Lyophobic colloids have no affinity for the dispersing medium and are not solvated.  They are stabilized by charge acquired by preferential adsorption of ions.  If water is the dispersing medium, they are known as hydrophobic colloids.

Lyophilic colloids are highly solvated as well as charged.  The charge usually is a result of ionization.  If water is the dispersing phase, they are called hydrophilic colloids.  Hydrophobic colloids will aggregate and precipitate if their electrical charge is removed; however hydrophilic colloids often remain dispersed and do not precipitate even after the charge is removed or neutralized.  Fortunately most pharmaceutical colloids are of the hydrophilic type and are stabilized by a dual mechanism.

The large surface of a lyophobic colloid preferentially adsorbs ions.  Trace ions will induce an opposit charge in a neighboring molecule and will then be held to the surface of the colloid by an ion-induced dipole attraction.  It is the adsorption of trace ions which gives a lyophobic colloid its charge; the charged particles repel each other and prevent aggregation and precipitation.  The adsorbed ions on the surface of the particle tend to attract oppositely charged ions, and two layers of oppositely charged electricity results.  The thickness of this double layer is small compared to the diameter of the colloidal particle, and the repuslive force of the charged colloid does not extend beyond the thickness of the double layer.

This double layer consists of two shells of ions of opposite charge.  The inner shell is narrow and compact, adhering tightly to the colloidal particle.  The outer shell is wide and diffused, with a high concentration of ions near the inner shell and a progressively lower concentration of ions as the distance from the surface of the particle to the bulk of the dispersing medium is increased.  The outer shell can be removed and reformed as the particle moves in the dispersion.

Between the surface of the colloid and the main body of the dispersion there exists a potential.  The total potential, E, may be divided into two parts.  The first is the potential between the inner shell and the surface of the colloid .  The second is called the "zeta" or electrokinetic potential and is the potential difference through the outer shell extending from the outer edge of the inner shell to the body of the solution. The greater the zeta potential of a lyophobic colloid the greater its stability.

The factors governing the selective adsorption of ions are not completely known.  For example, if dilute solutions of silver nitrate and sodium iodide are mixed, the colloidal silver iodide may be positive or negative.  It depends on the ions that are in excess.  If iodine is in excess, the particle is negatively charged and if the silver is in excess the particle will be positively charged.

Hydrophilic colloids acquire a charge by ionization.  Most of the gums used in pharmacy to make colloids like acacia are negatively charged due to the carboxyllic or sulfate groups on the carbohydrate polymer.  Proteins(gelatins) which are made of amino acids which are zwitter ions can acquire either a positive or negative charge based on the pH of the solution.  Pretreatment with acid or base can change the isoelectric point of the gelatin.  This results in acid treated gelatin being positively charged at pH 4 - 4.5 while basic treated gelatin is negatively charged at pH 8.

Hydrophobic colloids can be precipitated by small concentration of electrolytes or oppositely charged colloids.

When surfactants at very low concentrations are dispersed in water, they tend to become adsorbed as a closely packed monolayer at the air-water interface.  If additional surfactant is added, it cannot be accommodated at the surface and they agglomerate in the bulk of the solution forming aggregates called micelles.  Micelles of a given surfactant at a given concentration and temperature contain the same number of molecules ( 25 to 100).  The diameter of the micelles range between 30 and 80 angstroms.  This is called an association colloid.  In a hydrophobic sol the polar portions of the surfactant face inward and the non-polar face out.  In a hydrophilic sol the opposit is true.  The lowerset concentration at a given temperature and pressure at which micelles form is call the critical micell concentration (CMC).  These micelles can increase the solubility of poorly soluably compounds.

PROCEDURE:

Hydrophilic colloids are commonly used in pharmaceuticals.  Many have been suggested as protective colloids for hydrophobic colloidal particles.  Gelatin, acacia, agar, and sodium carboxymethylcellulose have been used in this capacity.  This laboratory consists of three parts.  One student will complete parts A, one student will complete part B, and  two students will complete part C. Part A & B require that you dry a portion of your product.  This is best done in a larger beaker over a steam bath.

A.  Acacia mucilage may be used as a protective colloid.  Dissolve 0.66g of potassium iodide and 0.66 g of silver nitrate in two separate 10 ml portions of water.  Add 15 ml of Acacia Mucilage to the potassium iodide solution.  To this mixture with constant stirring slowly add the silver nitrate solution.  Take about half of the colloid you just formed and evaporate it to dryness over a water bath.  Now try to reconstitute the dry colloid with 10 ml of distilled water.  Compare to the sample of the original colloid.
 

B. Pour one ml of benzoin tincture into 25 ml of water in a beaker.  Add the benzoin tincture very slowly with mixing.  Take half of the mixture and evaporate it to dryness on a water bath.  Try to reconstitute the dry colloid.  Compare the original colloid with the recostituted one.  Repeat the experiment using 10 ml of a 2% Sodium Carboxymethylcellulose solution and 2.5 ml of benzoin tincture.  Again take half and evaporate to dryness and reconstitute with water.  Compare the results.

C.  The entire class will work on the preparation of a three component phase diagram for peppermint oil-water-Tween 80 system.   Each group will be doing four titrations. Each group will have assigned different mixtures so that in effect we will be making 24 titrations. You will measure out the amount of peppermint oil and Tween 80 and place it into a 125 ml flask.  You will then titrate the mixture to a cloud point by adding water from the burette.  It will be your responsibility to record the amount of water used in each titration on the white board and to obtain the other groups numbers to be used in your graph in your groups report. Repeat each titration at least once.  The table below lists the number of grams of surfactant and the number of grams of oil to be used in each titration.  Your group is lettered from the north of the laboratory.  If you do not know what group or team you are ask the instructor.