[The Correlation Between Fatty Acid Chain Length and Lather Characteristics]
In Part 1, we looked at how the choice of fatty acid determines lather. But there's another fascinating fact.
Even when the exact same fatty acid is used, the soap ends up being a completely different type of soap depending on whether it is a sodium (Na) salt or a potassium (K) salt.
We'll dig into why the same fatty acid produces such different results, from the perspective of ion chemistry.
Revisiting the Saponification Reaction
R-COOH + NaOH → R-COO⁻Na⁺ + H₂O (sodium soap, hard)
R-COOH + KOH → R-COO⁻K⁺ + H₂O (potassium soap, soft)
The fatty acid chain (R-COO⁻) portion is completely identical.
The only difference is whether the counterion is Na⁺ or K⁺.
Yet this small difference completely splits apart the soap's hardness, solubility, and lather characteristics.
Why Does a Single Ion Make Such a Big Difference?
1) Differences in Ionic Radius and Hydration
Ion | Ionic Radius | Hydration Energy | Hydrated Ion Size |
Na⁺ | Small (0.102 nm) | High | Relatively large (strongly attracts water molecules) |
K⁺ | Large (0.138 nm) | Low | Relatively small |
Counterintuitively, although the Na⁺ ion itself is small, it attracts water molecules strongly (hydration) and actually forms a larger hydration shell.
This large, tight hydration shell binds strongly to the carboxylate anion (R-COO⁻), making the ionic lattice more densely packed. As a result:
- Na soap → strong lattice bonding, so it is hard and dissolves slowly in water
- K soap → weak lattice bonding, so it is soft and dissolves quickly in water
2) Differences in the Krafft Point
The Krafft Point is the minimum temperature at which a surfactant begins to form micelles.
Below this temperature, the surfactant exists in a crystalline state and produces almost no lather.
- Sodium stearate: relatively high Krafft Point (around 50°C)
- Potassium stearate: relatively low Krafft Point (noticeably lower than the Na salt)
This difference translates directly into the real-world user experience.
Pure Na soap doesn't dissolve well in cold or lukewarm water, making brush loading feel stiff, whereas K soap (or soap blended with K) activates easily even at low temperatures and builds lather quickly.
The phenomenon shaving enthusiasts often experience — "this soap needs a lot of water (a long loading time) before it lathers" — is, in many cases, actually caused by a high Na-salt ratio raising the Krafft Point.
3) Changes in Micelle Shape
The difference in ionic radius also affects the shape of the micelles the surfactant forms in water.
- Na soap: the strong electrostatic bonding of the counterion packs the surfactant head groups tightly → tends to form a lamellar/layered structure → creamy, thick lather
- K soap: weaker counterion bonding creates relatively greater repulsion between head groups → tends to form spherical micelles → light, voluminous lather that forms quickly
Why Do Shaving Soap Recipes Mix NaOH and KOH?
Understanding this principle makes it clear why premium shaving soap recipes use neither pure NaOH nor pure KOH, but a mixed ratio (e.g., NaOH x% + KOH y%).
Higher Na ratio → maintains puck form, hardness, creamy layered lather
Higher K ratio → fast activation, dissolves well even at low temperatures, abundant initial lather
- 100% Na → the puck is sturdy, but loading is too stiff and the initial bloom of bubbles is slow
- 100% K → lather comes quickly, but the puck is too soft to hold its shape (closer to a croap/stick type)
This is why most premium shaving soaps adopt a dual-lye approach, blending NaOH and KOH.
This follows exactly the same logical structure as the "lauric acid + stearic acid blending" strategy explained in Part 1 — striking a balance between the strengths of two different mechanisms to find the optimum.
A Quantitative Look — Trends Observed in Actual Batch Testing
The general trends I actually observe when designing recipes (with the fatty acid blend held fixed and only the alkali ratio varied) are roughly as follows:
KOH Ratio | Puck Hardness | Loading Speed | Lather Stability | Usage Character |
0% (Na only) | Very hard | Slow | Very stable | Classic hard puck |
10–15% | Hard | Moderate | Stable | Balanced, standard type |
20–30% | Medium | Fast | Good | Beginner-friendly, easy to load |
50%+ | Soft (croap-like) | Very fast | Abundant but somewhat unstable | Stick/croap type |
This table is meant to illustrate general tendencies rather than absolute figures; the actual optimal ratio needs to be adjusted together with the fatty acid composition, superfat percentage, and curing time.
The Two Variables Are Only Complete When Read Together
The fatty acid chain length covered in Part 1 and the type of alkali (Na/K) covered in this part are independent yet complementary variables.
- Fatty acid → determines the molecular cohesion of the surfactant film itself (the "structure" of the lather)
- Alkali → determines how easily, and in what form, that surfactant activates in water (the "speed of formation and solubility" of the lather)
Ultimately, a good shaving soap is built on the delicate balance of two axes: long-chain fatty acids that build the skeleton of the lather, and the Na/K ratio that tunes how readily that skeleton comes to life.
Next time you're lathering a soap and find yourself thinking, "the lather is coming up especially well today" — it might have less to do with chain length, and more to do with the counterion.
Dr. Dongju Jeon, Director of R&D at Hwayeon Soap (hwayeon.co.kr) and Terry's Soap
[The Correlation Between Fatty Acid Chain Length and Lather Characteristics]
In Part 1, we looked at how the choice of fatty acid determines lather. But there's another fascinating fact.
Even when the exact same fatty acid is used, the soap ends up being a completely different type of soap depending on whether it is a sodium (Na) salt or a potassium (K) salt.
We'll dig into why the same fatty acid produces such different results, from the perspective of ion chemistry.
Revisiting the Saponification Reaction
R-COOH + NaOH → R-COO⁻Na⁺ + H₂O (sodium soap, hard)
R-COOH + KOH → R-COO⁻K⁺ + H₂O (potassium soap, soft)
The fatty acid chain (R-COO⁻) portion is completely identical.
The only difference is whether the counterion is Na⁺ or K⁺.
Yet this small difference completely splits apart the soap's hardness, solubility, and lather characteristics.
Why Does a Single Ion Make Such a Big Difference?
1) Differences in Ionic Radius and Hydration
Ion
Ionic Radius
Hydration Energy
Hydrated Ion Size
Na⁺
Small (0.102 nm)
High
Relatively large (strongly attracts water molecules)
K⁺
Large (0.138 nm)
Low
Relatively small
Counterintuitively, although the Na⁺ ion itself is small, it attracts water molecules strongly (hydration) and actually forms a larger hydration shell.
This large, tight hydration shell binds strongly to the carboxylate anion (R-COO⁻), making the ionic lattice more densely packed. As a result:
2) Differences in the Krafft Point
The Krafft Point is the minimum temperature at which a surfactant begins to form micelles.
Below this temperature, the surfactant exists in a crystalline state and produces almost no lather.
This difference translates directly into the real-world user experience.
Pure Na soap doesn't dissolve well in cold or lukewarm water, making brush loading feel stiff, whereas K soap (or soap blended with K) activates easily even at low temperatures and builds lather quickly.
The phenomenon shaving enthusiasts often experience — "this soap needs a lot of water (a long loading time) before it lathers" — is, in many cases, actually caused by a high Na-salt ratio raising the Krafft Point.
3) Changes in Micelle Shape
The difference in ionic radius also affects the shape of the micelles the surfactant forms in water.
Why Do Shaving Soap Recipes Mix NaOH and KOH?
Understanding this principle makes it clear why premium shaving soap recipes use neither pure NaOH nor pure KOH, but a mixed ratio (e.g., NaOH x% + KOH y%).
Higher Na ratio → maintains puck form, hardness, creamy layered lather
Higher K ratio → fast activation, dissolves well even at low temperatures, abundant initial lather
This is why most premium shaving soaps adopt a dual-lye approach, blending NaOH and KOH.
This follows exactly the same logical structure as the "lauric acid + stearic acid blending" strategy explained in Part 1 — striking a balance between the strengths of two different mechanisms to find the optimum.
A Quantitative Look — Trends Observed in Actual Batch Testing
The general trends I actually observe when designing recipes (with the fatty acid blend held fixed and only the alkali ratio varied) are roughly as follows:
KOH Ratio
Puck Hardness
Loading Speed
Lather Stability
Usage Character
0% (Na only)
Very hard
Slow
Very stable
Classic hard puck
10–15%
Hard
Moderate
Stable
Balanced, standard type
20–30%
Medium
Fast
Good
Beginner-friendly, easy to load
50%+
Soft (croap-like)
Very fast
Abundant but somewhat unstable
Stick/croap type
This table is meant to illustrate general tendencies rather than absolute figures; the actual optimal ratio needs to be adjusted together with the fatty acid composition, superfat percentage, and curing time.
The Two Variables Are Only Complete When Read Together
The fatty acid chain length covered in Part 1 and the type of alkali (Na/K) covered in this part are independent yet complementary variables.
Ultimately, a good shaving soap is built on the delicate balance of two axes: long-chain fatty acids that build the skeleton of the lather, and the Na/K ratio that tunes how readily that skeleton comes to life.
Next time you're lathering a soap and find yourself thinking, "the lather is coming up especially well today" — it might have less to do with chain length, and more to do with the counterion.
Dr. Dongju Jeon, Director of R&D at Hwayeon Soap (hwayeon.co.kr) and Terry's Soap