It is known that in the presence of a hydrophobic powder, such as a hydrophobic silicon dioxide powder (silicone-coated silica powder), water can be dispersed into fine droplets and enveloped by the hydrophobic material, thus preventing the droplets from rejoining. Such material has been described as “dry water,” “powdered water,” or “powder-to-liquid” and can have a water content of over 95%. It is formed by the intensive mixing of water with hydrophobic material. During this process water droplets are sheathed by the solid particles and prevented from flowing together again. The first experiments on the use of “dry water” as a cosmetic base date from the 1960's. See U.S. Pat. No. 3,393,155. These free-flowing, fine powders liquefy when rubbed on the skin.
More recently, U.S. Pat. No. 6,290,941 describes cosmetic or pharmaceutical powder-to-liquid compositions comprising hydrophobically coated silica particles into which are incorporated water and a water soluble polymer, the composition containing substantially no oil. Such compositions are said to require less silica while retaining the water-holding capacity and permitting substantial elimination of added oil from the formula.
WO 2011/075418 discloses a powdery composition comprising a) at least one powder in the form of core-shell particles, the core comprising liquid water or a liquid aqueous phase and the shell comprising hydrophobic or hydrophobized particles, and b) at least one powder comprising carrier, and b1) at least partially water soluble liquid and/or b2) a water reactive substrate each located in and/or on the carrier.
Eshtiaghi et al., Powder Technology, Vol. 223, 2012, pages 65-76 describes a variety of powder-to-liquid materials and proposes mechanisms for their formation. Shell materials used included hydrophobic (silicone-coated) silica, hydrophobic glass beads and polytetrafluoroethylene (PTFE or TEFLON) powder. Core materials included water, glycerol, and polyethylene glycol (PEG). Reported particle sizes for materials containing glycerin were 1200 and 3400 microns.
US 2012/0315312 teaches core-shell particles, the shell of which includes aggregated, hydrophobicized silicon dioxide particles and the core of which includes a liquid phase. The ratio of the silicon dioxide particles to the liquid phase is 2:98 to 40:60 based on the total weight of the particles and 60-100% by weight of glycerol is present in the liquid phase.
U.S. application Ser. No. 13/719,649, filed Dec. 19, 2012 teaches a powder comprising core/shell particles having an average particle size of less than 1000 microns, each particle comprising: 1) a liquid core that is substantially free of water and comprises a polar liquid having a percent surface polarity of at least 24%, and 2) a shell comprising hydrophobic particles. The particles can include an active.
U.S. Pat. No. 6,946,120 teaches a pharmaceutical composition for topical administration, including, as the pharmaceutically active component, at least 5% by weight of minoxidil; an acid in an amount to completely solubilize the minoxidil; a solvent composition including at least two of water, a lower alcohol and a co-solvent selected from one or more of the group consisting of aromatic and polyhydric alcohols; wherein when the co-solvent includes propylene glycol, it is present in an amount of less than approximately 10% by weight.
Although water-based powder-to-liquids are commonly described, they are not suitable for formulating with active agents that are unstable or incompatible with water, e.g., plant extracts prone to oxidation and/or hydrolysis. In addition, water-containing particles generally lack structural stability and are prone to collapse or leak during storage, and allow evaporation of water from the core.
There is also a need for compositions that improve skin penetration of benefit agents. U.S. Pat. No. 6,419,913 teaches micellar compositions that enhance skin penetration. Although effective, these compositions can be difficult to manufacture and the cost of the products are relatively high.
Applicants have now discovered novel compositions and a method of enhancing the topical application of benefit agents. The compositions include powder-to-cream particles containing a core without water and at least one acidic solubility enhancer. Such particles are stable and useful for formulating with a variety of active agents, even those that are prone to oxidation and/or hydrolysis. Compositions containing such particles are also convenient to use while providing a cream-like, pleasant skin feel and skin substantivity (the ability to remain on the skin). The compositions can be used in cosmetic, skin care, wound care, dermatologic, and other personal care products, as well as in other applications and industries.