Colloids
There should be a more exciting name for the phenomena to be
discussed here. The dull term colloid that reminds us of glue
is, nevertheless, the accepted word. It was coined in the early 19th century by
the Father of Physical Chemistry, Thomas Graham (1805-1869), to
distinguish those materials in aqueous solution that would not pass through a parchment membrane from those that would. Glue was indeed a material that would not, and the Greek for glue is kolla, from which we also get "protocol" and "collagen." Those that would pass through were things like salt, and other soluble crystalline substances, which Graham calledcrystalloids. As we shall see, the field is much, much richer than this.
distinguish those materials in aqueous solution that would not pass through a parchment membrane from those that would. Glue was indeed a material that would not, and the Greek for glue is kolla, from which we also get "protocol" and "collagen." Those that would pass through were things like salt, and other soluble crystalline substances, which Graham calledcrystalloids. As we shall see, the field is much, much richer than this.
Colloids received little attention until the end of the century,
when van't Hoff, Oswald and Nernst founded modern physical chemistry and they,
and others, became fascinated by colloid phenomena. There had been famous
observations by Tyndall and others in the meantime, but chemists could not get
excited over glue. Then, in the 1920's and 1930's, the importance of colloids
to industrial processes and biochemistry changed everything. Colloids became a
hot field, and soon every elementary textbook said something about them. In
writing some of the other articles on the site recently, I became gradually
aware of the fascination of colloids, and recognized that my knowledge of them
was very deficient. This article is the result, and I hope it will serve as an
introduction to what colloids are all about, and demonstrate how interesting
and useful they are.
An interesting philosophical point was suggested by this study.
Colloids are often called a "fourth state of matter," and I wondered
just how meaningful this concept is. We shall find that it is very difficult to
encapsulate any concept concerning colloids in a word, though heaven knows
chemists have tried, and many words have been coined. It is necessary to name
things to think about them efficiently, and one thing scientists have done
assiduously is to assign names. Biology comes to mind, with endless terms and
names based only on surface appearances, at least until recently. Naming gives
the appearance of knowledge, where there is no real knowledge at all. The
antithesis to mere naming is mathematical analysis, which gives real
conclusions and effective knowledge. The danger of names comes when they are
regarded as real things and are used to delimit instead of simply to denote and
describe.
It is easy to recognize the three conventional states of matter in
ice, water and steam. The names solid, liquid and gas can be attached to
certain suites of properties, and makes a useful distinction. In a gas,
particles of the substance move freely and have to be stopped by walls. In a
liquid, the particles are sometimes associated, sometimes not, but always
occupy a certain volume. In a solid, the particles cannot move far relatively,
and can only vibrate. Many substances can be classified by these properties,
but the terms do not separate matter into three mutually exclusive boxes, and
may not be descriptive enough. Where is tar, for example, or jelly, or a
substance above its critical point? Colloids will give many examples of
substances for which the simple classification into three states is wholly
inadequate.
The properties of matter depend almost completely on its
structure. All metals are alike, to a good approximation. They are shiny, soft,
tough substances that conduct electricity. All ionic crystals are alike
(granted differences in crystal symmetry). They are hard, transparent and do
not conduct electricity in the solid state. They can usually be crushed into
white powders. The variations between metals, or between ionic crystals, are
very much less important than their similarities. Saying that a substance is a
metal, or an ionic crystal, says much more than simply that it is a solid.
Solidity is only a macroscopic appearance, of no fundamental significance, like
being green.
I have seen the definition of matter as "that which occupies
space." But what about gases? They occupy space, of course, but two or
more gases can occupy the same space, as far as appearances
go. The important thing is to use terms like solid, liquid, gas only as far as
they are useful descriptions, and not consider them as exclusive classifications
into which everything must fit. To see that this is not trivial, consider the
many sciences (not generally chemistry or physics) in which there are bitter
controversies about which named category to assign to some object or process.
We should not be limited by the arbitrary names we give our concepts.
I recalled that colloids were particles larger than molecules, but
smaller than grains of sand. This is true, and colloidal dimensions can be
considered to be from about 10 nm up to 1000 nm, or 1 μm, but mere size is not
the important thing about colloids. The overwhelmingly important property of
colloids is that they have very large surface area. To some degree, they
are all surface and their properties are those of their
surfaces. I do not remember appreciating this properly before, but I can assure
you of its significance. Incidentally, 1 μm is about the size of a bacterium. I
shall use the word "colloid" to refer to a substance of colloidal
dimensions, or to a colloidal system, indifferently.
To
see the significance of this observation, consider the cubic centimeter in the
diagram at the right. In this form, it has an area of 0.0006 m2. We
could say that it is almost all volume. Most of its molecules are safely
resident behind its surface, secure from disturbance or attack. Let us now
divide it into thin laminae, 10 nm thick, a colloidal distance. The cube
becomes a million laminae, with a total surface area of 200 m2.
Every molecule is now only a short distance from the cold outdoors, and the
material is all surface. We have turned the mass cube into a laminated
colloid by this delicate slicing alone.
Continuing, we now slice each of the million laminae into a
million fibers, and the surface area doubles. We still have a colloid, of
course, with two dimensions colloidal, but have not increased the area greatly,
not as we did in the first slicing. We can expect fibrillar colloids
as well as laminar ones. Finally, each fiber is chopped into a million bits,
giving a corpuscular colloid. This increases the surface area
only by 50%, to 600 m2. From the mass to the corpuscle, the surface
area has been increased by a factor of a million, which is typical of a
colloid. Note that most of this increase came with the first dimension to
"go colloidal," so we can call anything with any least dimension of
colloidal size to be a colloid. This was another thing that I did not
appreciate in my ignorance.
The large area emphasizes surface effects relative to volume
effects, giving colloids different properties than those of bulk matter. The
surface tension of a liquid is the free energy required per unit area to create
new surface. For water against air, its value at 20°C is 72.75 dyne/cm. For
mercury against air, it is 450 dyne/cm. For mercury against water, it is 375
dyne/cm, not surprisingly close to the difference of the other two values. The
ratio of surface to volume for a sphere of diameter d, or a cube of side d, is
6/d. The surface energy of small drops will strongly affect their properties.
If a cubic centimeter of water were divided into 10 nm cubes as above, the
coalescence of the cubes would release enough surface energy to heat the water
by about 10°C. A cubic centimeter of mercury would be warmed by 143°C by the
same procedure.
It is better to define a colloid as a system in which the surface
area is large and in which surface effects are predominant, rather than simply
in terms of particle size. Indeed, in foams there are no colloidal particles at
all--it is the thinness of the films that creates the colloidal behavior.
Similarly, in a gel the fibrous structure is what is colloidal. In any
colloidal system, there must be at least one structural dimension of colloidal
size in order for the large surface area to exist in a limited volume, however.
This broadened definition of colloid is not only reasonable, it is useful. A
colloid is a material system that is mainly surface.
The next important characteristic is that a colloid is a two-phase (at
least) system. A phase is a homogeneous component of a system,
in the sense of the Phase Rule. The Gibbs Phase Rule applies to systems in
which the phases have negligible surface energy, which is perfectly applicable
to phases that are "all volume" as our centimeter cube was, or even
to phases of microscopic dimensions (larger than 1μm). It does not apply to the
system of colloid phases, in which surface energy predominates. Therefore, be
careful when applying the Phase Rule to colloidal systems.
A colloidal system consists of an internal phase,
which is the material of colloidal dimensions, and an external phase,
which is the material in which the colloid is dispersed. These designations are
analagous to the terms solute and solvent used
for simple solutions (which form a single Gibbs phase). As the particles of a
corpuscular colloid become smaller and smaller, we go over imperceptibly from a
two-phase colloid to a single-phase solution, and there is no definite
boundary. This gives a hint as to why I discussed names and their significance
in the introductory paragraph.
The colloidal system that is most similar to a simple solution is
a dispersion of corpuscles, or particles, in a liquid. This is called a sol,
and the liquid is the external phase. This is the classical colloid as
described by Graham. If the external phase is a solid instead of a liquid, the
system is called a solid sol. The only difference is the mobility
of the molecules. In a solid sol, they can move only by diffusion. If the
external phase is a gas, usually air, instead of a liquid, we have an aerosol.
There is no definite boundary between a sol and a solution, but still they are
significantly different. There is also no definite boundary between a sol and a
coarse suspension. A coarse suspension will settle out rapidly, while a sol may
be permanent.
The particles that appear in a sol may be wetted by the liquid, or
may not. Wetting is a typical surface effect, and so is of paramount importance
in a colloid system. In the first case, the liquid is adsorbed on
the surface of the particle. The terms adsorb and absorb sound
alike, but are quite different. A substance that is absorbed is taken into the
volume of the absorbing substance, like water into sand. If is is adsorbed, it
attaches itself only to the surface. Since colloids are all surface, as we have
pointed out, adsorption is what is important with them. If the particle adsorbs
the external phase, it is called lyophilic, or hydrophilic,
if the external phase is water. The Greek verb "luo" means to
dissolve or destroy, and philic is from "philos," love. A lyophilic
colloid "loves the external phase." On the other hand, if the
particle does not adsorb the external phase, is is said to be lyophobic,
or "fears the external phase."
Sols that contain inorganic particles, such as metals, are mostly
lyophobic, as are most aerosols and solid sols. Lyophobic hydrosols are a very
common kind of colloid, and deserve detailed description. For example, consider
the hydrosol of gold with particles about 4 nm in size. This was one of the
first sols studied extensively, and has interesting properties. With about 0.1%
gold, the sol is a rich ruby red. The similar solid sol in glass makes ruby
glass. The gold particles absorb strongly in the green and blue, so the
transmitted light is red. There is a little yellow-green scattered light, but
mostly it is a case of absorption by the gold metal. If the gold particles
clump together, which they may do as time passes, the color of the solution
changes. When the particles are about 40 nm in diameter, the solution is blue,
with considerable scattered light. If the particles agglomerate further, the
color disappears and gold flakes settle out.
Bacteria, which are about 1 μm in diameter, can be suspended in
water to form a sol, which has all the classic properties. The Brownian motion,
the Tyndall effect (turbidity), and even electrophoresis are seen. The bacteria
act as a hydrophobic sol, peptized by their electrostatic charge. The
properties of a sol are largely independent of the nature of the internal
phase.
There are two interesting questions here about the stability of
the sol. First, what keeps the gold suspended in the red solution so that the
tiny particles do not settle out? Second, how are the particles kept from
agglomerating? Let's take the first question first. The gold particles fall
under gravity through the water. The terminal velocity v of their fall is given
by Stokes's equation, (mg - m'g) = 6πηav, where m is the mass of the particle,
a its radius, m' the mass of the water displaced, and η is the viscosity of
water (1.002 centipoise at 20°C). A correction factor (1 + Kλ/a) must be
applied for the small particles, where K is a constant, λ is the mean free path
of the liquid molecules (this factor really applies better to aerosols), and a
is the radius of the particle. On the other hand, the particles are subject to
the bombardment of the molecules of the liquid, which produces the Brownian
movement. For a sufficiently small particle, the upward diffusion produced by
the Brownian movement overcomes the gravitational fall, and an equilibrium is
reached, much like the equilibrium of gases in the atmosphere. Perrin first
verified this effect, although it is quite complicated in this case, the
gas-like effect only occurring close to the upper surface of the sol. There is
a critical sizefor a particle, below which it will not settle out.
At any rate, this is why colloidal-size particles do not settle out of a sol.
Now for the second question. Generally, if two colloidal particles
collide, they will stick together and make a bigger particle, because it is
usually favored by energy. Eventually, the particles get larger than the
critical size to be suspended by the Brownian movement, and they settle out.
There must be some good reason if this is not to happen. In most lyophobic
colloids, the particles are electrically charged with the same sign, and this
keeps them apart, since they repel one another. The particles are charged
mainly because they adsorb certain ions in the environment. In water, they may
be OH- ions, which are generally present, and give the
particles a negative charge. The H+ ions are hydrated, so are
not as easy to adsorb, but apparently some particles like them, and become
positively charged. If you try to make a hydrosol with particles of opposite
charges, they neutralize each other and the sol collapses. Since lyophobic sols
are stabilized by electric charge, adding electrolytes generally destroys the
sol. When rivers reach the sea with their loads of colloidal sediment, the ions
in sea water coagulate the sol, and the load is deposited in the delta. There are
other ways for a particle to become charged. A zinc particle, for example, may
become negative when some zinc dissolves according to Zn → Zn++ =
2e-, and the electrons remain on the particle.
How the charges are distributed is an interesting question. The
sol appears electrically neutral on the large scale. The particle with its
adsorbed charges is called a granule. The charges are in a thin
layer on the surface. They attract an atmosphere of opposite charges from the
external phase, just as an electron in a plasma surrounds itself with a
shielding positive charge by attracting positive ions and repelling electrons.
The whole neutral structure, granule plus mobile external charge, is called
a micelle. This, then, is what moves around, the charged particle and
its cloud of opposite charge. When we apply an electric field to the sol, the
cloud of charge is moved in one direction, and the granule moves in the other.
There is a local viscous flow about the granule, and the micelle moves toward
the anode, if the granule is positive, or toward the cathode, if it is
negative. This movement is called electrophoresis, and can be
practically useful. The particles of a sol do not repel one another until they
come quite close, and their micelles overlap, because of the shielding.
The mobility of the colloidal particle is its
velocity in a unit field. Helmholtz developed a formula for the mobility, M =
ζκ/4πη. Here, κ is the dielectric constant of the external phase, η its
viscosity, and ζ is the potential difference across the micelle from the
outside to the adsorbed charges on the particle, that is, through the fluid
that is sliding around the granule. From measurements of M, it is possible to
find ζ, which becomes a kind of "fudge factor" since it is hard to
calculate. For the small gold particles in the red sol, M = 4 x 10-4 cm2/s-V,
and ζ = -0.058 V. The negative sign indicates that the gold migrates to the
anode, and so has a negative charge. The Helmholtz equation gives M = 4.15 x 10-4,
which doesn't prove much except that we know how to use the equation. Since the
equation is written in esu, we must divide by (300)2 to convert
volt to statvolt.
In order to create a lyophobic sol, we must either reduce a mass
to colloidal size, called dispersion, or we must build the colloidal
particles from molecules, called condensation. In either case, a
third substance, apeptizing agent, may have to be added to
stabilize the sol. This agent can supply ions that will be adsorbed on the
particles resulting from dispersion or condensation to give them a stabilizing
charge. For clays, the OH- ion is a peptizing agent, which can
be supplied by alkalis. Dispersion can be done mechanically, in a colloid
mill that grinds the substance into small, equal particles. Another
method is with an electric arc. Metal electrodes are used, at a current of
5-10A and voltage of 30-40V. Bredig made particles of about 40 nm by this
method, and it was improved by Svedburg to obtain sols of many metals down to 5
nm particle size. Ultrasonics can also be used to disperse sols.
When a sol is created in a nonpolar solvent, the particles may not
be charged (and must be stabilized by some other means). They do not then
exhibit electrophoresis or other electrical phenomena depending on granule
charges.
A homogeneous phase or solution does not disturb the propagation
of light, except to change its phase velocity to v = c/n, where c is the speed
of light in vacuum and n is the index of refraction. In a gas, density
fluctuations that are a natural result of the free movement of the molecules
can scatter light. Scattering is the emission of light in all
directions, which decreases the intensity of the ordered beam. This Rayleigh
scattering is proportional to the inverse fourth power of the wavelength, so
blue light is scattered more than red, giving the blue color of the clear sky.
Scattering should be distinguished from absorption, which is the
conversion of the energy of the light to other forms. Both scattering and
absorption cause attenuation of the light beam. The
transmitted beam and the scattered light may be colored if the scattering or
absorption is not constant with wavelength. The blue sky and the orange sunset
colors have the same cause, Rayleigh scattering by density fluctuations.
A colloidal system contains particles that affect a light beam by
scattering and absorption. If the particles are of a size comparable to the
wavelength of light or larger, they scatter or absorb light independently. The
same thing happens if they are separated by distances comparable to or greater
than the wavelength of light. The wavelength of visible light is 400-700 nm,
with the maximum sensitivity at 555 nm. This is in the middle of the range of
colloidal dimensions, so colloids can be expected to have significant effects
on a light beam.
One
common effect of colloids is turbidity, an effect like that of
stirring up mud in water. Slight turbidity may not be noticed until a beam of
light passes through the colloid. Colloidal systems need not be turbid: a gel
may be quite transparent when the particles are small. Solutions, as
homogeneous phases, are not turbid. The turbidity causes scattering so that the
path of the light beam can be clearly seen. This is called the Tyndall
effect, and the observed scattered light is called the Tyndall cone.
John Tyndall (1820-1893) was Faraday's successor at the Royal Institution. He
suggested that the blue sky was caused by scattering by dust particles, but
Rayleigh later found the true cause, showing that the sky would be blue even if
the air were pure. The scattered light is polarized perpendicularly to the
direction of the beam if the particles are approximately spherical and small.
If the particles are nonspherical, or larger than a wavelength, the scattered
light will be partially polarized.
The Tyndall effect is a common atmospheric phenomenon.
Searchlights produce Tyndall cones in slightly hazy air. Smoke is often blue
when seen in scattered light, orange in transmitted light. This was quite clear
in the summer of 2002, when forest fires near Denver put smoke in the air, and
the sunlight became a strange reddish color. The crepuscular rays seen at
sunset, which seem to radiate from the position of the sun, are parallel
Tyndall cones. A laser beam may make a distinct Tyndall cone in dusty air. If
you have a piece of polarizing filter, try to determine the polarization of any
Tyndall cones you may observe. All the Tyndall cones that you see are evidence
of lyophobic sols.
If the sol is composed of transparent particles with an index of
refraction considerably different from that of the external phase, and they are
present in sufficient concentration, the sol wil be come opaque and white, the
limit of turbidity. If the particles have the same index of refraction, the
Tyndall effect will be small. This property is used in enamels, which are
opaque glasses fused onto a metal substrate, and in pottery glazes. Stannic
oxide, for example, gives a white enamel or glaze.
A beam of bright sunlight entering through a window may be marked
with moving bright specks that are light scattered from colloidal dust grains
that are always present in the air. The grains themselves, which may be
submicroscopic, are not seen, only the light they scatter. This principle is
used in the ultramicroscope that allows individual, submicroscopic
particles to be observed. Only the light from them is detected against a dark
background, so that they can be counted and their motion observed; no image of
the particles can be formed. The instrument consists of a bright light source,
a slit and optics to focus the slit on the sample, and a microscope. A thin
slice of the sample is illuminated by the slit. By turning the slit 90° the
depth of the area viewed can be determined. The direction of viewing is at
right angles to the illumination. Some ultramicroscopes are coaxial and use a
different illumination method. If the number of particles per unit volume is
found in this way, by counting using a squared graticule, and the weight of
colloid per unit volume is known, then the size of the particles can be
determined.
Colloids can produce color. The red of a gold sol or ruby glass
has already been mentioned, where the color is due to the wavelength dependence
of scattering and absorption. Color can also be produced by the interference of
white light, especially if there are thin films or a periodic regularity in the
density. The colors in thin oil films are familiar, and the films are, of
course, colloidal in thickness. Color produced by such means is called structural.
Color produced by absorption by colored pigments is called pigmental.
The blue color seen in blue eyes and birds' feathers is structural, caused by
scattering by fibers. Brown pigments in the iris modify the blue color to
green, then overwhelm it to make brown eyes. Much color in the insect world is
structural, such as the colors of a butterfly's wings, often combined with
pigmental color to produce a great variety. The greenish colors of crude oil
and its products are a result of colloidal suspensions.
An aerosol of colloidal solid particles may be called a smoke,
while if the particles are liquid, it is a fog. Sometimes the two
are combined, in a suspension of solid particles with an adsorbed liquid film
on the surface. The original smog was a smoke with a liquid
film of sulphuric acid, which made it excessively unpleasant to breathe. The
Great Smog occurred in London on 5 December 1952, killing nearly 4000 people
who had respiratory problems, and stimulating clean air legislation. Now it
usually lacks the smoke, and is a fog of some unpleasant liquid coming from
motorcars instead. Aerosols have the usual characteristics of lyophobic sols: a
strong Brownian movement, the scattered blue light of the Tyndall effect, and
stabilization with particle charges.
Clouds are aerosols of water particles, supported by the Brownian
motion like any sol. The droplets are produced by condensation of water vapor
on condensation nuclei, which are usually hygroscopic particles of
dust, or positive ions. The mist above splashing water is positively charged,
with compensating negative charge in the form of unhydrated negative ions. For
condensation to occur, the air must be supersaturated for water vapor. The
radius of the droplets is large for colloidal particles, and they are often
supported by updrafts more than by Brownian motion. The radius of cirrus cloud
droplets may be 2 μm. When they reach a radius of about 0.04 mm, the droplets
fall as rain, often coalescing with others. The largest raindrops have a radius
of about 3.6 mm, and such large drops are rare.
Raindrops can be blown upwards into freezing air by updrafts,
gathering water by coalescence with others, and freezing to a solid ball. This
can happen repeatedly, forming large hailstones that eventually fall. The
largest hail reported may be the 3" diameter hail that fell in
Bloomington, Ind. in 1917. A farmer was killed by hail near Lubbock, Texas in
the 1930's, but this is the only reported fatality.
Saturated air can be expanded adiabatically to cool it and achieve
supersaturation. This can be done in the laboratory with the Wilson cloud
chamber with an adiabatic expansion against a piston or the equivalent, which
does work that cools the air. The cooling is given by T2/T1 =
(V1/V2)k - 1, where k is the ratio of the
specific heat at constant pressure to the specific heat at constant volume (1.4
for air). In clean air, condensation occurs for a volume ratio greater than
1.25. From 1.25 to 1.34, the condensation falls as fine rain. For 1.25 to 1.28
the condensation is on negative ions, and the particle radius is 200μm. For
1.28 to 1.34 the condensation is on the positive ions, and the particle radius
is 20μm. For larger expansion ratios, the mists can be colored: 1.408 gives
green, 1.422 purple, and 1.429 red.
At the lower expansion ratios, condensation is difficult enough
that it may occur along the tracks of alpha particles, which ionize strongly,
making the tracks visible. The condensation soon settles out, since the
droplets are large. The chamber can work with alcohol vapor as well as water vapor.
The vapor pressure p' of a small drop of radius r is greater than
the vapor pressure p of a plane surface of the liquid. The vapor pressure p' is
given by ln (p'/p) = (1/RTd)(2γ/r - q2/8πr4), if q is the
charge density on the surface. γ is the surface tension of the liquid. This
shows that a charge of either sign will stabilize a small droplet against
evaporation. In the absence of a charge, small droplets will evaporate in favor
of large drops. Electric charges may explain the stability of clouds, and the
fact that rain may not fall from them.
An aerosol of starch grains, with a density of 100g/m3,
has a very large surface area, and adsorbs O2 from the
atmosphere. Starch grains in flour are from 5 nm to 200 nm in diameter. It is
no wonder that it is a violet explosive. Coal ground to pass through a 200-300
mesh sieve can be blown directly into a fire as a convenient fuel. It can also
make a sol with fuel oil to form a colloidal fuel that can be
used exactly like fuel oil. Coal dust is reputed to be a fire hazard, but it is
not as dangerous as starch dust. Many "coal dust" explosions may have
another cause.
Smudge pots are used to protect agricultural plants from frost.
They are small fires producing dense smoke. The heat produced by the fires is
probably one of the most important effects. The smoke may provide condensation
nuclei for the dews of evening, increasing the heat by the latent heat of the
condensed vapor. The aerosol blanket slows loss of heat by radiation, which can
be very important into a clear night sky.
Smoke is also used for military purposes, for concealment or
signalling. White phosphorus burns to hygroscopic P2O5,
which forms a dense white fog in humid air. Silicon tetrachloride mixed with
ammonium hydroxide gives a dense white smoke of ammonium chloride and
metasilicic acid. This smoke is used in skywriting, since it is easily made
from liquids when needed. The navy's smoke for concealment was made by
restricting the air supply to the boilers, producing thick black smoke from the
funnels. Radar and the end of gunnery has rendered smoke screens useless.
Colored smoke may be used for signalling.
An emulsion is a colloidal system in which both
phases are liquid. If the liquids were miscible, they would form a solution, so
emulsions are lyophobic colloids. The typical example is water and oil. The
internal phase is determined by which component has the higher surface tension.
This component will form spherical bubbles immersed in the other, which will be
a continuous phase. The granules of an emulsion may be large, even microscopic.
An emulsifying agent is usually required to form a stable emulsion. The emulsifying
agent, or protective colloid, is surface-active,
meaning that it reduces the surface tension of the liquid, and so tends to
concentrate in boundary films. In the case of water and oil, sodium oleate,
a soap reduces the water surface tension and raises that of
the oil, so that the emulsion will be oil droplets in water, and quite stable.
There are many other detergents, but sodium oleate will serve as a
good example.
Oleic acid is a fatty acid with the formula CH3(CH2)7CH=CH(CH2)7COOH,
a monounsaturated carboxylic acid with a long hydrocarbon chain. Fats are
glyceryl esters of this acid. Glycerol has three OH groups, each of which can
take the H off the end of the oleic acid and stick the rest to the glycerol
framework, making a triglyceride. Boiling the fat with NaOH makes
three molecules of sodium oleate, a liquid soap. The hydrocarbon chain nestles
up to the oil, the sodium to the water, and peptizes the oil. The emulsion can
be made by simply shaking oil, water and soap together, but this will not make
droplets of a uniform size. The larger droplets may "cream off" in
this case, and float to the surface of the emulsion. The emulsion may be homogenized by
blowing it through small orifices under a pressure of 4000 to 5000 psi. The
small, uniform drops that result will make a stable, long-lasting emulsion.
Milk is an emulsion of oil ("butterfat") in a watery sol
of the hydrophilic protein casein in which the external phase is a solution of
lactose and various salts. Milk will "cream" unless homogenized, since
the fat globules range from 100 nm to 22 μm in diameter. Cream can contain from
29% to 56% fat, in packed globules stabilized by casein. Human milk contains
albumin in addition to casein. Mayonnaise is an emulsion of oil in water, using
egg yolks as an emulsifying agent. Hollandaise sauce is another emulsion, of
butter in lemon juice, again using egg yolks as an emulsifying agent. Butter
itself is an emulsion, this time of water droplets in oil. The cosmetic
"vanishing cream" is an oil-in-water emulsion. If this is reversed,
"nourishing cream" is a water-in-oil emulsion. Crude oil as it comes
from the well is often emulsified with water. In this case, heating is
sufficient to "break" the emulsion, and separate the oil from the
salt water. Many insecticides are oil-in-water emulsions for spraying. The oil
wets the oily leaf surfaces and sticks, while the water carrying the poison
evaporates. Lubricating grease is a water-in-oil emulsion. The emulsifying
agent, calcium oleate, is soluble in oil, not in water, and so makes the water
the internal phase. The idea here is to make the lubricant stiff, so it will
not drip off.
Whether a component of an emulsion is the internal or external
phase is determined by the relative surface tensions, not be the amounts of the
components. Equal-sized globules can close-pack like spheres to occupy 74% of
the volume. The internal phase can be even higher in concentration if the
globules are not all the same size, so the smaller can huddle in the voids left
by the larger. Because of the presence of an emulsifying agent, emulsions are
very stable, and "breaking" them when required can be difficult.
To find out whether an emulsion is oil-in-water or water-in-oil,
the effect of adding a small amount of either oil or water to a sample of the
emulsion on a microscope slide is observed. If you add the external phase, it
will mix easily and quickly, but the internal phase will not mix and remain a
drop. This is called the dilution test.
Foams
A foam may be an internal phase of gas in an external phase of
liquid or solid. In a liquid foam, a colloidal adsorptive agent forms a film
that bounds the gas bubble. Bubbles blown with soap solution are related to
foams, but are quite large and have an independent existence. Smaller bubbles
in a mass form the more usual foam. People washing dishes or clothes are
reassured by a thick layer of soap foam on the water, showing that there is
still detergent left to emulsify additional oil. Such foams are mainly air,
with very little liquid. The colloidal dimension in a foam is the thickness of
the film, not the size of the bubble. The bubble is lighter than its
surroundings, and will rise to the top, where it joins the foam. In beer, the
foam is stabilized by albumin and by the hop resins added to the beer. When
carbon dioxide is released, it uses the adsorptive agent it finds to make the
foam. This "head" on Guinness stout is famous and creamy--and
carefully engineered. Meringue is a dried foam using egg albumin. Marshmallows
use sugared gelatin for the same purpose.
Ore flotation depends on the property of the adsorptive agent to
wet the valuable metallic sulphides or other ore, but not the silicates, which
are preferentially wetted by water. Note again the central role played by
surface chemistry. A froth is made with water and the adsorptive agent, and
mixed with the ore to be beneficiated. The froth floats to the surface, where
it is skimmed off, together with the enriched ore. In a few special cases, the
valuable mineral is wetted by the water, while the gangue sticks to the oil. We
use the buoyancy of the bubbles to effect the separation.
A fire-fighting foam is made from mixing water, aluminium sulphate
and sodium bicarbonate with an adsorptive agent. The carbon dioxide that is
released makes a dry foam, while the other ingredients form a kind of gel. This
foam can be used on all kinds of fires, including burning oils.
Examples of solid foams are pumice, meerschaum, and Ivory® soap.
The white soap has colloidal air beaten into it so that it will float.
Meerschaum, "sea foam" in German, is a light-colored metamorphic rock
associated with serpentine, and is a magnesium silicate, also known as sepiolite from
its resemblance to light cuttlefish (sepia) bone. It is soft, smooth, light and
translucent, used mainly for carving smoking pipes. It is fibrous and porous,
with gaseous inclusions, apparently a dried gel. It is so porous that it floats
on water, in spite of its mass density of 2.0 g/cc. Pumice, an extrusive
igneous rock, is a solidifed foam of volcanic glass, usually obsidian. It also
floats on water, and makes a good gentle abrasive. Diatomaceous earth consists
of the microscopic shells of diatoms, very common marine plants, made of
opaline silica and very porous. It adsorbs nitroglycerine to make dynamite,
rendering it much less sensitive and much safer to handle. Bread is, of course,
a dried foam as well. The protein gluten makes the film that
surrounds the CO2bubbles produced by the enzyme zymase secreted by
the yeast. Zymase acts on the sugars (hexose) produced from starch by other
enzymes, and also makes ethyl alcohol at the same time as the carbon dioxide.
The alcohol is often the desired product! Hydrophilic gluten and water make a
good gel in "strong" flours that include as much CO2 as
possible and hold the starch granules. Rye flour has little gluten, and will
not make light bread by itself. Foams aid digestion by providing as large a
surface area as possible.
So far we have looked at lyophobic colloids, which will have
nothing to do with the other phase. A lyophilic colloid, by contrast, actively
seeks out the other phase and adsorbs it strongly. In most cases, the other
phase is water, so we are dealing with hydrophilic colloids. Aluminum hydroxide
and orthosilicic acid are inorganic examples. Most organic colloids are
hydrophilic. Gelatin is of animal origin, derived from protein, made by boiling
bones and horny parts. Gums are of vegetable origin. Gums are branching
polysaccharides, soluble in water. They include the mucilages carrageenan
(from Irish moss, a seaweed) and agar-agar (also from seaweed), which are
sulphate esters. Gum arabic is from the acacia tree, and gum mastic is from
the Pistacia lentiscus. Gums are secreted by trees to protect and
seal wounds. When you find any of these in a list of food ingredients, they are
for the purpose of making a gel. Ice cream may contain guar gum, cellulose gum,
locust bean gum and carrageenan. These gums are not digestible.
A resin is similar to a gum in purpose, but is
insoluble in water. Resins are notably secreted by evergreens, and are terpene
derivatives, soluble in turpentine and similar solvents. Rosin is the residue
when turpentine is distilled. Amber and copal are other natural resins, while
alkyd and phenolic resins are artificial, used in plastics and paint.
Gelatin or gums dissolve in hot water. As the clear colloidal
system cools, its viscosity increases steadily. The viscosity of the external
phase is not affected in hydrophobic sols, by distinction. At some point, the
system gels, forming a wobbly but definitely solid body. This is
really an extraordinary thing to happen. If you heat the gel, it will melt and
form a viscous liquid. On cooling, it will gel again. If you dry it out, it
will shrink and look horrible. On adding water, it will plump up again into a
wiggly gel. These colloids are called reversible or elastic.
The gels formed from inorganic hydroxides will not reform a gel once they have
dried out, and the dry form will be brittle. The pore spaces will still be
there, however, and will absorb moisture and other substances. Silica gel is a
widely used substance. Though it can be renewed by heating and live to absorb
again, it will never again be wobbly and gelatinous. I say absorb, since it
will appear to be this, but on a microscopic scale it is still adsorption, of
course.
Gels are used as culture media for microorganisms. Gelatin was
originally used, but it melts at 37°C, and so cannot be used to study microorganisms
at body temperature. Agar-agar makes a gel that can stand higher temperatures,
so it is used in preference as a culture medium, poured into the familiar Petri
dish. It was first used by Robert Koch.
The colloidal phase in a gelatin is fibrillar, composed of fibers
of colloidal cross-section. When a gel sets, these fibers form a tangled mass
like a pile of brush, that holds the system together. Droplets in a gel are
lens-shaped, showing the packing. There has been quite a bit of controversy
over the structure of gels, but the fibrillar structure seems correct. The
original idea of a cellular structure is untenable. The fibers adsorb large
quantities of water, and there are also droplets of water, so that the gel is
mainly water, given a doubtful rigidity by the stacks of fibers.
Pectin is a gelatin-like protein substance found in the rind of
citrus fruit, in apples and generally in fruits. If a slightly acid solution of
pectin is made 65% or 70% sucrose, it will gel. This is the reaction used to
make jams. The verb pectize is used to describe the creation
of a gel, as peptize is to create a sol.
Gels have some curious properties. As they age, syneresis may
occur, which is the loss of liquid. This is a result of the closer
agglomeration of the colloid. The reduced active area requires less water, so
the excess water is eliminated. It does not mean that the gel is deteriorating.
Another property is thixotropy. On agitation, the gel becomes
fluid, but reverts to the gel when left alone. Fresh gelatin gels are quite
thixotropic, and advantage can be taken of this to add ingredients to a jelly.
The "thixo-" comes from the future of the Greek verb for
"touch," thixomai, and a thixotropic substance would be
one whose state changes with touching.
Let's
review the phenomenon of osmosis. In the diagram, we have put some
solution into a container closed off by a semipermeable membrane,
and put the container into a vessel containing the pure solvent. Consider the
dots as representing the dissolved substance. This applies to a sol, as well as
to a solution, but there will be more "solute" particles in the
solution. The membrane can be cellophane, but ordinary wrapping cellophane
cannot be used, since it has been lacquered to close the pores. The pores must
be small enough to prevent the solute from passing through. The solvent is
found to cross the membrane and enter the solution, making it more dilute. The
level of the solution rises to some height h in equilibrium. If d is the
density, then π = dgh is the osmotic pressure. This is the pressure
required on the solution side to make the rate of movement of the solvent the
same in both directions across the membrane. If the solution is dilute, van't
Hoff's equation gives the osmotic pressure: πV = nRT, where the volume V of the
solution contains n moles of solute.
The osmotic pressure of a solution containing 1 mole of solute per
1 kg of solvent, called a 1 molal solution, is quite large, about 25
atmospheres (as we can estimate from van't Hoff's equation). Not only is this
hard on cellophane membranes, but would require a liquid column 825 feet high,
which is inconvenient. Nevertheless, osmotic pressures are high enough to push
fluids to the tops of high trees (this is actually a complex subject about
which there is controversy). Strong semipermeable membranes can be made by
precipitating insoluble salts in the pores of unglazed porcelain. Sols will
exert quite modest osmotic pressures, since there are many fewer particles per
unit volume. In fact, the pressures are so low that they cannot be used in most
cases to find the number of particles per unit volume. However, the molecular
weight of haemoglobin was determined in this way. Cell walls are semipermeable
membranes. If an erythrocyte (red blood cell) is put into pure water, it will
swell and burst because it contains a solution of electrolytes in a
semipermeable membrane. If put into strong salt solution, it will become
dehydrated and shrivel. Solutions of the same osmotic pressure are called isotonic.
Isotonic solutions are used to prevent damage to biological systems.
Erythrocytes are about 8.6 μm in diameter and 2.6 μm thick, and slightly
concave on the faces. They are a bit large for colloidal particles, though the
blood plasma is a colloid that can gel under certain circumstances. Like
platelets, which are definitely colloidal, they are not living cells like the
leucocytes that accompany them.
The separation of colloids and crystalloids is called dialysis,
and can be carried out with a suitable semipermeable membrane, just as in
osmosis. Chemists actually distingush dialysis, in which the components diffuse
at different rates across a membrane, from ultrafiltration, in
which larger particles are mechanically stopped while smaller ones are allowed
to pass. Ultrafiltration can occur through gels, while dialysis uses dialytic
membranes. There is no fundamental difference between the processes, however.
Dialysis is carried out in the body by the kidneys, which separate
the crystalloids urea, uric acid, hippuric acid and ammonia compounds from the
colloidal albumin and other proteins of the blood. The crystalloids diffuse
more rapidly across the interface than the larger particles. In the laboratory,
we can put the sample to be dialyzed into a container like that used for
osmosis, and change the solvent as it becomes concentrated in the crystalloids.
Parchment paper can be used for the dialytic membrane. A bag of parchment paper
can be filled with the sample to be dialyzed, and it can be suspended in a bath
of moving warm water for rapid dialysis. Membranes can also be prepared from
other animal membranes, or collodion (nitrocellulose dissolved in alcohol and
ether), or artificial sheet polymers. Electrodialysis can also be used, taking
advantage of ion migration in an electric field.
Dialysis has many industrial applications. Sugar is extracted from
sugar beets by using the cell walls as dialytic membranes, washing cut beets in
warm water. Dialysis is used in the artificial fiber industry to separate
alkali from the colloidal fiber material, and in the pharmaceutical industry
for the purification of colloidal medicines. Dialysis is used in artificial
kidney machines to simulate the action of the kidneys.
As has been pointed out, absorption is a volume effect, while
adsorption is a surface effect. Colloids, having large surface areas for a
given volume, are excellent at adsorption. A cubic centimeter of charcoal can
have a surface area of 1000 m2, so a little charcoal can do a lot of
adsorbing. Adsorption may be specific. For example, Ni, Pt and Pd in colloidal
form adsorb H2 up to 1000 or 3000 times their volume. Palladium
is the best at this. At red heat, palladium metal will absorb hydrogen readily,
and release it at even higher temperature. Black palladium powder can be made
by reducing PdCl2 in solution. Because of the adsorption, these
metals, in colloidal form, make excellent catalysts for hydrogenation.
Usually, adsorption is not very specific, and a wide variety of
substances can be adsorbed on a certain medium. Adsorption is
temperature-sensitive, being much more effective at low temperatures, and
evolving the adsorbed substances at higher temperatures. Carbon at room
temperature does not adsorb oxygen and nitrogen, but does so at liquid-nitrogen
temperatures. Adsorption is generally accompanied by a negative change in
enthalpy, so it is exothermic. The temperature dependence is a
consequence of LeChatelier's Principle.
The three most commonly used adsorbents are carbon as charcoal,
alumina and silica. When specially prepared as adsorbents, they are
called activated. Activation is usually a matter mainly of heating,
and perhaps some chemical cleaning. A saturated adsorbent may be re-activated
by heating, say at 175°C for 6-8 hours in air. Charcoal may be prepared from
wood, bone, blood and sugar. The charcoal from different sources has different
impurities and therefore somewhat different characteristics. Charcoal is
a nonpolar adsorbent that is good for organic vapors and nonpolar
substances in general. Alumina and Silica are polaradsorbents and
are best for polar substances, like water.
Charcoal is used in gas masks, since it readily adsorbs toxic
organic vapors rather indiscriminately. It does not adsorb carbon monoxide or
ammonia well, so specal adsorbents for these must be included. Carbon monoxide
is usually oxidized to the dioxide, and ammonia by silica gel. Therefore, a
complete gas mask canister contains a mechanical filter, charcoal, silica gel,
and an oxidant for CO. Charcoal can be used to produce ultra-high vacuum by
cooling it to liquid nitrogen temperatures. If charcoal in a stout glass tube
in a U-shape is saturated with chlorine or sulphur dioxide, and then sealed
off, the gas is liquefied when the charcoal end of the tube is heated, and the
other end is cooled. Natural gas may be filtered through charcoal to remove
higher alkanes. Pentane and hexane are more strongly adsorbed than the lighter
methane and ethane. Charcoal is, in general, used when organic substances are
to be removed from a gas.
Alumina gel and silica gel are produced by drying the gel produced
by precipitating the hydroxides in water. Alumina gel is good for water vapor,
carbon dioxide and alcohol. It makes an excellent laboratory dessicant. A small
amount of colorimetric indicator is added that is blue when the alumina is dry,
but turns pink when it has adsorbed water and is alkaline. The dessicant can
then be regenerated by heating in an oven. Alumina gel is generally used for
drying organic liquids, which it also decolorizes, and for removing acids from
oils. It also removes oil vapor from compressed air. Alumina gel is a good
example of the nonspecifity of adsorption.
Silica gel is the most generally useful adsorbent. It is
especially good at adsorbing benzene, for example from coke-oven gas. Silica
gel is used to dry carbon dioxide, hydrogen and oxygen before they are
liquefied or solidified. It is often found in small bags in sealed packages
that must be protected from humidity. These bags are easily re-activated by
heating. Silica gel can be used to dry natural gas for pipeline transport, to
prevent the formation of ice clathrates at low temperatures. These clathrates
can form above the freezing point and easily clog pipelines.
Other colloidal adsorbents found naturally are diatomite,
also known as diatomaceous earth or kieselguhr, which is composed of diatom
shells made from opaline silica, and bentonite, a strongly
hydrophilic colloidal clay consisting mainly of montmorillonite. Bentonite can
clarify and deodorize petroleum, purify and soften water, make drilling mud,
plug leaks as a grout, improve cleaning powders, and destroy building
foundations by swelling. Diatomite has perhaps even more uses than bentonite.
Both are valuable colloidal minerals.
Adsorbents are imporant in dyeing fabrics. Often the fabric will
not adsorb the dye directly, since the dye may be polar and the fiber nonpolar.
However, gels like amphoteric metal hydroxides, especially aluminium hydroxide,
may cling to the fibers while strongly adsorbing the dyes. Such intermediates
are called mordants, which are usually colloids. The dye and the
mordant together, without the fiber, is called a lake (from
the same word that gave "lacquer"). Purple of Cassius is a famous
lake, formed on stannic hydroxide gel by colloidal gold.
Colloids are mixtures whose particles are larger than the size of
a molecule but smaller than particles that can be seen with the naked eye.
Colloids are one of three major types of mixtures, the other two being
solutions and suspensions. The three kinds of mixtures are distinguished by the
size of the particles that make them up. The particles in a solution are about
the size of molecules, approximately 1 nanometer (1 billionth of a meter) in
diameter. Those that make up suspensions are larger than 1,000 nanometers.
Finally, colloidal particles range in size between 1 and 1,000 nanometers.
Colloids are also called colloidal dispersions because the particles of which
they are made are dispersed, or spread out, through the mixture.
Types of colloids
Colloids are common in everyday life. Some examples include
whipped cream, mayonnaise, milk, butter, gelatin, jelly, muddy water, plaster,
colored glass, and paper.
Every colloid consists of two parts: colloidal particles and the
dispersing medium. The dispersing medium is the substance in which the
colloidal particles are distributed. In muddy water, for example, the colloidal
particles are tiny grains of sand, silt, and clay. The dispersing medium is the
water in which these particles are suspended.
Colloids can be made from almost any combination of gas, liquid,
and solid. The particles of which the colloid is made are called the dispersed
material. Any colloid consisting of a solid dispersed in a gas is called a
smoke. A liquid dispersed in a gas is referred to as a fog.
ies of colloids
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Each type of mixture has special properties by which it can be
identified. For example, a suspension always settles out after a certain period
of time. That is, the particles that make up the suspension separate from the
medium in which they are suspended and fall to the bottom of a container. In
contrast, colloidal particles typically do not settle out. Like the particles
in a solution, they remain in suspension within the medium that contains them.
Colloids also exhibit Brownian movement. Brownian movement is the
random zigzag motion of particles that can be seen under a microscope. The
motion is caused by the collision of molecules with colloid particles in the
dispersing medium. In addition, colloids display the Tyndall effect. When a
strong light is shone through a colloidal dispersion, the light beam becomes
visible, like a column of light. A common example of this effect can be seen
when a spotlight is turned on during a foggy night. You can see the spotlight
beam because of the fuzzy trace it makes in the fog (a colloid).
Light shining through a
solution of sodium hydroxide (left) and a colloidal mixture. The size of
colloidal particles makes the mixture, which is neither a solution nor a
suspension, appear cloudy.
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