one side of membrane

• osmotic pressure is measured by the force applied for balance to occur on both sides

• colligative property - only one that is actually useful and practical for studying colloidal particles

• depends on number of molecules in solution and vapour pressure, freezing point, bp

• can only be used for MW of 100000-10000000 - if below - too small to pass, if beyond, too small to measure

• eliminated effect of small molecules to which is membrane permeable

osmotic pressure

• pie x V = nRT

• osmotic pressure as a function of no. of moles of solute in the solution volume of V.

• CxRT/M - M - molecular weight - dilute solution

• colloid - piex C = RT/M + BC - B - constant

measuring osmotic pressure

• the rate of sedimentation can be measured, colloidal systems with a rate of sedimentation higher correlate to a higher molecular weight

• colloids not sediment - as very small and kept afloat by brownian motion - only occurs if particle is less than or 0.5 microns

• useful for particle sizing

sedimentation

• instead of g replaced by w2x - angular velocity and x is distance of particle from centre of rotation

• useful for determining micelle size and molecular weight of substances

change of equation due to ultracentriguation

• resistance to flow of a system under applied stress

• viscosity = n0 (viscosity of medium) x (1 + 2.5 x volume of fraction particles)

• 2.5 - spherical particles

• relative viscosity - ratio of the viscosity of a solution (such as a polymer or salt solution) to the viscosity of the pure solvent

• specific viscosity - the fractional increase in the viscosity of a solvent caused by the addition of a solute

• large molecular weight molecules have longer polymer chains and more branched structures - causing an increase in viscosity - the total viscosity can be used to find the intrinsic viscosity of the particle - correlating to the molecular weight

viscosity

• viscosity and shape of disperse particles - reflect degree of solvation

• linear particles - more viscous

• spherical particles - reduced viscosity

• interactions of long chained molecules with solvent and concentration of disperse phase - makes it more viscous

• used to detect shape of flexible colloidal particles and macromolecules

• avg weight of dextrans

• high viscosity - gel formations

kinetic colloid properties

• hydrated, chains entangled by brownian motion

• hydration sheath (layer of water molecules that surrounds a particle) allows the chains to slip past without bonding

• conditions change to reduce amount of hydration, neighbouring chains will attract one another and form reversible bonds - van der wall or h bonding

• dehydrated - folding up of particle or attracted to other same partciles

• polymer may precipitate out, form a colloid rich viscous layer or set into semi-solid gel netwiork

viscosity of hydrophilic colloids

• temp

• conc

• molecular weight

• nature of polymer

• solvent

• can separation, precepitation or gelation (more likely in higher MW polymers, increase con and reduced solubility of polymer)

factors that affect what occurs to a colloid

• hydrophobic

gold dispersion type of colloid

• assosicated

type of colloid that becomes solution when diluted

• prescence of metallic ions - which interact with buffers

• ph modifying agents - decrease zeta potential

• opposite charged colloidal particles

• addition of incompatible solvent

ingredients which destabilises the colloids