KOH: (why is K+ > Na+ when it comes to liquid soap? Ionic conc, solubility effects, etc...)
Brace yourself, this may hurt a little
Short answer
Potassium fatty acid salts tend to be more water soluble than their Sodium counterparts [1].
Long answer
Soaps are the water-soluble sodium (Na) or potassium (K) salts of fatty acids (FA). As such they are anionic surfactants. Soaps are, for our purposes, made by the base catalysed hydrolysis of triacyl glycerides [2] using strong alkali.
Using ‘caustic soda’ (NaOH) to hydrolyse the oils and fats produces Na fatty acid salts (Na-FA) while using ‘caustic potash’ [3] (KOH) produces K fatty acid salts [4]. K-FA based soaps are used more widely for liquid soap products than Na-FA based soaps.
Given the oils used in soap-making we’re mostly dealing with 12 - 18 Carbon chain length fatty acids.
K-FA salts are more water soluble than Na-FA salts. This is the core reason that liquid soaps are generally, although not exclusively, made using KOH.
Water solubility is the result of several interactions: cation-anion attraction (lattice energy); cation-water and anion-water interactions (hydration energy); and water-water interactions [5]. The solubility of ionic solids in water depends largely on two things: the energy change that occurs when the ionic solid goes into solution as hydrated ions, and the effect of the hydrated ions on the arrangement of the surrounding water molecules, measured by the organization energy.
Some of the contributory factors:
Ion charge. Smaller ion charge tends to favour solubility. Not an issue in this discussion because K and Na have the same +1 ion charge.
Ionic radius. Larger ions tend to be more soluble. The fact of K having a greater ionic radius than Na, 133 v 95, favours solubility by reduction in lattice enthalpy … more energy is released in the hydration process than is stored in the cation-anion bonds. Cations tend to have more influence on this than the anions, because they are much smaller and there’s a 1/r^2 relationship with hydration energy. Perhaps paradoxically increasing fatty acid chain length (alkyl) tends to correlate with decreasing solubility of a soap – coconut oil, lauric acid (C12) [6], and soap that will lather in seawater.
Soap mixtures tend to be more soluble than pure soaps. For that matter, soap mixtures tend to do everything soapy better than pure soaps [7].
Fun fact: K-FA salts are widely used as insecticides, herbicides, fungicides, and algaecides. I’ve used liquid soap as an aphid spray with effect. They’re more effective against soft-bodied bugs, like aphids. They kill bugs via desiccation, as does diatomaceous earth, but they achieve the desiccation slightly differently. The structure of those K-FA salts provides water solubility for the fatty acids. The lipophilic carbon chains of the fatty acids penetrate and disrupt the lipoprotein matrix of the insects’ cellular membranes. The membrane disruption leads to evacuation of cellular contents, causing the cell to dehydrate and die. Fatty acid toxicity increases with increasing carbon chain length, typically peaking at C10, and then decreasing. Fatty acid chain lengths of 18 carbons with one or two double bonds (unsaturated) also display insecticidal activity
Hmmm, that didn't end up as long, or as rambling, as I thought it might. I hope it makes some sense.
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Footnotes
1. In contrast Na halides (e.g. NaCl) are more water soluble than their K counterparts … for #reasons.
2. Triacylglycerides, or triglycerides, are esters of glycerol (a polyol alcohol) and fatty acids. One glycerol molecule, because of its three hydroxyl groups, can esterise with three fatty acid molecules – hence
triglyceride. The oils and fats used in soap-making contain various triglycerides in differing combinations and proportions.
Fatty acids themselves are poorly soluble in water, but their Na and K salts are relatively hydrophilic.
3. The word potassium derives from
potash. The word potash (
potaschen - mid 15th century Middle Dutch etymology) derives from the olden-days method of manufacturing potassium carbonate: water leaching the
ashes of burnt plant material in large
pots. Caustic potash was then made by treating that potash with slaked lime. cOver time the term potash also came to be applied to mined sources of potassium salts - mainly KCl.
Potassium was first isolated in the early19th century via the electrolysis of caustic potash.
Along the same lines
soda ash, ash from the burning of marine plants, was originally used to make caustic soda using a similar reaction with slaked lime. Soda ash, mostly Na2CO3, is also the ash that forms on the surface of some handmade soaps.
Early soaps were made using potash and then converted to harder sodium salts by treating the soft potassium soaps with salt (NaCL) solution.
4. Other cations can be used to make fatty acid salts. Calcium (Ca) and Magnesium (Mg), along with K and Na, salts are used as food additives. Ca-FA and Mg-FA salts are insoluble in water and part of the scum that forms when many soaps are used in ‘hard’ water. Lead and Magnesium soaps have a range of industrial uses, including an additive to help paints and other surface finishes dry. Calcium soaps are used in synthetic lubricating greases. Zinc soaps are used in talcum powder, and other cosmetic, manufacture.
5. It gets messier and less perfect when you start looking at ion pair formation, incomplete dissociation, the formation of complex ions, and pH changes. The fact that we’re talking about soaps, with their micelle-forming behaviour, also reduces the validity of applying purist chemistry reasoning. It gets messy and it probably gets quantum.
6. Make soap with shorter chain fatty acids and things can start to smell bad.
7. C7 soaps wet better, while C13 soaps best solubilise fats and oils. Differing chain length also helps emulsions maintain their stability, and good thing for soap function. The C7 saturated FA, heptanoic OFC, has an unpleasant rancid smell though.
