Castile soap, originating from Spain, is a plant-oil based soap known for its natural composition and broad utility. Traditionally crafted from olive oil, it now includes other vegetable oils like coconut and hemp. This cleanser serves various purposes, from personal hygiene to household cleaning, making it a popular choice for those seeking natural alternatives.
The pH of Castile Soap
Castile soap is inherently alkaline, typically exhibiting a pH ranging from 8.5 to 11.5, with many formulations falling between 9 and 10. This alkalinity results from the saponification process, the chemical reaction that creates soap. Saponification involves combining vegetable oils with a strong alkali like lye (sodium hydroxide for bar soap or potassium hydroxide for liquid soap). This reaction transforms the oils and alkali into soap and glycerin.
The pH scale measures the acidity or alkalinity of an aqueous solution, ranging from 0 to 14. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are alkaline. Each whole number increment on the pH scale represents a tenfold difference in acidity or alkalinity. For example, a substance with a pH of 9 is ten times more alkaline than one with a pH of 8.
Impact of Castile Soap’s pH
The alkaline nature of Castile soap significantly influences its interactions with skin, hair, and various surfaces. On the skin, its high pH can temporarily raise the skin’s natural pH, which typically ranges from 4.5 to 5.5, a slightly acidic state known as the acid mantle. While healthy skin can often rebalance its pH quickly, prolonged or frequent exposure to highly alkaline products may disrupt this protective barrier, potentially leading to dryness, irritation, or increased susceptibility to bacteria. This can result in a “squeaky clean” feeling, as the soap effectively removes natural oils.
When used on hair, Castile soap’s alkalinity can cause hair cuticles, which are like overlapping scales, to lift. This can lead to hair feeling tangly, matted, or appearing dull. If used with hard water, which contains dissolved minerals, Castile soap can react to form insoluble precipitates, commonly known as soap scum. This reaction can leave a waxy or sticky residue on hair and surfaces, contributing to dullness or build-up.
Despite these considerations, Castile soap’s alkalinity also contributes to its effectiveness as a cleaner. Alkaline solutions are efficient at breaking down and emulsifying oils, fats, and grease, making Castile soap an effective degreaser for dishes and other household surfaces. However, its alkaline properties mean it can etch or damage certain materials, particularly unsealed porous surfaces like natural stone, if not used appropriately.
Best Practices for Using Castile Soap
Given its concentrated and alkaline nature, proper usage of Castile soap is key. Dilution is necessary, as the soap is potent. For instance, a common dilution for foaming hand soap is one part soap to five parts water, and for general cleaning sprays, about two tablespoons per 16 ounces of water. Dilute only the amount needed for immediate use, usually within two weeks to a month, as diluted solutions may not contain preservatives.
Thorough rinsing is important, especially for personal care, to prevent residue that might contribute to dryness or a sticky feeling. Following a hair wash with an acidic rinse, such as diluted apple cider vinegar or lemon juice, can help smooth hair cuticles, reduce tangles, and restore the hair’s natural pH balance. This acidic rinse counteracts the soap’s alkalinity, promoting a softer feel.
Avoid mixing Castile soap directly with acidic ingredients like vinegar or lemon juice. The alkaline soap and acidic ingredients will neutralize each other, causing the soap to “unsaponify” or revert to its original oil components. This reaction results in a curdled, greasy substance that loses its cleaning efficacy. Instead, use these products separately, with a rinse in between applications if both are desired.