Colloidal dispersions: an overview: the cosmetic scientist must consider various repellent and attractive forces involved in these systems.

NATURAL COLLOIDS have been in existence from time immemorial, but it was only recently that they began to appear worthy of serious study. Widespread throughout nature in the form of gases and dust particles, colloidal systems are very important in modern chemistry, medicine, food, dairy and engineering (metallurgy). Colloidal systems and processes play a role in meteorological phenomena such as clouds and mists, as well as the formation of rocks and minerals. A cosmetic chemist in his day-to-day work regularly encounters many areas of colloid science that involve particles in the colloidal dimension, range and size. Many major operations in cosmetic science are essentially colloidal processes. For example, nail lacquers are pigment suspensions and the color and coverage ability of lacquers greatly depends on the size of pigment particles. The preparation of emulsion, cream and ointment consistency is extremely important in cosmetics, involving the dispersion of various materials in colloidal dimensions. The making of powders, pigments, thickening agents and lubricants, soaps and detergents, adhesives, eye liners, aerosols, foams, fibers and liquid crystals also includes the chemistry of colloids. Similarly, colloid science involves the biochemistry of plants and molecular biology of amino acids and proteins and an improved delivery of cosmetic actives.

A colloid is a dispersion of small particles of one material in another. "Small" means less than 500nm in diameter (about the wavelength of visible light). The particles in colloids are dispersed without appreciable bonding to solvent molecules, and do not settle out. However, there are some exceptions where no molecule bonds are involved between the dispersant and dispersing solvent. In general, colloidal particles are aggregates of numerous atoms or molecules, but are too small to be seen with the naked eye. They pass through most filter papers, but can be detected by light scattering, sedimentation and osmosis. Colloids are relevant to a discussion of surfaces because the ratio of surface area to volume is so large that their properties are dominated by events at their surfaces. For example, a cube of material of side 1 cm has a surface area of 6 [cm.sup.2], but when it is dispersed as 1018 little cubes of side 10nm, the total surface area is 6 x 106 [cm.sup.2] (the size of a tennis court).

Solution and suspension are two extreme means in which two substances can be mixed. In a solution, the solute particles are so small that they cannot be seen. In a suspension (noncolloidal) of a solid in a liquid, the particles of solid are large enough to be visible and settle after a while due to gravity. Particle size is the difference between a solution and a suspension. Generally in a solution, solute particles are smaller than 1nm in diameter. In a suspension, solid particles are larger than 1000nm in diameter. There are mixtures in which the size of the particles is between the upper limit for solutions and the lower limit for suspensions. These mixtures often have properties of both suspensions and solutions and are perceived as colloids.

The term "colloid" implies a certain state of matter, having a certain set of properties specific to it and by which it may be recognized. These properties are developed when the individual particles of the substance fall within a certain range. A substance cannot be categorized as a colloid or noncolloid merely by virtue of its chemical nature. The same substance may be a colloid under one set of conditions and not under another. A good example, crystalline material sodium chloride, has been obtained in colloidal condition, as are the protein molecules pepsin, gelatin and albumin. Similarly, ordinary soap, when dissolved in water, diffuses very slowly and penetrates a membrane, showing it to be a colloid. But in alcohol, the same soap possesses the properties of a crystal. Substances in the colloidal condition are in a state of subdivision, intermediate between the molecular dispersion of true solutions and the coarse dispersion of suspensions. Some colloids consist of well-defined molecules, with constant molecular weight and definite molecular shape, permitting them to be ordered in a crystalline array. Proteins have molecular masses ranging from about 10,000 to several hundred thousand. Most colloid particles do not settle out easily because they are too small.

Classification of Colloids

Colloids are classified by the state of their components and the degree of attraction between the dispersed phase and dispersion medium. Colloids may be divided as a dispersed phase and a dispersion medium. The dispersion medium in many colloids is a liquid. If the dispersed phase is a solid and the system appears to be liquid and flows, it is called a sol. If it has a solid-like structure that prevents it from flowing, it is called a gel. A gel is a semi-rigid mass of a lyophilic sol in which all the dispersion medium has penetrated into the sol particles. Milk of Magnesia is a sol. Jell-O is a gel when it is cool but a sol when it is warm enough to flow. A further classification of colloids is as lyophilic (solvent-attracting) and lyophobic (solvent-repelling). If the solvent is water, the terms hydrophilic (attracting) and hydrophobic (repelling) are used. Lyophobic colloids are irreversible colloids or suspensoids, while lyophilic colloids are known as reversible colloids or emulsoids. Lyophobic colloids include the metal sols. Lyophilic colloids generally have some chemical similarity to the solvent; for example, -OH groups able to form hydrogen bonds.

For the dispersed phase of a polymeric nature the dispersion is called latex. A colloidal particle exists as corpuscular, laminar or linear; however, the exact shape may be complex. The corpuscular particle includes many proteins and iron ([Fe.sub.3]) and clay suspensions are systems containing plate-like particles. High polymer materials exist in the form of long thread-like straight or branched-chain molecules, and as a result, inter-chain attraction or cross-linkage provides mechanical strength and durability. This is not possible when the particles are corpuscular or laminar. In nature, thread-like polymeric materials fulfill an essential structural role. Plant life is built mainly from cellulose fibers. Animal life is built from linear protein materials such as collagen in skin, sinew and bone, myosin in muscle and keratin in nails and hair. The coiled polypeptide chains of the so-called globular proteins that circulate in the body fluids are folded to create corpuscular particles. When particles aggregate together, many different shapes can be formed, and these do not correspond to the shape of the primary particles.

Colloidal systems are often colored. The color of precious or semiprecious stones is determined by the presence of negligible amounts of heavy metals and their oxides in a state of a colloidal extent of disintegration. For example, admixtures are iron compounds in natural rubies and chromium compounds in emeralds. Clay particles along with other materials of colloidal dimensions are added to foundations, mascaras and liquid eyeliner. However, inorganic pigments in cosmetic foundations, such as iron oxides, white titanium dioxide, titanium dioxide coated mica, dyes and lakes, are pre-dispersed suspensions.

Metallic sols have especially bright color, owing to the great difference in densities and, consequently, in the refraction indices of the dispersed phase and the dispersion medium. In general, the color of sols may be very intense, even more so than the color of molecular solutions. For example, when the content of the dispersed phase is the same, the color of the gold sol is 400 times more intense than that of the brilliant bluish red fuchsine dye solution. The intense color of colloidal solutions makes it possible to determine negligible amounts of the dispersed phase of a colloid. The color of a colloidal system may also depend on the technique of preparing sol and on the conditions of its observation (in transmitted or reflected light). Colorless (white) non-metallic sols do not exhibit selective light absorption and usually reveal an orange color in transmitted light and bluish opalescence in reflected light. However, there are many sols with non-metallic particles that have a specific color which depends on the selective absorption of light rays by their particles.

Many other factors determine the color of a colloidal system. Due to the scattering and absorption of light, color is affected not only by the nature of the dispersed phase and the dispersion medium, but also by the degree of particle dispersion, shape and structure. The color in sols containing metallic particles can cause problems for formulators. The color of metallic sols is determined by the true absorption of light. Some of the light energy is transformed into thermal energy, causing a color change. In addition, the color is affected by light scattering. Sols of the same metal may exhibit a different color because light absorption and scattering pass through a maximum as the particle size and light wavelength increase. For example, coarsely dispersed gold sols are characterized by comparatively low true absorption that is shifted to the red spherical region, and strongly scatter light with the maximum in the same spectral region. These sols usually have a blue color in transmitted light and red opalescence in scattered light. Conversely, highly dispersed gold sols are usually red with blue opalescence due to their strong ability to absorb light with a sharp maximum in the yellow-green region of the spectrum. Gold sols are yellow with more dispersion and the color is similar to the solutions of gold chloride, which is a molecular dispersed system.

Preparations of Colloids

The preparation of aerosol colloids can be as simple as sneezing (which produces an imperfect aerosol). Laboratory and commercial methods make use of several techniques. Material (e.g. quartz) may be ground in the presence of the dispersion medium. Passing a heavy electric current through a cell may lead to the sputtering (crumbling) of an electrode into colloidal particles. Arching between electrodes immersed in the support medium also produces a colloid. Chemical precipitation sometimes results in a colloid. A precipitation (e.g. silver iodide) already formed may be dispersed by the addition of a peptizing agent (e.g. potassium iodide). Clay may be peptized by alkalis, the OH- ion being the active agent.

Colloids can be prepared by dispersing large particles or condensing small particles. Preparation of sols requires the disintegration of materials in mass into particles of colloidal dimension and subsequent suspension in a suitable medium. This may be affected by grinding coarse particles in the presence of a liquid in a colloid mill.

Emulsions are normally prepared by vigorously shaking the two components together, and an emulsifying agent is added for stability. This emulsifying agent may be a soap (a long-chain carboxylic acid), a surfactant or a lyophlic sol that forms a protective film around the dispersed phase.

One way to form an aerosol is to tear apart a spray of liquid with a jet of gas. If a charge is applied to the liquid, then electrostatic repulsions help to blast it apart into droplets. This procedure may also be used to produce emulsions, as the charged liquid phase may be directed into another liquid.

Colloidal dispersions are also being prepared by treatment with ultrasonic waves. In this method, high frequency sound waves cause the disruption and dispersion of liquids into droplets of colloidal size, forming emulsions in water of such substances as oil. Solid-liquid and liquid-gas systems have also been prepared by ultrasonic methods.

Condensation is another preparation method that usually involves substances in true solution and causes a chemical or physical change. Condensation takes place in two stages. In the first, molecules or ions condense to form invisible nuclei. In the second, the nuclei grow until they are large enough to precipitate. The more nuclei, the smaller the individual particle will be; the rate of nucleus formation is related to the degree of super-saturation. This is commonly seen in crystal growth. However, unless special precautions are taken, the colloidal particles agglomerate and eventually precipitate. Carefully controlling the conditions will halt the growth of particles without transforming into precipitation while producing and maintaining a colloidal dispersion.

This may be prevented by avoiding the presence of electrolytes in the dispersion medium or by keeping their concentration at a minimum. Therefore, it is important to keep this factor in mind when choosing a chemical reaction suitable for the production of a colloidal dispersion. For example, arsenous sulfide can be produced by either of the following reactions:

2As[Cl.sub.3] + 3[H.sub.2]S = [As.sub.2][S.sub.3] + 6HCI As[O.sub.3] + 3[H.sub.2]S = [As.sub.2][S.sub.3] + 3[H.sub.2]O

The first will result in a precipitate because the hydrochloric acid causes the agglomeration of the arsenous sulfide particles; the second reaction forms a colloidal dispersion of arsenous sulfide stabilized by hydrogen sulfide.

Properties of Colloids

Colloidal particles dispersed in a liquid medium are either single, large molecules of higher molecular weight (macromolecules) or aggregates of smaller molecules. In either case they nearly always possess an electric charge which is responsible for their stability. Electrostatic repulsion, which occurs when particles of like charge approach each other, operates against the formation of larger particles and eventual fiocculation. The charge on the particle may be acquired by adsorbing selective ions from the surrounding medium. The electrolyte that furnishes these ions is known as the stabilizing electrolyte. First, ions of one charge type are strongly adsorbed in a layer firmly attached to the colloidal particle and give the particle its charge. These ions are the potential determining ions. Then, the ions of opposite charge are attracted to the vicinity of the charged particle and form a second, more diffuse layer. These ions are called the counter ions. The entire dispersed particle, with the attached double layer of ions, has to be a micelle-like structure. Particles of colloidal dimensions, suspended in a transparent medium, scatter and, to a certain extent, polarize light that falls upon them. If a beam of light passes through a colloidal dispersion, the path of the beam is visible. The light beam is invisible in a true solution and is described as "optically empty." This colloid behavior, the Tyndall phenomenon (the motion of colloidal particles in medium), is of great importance in clear cosmetic products.

Colloids are one way to stabilize an active or a particular chemical compound in cosmetics or pharmaceuticals. Therefore, stability is an important consideration in the study and application of colloids. Colloids are thermodynamically unstable due to their greater surface area. Lyophobic colloids maintain their stability through the electrical charge on the particles. If this charge is removed or reduced below a critical value, the colloidal particles cling together and rapid coagulation or precipitation occurs. The addition of excess electrolytes may destabilize the colloid system. Lyophilic colloids are stabilized by solvation (hydration) and electric charge; so they are usually more stable than lypophobic colloids.

Physical Properties or Benefits

Increased surface area of materials demonstrates an increase in solubility and vapor pressure, and a decrease in melting point. Increased surface area of certain materials can cause color to change. Examples include antimony trisulfide, changing from yellow to red.

Adsorption from solutions is an important physicochemical process in plants and animals. Adsorption from a solution onto a solid surface is most important to the colloid system, especially for the formation, stabilization and disintegration of lyosols. The surface phenomenon involves the concentration or accumulation of gas, liquid or solid on the surface of a liquid or solid while it is in contact. In considering adsorption from a solution on a solid, two types are distinguishable: nonionic and ionic electrolytes. The adsorption of ionic electrolyte has a stronger affinity to the surface. This is an important tool as an ion-exchange adsorption in skin biology. Most coated raw materials in cosmetics utilize this phenomenon for better feel and stability.

Wetting phenomenon involves the reduction of the contact angle between solid surface and liquid. It is similar to the adsorption phenomena that are determined by the intensity of interaction between molecules of different substances.

Electrophoresis studies are an important tool for determining zeta potential, the sign and magnitude of the charge on colloidal particles. The electric charges on the colloidal particles will facilitate themselves to move in the dispersion medium under the influence of an applied electric field. A potential, E, exists between the surface of the colloid and the main body of the dispersion. The potential, E, may be divided into two parts. The first is the potential between the colloid's inner shell and surface. The second is the zeta or electrokinetic potential, and is the potential difference between the colloidal particles and the surrounding medium. The greater zeta potential value provides greater stability to the lyphobic colloid. This technique is also used for the purification of proteins and other actives in cosmetic products. By the same token, dialysis is another technique for the purification of colloids. The aim is to remove much (but not all) of the unwanted ionic materials from the preparation. Cellulose or a similar membrane allows the permeation of solvents and ions, but not the colloid particles.

Hydrophilic colloids, such as gum acacia, present charges in the aqueous medium due to the carboxylic or sulfate groups on the carbohydrate polymer. Proteins (gelatins) are made up of amino acids, which are zwitterions and can posses either a positive or negative charge based on the pH of the solution. Pretreatment with an acid or base can change the isoelectric point of the gelatin, resulting in a positive charge at pH 4-4.5. A basic treated gelatin is negatively charged at pH 8.

Emulsions

Emulsions are colloidal dispersions of one liquid in another. The conditions required for forming emulsions are similar to those needed for obtaining colloidal systems. Emulsions deliver cosmetic actives, emollients and moisturizers. Sustained or prolonged release of skin beneficial materials is theoretically possible by the development of special emulsion systems.

During emulsion preparation, large blobs of the dispersed phase are elongated under shearing forces, necking and finally separating into smaller drops. Emulsification will take place if the interfacial tension between two immiscible liquids is sufficiently low. The adsorption of a surface film with lowering of [gamma] (critical surface tension) and increase in surface area ([eta]s) clearly enter the picture. To expand a surface of a liquid, energy in the form of work is exerted to overcome the tendency of the surface molecules to move into the bulk liquid. This statement is quantified in the expression:

[gamma] = W/([eta]A)

Where [gamma] is the surface tension, W is the work required to expand a surface, and [eta]A is the increase in the surface area.

If W is expressed in ergs and [eta]A is given in [cm.sup.2], the surface tension ([gamma]) would be expressed as ergs/[cm.sup.2]. Surface tension thus may be defined as the work in ergs necessary to generate one square centimeter of surface. Even though the surface tension as erg/[cm.sup.2] is quite acceptable, the more conventional unit, dynes/cm is used today. It is possible to alter the surface tension or interfacial tension of a liquid by the addition of a solute. The surface tension of water may be increased by the addition of inorganic salts to a slight degree or decreased appreciably by certain organic compounds such as soap. Gibbs' surface adsorption demonstrates the mathematical relationship between the influences of surface concentration of a solute to the surface-tension change produced at different activities of a solution. The derivation of this equation is beyond the scope of this discussion, however it arises from a classical thermodynamic treatment of the change in free energy when molecules concentrate at the boundary between two phases. The equation is expressed:

[GAMMA] = - a/RT d[gamma]/da

Where [GAMMA] is the moles of solute adsorbed/unit area; R is the gas con stant; T is the absolute temperature and d[gamma] is the change in the surface tension with a change in solute activity, da, at activity, a.

For dilute solutions of non-electrolytes, or electrolytes when the Debye-Huckel equation for activity coefficient is applicable, the value of a may be replaced by solute as concentration c. Since the term dc/c is equal to dInc, the Gibbs equation is often written:

[GAMMA] = - 1/RT d[gamma]/d Inc

In this way, the slope of a plot of g vs. Inc multiplied by 1/RT should give F at a particular value of c. This equation enables the extent of adsorption at a liquid surface to be estimated from surface tension data. This equation has been successfully applied to extremely simple systems and in those instances where the interface is not curved. This concept resulted in the inception of "oriented wedge theory;" i.e., surface orientation of the emulsifying molecule at the interface, predicts o/w or w/o type of emulsion. However, this theory has come under criticism and current knowledge does little to support the "oriented wedge theory."

The Role of Film Properties

Dynamic and equilibrium film properties play a considerable role in the formation of emulsions and foams. The monomolecular film, resulting from the adsorption of the emulsifier at the interface of an oil and water, should be strong but pliable because it imparts maximum stability of the emulsion system. It should be emphasized, however, that sufficient concentration of the emulsifying agent must be included in the emulsion system to cover all the dispersed globules. The emulsifying agents form a film around the suspended oil droplet and strengthening this film can create greater stability. The emulsifiers are usually amphipathic molecules that possess the ability to adsorb at the interface, lowering the interfacial tension and increasing the interfacial viscosity. The ionic emulsitiers make emulsions stable due to the electrostatic repulsion of the particles and/or steric hindrance.

Hydrogen bonding works in the case of nonionic emulsifiers. The emulsifying agent should rapidly form a film to prevent the coalescence of the dispersed globules. Mixed blends of emulsifying agents in most instances produce complex films, which in turn give more desirable films and hence greater emulsion stability. A combination of triethanolamine stearate and stearic acid provides a stronger interfacial film compared to an emulsion made with triethanolamine stearate alone. The same theory applies to the combination of sodium cetyl sulphate with cetyl alcohol, steareth-20 with ceteth-2 and sodium cetyl sulphate with cholesterol.

Solutions of high surface-active materials exhibit unusual physical properties. In a dilute solution, the surfactant acts as a normal solute and, in the case of ionic surfactants, normal electrolyte behavior is observed. However, at fairly well-defined concentrations, abrupt changes in several properties, such as osmotic pressure, turbidity, electrical conductance and surface tension, take place. The rate at which osmotic pressure increases with concentration becomes abnormally low and the rate of increase of turbidity with concentration is much enhanced. The conductance of ionic surfactant solutions, however, remains relatively high, which shows that ionic dissociation is still in force. McBain pointed out that this seemingly anomalous behavior could be explained in terms of organized aggregates; or micelles, of the surfactant ions in which the lipophilic hydrocarbon chains are oriented toward the interior of the micelle, leaving the hydrophilic groups in contact with the aqueous medium. The concentration, above which micelle formation becomes appreciable, is termed the critical micelle concentration (CMC). CMC value can be determined by surface tension, electrical conductivity and dye solubilization measurements. Possible micelle structures include the spherical laminar and cylindrical arrangements. Information concerning the sizes and shapes of micelles can be obtained from dynamic light-scattering, ultra-centrifugation, viscosity and low angle X-ray scattering.

Surfactant solutions above the CMC can solubilize otherwise insoluble organic material by incorporating it into the interior of the micelles. For example, the dye xylenol orange dissolves only sparingly in pure water but gives a deep red solution with sodium dodecyl sulphate present above its CMC. This solubilization is of practical importance in treatment cosmetics formulations which contain water-insoluble ingredients and detergents, where it plays a major role in the removal of oily soil, emulsion polymerization and micelle catalysis of organic reactions.

Surfactants used for cleaning are called detergents or soaps. They are effective as cleaning agents due to their ability to emulsify fats and oils in water. The fatty acid anions, such as palmitate ions, form a layer around an oil droplet. The sodium ions dissolve in the water and the negatively-charged carboxylate ends of the fatty acid anions remain in the aqueous phase. There are many kinds of synthetic detergents in most household and industrial products. A common detergent has the sulfonate ion group, -[So.sub.3] in place of the carboxylate group, - [Co.sub.2]. Ordinary soap forms a greasy precipitate of the calcium salt of the fatty acid with hard water. Sulfonate detergents have the advantage of not forming this precipitate, because of the greater solubility of the calcium salts. Soaps and other detergents have valuable antiseptic properties. The cationic surfactants (cetylpyridinium chloride) have exceptional bacteriostatic and bactericidal properties.

Clear transparent emulsions (microemulsions), in particular, have a high profile with personal care products. Micro-emulsions are colloidal systems of one liquid dispersed in another liquid. The particle size of the dispersed particles is probably in the range of 100-600 A [degrees] and thus, much smaller than light rays. This very small size produces transparency.

Emulsion Instability

The first step in breaking an emulsion is the joining of individual drops. The encountering charges determine whether the drops aggregate into a flock or go their separate ways. These involve rupture of the thin continuous phase film, aggregation of droplets into a single kinetic unit, and eventually coalescence into a single geometrical unit. Again, surface tension and surface viscosity are certainly pertinent to the coalescence process.

The aggregative stability of emulsions is considerably affected by the nature of the emulsifier and its concentration in a system. In other words, an emulsifier, while being adsorbed at the interface, reduces interfacial tension and, in some cases, can determine the stability of colloidal systems. According to Bancroft's rule, hydrophilic emulsitiers, which dissolve better in oils and hydrocarbons, favor the formation of a w/o emulsion. An emulsifier hinders the coalescence of droplets when it dissolves better in the dispersion medium. A special number denotes the effectiveness of an emulsifier: the hydrophilelipophile balance (HLB). For an HLB number between 3-6, a w/o emulsion is formed. Emulsifiers having an HLB number of 8-13 produce an o/w emulsion.

Repellent forces (the energy barrier) between droplets at the interface can also determine the emulsion stability when an emulsifier is present. For example, ionic soap stabilizes itself by a double electric layer (two shells of ions of opposite charges) on the surface of emulsion drops. The inner shell is narrow and compact, adhering tightly to the colloid 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. This double layer makes the emulsion stable. Therefore, first-order emulsions, stabilized by ionic soaps, have all the properties inherent in typical hydrosols; i.e., the Schulze-Hardy rule is valid and the emulsion's particle charge may be reversed with polyvalent ions. For an organic ion to be adsorbed, it must be adsorbed well by the dispersed phase; i.e., it must have a sufficiently long hydrocarbon chain. Therefore, o/w emulsions can be stabilized only by soaps of rather high molecular weight (alkaline salts of stearic acid and other fatty acids of a higher molecular weight). The second factor of stability of concentrated o/w emulsions is the formation of crosslinked gel layers of an emulsifier on the surface of their droplets; these layers have high structural viscosity and strength, and are hydrated.

Stabilizing o/w Emulsions

The stability of o/w emulsions with nonionic emulsifiers can be explained as diphilic molecules of an emulsifier which are oriented at the interface in a way that the hydrocarbon parts are hindered from the polar-dispersed phase and greatly hydrated groups are in water. They either form a sufficiently thick hydrate layer which causes dissociation pressure, or undergo microBrownian motion (entropy factor of stability). The stabilization of w/o emulsions with soaps having polyvalent cations is due primarily to the presence of a structural mechanical barrier on the surface emulsion droplets. The explanation that w/o emulsions are stable because an electric double layer exists at the interface seems untenable since the dielectric constant of the dispersion medium is low. However, emulsifter molecules may dissociate even in non-polar media.

The second factor that stabilizes w/o emulsions having a polyvalent cation as the emulsifier, has been explained as follows: The polar part of polyvalent cations are adsorbed or linked on the surface of water droplets through hydrogen bonding. While the non-polar hydrocarbon tail end molecules are solubilized in the oil non-polar phase. In considering this as an individual particle, it has been described to be capable of undergoing Brownian motion.

In this case, stability is apparently determined by thermal motion and the mutual repulsion of hydrocarbon radicals; i.e., by a factor which has an entropic nature. The emulsifying action of both ionic and nonionic surfactants is more effective because there is a better balance between the polar and non-polar phases. This means that the diphilic molecule of a good emulsifier must have affinity for both polar and non-polar media. In principle, o/w emulsions are formed with emulsifiers where the action of polar parts prevails over the action of non-polar ones, and which dissolve better in water. Conversely, w/o emulsions are produced with emulsifiers in which the action of non-polar groups of molecules predominates over that of polar groups which dissolve better in hydrocarbons.

The emulsifying action of substances of high molecular weight, such as gelatin, casein, polymethacrylic acid, methylcellulose and polyvinyl alcohol, and their action as protective colloids, can be best explained by the colloid dispersion. According to Stoke's law, the less difference in density between two phases, the more stable the emulsion.

Emulsion type is established easily by determining the properties of its dispersion medium: the ability of an emulsion to wet a hydrophobic surface; its potential to be diluted with water; the ability of an emulsion to be colored when dye, which dissolves in the dispersion medium, is introduced and the electric conductivity of an emulsion. An emulsion is of the o/w type if it does not wet a hydrophobic surface, is diluted with water, is colored with a water-soluble dye (e.g., methylene blue) and exhibits rather high electric conductivity. Conversely, an emulsion is of the w/o type if it wets a hydrophobic surface, is not colored with a water-soluble dye (or is colored with an oil-soluble dye (e.g., sudan 111), and does not exhibit noticeable electric conductivity.

Colloid and interface science is an important field of research and application in the cosmetic industry. There has been renewed interest in colloids due to the many new methods incorporated to develop cosmetic products in the R&D laboratory. Today, the cosmetic scientist must consider the various repellent and attractive forces involved in these dispersed systems. Chemists must understand both electrostatic and steric stabilization and the properties of surfactants that are used to prepare these systems so that they can control the interactions that might occur in such complicated mixtures and situations. Some typical colloidal systems in cosmetics are: aerosols of liquid droplets or solid particles, foams, emulsions, sols or suspensions, solid foams, microemulsions and gels.

Colloid and interface science deals with multi-phase systems that are common in many cosmetics where one or more phases are dispersed in a continuous phase of different composition or state. In classic colloid science, dispersions have at least one dimension of a dispersed phase that falls within the range of 1-1000nm. However, in applied colloid science, the upper size limit is commonly extended to at least 10,000-100,000nm. Interface science deals with dispersions in which there is an extremely large interfacial area between two of the phases. The dispersed phases may be particles, droplets or bubbles.