The Colloidal State

[Music] all of the following things have something in common a cloud a rubber tire a piece of paper some plaster some fresh paint these totally different things are all examples of colloids materials in the colloidal State the colloidal state is only one type of dispersion in which small particles of one material are scattered or dispersed throughout a second material one way to define a colloid is by comparison with other dispersions if we mix some copper sulfate crystals in water they dissolve they separate into particles of molecular or ionic size the chemist calls this kind of dispersion a true solution in a solution the particles will not settle out now we'll make another kind of dispersion we'll mix white coarse sand in the water it doesn't dissolve but settles out in a short time we call this kind of dispersion a suspension but if a material is ground until the particles are very fine like this white clay then it neither dissolves nor settles out this condition is called a colloidal dispersion which is somewhere between a solution and a suspension to learn more about the relative sizes of particles that form a colloidal dispersion let's see what we can learn by filtering the solution the colloid and the suspension we find that the solution comes through the filter paper the colloidal dispersion too comes through the filter but the corser matter of the suspension the sand in the water does not come through the filter evidently the suspension contains the largest particles of our three examples both the solution and the colloidal dispersion pass through the filter are colloidal particles then as small as the molecular particles of solutions perhaps we can answer the question with this experiment let's mix a salt solution and a colloidal dispersion of starch we'll place the mixture in a bag made of a semi-permeable membrane we suspend the bag in water then we pour in our mixture of a solution and a colloid next we are going to test to see what passed through the membrane into the water when iodine a reagent used to test for starch is added to the bag it turns blue showing that starch is still in the bag iodine in the surrounding water shows no blue reaction so starch didn't pass through the membrane silver nitrate is a reagent used to test for the chloride ion of salt it does show the presence of salt in the water so the salt in solution passed through the membrane the starch in the colloidal state did not colloidal particles then must be larger than the molecular sized particles of solutions and so particles in a colloidal dispersion are between the smaller size of particles in Solutions and the larger particles in suspensions we have other indications of the relative size of colloidal particles if we shine a strong Light Beam through a solution we can scarcely see the path of the beam in a colloidal dispersion the path of the beam is readily visible reflected by the larger particles of the colloid this is called the Tindle effect we can use the effect of light in another demonstration we can't see colloidal particles under a microscope but a light beam shining sideways through a drop of colloidal material reveals tiny Points of Light reflected by the particles we notice too that these points are in constant irregular movement this is called Brownian movement the reason colloidal particles showed brownie in movement is that smaller particles molecules were constantly striking them something like this causing them to remain in suspension colloidal and other particles are measured in extremely small units microns a millimicron is 1 millionth of a millimeter or 125 millionth of an inch particles about 250 microns or more in diameter show the properties of a suspension particles 1 Micron or less are molecular and show the properties of a solution between the two say between 1 and 100 microns are colloidal particles here's another way we can get some idea of the relative size of colloidal particles here are thousands of grains of sand if we were to take just a single grain of sand and look at it under magnification we'd see this within this one grain we could put hundreds of colloidal particles so far we've discussed colloids which were solid particles dispersed in liquids but as we saw earlier there are other types of colloids let's see the different ways that matter can form colloidal dispersions we know that matter can exist in three states as solids liquids or gases any one state can be dispersed in any other so there are nine possible pairs of dispersions solids dispersed in solids in liquids in gases liquids in solids in liquids and in gases gases in solids in liquids and in gases let's see examples of each pair of dispersions carbon black particles dispersed in the rubber of a tire are an example of a solid dispersed in a solid paint is an example of a solid in a liquid a solid in a gas is smoke tiny solid particles dispersed in air butter is a liquid in a solid that is water in butter fat many hand lotions are liquids in liquids such as oils in water some very fine Grays such as aerosols are liquids dispersed in Gases such as oil in air plaster of Paris is a gas air dispersed in the solid calcium sulfate when you whip cream you're making a colloidal dispersion of a gas air in a liquid cream what about a gas dispersed in a gas here we're going to mix chlorine and bromine in the mixing flask the two gases are forming a dispersion but we should remember that this is a mixture of molecular sized particles and that defines a solution not a colloid therefore a mixture of two gases is not a colloidal dispersion so we have then eight possible combinations of matter in different states that may produce U colloidal dispersions we saw some of the eight combinations that can give us a variety of colloids but they are all made in two principal ways we know that particles too fine to settle out of their dispersing medium will form a colloid so one General way to make a colloidal dispersion is to break matter down to particles of colloidal size a great many colloids are made by mechanical means in this Mill coarse solid particles are ground down to colloidal size small enough to be dispersed thoroughly in a liquid medium this Mill grinds pigments mixed with an oil base to form a colloidal paint sometimes in dispersing one liquid in another we run into difficulty you've probably seen vinegar and salad oil mixed together it is difficult to break up the oil droplets finely enough to stay suspended they tend to merge together to coales to larger size and come out of suspension to make them stay suspended we need an emulsifier in this case we're going to use egg yolk as our emulsifier when we add the emulsifier to the two liquids vinegar and oil we find that the oil will stay in suspension the emulsifier forms a protective film around the oil droplets preventing them from coalescing into larger than colloidal size this particular Emulsion or colloidal dispersion of two liquids is a familiar one it's mayonnaise oil and vinegar emulsified with egg yolk just as we can prepare colloidal dispersions by grinding matter down to extremely fine size we can make colloidal particles in a second way instead of breaking down particles we can also build them up in a solution we can coagulate or build up molecular sized particles to larger colloidal sized particles when we add feric chloride to boiling water for instance reddish hydrated feric this colloidal dispersion was made chemically by combining molecular sized particles to form the larger colloidal sized particles the colloidal particles of the same substance have the same electrical charges in this case they are the positive charges since the positively charged particles repel each other they stay separated in suspension however if we neutralize the positive charges the particles could become larger than colloidal size and precipitate out in other words having made a colloid chemically we can destroy it chemically one way to do this is to add an electrolyte to the colloidal display ersion negatively charged ions in this electrolyte will be absorbed by the colloidal particles and neutralize their charges Watch What Happens as we neutralize the positive charges on the colloidal particles as the particles are neutralized they Clump together or coales and begin to precipitate out so it is possible to destroy a colloid chemically later we'll see what value this has another way to destroy a colloid is by use of a strong electric field which we can apply in this tube we introduce smoke which is a colloid with electrically charged particles a current Through the Wire neutralizes the charges on the smoke particles they drop out of suspension it's obvious what use this process might have the problem of industrial smoke abatement is being solved by the principle we just saw by electrical precipitation of the colloid which is smoke but this is only one way in which an understanding of colloids is useful to us for instance tanning leather involves precipitating colloids chemically the hides which are made into our shoes and other leather goods are largely prot in colloids liquid latex another colloid is coagulated in the process of making rubber in another phase of rubber manufacturer liquid latex is precipitated electrically onto rubber forms once more precipitation of a colloid a component of milk casine can be precipitated chemically by acids or by an enzyme called renit the precipitated casine can be dried as we see here casine has important industrial uses much of the casine made today is used as a coating for paper to improve the quality a more recent and growing use of casine is in the preparation of high protein foods such as this cereal edible casine is also added to certain kinds of ice cream casine is used as a binding medium for some of our household paints we constantly use the products and principles of colloidal dispersions in hundreds of practical ways soaps and detergents for instance are emulsifiers washing involves forming an Emulsion of oily dirt particles and water an Emulsion that carries the dirt off the skin many of our foods are colloids homogenized milk is milk in which the fat particles have been broken down to colloidal size so that the cream doesn't separate out the dyes that color our Fabrics owe much of their tinting power to colloidal properties these are only a few of the thousands of ways we put to use our knowledge of materials in the colloidal state [Music]