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Dialogs about physico-chemistry
applied to arts
Chapter I
About binders |
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dial dial
dial
[Translation:
Anne Clerget]
French text
Let us start the Dialogs at
Dotapea with a discussion between Jean-Louis, physico-chemist at the CNRS,
Anne, soapmaker and professional cook -
see her website
- and a candid, Emmanuel.
The personages are real, the
discussion too. It can resume any time and this text will lengthen.
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Emmanuel:
(to Jean-Louis) I am going to ask you a question that
occurred to me while I was making a mayonnaise. I warn you: I already
asked the same question to an experienced cook to have another point of
view.
The cooks
mention
binders which indeed are also present in painting (egg, flour,
gelatin, etc.); and nothing prevent you from
painting with mayonnaise
or from making a
tempera « sauce poulette ». Sometimes,
however, the cooks' binding agent doesn't bind anything in particular.
It does not seem to coat but rather to come in between in the manner of
a simple
colloidal charge (I
wonder what would give a "colloidal silica Blanquette"), whereas in
painting it coats the pigment, holds it in place. On the other hand, in
both disciplines, the binding agent seems to have some
plastic properties.
I am starting to wonder what really defines the term "binder". Its
plastic properties, its sticking and colloidal properties, with which
physical reality does it, match, finally? Unless it is only a
common term, a holdall. And yet, we find
approximately the same products in these disciplines.
What does
an interfaces specialist say to that?
And an experienced cook? What is a culinary binding agent?
Anne :
Hum, the binder allows to bind, do you see? To bind two or more elements
that, without its help, would refuse to mix together. The binder can be
a
surfactant, an
emulsifier (egg yolk
for instance). But, the binding agent can be also what gives body to a
preparation. A thickener like flour or corn flour.
This is what is coming to my mind.
Jean-Louis :
As always, nothing is simple. The term alone of
binder does not give an account of all what happens "behind". To sum up,
we may say that there are two categories of problems: 1) making miscible
two products that are not (ex. water and oil) or stabilizing suspensions
(ex.
Indian ink); 2) gluing
(i.e.: binding, sticking together, coating ...) solid grains on a
surface (paint).
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First
problem:
The very well-known example of the vinaigrette: if you mix
oil
and
vinegar,
the two components separate as soon as you stop stirring. Magic: when
you add mustard, it is already more stable. With
egg yolk
you will even get a mayonnaise. Another famous example: if you scatter
soot into water you obtain an outstanding ink, but very quickly, the
soot flakes stick together, fall down to the bottom of the inkwell and
it is lost.
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I. Miscibility :
Love, hate
and amphiphiles |
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Magic: if you add
Arabic gum,
the mix remains stable (Indian
ink - see also
lamp black and soot black).
Why?
Empedocles
(450 BC), then Aristotle assumed that two forces, Love and Hate, were
sufficient to account for all natural phenomenons: Love causing the
rapprochement of the objects (ex. magnetite); Hate causing them moving
away. This theory has been meeting success during several
centuries.
And today still, we have to admit that it could remain tempting, because
the molecular interactions often sum up to attractive or repulsive
interactions, and there are two part molecules, made with love and hate,
the
soaps
or amphiphiles ( from the
Greek : who likes both).
An amphiphiles is a molecular compound of which one part of the
structure is
polar
(or hydrophilic, who likes water and the liquids known as polar) and
another one likes lipids - that are apolar -, known as lipophilic or
hydrophobic.
Mustard
grains,
egg yolk, contain such amphiphiles molecules.
When mixing, amphiphiles place themselves at the interface between water
and oil, and it stabilizes the sauce; at that time, this is an
emulsion.
We generally emulsify oil in water: and we have then the following
structure :
The amphiphiles (in food research the term emulsifier is often used) of
the egg yolk is
lecithin.
Found also in
soybean,
this is a key product for many preparations (read the labels!), of
sauces, mayonnaise and other chocolates..
Soaps are amphiphiles: their fatty part solubilizes dirt (which is often
fatty) and the forming of
micelles
allows dispersion in water
Because of their own nature, amphiphiles are molecules that prefer
interfaces. Oppositely to what we could think, soap is not very soluble
in water, and its molecules place themselves rather on the surface.
Soap are surfactants
[link],
they alter the superficial tension of the liquids in which they are
dispersed. This is the reason why we can then make bubbles....
In the case of Indian ink, the problem originating the
flocculation
is that the ink particles are electrically charged and attract each other.
Arabic gum
(extracted from the sap of an acacia tree) is a water-soluble polymer
playing at once a role of neutralization of the electric charges (the
polymer
winds around particles) and of solubilization (Arabic gum is also used in
industry as an emulsifier.
Cf.
apega.bf/gomar.htm).
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[note : we do not know well the binder of Indian ink - see
text -,
but Arabic gum can give out a very good ink] |
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Second
problem :
To make hold on a surface a product that is generally
pulverulent: the
pigment. In case that the
solid base is human skin, one generally seeks a reversible decorative
action, and therefore, the binder is an ordinary
lipid.
If durability is sought, one will use a binding agent capable of becoming
indissoluble. Several possibilities!
By being a bit simplistic, it
is possible to say that the physical phenomenon carried out is
reticulation.
Reticulation is the
phenomenon whereby polymer chains which are initially independent from each
other (they "flow") bond together to form a solid or an extremely viscous
mass (does not flow anymore).One differentiates physical (reversible) and
chemical (irreversible) reticulation.
Physical reticulation: often natural polymers (gelatin,
skin glue,
bone glue,
nerve glue ...). While cooling, a solution of gelatin gets thicker and
becomes a gel which does not run anymore.
Ibid for
animal glues.
When reheating, the polymer chains take their freedom back, these glues are
reversible.
Chemical reticulation: under the action of air, ultraviolet rays,
temperature, the polymer chains are bonding together (true chemical bond).
Example:
linseed oil.
It is not reversible anymore; the structure must be literally broken to come
back to a previous state.
Some paints
(ex.
acrylic,
vinylic,
epoxy)
can be put into this category, although in the strictest sense the reaction
carried out is then a polymerization and not a reticulation. Reticulation:
long polymer chains, initially independent bond together to form a
three-dimensional network (the terms of gel or elastomer will be used). Very
often, individual chains are always fluid in the reticulated state, but the
fact that they are all interdependent prevents them from flowing.
Polymerization: small molecules (monomers)
place themselves end to end to form chains that are longer and longer
(polymer). The increasing of the size leads to an increase of the
viscosity,
eventually it is so viscous that it is like a solid. Most of plastic
materials around us are such "viscous liquids". (Plate-
glass
is not formed by a polymer but it is a viscous liquid!)
I
get back to the paint: the painter will want to disperse his pigment in a
material able to "harden" (in the widest sense). He needs then at the least
a pigment and a medium which is apt to congeal (gelatin), to reticulate
(oil) or to polymerize (latex, acrylic...). Often, in order to improve the
dispersion of his pigment in the matrix the painter will have to add a
surfactant, but this is not essential. We may then add texturing products
which will give some substance to the paint; for example the
silica
particles: they prevent the pictorial layer from dripping (thixotropy)
and allow keeping the stroke.
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II. Reticulation, polymerization : a pigment problematic at the border
between fluid and solid |
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The case of frescoes: this
is a bit particular, since in this case the colours are applied over a
fresh rendered surface, i.e. which has not finished to harden, to set.
When the mineral matter of the coating (lime,
plaster,
cement) hardens, it
traps the particles of pigment that have been applied and which diffuse
on a few tenths of millimeter. We can say that the medium, the binder,
is the wall!
For this kind of paints, you will evidently use water-based binders so
that the paints will be compatible with the coating which is also
water-based. The painter will often add
egg
or casein,
to get a good paint holding when starting the pictorial work (you don't
paint on the walls with
watercolour), to improve the density of colours and to avoid a too deep or too
uneven diffusion within the coating. |
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[note : painting a mural with watercolour is not common but
Kevin Mc Cloud
reports a process with Arabic gum] |
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Emmanuel : Let's get back now to the
mayonnaise.
Upon Anne's latest advice, I read an article where a cook calls on Hervé
This. I was trying to find some answers to the question: "is it
necessary to whip it?" (Or "can we whip it?"), which does not really go
without saying, depending on the sources.
Well, yes,
we whip it.
See
the "parent" article
and the
actual recipe
(external links, new windows).
This is
quite related to our subject.
Hervé
This claims that this is not the
phospholipids which are
responsible for the good consistency of the emulsion, but the
proteins. What do you
think of that?
Jean-Louis: If he is saying so. Most of the
organic molecules and especially the very big ones like proteins have
hydrophile and hydrophobe parts. They can therefore be used as a
surfactant in the same way as the phospholipids.
Read a
more detailed passage
in Chapter III
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Where we get back to the mayonnaise |
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Emmanuel : It is the second time that you are
using the term "surfactant". How do you define it?
Jean-Louis :
A quasi-philosophical question. This is a molecule able to modify the
interfacial energies, the
surface tensions.
So, a molecule which likes the interfaces between two bodies. The
molecules of a given body, the easiest way is to imagine it a liquid
body, enjoy staying with each other. A molecule located at the core of
the liquid is happy, all her neighbours are like her; a molecule located
"at the edge" is not happy, half of her neighbours are of a different
nature. So, the molecules from the surface whish that their friends come
and surround them; and this attractive force creates the surface
tension.
On Earth, gravity counteracts their efforts; the surface of liquids is
flat. In the absence of gravity, liquids form spheres because for a
given volume of liquid, the geometrical shape with the smallest surface
is the sphere. This is the shape that
generates the smallest number of "unhappy" surface molecules.
The surface tension (see
picture)
is what allows making soap bubbles. The surface tension is also
responsible of the capillarity phenomenon, which, among other things,
enables the plants to make the sap circulate.
A surfactant is a molecule
generally
amphiphiles that
changes the « force ratio » between two bodies in contact: liquid-liquid
(emulsion), liquid-gas (bubbles, foams), liquid-solid (wetting and
capillarity).
But what you are trying to
do, finally, is to evaluate the siccativity of mayonnaise...?
Emmanuel :
Yes, and to explore similar issues at the same time. We'll get back to
it in the next chapter. |
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The surfactant,
a story of elective affinities |
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Next chapter (english) |
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