Diminishing hyperpigmented spots, enhancing skin radiance, preventing and protecting against the damage caused by free radicals, firming the skin, reducing the appearance of wrinkles... the vitamin C (INCI name: Ascorbic Acid) is an organic compound known for improving the appearance, texture, and complexion of the skin. Unfortunately, its chemical structure predisposes vitamin C to oxidation and instability in aerobic or anaerobic conditions, despite its proven effectiveness.
Vitamin C is an unstable compound in aqueous solution because it is prone to oxidation, identifiable by its colour change. In its ionised form, it can rapidly degrade when exposed to light, oxygen, high temperatures, high pH, and humidity, making it challenging to formulate. A few months after opening, a serum containing pure vitamin C may darken and turn deep orange or brown, indicating a decrease in its antioxidant effects. To stabilise it, it must be in an anhydrous state, in a reduced form, or formulated at a pH lower than 3.5...
Its hydrophilic nature does not make it optimal for penetrating the skin barrier, as the skin is protected by a lipid layer that makes it hydrophobic. This is why vitamin C requires specific conditions to effectively penetrate the skin and deliver its benefits, namely a very low pH. Indeed, pH plays a crucial role in the absorption of vitamin C.
Its acidic nature (pH < 3.5) can cause redness, tingling, and sometimes irritations as side effects in some individuals whose skin barrier is damaged, such as those with sensitive, dry, and dehydrated skin. However, this condition is essential if one wants to use vitamin C. Using an ascorbic acid serum with a less acidic pH will be ineffective.
A concentration of 20% Vitamin C is considered optimal for its penetration into the skin and to reduce the extent of its degradation. However, such a dosage can prove to be irritating.
To address these issues, derivatives of vitamin C have been synthesised and can be selected to maintain the physiological pH of the skin around 5.5 - 6. It can be combined with other molecules to form a stable compound that can be used in skincare formulas for a long shelf life and to be less irritating as the cells must break it down to release the pure vitamin C. Here are a few examples.
Tetrahexyldecyl Ascorbate (INCI name: Tetrahexyldeycl Ascorbate): This is a modified form of Vitamin C that is lipid-soluble and has a very low potential for irritability. This property makes it much more versatile. Classified as a quasi-drug in Japan and South Korea at a concentration of 2 to 3%, it is very stable in solution (it does not degrade easily) and is compatible with most skin types. THD Ascorbate also promotes better skin penetration at a pH > 5 due to its chemical modification, but it must first be converted into ascorbic acid.
Magnesium Ascorbyl Phosphate (INCI name: Magnesium Ascorbyl Phosphate): MAP is a water-soluble esterified derivative of vitamin C. It is highly stable in formulations with a pH > 7 and penetrates the skin quite well, where it is metabolised into ascorbic acid. It acts as a brightening and antioxidant agent to slow down the deterioration caused by exposure to air and also to control the pH of the finished product.
Sodium Ascorbyl Phosphate (INCI name: Sodium Ascorbyl Phosphate): This is the mineral form of vitamin C, which is growing in popularity. It converts into ascorbic acid when it penetrates the skin and has a lower irritant power, making it more suitable for acne-prone, sensitive, and damaged skin. However, its ability to penetrate the skin through topical application appears to be very limited due to its hydrophilic nature. As a result, it must be converted by a hydrolytic enzymatic process, presumably accomplished by an alkaline phosphatase present in the skin, before it can penetrate. Furthermore, SAP exhibits much higher stability and thus protects the formulation from oxidation, a result of its chemical structure with the introduction of a phosphate group in position 2 of the cycle. It is stable in solution at ~ pH 7.
Ascorbyl Glucoside (INCI name Ascorbyl Glucoside): This is a stable derivative of vitamin C. However, to perform its functions, it must be hydrolysed by cellular alpha-glucosidase to release ascorbic acid. It appears to penetrate the skin effectively, but its rate of penetration and conversion in vivo into vitamin C has not yet been determined.
Ascorbyl Palmitate (INCI name Ascorbyl Palmitate): This is a fat-soluble fatty acid ester, more specifically a residue of palmitic acid has been added to ascorbic acid. Due to its lipophilic property, it appears to penetrate the skin more easily. At concentrations of 0.05% to 0.1%, it is used to protect the formulation from oxidation and stabilise oxygen-sensitive ingredients in cosmetic formulations. However, at concentrations of 1 - 2%, it is used as an active agent. On the other hand, its chemical modification on carbon 6 of the cycle offers no protection to vitamin C against oxidative degradation: it is therefore less stable in the long term than other forms in topical formulations.
Ascorbyl Tetraisopalmitate (INCI name: Ascorbyl Tetraisopalmitate): This is a synthetic, fat-soluble form of vitamin C, where all its hydroxy groups (active sites of vitamin C) have been esterified with isopalmitic acid. When applied topically, it has shown the same physiological effects as ascorbic acid. To exert its effects, it penetrates the skin and is converted in vitro into ascorbic acid by an enzymatic reaction carried out by cytosolic esterase. It has been found to act as a precursor to vitamin C. In addition, it can be used at high concentrations in formulas without risk. In formulations, it is only stable when the pH is above 5.
3-O ethyl ascorbate (INCI name: 3-O Ethyl Ascorbic Acid): This is a stable alternative to ascorbic acid with an ethyl group in position 3 on the cycle. It is soluble in both water and oil. This structural modification protects the 3-OH group from ionisation and thus the molecule from oxidation, but it does result in changes to its physicochemical properties. In terms of its safety for use, it is generally well tolerated: only two cases of allergic contact dermatitis have been reported in the literature to date.
In our vitamin C-based treatments, we specifically use derivatives, namely sodium ascorbyl phosphate and tetraisopalmitate ascorbyl.
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