It is commonly observed that in different geographic locations the same soap can produce abundant leather or nor at all, and that it can leave a slippery feeling when rinsing or not. This has more to do with the water and the soap than with the latitude and the longitude of the site. 

Soap was invented in Gaul (today’s France) and has been produced over the millennia by boiling animal fat in calcium hydroxide, thus generating alkaline salts of multi-carbon-atoms fatty acids, usually called anionic detergents. Water can contain varying degrees of different salts: when it contains calcium or magnesium ions, it is called “hard” and when those ions are present at negligible concentration, it is called “soft.” 

There is no judgment of value attached to the adjectives hard and soft: the adjectives are just a quick way to characterize the ionic status of a specific water. It so happens that hard water is a poor solvent of soap. Why is that?

WATER AND DETERGENT

Detergents can be anionic, with a negatively charged “head” and an aliphatic “tail” as in the case of sodium dodecyl sulfate or cationic, with a positively charged head and an aliphatic tail as in the case of benzalkonium chloride. Detergents can also have a non-ionic head or a zwitterionic head or no charge at all. Above a defined concentration called critical micelle concentration, detergents tend to aggregate their aliphatic tails and form supramolecular particles having the electrically charged heads on the outside.

The addition of calcium (even in small quantities) contributes to the screening of the electrostatic repulsion between the anionic headgroups, allowing the anionic detergents to aggregate more easily. The presence of calcium reduces the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) facilitating micelle formation at lower concentrations. Pure SDS has a CMC of ~8 mM at 25°C, adding calcium leads to a much lower, calcium-dependent CMC: ~ 1.3 mM. At low surfactant concentrations, calcium dodecyl sulfate precipitates. As surfactant concentration increases, micellization can occur, sometimes in competition with the precipitation process. In hard water, the effective CMC of SDS is lowered, but the overall surfactant efficiency decreases due to the limited solubility of the resulting calcium-surfactant moiety. The consequence is that when washing hands, hair or body with soap and with hard water, the precipitate tends to stick to the skin and the hair, which provokes a disagreeable feeling. 

On the other hand (the pun is not voluntary), when using soft water, the rinsing of the soap takes time, and during the rinsing one can feel a prolonged fastidious slippery feeling.

WATER AND THE SKIN

Many consumers worry about the negative effects of hard water on their skin, and are prone to equip the water pipes of their homes with water filters to remove impurities. Is this reasonable?

The fact is that tap water does contain several types of “impurities.” Often the people who have the responsibility of delivering water do add some bleach (~4mg/liter) to keep it free from potentially contaminating microorganisms. While this concentration is absolutely safe, the odor that comes with it might not be of the liking of everybody, and this justifies the filtering of tap water for personal consumption. And the precipitate that soap forms in hard water can clog pores and create some unwanted skin conditions. An alternative proposed by somebody is to use distilled water to rinse soap when soft water is not readily available. Given the dimensions of a water distillation unit or the cost of a purification pack having a very small flow, I leave the decision to thecommon sense of the consumer.

WHAT ARE THE OPTIONS?

The drawbacks of hard water are the consequence of the chemistry of the anionic surfactants present in the soap prepared according to the ancient Gallic technology. 

One way to avoid the disagreements associated to hard water is to prepare the soap using detergents that are not affected by the presence of divalent cations, such as zwitterionic surfactants and non-ionic surfactants. Among these one finds alcohol etoxylates, alkylphenol etoxylates, alkyl polyglycosides, etc. 

Alcohol ethoxylates (AEs) offer the best performance in hard water and are highly biodegradable. Unfortunately they have several significant drawbacks related to human health and environmental toxicity. For instance the manufacturing process to achieve ethoxylation can contaminate the final product with trace amounts of 1,4-dioxane, that is considered a probable human carcinogen that with long-term exposure may also cause damage to the liver and the kidneys.

Safer alternatives to alcohol ethoxylates (AEs) can be found among alkyl polyglucosides (APGs), biosurfactants and zwitterionic surfactants.

APGs are derived from renewable sugars and fatty alcohols. They are excellent in hard water and highly stable in alkaline environments. They are 100% biodegradable and are extremely mild on skin. Decyl glucoside, lauryl glucoside and coco glucoside are examples of APGs. 

Biosurfactants are produced through natural fermentation processes using microorganisms. Sophorolipids are produced by Candida bombicola. They can outperform AEs in cleaning power while being significantly gentler on the environment. Rhamnolipids are produced by Pseudomonas aeruginosa via the fermentation of sugars. 

Zwitterionic surfactants that perform very well in hard water are cocamidopropyl betaine and cetyl betaine.

Personal care products resistant to hard water include syndet (synthetic detergent) bars, liquid body washes and soaps containing chelating agents like EDTA or citric acid. These products prevent divalent cation to interacy with the heads of anionic surfactants and avoid mineral buildup, scum formation and skin dryness. 

Consumers who would like to avoid the disagreements provoked by hard water and traditional soap can look for products containing chelating agents, mild surfactants or moisturizers to combat the drying effects of minerals. 

Paolo Giacomoni, PhD of Insight Analysis Consulting acts as an independent consultant to the skin care industry. He served as executive director of Research at Estée Lauder and was head of the department of biology with L’Oréal. He has built a record of achievements through research on DNA damage and metabolic impairment induced by UV radiation as well as on the positive effects of vitamins and antioxidants. He has authored more than 100 peer-reviewed publications and has more than 20 patents. He is presently head of R&D with L.RAPHAEL—The science of beauty—Geneva, Switzerland. His email is: [email protected].