Colloids.pdf

Vol. 9

COLLOIDS

235

COLLOIDS

Introduction

1000 g/mol and

measurable by freezing point depression. Macroscopic particles fall into the realm

of classical physics and can be understood in terms of physical mechanics. Residing

between these extremes is the colloidal size range of particles whose small sizes

and high surface area-to-volume ratios make the properties of their surfaces very

important and lead to some unique physical properties. Their solutions may have

undetectable freezing point depressions, and their dispersions, even if very dilute,

may sediment out very slowly, and not be well described by Stokes’ law. Whereas

the particles of classical chemistry may have one or a few electrical charges, col-

loidal particles may carry thousands of charges. With such strong electrical forces,

complete dissociation is the rule rather than the exception. In addition, the elec-

tric ﬁelds can strongly inﬂuence the actions of neighboring particles. In industrial

practice it is very common to encounter problems associated with colloidal sized

particles, droplets, or bubbles.

The ﬁeld began to acquire its own identity when Graham coined the term col-

loid in 1861 (1–3). Since that time the language of colloid science has evolved con-

siderably (4–6) and makes two principal distinctions: lyophobic (thermodynam-

ically unstable) and lyophilic (thermodynamically stable) colloidal dispersions.

Examples of lyophobic and lyophilic colloidal dispersions are suspensions of gold

particles and surfactant micelles in solution, respectively. Colloidal particles (or

droplets or bubbles) are deﬁned as those having at least one dimension between

∼

m. In dealing with practical applications, the upper size limit is

frequently extended to tens or even hundreds of micrometers. For example, the

principles of colloid science can be usefully applied to emulsions whose droplets

exceed the 1-

m size limit by several orders of magnitude (ie, in cases for which the

surface properties of the dispersed phase dominate). Simple colloidal dispersions

are two-phase systems, comprising a dispersed phase of small particles, droplets,

or bubbles, and a dispersion medium (or dispersing phase) surrounding them.

In modern practice, the terms lyophilic and lyophobic (especially hydrophilic and

hydrophobic) are often used to characterize surfaces in addition to colloidal dis-

persions. This sometimes leads to confusing usage. For example, a clay dispersion

in water could be classiﬁed as a lyophobic colloid with hydrophilic surfaces.

Various types of colloidal dispersions occur, as illustrated in Table 1. In prac-

tice, many colloidal dispersions are more complex and are characterized by the

nature of the continuous phase and a primary dispersed phase, according to the

designations in Table 1.

One reason for the importance of colloidal systems is that they appear in

a wide variety of practical disciplines, products, and processes. The colloidal in-

volvement in a process may be desirable, as in the stabilizing of emulsions in

mayonnaise preparation, or undesirable, as in the tendency of very ﬁnely divided

and highly charged particles to resist settling and ﬁltration in water treatment

Matter of colloidal size, just above atomic dimensions, exhibits physicochemical

properties that differ from those of the constituent atoms or molecules yet are

also different from macroscopic material. The atoms and molecules of classical

chemistry are extremely small, usually having molar masses

1 nm and 1

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COLLOIDS

Vol. 9

Table 1. Types of Colloidal Dispersion

Dispersed Dispersion

phase

medium

Name

Examples

Liquid

Gas

Liquid aerosol

Fog, mist

Solid

Gas

Solid aerosol

Smoke, dust

Gas

Liquid

Foam

Soap suds

Liquid

Emulsion

Milk, mayonnaise

Solid

Liquid

Sol, suspension

Ink, paint, gel

Gas

Solid

Solid foam

Polystyrene foam, pumice stone

Liquid

Solid

Solid emulsion

Opal, pearl

Solid

Solid suspension Alloy, ruby-stained glass

plants. Examples of the variety of practical problems in colloid chemistry include

control of ﬁltration operations, breaking of emulsions, regulating foams, preparing

catalysts, managing ﬂuid ﬂow, and cleaning surfaces (see Table 2).

The variety of systems represented or suggested by Tables 1 and 2 under-

scores the fact that the problems associated with colloids are usually interdisci-

plinary in nature and that a broad scientiﬁc base is required to understand them

completely. A wealth of literature exists on the topic of colloidal dispersions, in-

cluding a range of basic reference texts (7–11), dictionaries (4–6,12), and treatises

on the myriad of applied aspects, of which only a few are cited here (13–24).

Preparation and Stability of Dispersions

Preparation. Colloidal dispersions can be formed either by nucleation with

subsequent growth or by subdivision processes (7,8,11,25,26). The nucleation pro-

cess requires a phase change, such as condensation of vapor to yield liquid or

solid, or precipitation from solution. Some mechanisms of such colloid formation

are listed in Table 3.

The subdivision process refers to the comminution of particles, droplets, or

bubbles into smaller sizes by applying high shearing forces, using devices such as

a propeller-style mixer, colloid mill, or ultrasound generator. A complex technol-

ogy has developed to conduct and to control comminution and size-fractionation

processes. Mathematical models are available to describe changes in particle size

distribution during comminution, but these are generally restricted to speciﬁc pro-

cesses. Comprehensive reviews of the developments in preparing colloidal solids

by subdivision should be consulted for further details (27,28). A wide range of

techniques is now available, including, eg, atomizers and nebulizers of various

designs used, to produce colloidal liquid or solid aerosols, and emulsions hav-

ing relatively narrow size distributions. Monosized powders and monodispersed

colloidal sols are frequently used in many products, eg, pigments, coatings, and

pharmaceuticals.

Colloidal suspensions of uniform chemical and phase composition, particle

size, and shape are now available for many elements (including sulfur, gold,

selenium, and silver, carbon, cobalt, and nickel), many inorganic compounds

Table 2. Some Occurrences of Colloids

Liquid

Solid

Field

aerosol

Foam

Emulsion

Suspension Solid foam

emulsion

suspension

Environment

and

meteorology

Fog, mist,

cloud,

smog

Volcanic

smoke,

dust,

smog

Polluted river

foams

Water/sewage

treatment

emulsions, oil

spill

emulsions

River

water,

glacial

runoff

Foods

Champagne,

soda and

beer heads,

whipped

cream,

meringue

Milk, butter,

mayonnaise,

cheese, cream

liqueurs

Jellies

Leavened

breads

Geology,

agriculture,

and soil

science

Crop sprays

Foam

fumigant,

insecticide

and

herbicide

blankets

Insecticides and

herbicides

Quicksand,

clay soil

suspen-

sions

Pumice stone,

zeolites

Opal, pearl Pearl

Manufacturing

and

materials

science

Paint sprays Sand

blasting

Foam frac-

tionation,

pulping

black liquor

foam

Polishes

Ink, gel,

paints,

ﬁber sus-

pensions

Polystyrene

foam,

polyurethane

foam

High

impact

plastics,

alkaline

battery

ﬁll

Stained glass,

ceramics,

cement,

plastics,

catalysts,

alloys,

composites

Biology and

medicine

Nasal

sprays

Airborne

pollen,

inhalant

drugs

Vacuoles,

insect

excretions,

contracep-

tive

foam

Soluble vitamin

and hormone

products,

biological

membranes,

blood

Liniment

suspen-

sions,

proteins,

viruses

Loofah plant

Wood, bone

Table 2. (Continued)

Liquid

Solid

Field

aerosol

Foam

Emulsion

Suspension Solid foam

emulsion

suspension

Petroleum

production

and mineral

processing

Reﬁnery

foams,

ﬂotation

froths, ﬁre

extinguish-

ing foams,

explosion

suppressant

foam

Oilﬁeld

emulsions,

asphalt

emulsion

Drilling

ﬂuids,

drill

cuttings,

mineral

slurries,

process

tailings

Oil

reservoir

Home and

personal care

products

Hair spray

Shampoo

suds,

shaving

cream, con-

traceptive

foams,

bubble bath

foam

Hair and skin

creams and

lotions

Sponges, carpet

underlay,

cellular foam

insulation

Bakelite

products

Vol. 9

COLLOIDS

239

Table 3. Industrially Produced Colloidal Materials and Related Processes

Mechanism

Examples a

Vapor

→

liquid

→

solid

↓→→→↑

Oxides, carbides via high intensity

arc; metallic powders via vaccum

or catalytic reactions

Vapor

vapor

→

solid

Chemical vapor deposition,

radio-frequency-induced plasma,

laser-induced precipitation

Liquid → solid

Ferrites, titanates, aluminates,

zirconates, molybdates via

precipitation

Solid → solid

Oxides, carbides via thermal

decomposition

a Refs. 27 and 28.

(including halide salts, sulfates, oxides, hydroxides, and sulﬁdes), and many

organic compounds [including, poly(vinyl acetate), polystyrene, poly(vinyl chlo-

ride), styrene–butadiene rubber, poly(acrylic acid), polyurea, poly-styrene–

poly(acrylate), and poly(methacrylate)–poly(acrylate)]. (see V INYL A CETATE P OLY -

MERS ;S TYRENE P OLYMERS ;V INYL C HLORIDE P OLYMERS ;S TYRENE -B UTADIENE C OPOLY -

MERS ;A CRYLIC E STER P OLYMERS ;M ETHACRYLIC E STER P OLYMERS ;A CRYLIC ( AND

M ETHACRYLIC )A CID P OLYMERS ).

Stability. A complete characterization of colloid stability requires con-

sideration of the different processes through which dispersed species can en-

counter each other: sedimentation (creaming), aggregation, and coalescence. Sed-

imentation results from a density difference between the dispersed and contin-

uous phases and produces two separate layers of dispersion that have differ-

ent dispersed-phase concentrations. One of the layers will contain an enhanced

concentration of dispersed phase, which may promote aggregation. Aggregation is

when two or more dispersed species clump together under the inﬂuence of Brown-

ian motion, sedimentation, or stirring, possibly touching at some points, and with

virtually no change in total surface area. Aggregation is sometimes referred to as

ﬂocculation or coagulation (although in speciﬁc situations these latter terms can

have slightly different meanings). In aggregation, the species retain their identity

but lose their kinetic independence since the aggregate moves as a single unit. Ag-

gregation of droplets may lead to coalescence and the formation of larger droplets

until the phases become separated. In coalescence thin ﬁlm drainage occurs, lead-

ing to rupture of the separating ﬁlm, and two or more particles, droplets, or bubbles

fuse together to form a single larger unit, reducing the total surface area. In this

case the original species lose their identity and become part of a new species. In

emulsions, inversion can take place, in which the emulsion suddenly changes form,

from oil-in-water (O/W) to water-in-oil (W/O), or vice versa. For example, butter

results from the creaming, breaking, and inversion of emulsiﬁed fat droplets in

milk. Kinetic stability can thus have different meanings. A colloidal dispersion

can be kinetically stable with respect to coalescence but unstable with respect to

aggregation. Or, a system could be kinetically stable with respect to aggregation

but unstable with respect to sedimentation. In summary, lyophobic colloids are

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