Hydrophilic and hydrophobic moisturizing
The primary function of moisturizers is to delay the loss of moisture especially on damaged skin. Sustained delivery of moisture to the skin greatly assists the repair of the barrier function. There are two primary physico-chemical mechanisms for rehydration of the horny layer (stratum corneum): the use of a hydrophobic protective barrier in the form of "occlusion" and the use of hydrophilic humectants. Skin care products specifically formulated for the purpose of moisturizing dry skin therefore contain humectants (e.g. glycerol, propylene glycol or sorbitol) or hydrophobic moisturizers (e.g. petrolatum, mineral oil, lanolin, vegetable oils, dimethicone, etc.) with the aim of maintaining the hydration status of the horny layer. From the formulation developer's viewpoint, humectants are used to maintain moisture in O/W emulsions and gels and keep the products from drying out, an effect that could possibly result in blockage of tube apertures or pump dispensers. The function of the markedly hydrophobic lipids, such as hydrocarbons like vaseline, beeswax etc. is generally understood as a type of effect based on the formation of an inert, occlusive epicutaneous film or a membrane which reduces transepidermal loss of water by decreasing the evaporation of moisture. It has also been shown, however, that topically applied lipids can penetrate into the living epidermis and become incorporated in the lamellar bilayers of the horny layer This can result in the formation of a water vapour impermeable coating which inhibits de novo synthesis of intercellular lipids.
The function of hydrophilic humectants lies in their ability to attract water. In so doing they assume the role of the dermal hydrophilic components of the horny layer by being absorbed from the formulation by the horny layer and exerting a hydrating effect by binding water. Humectants withdraw water especially from the dermis into the epidermis and less from the environment when the relative atmospheric humidity exceeds 70%. Under certain conditions, substances such as propylene glycol, for example, can even increase transdermal water depletion, especially when atmospheric humidity is very low.
Generally, moisturizers can change the moisture content of the skin and its skin barrier function. The quality and quantity of humectants and hydrophobic moisturizers in their composition in the formulation can influence the effectiveness of the product. The diagnosis of the underlying skin disease is a factor of equal importance in selecting the suitable moisturizing product.