Dialogs about physico-chemistry
applied to arts

Chapter III

Emmanuel : In cookery we talk about "chemical baking”. It is intriguing and I think that it can open up to things that will interest artists and decorators.

To what does this "baking" correspond in interfaces physico-chemistry?

Jean-Louis : To a reticulation/polymerization maybe. For example, if you add vinegar to creme fraiche, it will coagulate without heating because vinegar polymerizes milk proteins.

Emmanuel : How does it do that?

Jean-Louis : In milk or creme, casein forms micelles whose use is to stabilize calcium. These micelles are negatively charged and thus reject each other under the action of electrostatic forces. Under the action of acids (positively charged) (for example, lactic acid which is produced by the bacteria "eating" the lactose) the micelles will loose gradually their charge. When they become uncharged they agglutinate.

There is another way to coagulate milk proteins.
Under the action of enzymes (in our stomach or in a veal stomach), micelles are partially destabilized. The molecules of casein unwind partly (it is a big molecule) and, with the essential help of calcium ions, "bridges" of water and calcium molecules are formed between contiguous micelles and then, the milk or creme coagulates.

Emmanuel : In margin of these questions, what is the part of phosphorus (that we find back in casein) in lactic "phospholipids"?

Jean-Louis : Phosphorus is no use! It is only the element upon which the whole molecule articulates. Phosphorus bears the charged polar part and the fat hydrophobic chains. Cf. molecule below.

Note : in this  representation of a dioleoyl-phosphatidylethanolamine molecule, we find the polar part on the left, followed by the"central" phosphorus, then by the fat chains on the right.

Emmanuel : All right. Phosphorus can be in the center of this type of structure. But eventually, how many elements can fulfil this kind of role?

Jean-Louis : Heaps, mon bon monsieur. But life has chosen phosphorus, doubtlessly because it is available in plenty.
 

Emmanuel : Some mention a finally rather minor role of the phospholipids in comparison with the one of proteins in egg emulsions. What do you think of that?
 

Jean-Louis : To be frank, I do not know. If we look for surfactants, phospholipids are ideal candidates since they are omnipresent in every living structure. But proteins can also play a part of a surfactant. So then,  knowing who does what.... In the case of the egg, phospholipids are surely minority, in mass at least. That being said, what is proper to a surfactant is to know how to position at the right place (interfaces), and very little is eventually needed. Just like in the ad, "a few drops suffice" !

Read passage in Chapter I

Emmanuel   A Dotapea reader brought up an interesting questioning about casein: « I bought a book,” Vegetable colours : dyes, pigments and inks", at Les Ateliers de Provence,  announcing in  its  ingredients board, in  the

article about borax: "this is sodium borate, an alkali transforming casein into very adherent (sic) glue ". And in the same collection, the book "Ochres and decorative paints from Provence", gives lots of recipes for casein paints, either with boron, or lime, or ammonia water.
In short, there is nothing very scientific there, but, this casein/boron association comes back often, definitely. »

He is not wrong : nothing very scientific there, and yet, this theme often comes back indeed.
There is a thing that bothers me (and this is what I am getting at, something more general) : how and why can a salt be used as an alkali ?

Casein and

Jean-Louis : A salt results from the action of an  acid on a base (or the opposite, it is the same). We distinguish strong and weak  bases and acids. This has nothing to do with their corrosive power; adjectives are historical and have no relation with this power. It is a matter of dissociation. If in the solution there is the original molecule AND both dissociated ends, it is weak, if the solution contains only dissociated ends, it is strong. Ammonia water and acetic acid are "weak" but I would not advise you to put your fingers in.

Muriatic acid HCl is strong because in its solution, 100% of the HCl molecules are dissociated in H⁺ and Cl^-   ions. Acetic acid CH₃COOH is weak because its solution contains whole molecules, CH₃COO^- ions and

 H⁺   ions.

Emmanuel : So, a strong  acid is defined by the fact that when we put it into a solution, it dissociates in simple elements, is that right ?

Jean-Louis :Not at all. The fact that they dissociate is not proper to acids or bases. Cooking salt, NaCl, dissociates in water into Na⁺ et Cl^-. Muriatic acid into H⁺ and Cl^-, sulphuric acid into H⁺ and SO₄^--. Ammonia water (a base) is a solution containing OH^- et NH₄⁺. The strong or weak term has unfortunately a connotation of "power" which is mistaken. For the chemist, weak means that in the acid solution (for example in an acetic acid solution), there is a blend of CH₃COOH (non-dissociated), of H⁺ and of CH₃COO^- (dissociated). Strong means that the non-dissociated acid (or base) does not exist anymore in the solution.

Emmanuel : My question was badly asked. But let us talk now about strong or weak bases, then about salts.

Jean-Louis : This is the same thing for the bases : NaOH (soda-lime) is a strong base because in solution it contains only Na⁺ and OH^-  ions. Ammonia water is a weak base because non-dissociated molecules remain.

Strength and

Weakness

Then, the action of a strong base on a strong acid gives a neutral salt : HCl + NaOH gives NaCl whose pH  is neutral (pH=7), the action of a weak base on a strong acid will generate a rather acid salt, the action of a strong base on a weak acid will give a basic salt. NaOH in being a strong base, boric acid in being weak, sodium borate or borax is basic. Its pH is basic (pH~9). We can therefore consider the borax as a base, an alkali. Let us be clear, however, a salt is not a base, even if it makes rise the pH of a solution.

Understanding

the salts

Emmanuel : What is the measure of the dissociations at stake, how far does it go? Is there anymore bonds? It is difficult for me to picture how it goes.

Jean-Louis : Referring to HCl or NaCl, both fractions (H⁺ - Cl^-)and(Na⁺ - Cl^-) simply coexist in water. I say “simply”, but this is actually very complicated. But it is to say that the fragments are not chemically bound anymore. In the case of NaCl, if we remove the water we re-form the salt from the beginning. We do have dissociation, not a breaking down. In the case of weak acids/bases/salts, there is a chemical equilibrium. The dissociation of acetic acid in water is limited. At this moment we get a stabile and known proportion of CH₃COOH, of H⁺ and of CH₃COO^-.Not one more, not one less. The "dissociation constant" depends on the body, the temperature, the solvent, etc etc.
 

Let us get back now to casein and to the role of borax.

Casein is one of the main milk proteins (80%). In the milk, casein forms micelles which are small spherical structures whose inner surface is hydrophobic, and external surface is hydrophilic.

It can be used as a cleansing or bleaching agent. I think it is also used to pull down the melting point (fondant) of some enamels.

An attempt for a synthesis about casein, after some researches.
Casein is indissoluble. We obtain it by the action of an acid and heat on milk. It can be also obtained naturally by bacterial fermentation. It takes only more time.

This acid, indissoluble casein is transformed into caseinate by the action of a base. Soda (NaOH), lime (CaO), potash (KOH), etc... It is then soluble but its properties depend on the chosen base. Sodium caseinate has a more important viscosity than calcium caseinate. Lime and casein-based glues have been used during a long time, notably in aviation, to glue wood.

Blends of casein + Na (brought by borax) or casein + Ca (brought by lime) are used in the a tempera paints.