Why does ink stick to paper?

I often use water-based ink on paper as my drawing material. I sometimes categorise my drawing materials into those that flow in water as if in a primordial sea and those that are scraped off on hard surfaces like some form of geological attrition. There is for myself a rightness about the flow of ink, it feels as if I am tapped into working with something akin to my own life blood.

But in order to explain this I need to explore some very basic things about drawing with water based inks, to look at the underlying processes that allow drawings to ‘stick’ to the paper they are made on.

I usually choose to use water based or hydrophilic inks, which are in chemical terms called polar inks, i.e. their molecules have a high attraction for certain papers, that causes them to absorb and/or be receptive to water.

I use paper that is made of cellulose fibres, and cellulose is a polymer of the simple sugar, glucose.

The key point about cellulose is that the polymer chains that bind cellulose together have -OH or hydroxyl groups sticking out all around them; they look as if they are wanting to bond with something. Hydroxyl groups are often found in organic or ‘life’ chemistry and are vital to the formation of sugars and alcohols. A hydroxyl group consists of one hydrogen and one oxygen atom, which is of course closely related to the structure of water, which consists of two hydrogen atoms bonded with one oxygen atom. Hydroxyl groups are polar, and the oxygen side is always negative, while the hydrogen side is always positive. All a hydroxyl molecule needs to become a water molecule is another hydrogen molecule, therefore it has a high capacity to form a hydrogen bond. The capacity of a hydroxy group to form hydrogen bonds imparts water solubility and internal structural stability to proteins, including amino acids and hydroxy groups also participate in the dehydration reactions that link simple biological molecules into long chains. I.e. they allow some of the fundamental life processes to be maintained.

So when my inks do attach happily to their paper surface, they will attach initially using a hydrogen bond, which is “the electrostatic attraction between two polar groups that occurs when a hydrogen (H) atom covalently bound to a highly electronegative atom such as oxygen (O) experiences the electrostatic field of another highly electronegative atom nearby.”

As I use my water based or hydrophilic inks, they will create molecular bonds within the thin layer of water that I have created around the cellulose fibres as I drag my ink laden pen or brush across the surface of the paper. Put into very basic terms, my water based ink sticks to paper because it has a high affinity for the medium it's being applied to.

On the other hand there are hydrophobic surfaces. When water based ink is drawn onto a surface that it doesn't like, it aggregates, it sort of clumps together, as if it is trying to avoid any interaction with the surface. Just rub some butter or lard on a sheet of paper and try and draw on it, you will see what I mean.

This is not about repulsion between individual water molecules and lipids, it is because of the strong attraction of water molecules for one another in comparison with the slight lipid attraction.

When water soluble ink begins to interact with lipids, (organic compounds that are insoluble in water, such as fatty acids or grease), the water forms ordered cages around the hydrophobic elements, (the lipids), because of the way the water molecules are more attracted to each other than the lipids. These cages cause a decrease in entropy, and by the 2nd law of thermodynamics, this is unfavourable. I.e. the system cannot pass from one stage of thermodynamic equilibrium to another, therefore the internal structural stability that hydroxyl groups give to proteins, would not be possible.

In the diagram above you can think of the ink as the water, and the hydrocarbon chain of the lipid as the greasy surface.

For me this is a deep metaphor, the difficulty of trying to draw with water based inks on a greasy surface, is reflected in the fact that the molecules cannot interact positively and the ease in which my ink flows and bonds with paper, reflects the way that water operates as a life enhancing medium, the complex molecular bonds that are needed to create amino acids being not that dissimilar to the way that ink sticks to paper via intermolecular attraction.

The fact that I can think of my ink as being like blood is not too far fetched, in fact T S Elliot stated that the purpose of literature was to turn blood into ink. An ink very like blood in appearance is oak or iron gall ink. Iron gall ink is a purple-black or brown-black ink made from iron salts and vegetable sourced tannic acids (usually oak galls), and was the standard drawing ink used in Europe from the 5th century to the 19th century, remaining in use well into the 20th century. 

The ink stuck so well because iron sulphate, (FeSO₄) added to tannic acid (C_6H2(OH₃COOH) had a very strong affinity with the cellulose fibres of vellum and paper. In fact the bond was so strong the only way to erase a mark was to sand the whole surface down.

The other area of intermolecular attraction that is also helping to hold the ink on paper, (London forces), also operates as one of the main forces when holding marks on paper when drawing with chalks, charcoal and graphite, so I will look at this in a future post, when I will try and open out some of the more geological aspects to leaving a mark on paper.  

There is also capillary action to consider. If you pour mercury over your paper it will not wet it, mercury's surface tension is so great it adheres to itself. But if you pour water onto paper it will wet it, even if the paper is sized, because the surface tension of water is much less. If however you clean the surface of a metal, such as aluminium, mercury will 'wet' that. 

At a level below all this is the behaviour of sub-atomic forces and the fact that these are shaping everything we do is sometimes forgotten. If you want to explore these in more detail go to my posts of drawing and quantum theory. 

Drawing and quantum theory one
Drawing and quantum theory two

I did refer to London Forces briefly at the end of an earlier post and you can find that here.

See also:

For more on oak gall ink

For thoughts on hard dry drawing materials

Chinese ink drawing

Drawing in pen and ink

Drawing using pigments suspended in water