The research, which focused on a commercial face cleanser and a hair shampoo, involved an integrated analysis using macroscopic, microscopic, rheological, and tribological techniques to quantify the damage hard water could inflict on cosmetic foams.
It found that the presence of hard water ions, primarily calcium and magnesium, caused a critical loss of foam volume, accelerated liquid drainage, and shifted the foam’s mechanical properties from an elastic, solid-like state to a fragile, unstable liquid.
Rapid collapse of foam volume and drainage
At the macroscopic level, the results showed a significant and immediate degradation of foam stability when products were mixed with hard water. The face cleanser, for instance, exhibited a substantial foam decay of approximately 38% in hard water over a 10-minute period. This was nearly 2.4 times greater than the 16% decay measured in soft water.
The hair shampoo exhibited a similar pattern, with a 35% foam decay in hard water, approximately 1.8 times higher than the 19% seen in soft water. This collapse was also reflected in the foam’s liquid retention ability, measured by the drainage half-life — a shorter half-life meant rapid liquid content loss in the foam, directly impacting its longevity during usage.
In soft water, the face cleanser maintained its structure for an average of 165 seconds before half of its liquid drained. In hard water, this plummeted to 48.9 seconds, a nearly 3.4-fold decrease. The shampoo showed similar instability.
The researchers attributed this instability to the direct interaction between divalent cations, such as calcium and magnesium, and the anionic surfactants used to create the lather.
These multivalent ions readily bound with negatively charged surfactant molecules, leading to the formation of insoluble precipitates and reducing the concentration of active surfactants available to stabilise the air-water interfaces.
Microscopic bubble structure destroyed
Beyond the visible volume loss, the study confirmed that hard water fundamentally compromised the microscopic integrity of the foam. Researchers analysed the density and size of the bubbles over time, which directly correlated to the consumer’s perception of “richness” and “creaminess”.
Upon initial formation, both products produced a fine-textured foam. However, the presence of hard water caused a “dramatic reduction” in bubble count and a “massive increase” in mean bubble area over 600 seconds. This process, known as bubble coalescence, entails individual bubbles merging into much larger, coarser ones due to film rupture.
The face cleanser in hard water saw its bubble count drop from an initial 75.9 to a mere 5.06 bubbles per square millimetre, while its mean bubble area swelled by approximately 15 times. The hair shampoo saw an even more dramatic reaction, with its mean bubble area increasing nearly 22-fold.
Conversely, soft water conditions preserved the fine bubble architecture much more effectively. The cleanser in soft water retained a substantial 52.8 bubbles per square millimetre. This clearly showed that hard water not only reduced volume but compromised the delicate bubble structure, resulting in a coarser, less luxurious lather.
Compromised elasticity
The L’Oréal team also used rheology to analyse the foam’s mechanical properties, focusing on its viscoelastic behaviour — its ability to resist and recover from stress.
The analysis of the storage modulus and loss modulus showed a stark contrast in the foam’s structural integrity. The storage modulus represents the elastic, solid-like component, while the loss modulus reflects the viscous, liquid-like component.
In soft water, both the cleanser and shampoo foams exhibited high storage modulus values that dominated loss modulus. This signified a predominantly elastic, solid-like network with strong structural integrity, meaning the foam was resilient and could resist deformation.
In hard water, however, the loss modulus either became comparable to or rapidly surpassed the storage modulus at much lower oscillation stresses. This early crossover of loss modulus over storage modulus marked a swift transition to a predominantly viscous, liquid-like behaviour.
The foam dissipated more energy than it stored, and was highly prone to structural breakdown and flow under minimal stress.
Further evidence came from the phase angle delta analysis, with a lower phase angle signifying a more elastic and stable foam. Foams in soft water maintained low phase angles, reflecting their highly elastic, resilient nature.
However, hard water foams exhibited significantly higher initial phase angles and a rapid increase towards purely viscous behaviours as stress increased. This unequivocally demonstrated a profound and rapid loss of the foam’s elastic character.
Interestingly, the study noted that water hardness did not substantially alter the foam’s controlled rate viscosity profile, meaning the foam’s initial perceived “thickness” or “spreadability” might be the same across different water types, even as its stability dramatically differs.
Friction and lubrication also affected
The final component of the research explored the tribological properties, which related to the frictional feel of the foam on the skin and hair, also known as the “slipperiness” or “rinse-feel”. Tribology measures the coefficient of friction (CoF), or the drag between two surfaces.
For the face cleanser, the presence of hard water consistently resulted in a CoF across the measured speed range. In hard water, it exhibited a CoF of 0.076 at 1mm/s, which was approximately 105% higher than the 0.037 observed in soft water at the same speed.
The formation of insoluble surfactant-cation complexes in hard water reduced the foam’s lubricating efficiency, shifting it toward a higher-friction, “less slippery” feel.
In contrast, the hair shampoo exhibited a more complex and speed-dependent frictional profile, where the difference in CoF between hard and soft water was less pronounced.
The researchers suggested that this resilience was due to the shampoo’s specific formulation, which may contain a more robust or less ion-sensitive surfactant system, polymers, or conditioning agents.
Critical necessity for formulation chemists
The researchers concluded that the study’s insights were “invaluable for developing next-generation cosmetic products” that could deliver consistent efficacy and superior sensory attributes in diverse global water conditions. This would improve user satisfaction and minimise product waste due to poor performance.
They added: “These findings also emphasise the critical importance for researchers to consider water quality, particularly hardness, when developing cleansing products to ensure optimal product efficacy.
“Additionally, leveraging foam boosters and rheology modifiers can enhance foam stability and integrity to ensure consumer satisfaction and product performance across diverse water qualities.”
Source: Cosmetics
“Cosmetic Foams: The Rheo-Tribological and Microstructural Effects of Hard Water”
https://doi.org/10.3390/cosmetics12060270
Authors: Richa Mehta, et al.
