ERNST A. HAUSER Massachusetts Institute of Technology, Cambridge, Massachusetts
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increasing interest in colloid science and the realization that this branch of chemistry is gaining in importance in biology, medicine, ceramics, photography, and many more branches of science and technology, may no longer be overlooked. It therefore seems advisable to collect the most important and most accepted nomenclature pertaining to colloid chemistry and to offer a simple definition for every term, so that those not specialized in colloid chemistry may use this list for their own writings, or when studying original colloid chemical literature. The most accepted terms will be listed alphabetically. Less common, but synonymous nomenclatures will be listed with reference to the now more accepted terminology, in the alphabetical sequence. Absorption: If the substance adsorbed (see adsorpt w n ) a t the interface does not remain there but enters into the adsorbent, one speaks of absorption. Absorp tion is a volume-increasing imbibition or binding, in . contrast to adsorption. Adsorbendurn: The substance taken up by adsorption (see adsovption). The dissolved substances are taken up in various concentrations in which the solvent plays an important part. Surface inactive substances in organic solvents are poorly adsorbed, whereas surface active substances are well adsorbed. Adsorbent: The substance on which another substance is adsorbed. The activity of an adsorbent depends primarily on its surface development and camposition. The adsorption capacity is, in general, more pronounced with an amorphous adsorbenti'than with a crystalline one. Adsorbentia, hydrophilic: Substances which preferentially adsorb water, as, for example, silica gels and colloidal metal hydroxides. Adsorbentia, hydrophobic: Water-repelling adsorbents, as, for example, activated carbon. Adsorption: An increase in concentration of a substance a t an interface. The substance to be adsorbed can be of molecular, colloidal, or coarse dispersion. For the degree of adsorption, the dimensions of the interface are of predominant importance. Adsorption, negative: This occurs when the concentration of the substance to be adsorbed is less in the interface than in the bulk of the solution. This is specifically the case when dealing with adsorption from solutions in which the solvent itself is strongly adsorbed. Adsorption, oriented: According to the theory postulated by Hardy, Langmuir, and Harkins, unsymmetrical molecules will, if adsorbed in an int.erface,
orient themselves in such a fashion that the active groups face that phase to which they have the greater affinity. Aerodispersoid: Substance, the dispersion medium of which is a gas. Aerosol: Substance, the dispersion medium of which is a gas. Alcogel: A gel obtained from an alcosol, its dispersion medium being alcohol. Alcosol: A disperse system the disperse phase of which is of colloidal dimensions and the dispersion medium is alcohol. Amicron: Particle of a disperse system of such dimensions that it is not even visible in an nltramicroscope. Anisotropy of colloids: Refers to col'loidal systems, the disperse phases of which show anisotropic particles. Aurum potabile: A gold sol obtained by a reduction reaction of a gold chloride solution (see gold sol). Azimuth stop: A stop introduced into the ultramicroscope condenser which permits the light to enter the preparation only in certain regions, so that the particles of a colloidal sol are illuminated only if they lie perpendicular to the incoming beari bf light. Used to . ascertain particle &ape. Brownian motion: The irregular movement of dispersed colloidal particles caused by impacts of the molecules on the dispersed particles. Capillary condensation: Condensation of vapor on the surface of a wettable adsorbent (activated carbon, silica gel). Capillal y-inactive: See surface-inactive. Coaceruatwn: Coagulation of lyophilic colloids by causing demixing of two liquids insoluble in each other (gelatin in water a t 50°C. or gelatin to which alcohol is added). Coagel: A gelatinous precipitate obtained from a sol by coagulation. Coagulation: The change of a colloidal sol into a coarsely dispersed system. The ultramicrons aggregate to form larger particles until they finally become visible as flocks and settle out. Coagulation, causes of: Addition of electrolytes or desolvatizing agents; freezing or boiling; removal by dialysis of electrolytes needed for stability; by mechanical means. Couqulation of lyophilic sols: In contrast to lyophobic sols, lyophilic sols can be coagulated only by high concentrations of electrolyte or dehydrating agents. Coagulation of lyophobic sols: Decrease of the elec-
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trokinetic potential by the addkion of appropriate types and concentrations of electolytes (see HardySchulze rule) until the sol separates and the disperse phase settles out. The sediment is generally termed a precipitate, although there is some tendency t,o avoid the general use of the term. Colloidal dispersion: A disperse system, the particles of the disperse phase ranging in dimensions between 1000 and 20 mfi. Colloid mill: Mechanical equipment which permits the disintegration of coarsematt,er into the colloidal range of dimensions. The percentage of matter in the colloidal range of diiensions recovered after milling nil1 depend on the composition of the substance to be ground and the construction of the mill. It is never possible to obtain only particles n-ithin the colloidal range of dimensions. Colloids: Substances which exhibit colloidal phenomena. The term colloid^' does not refer to the properties of specific substances but to a specific state of matter. Condensation methods: Methods which permit the aggregation of matter present in true solution or in gaseous form into the colloidal range of diiensions (reduction of solubility, chemical precipitation). Creaming: A reaction used to cause separation of a disperse phase of a colloidal sol by introducing a protective colloid which, by adsorption, will cause the conoidal particles to concentrate in the surface or at the bottom of the system, depending on their specific gravity (used, for example, for the concentration of natural and synthetic rubber lattices. Ore flotation is also based on this phenomenon). Ciyogel: Colloidal dispersions obtained by rapid freezing of supersaturated solutions. Cryosol: A colloid-disperse system which is stable only at low temperatures (colloidal ice in benzol). Dehydration: Withdrawal of water from a solvatized colloid by the introduction of 'water-removing substances like salts, alcohols, etc. c Desoluation: Removal of the solvating liquid from a colloid. Dialysis: Passage of a substance, present in dissolved state, 'through a membrane into the solvent. Dialyzer: Instrument for accomplishing dialysis. Dialyzing membrane: A membrane used to perform dialysis; for example, parchment paper, cellophane, fish bladders. Difform systems: Systems characterized by having one or two of their dimensions developed more pronouncedly than the third; for example, threads are termed fibrillar difform, films are classified as laminar difform. Diffuse double layer (Gouy): The diffuse double laver is the location of the electric charges of colloids. 1t"extendsfrom the surface of the colloidil particle into the dispersion medium. Dilatancy: The phenomenon of dilatancy is the exact reverse of thixotropy. A system is called dilatant if it will set to a solid upon agitation, pressure, or rapid
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expansion. (Wet sea sand when trodden upon wi!l be quite dry, starch pastes will crumble when rapidly agitated and so will kaolin pastes. Certain silicone resins known as "bouncing putty" will become dry when rapidly expanded or subjected to agitation, but return to their fluid condition when a t rest,.) Disperse system: A disperse system is composed of the dispersion medium, which is the continuous phase, and the disperse phase, which is the discontinuous. Dispersion, chemical: The change of a precipitate into a sol either by washing out excessive elrctrolytes or by adding an appropriate electrolyte. Dispersion, electrical: Forming a colloidal sol of a conductive substance by passing an electric current between two electrodes. The electrodes are thereby dispersed into the colloidal range of dimensions. Dispersion, mechanical: Subdividing coarse matter into a colloidal dispersion by mechanic~lmeans, as, for example, by the use of a colloid mill. Dispersion, degree of: Average diameter of particles composing the disperse phase of a dispersion. Dispersion medium: The continuous phasc of a dispersion. Dispersion methods: See dispersion. Dissolution: The change of a colloidal sol into a molecular disperse or true solution. Donnan equilibrium: The distribution of an electrolyte present in a sol when this is in contact vith the pure electrolyte through a membrane which does not permit the passage of particles of colloidal size. Double refraction of colloids: A phenomenon exhibited by colloidal sols and gels. Pronounced with sols composed of anisometric particks when they are caused to flowor if oriefitation is obtained by imposing a magnetic or electric field. Electrocratic sol: Sols which o1m their stability to their electric charge and which can he destabilized by the addition of electrolytes. Electrodecantation: When elect&ialyzing a colloidal sol, the system separates into two layers, one of them being free of colloidal matter. The top layer, free of colloids, is formed on the membrane and rises due to its lower specific gravity. This phenomenon is used for the concentration of sols. Electrodialysis: Dialysis combined with electrophoresis. Used for the purpose of speeding up standard.dialysis. Electrokinetic potential (zeta potential) : The potential between the liquid layer adsorbed on the colloidal particle and the dispersion medium. Electroosmosis: Movement of the dispersion medium out of a coherent plug of sand,. clay, . . cotton, etc.,. by. the application of an &ec& field. Elect~ophoresis: Movement of colloidal particles in a sol. caused bv an electric field. ~lectroviscouseffect: Increase of the viscosity of an electrically charged sol by the effect of the charged particles on the surrounding dispersion medium. Electroultrafiltration: A method consisting of placing electrodes outside the membrane, between which has
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been deposited the sol to be filtered. Then an electric current is passed through the system. Emulsifiers (emulgators): Substances which facilitate the formation of stable emulsions. Emulsion: A disperse system consisting of two immiscible liquids, one being dispersed in the other. Distinction is made between water-in-oil and oil-inwater types of emulsion. Emulsion reversion. An oil-in-water emulsion can be changed to a water-in-oil emulsion by changing the emulsifying agenf,used. For example, if one is dealing with an oil-in-water emulsion with sodium oleate as emulsifier, the addition of calcium chloride will cause a change into a water-in-oil emulsion, because the formed calcium oleate uill be preferentially adsorbed by the oil phase. Emulsoirl: A colloidal system the disperse phase of which is a liquid of colloidal dimensions. Eucolloid: Colloids which by the action of primary valences have reached colloidal dimensions. When put into solution, they form colloidal systems. Gelatin, rubber, albumen, are typical examples. Flocculation: See coagulation. Gel: Disperse systems of varying degrees of dispersion which exhibit physical properties corresponding to solid bodies when subjected to sudden impacts or stretch, but resemble a liquid of high viscosity if this reaction is time-consuming (elasticity of gels). One can differentiate between gels of the following composition: liquid/liquid; solid/liquid; liquid/solid, etc. Gelation: The formation of a gel from a sol. Gels, nonswelling: Gels which imbibe liquid without increase of volume (silica gel). Gels, swelling: Gels which when imbibing liquid show an increase in volume (gelatin in water, rubber in organic solvents). Gold sol: A dispersion of elementary gold particles of colloidal dimensions in water. Gold-ruby glass: A colloidal dispersion of elementary gold in glass (this was famous in alchemiGic days). Hardy-Schulze rule: A colloidal sol is precipitated by the addition ofions carrying a charge opposite to that of the disperse phase. The effectiveness of the precipitab ing ion increases with its valency. The ratio of chloride of K, Ca, and Al is 1 to 100 to 1000. Helmholtz double layer: This concept assumed that the double layer is composed of the actual surface charge of the colloidal particle surrounded in the dispersion medium by ions of opposite charge. (This theory is outmoded. See diffuse double layer.) Hofmeister's ionic series: The coagulation ability of neutral salts on lyophilic sols follows this series. For acid sols: nitrates, chlorides, acetates, sulfatesin increasing efficiency. In alkaline solutions, the series is reversed. Alkali cations show this series: Li, Cs, Xa, Rb, K. Hydration: Adsorption of the water on a colloidal particle. Hydrogel: A gel the dispersion medium of which is water.
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Hydrophilic sol: A sol the disperse part of which adsorbs water. Hydrophobic sol: A sol the disperse phase of which has no affinity for water. Hydrosol: A liquid colloidal system having water as the dispersion medium. Jelly: A term frequently used synonymously with the term "gel." To he specific, the term jelly should only be used for transparent, elastic substances like gelatin. Flocked precipitates or gelatinous precipitates should he termed gels. Laminar sl/stems: See d i f f o m systems. Lyogel: Gel obtained by cooling a solvatized sol. Lyophilic colloid: A colloid the disperse phase of which has a great affinity for the dispersion medium. Lyophobic colloid: A colloid the disperse phase of which has no affinity for the dispersion medium. Lyosorption: The adsorption of the dispersion medium when it wets the dispersed phase. The soformed liquid layer is known as the lyosphere. Mevbrane equilibrium: See Donnan equilibrium. Micelle: The actual colloidal particle plus the firmly attached layer of the dispersion medium containing the ions which compose the diffuse electric double layer. Monodisperse sol: Sol in which the disperse phase is composed only of particles of practically uniform dimensions. Organogel: A gel of such colloids as have an organic dispersion medium. Organosol: A colloid-disperse system, the dispersion medium being an organic solution. Peptization: The change of a precipitate of an electrocratic colloid into a sol by the,addition of electrolytes which will increaSe the electrokinetic potential; or the change of a gel of a lyocratic colloid into a sol by the addition of the appropriate dispersion medium. Precipitate: The addition of an electrolyte to an electrocratic sol causes its destabilization and results in the disperse phase settlmg out in the form of noncoherent aggregates of particles. (See coagulation of lyophobie sols.) Precipitation: The reaction causing the formation of a precipitate. Protective colloids: Lyophillic colloids which protect lyophobic sols from coagulation.Rheopezy: The reverse of thixotropy. A colloidal sol can be set to a gel by rhythmic tapping or swinging, but will liquefy as soon as such movement is stopped. Salting out: Coagulation of hydrophilic sols by the use of high salt concentrations (see coagulation). Slit ultramicroscope: A microscope the illumination of which enters the preparation a t a right angle through a fine slit. Solvation: The adsorption of liquids by all colloids which are characterized by high viscosity and great stability toward electrolytes. Solvatized sol: Sols which exhibit a strong adsorption of dispersion medium on the disperse phase. Spumoids: Foams, i. e., disperse systems of a gas/ liquid composition.
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Stalagmometer: Instrument used to determine surface tension by counting the drops formed from a specified volume (developed by I. Traube). Stream double refraction: Double refraction caused by the parallel orientation of fibrillar or laminar shaped colloidal particles. This can be accomplished by causing a sol containing such particles to flow in one direction. This technique has already been successfully applied with clay dispersions in the study of flow. Surjace-active: Substances which reduce the surface tension of the medium in which they are dissolved (surface positive). Surface-inactive: Substances which increase the surface tension of the medium in which they are dissolved (surface negative). Suspensions: Systems composed of a liquid dispersion medium and a solid disperse phase with particle sizes above 0 . 2 ~ . Suspensoids: Disperse systems composed of a liquid dispersion medium and a solid disperse phase, the particles being of colloidal dimensions. Syneresis: The expulsion of liquid from a gel during aging, with resulting contraction. Tactoid: Location of parallel aligned, nonspherical particles of a colloidal tactosol. Tactosol: Sol with nonspherical particles, in which aging has caused parallel orientation in zones of high concentrations. Thixotropy: Transformation of gelatinous colloidal
systems into sols by shaking or stirring. When t,he movement is stopped, the system reverts to a gel., Twinkling phenomenon of colloids: Twinkling effect exhibited by nonspherical colloidal particles in the slit, ultramicroscope or in an ultramicroscope equipped ~vith an azimuth stop. Tyndall phenomenon: If a beam of light is passed through a colloidal sol and observed perpendicularly to the direction of illuminat.ion, a diffuse cone of light becomes observable. Uliracentrifuge: A centrifuge which permits the separation of colloidal particles from the dispersion medium. Ultrafilter: Filter consisting of membranes which do not permit passage of colloidally sized particles, but only of fully ionized chemicals, or filters of such porosity t.hat they permit stepwise fractionation of colloidal particles of different size. Ultrafiltration: Filtration of colloidal solutions through ultrafilter membranes which permit passage of water and molecularly dispersed substances but not of colloids. Ultramicroscope: Microscope used to study colloidal sols by applying the Tyndall phenomenon for illumination. Wetting: A solid surface can be completely, partially, or not at all wet,ted. The lower their surface tension the more readily will solutions cause wetting. Zeta potential: See electrokinetic potential.
