Conducted by researchers at A*STAR Skin Research Labs in Singapore, the study used streaming potential and zeta-potential measurements to track in real time how conditioner actives moved onto and off the hair surface.
They stated that this gave formulators a more quantitative way to tune deposition, rinsability, and in-use performance.
Conventional analytical tools such as ICP-OES, ToF-SIMS or XRF can show if a surfactant is present on hair, but they do not capture how fast it deposits, how it builds up, or how easily it rinses away. They also struggle with hair’s low hydrocarbon signal and complex, fibrous structure.
As such, the research team used electrokinetic analysis instead. By pushing an electrolyte through a packed plug of hair or through a microchannel containing hair fibres, they measured the streaming potential. They then converted it into zeta-potential, which reflects the effective surface charge an approaching molecule “feels” at the interface.
The study tested four widely used quaternary ammonium surfactants at 1% active in simple conditioner bases: behentrimonium chloride (BTMC), behentrimonium methosulfate (BTMS), hexadecyltrimethylammonium chloride (CTAC), and stearylalkonium chloride (STAC).
The researchers ran zeta-potential as a function of concentration, time and pH on both healthy (virgin) and bleached (damaged) hair, then linked these data to wet combing and ATR-FTIR results.
Damaged hair’s unexpected behaviour
Bleaching introduces sulfonic acid groups, and is usually assumed to make hair more negatively charged and more attractive to cationic surfactants. However, the team found the opposite at the measurement interface. At pH 7.4, healthy hair showed a zeta-potential of around −29 mV, while bleached hair was close to neutral.
The authors explained this by separating “intrinsic” surface charge from the apparent zeta-potential — bleached hair is more oxidised and intrinsically more negative but also more hydrophilic, swollen and porous, so water and ions penetrate deeper into the fibre.
Long-chain conditioning ingredients stick better to your hair
The researchers found that large, bulky chemical groups on the end of conditioning molecules could actually harm how well they worked. STAC, which has one of these big ring-shaped groups, gave them the weakest change in the hair’s electrical charge.
This large end group physically blocks the ingredient’s positively charged part, stopping it from getting close to the negatively charged hair surface and packing neatly onto it.
When the team ran tests over time, they saw that CTAC was quick to stick to the hair but also quick to fall off. Because it has a shorter chemical tail, it moved quickly in the solution, allowing it to stick on fast and create a large temporary change in the hair’s charge. However, it did not anchor strongly, meaning too much of it washed away during rinsing.
On the other hand, BTMC and BTMS stuck on more slowly but remained persistent. They lost very little of their conditioning effect, especially on healthy hair, even after the researchers had rinsed them thoroughly.
Silicon wafers offer a simple screening platform
To simplify their testing, the researchers swapped complicated, fibrous hair for flat silicon wafers coated in silicon oxide.
Although these flat, negatively charged wafers could not perfectly copy how hair builds up an electrical charge, they gave the research team a clean, repeatable surface to test on.
Under the same conditions, the wafers showed a much stronger negative charge than hair and needed less conditioner to reverse that charge, simply because they were smoother and smaller.
Crucially, the order of how well the four conditioners stuck to the wafers matched the researchers’ findings on hair: BTMC and BTMS stuck the best, followed by CTAC, then STAC.
This means testing conditioners on wafers is a quick and practical way to screen ingredients before moving onto more time-consuming work using actual hair.
Combability improves but damage remains
The research team wanted to link their electrical data to a familiar metric, so they measured how much effort it took to comb wet, bleached European hair treated with each formulation.
Although all four conditioners reduced the work needed to comb the hair compared to the bleached starting point, clear differences emerged again.
BTMC resulted in the largest reduction in combing force on both healthy and damaged hair sections, while BTMS worked slightly less well than BTMC — consistent with the fact that less of it stuck to the hair. CTAC and especially STAC showed smaller improvements, falling in line with their weaker sticking power and lower electrical charge changes.
These findings support the idea that conditioning ingredients with longer chemical tails create tougher, better layers that reduce friction between hair strands, especially on damaged hair where swelling and rough cuticles worsen drag.
However, another type of analysis told a different story about actual repair. The level of cysteic acid, which the researchers used as a marker of damage from bleaching, remained high in the hair and was completely unchanged by any of the treatments.
The authors stressed that while these conditioners made hair feel much better and easier to manage, they did not actually reverse the chemical damage to the hair fibre.
To conclude, they stated: “Beyond the scope of this work, further studies could include dry combing data in the context of leave-in formulations or products designed for dry hair management. Nevertheless, these findings advance our understanding of cationic surfactant behaviour and provide practical guidance for optimising hair care formulations.
“Furthermore, by leveraging streaming potential, formulators can better predict and enhance the performance of conditioning agents, ultimately improving the efficacy of hair care products.”
Source: International Journal of Cosmetic Science
“Electrokinetic analysis reveals common conditioner ingredient interactions with human hairs”
https://doi.org/10.1111/ics.70038
Authors: Huijun Phoebe Tham, et al.
