How does penetration of ingredient from an emulsion to skin happens?

If an emulsion for example has 10 ingredients in different ratios, when applied to skin and absorbed, does the emulsion droplet or the smallest particle which has all the ingredient in the same ratio as other droplets absorb as a whole or the emulsion breaks down and each ingredient absorb at different ratio and speed individually if they absorb at all? 

Same question for other forms like serum, gel and ECT. 

Comments

  • This is too complex question to give a short answer and the one of the major aspects of studies in dermatology. There are many factors that define skin penetration. The skin is protective organ, hence it defends the body from nearly any outer substances including those that we consider "good" for us. However, there are two most important factors - the structure of the Stratum Corneum (lipids+water with the components of the Natural Moisturizing Factor), and the molecular weight of a compound that is supposed to be penetrated. There is even the "rule" in Dermatology - the rule of the 500 Daltons. According to the rule, molecules greater than 500 Daltons cannot penetrate the skin barrier. In fact, the molecules greater than 500 Da can penetrate with the aid of enhancers (for instance glycols, solvents, surfactants or Urea). All the penetration enhancer affect the structures of the Stratum Corneum. Basically, the lipophilic ingredients have much better penetration potential than water soluble, the w/o emulsions and lamellar emulsions are most efficient form for the delivery and penetration than o/w emulsions while water based gels, water solutions are the least efficacious due to their nature. 
    You may find out more if you read the special literature. I recommend to study works by Albert Kligman first, the father of the modern Cosmetology and Dermatology. He also found a doctrine of Corneology and Corneotherapy - the science that researches the Stratum Corneum. I bet this knowledge enormously helps any formulators and cosmetic chemists/pharmaceutical chemists in their work. 
  • @vitalys thanks a lot for detailed information. 
    You didn't say anything about This one.
    Does ingredient from an emulsion penetrate the skin in the same ratio as it is in emulsion or the emulsion breaks down and each ingredient penetrate at different rate?

    For example 
    A lotion 
    5% petrolatum
    1% beeswax
    0.3% retinol 
    0.3% xanthan gum 
    2% glyceryl stearate
    2% Polyglyceryl 6 distearate
    2% cetyl alcohol 
    1% glyceryl oleate
    1% phenoxyethanol+caprylyl Glycol
    Citric acid
    Water 
    pH 5

    If penetration happened, does these ingredients from this lotion all penetrate in this ratio that they are in the emulsion or the emulsion breaks down and some ingredient like retinol and glyceryl oleate penetrate more and others like xanthan gum, petrolatum and beeswax penetrate less if at all?
  • @Abdullah Yes, the emulsion is going to break down at the moment when it contacts the skin. The ingredients will be absorbed according to their lipophilic/hydrophilic properties. The ingredients, which have the minimal molecular mass will penetrate first. The compounds of higher weight will stay on the skin surface and create the kind of occlusion - petrolatum, for instance. 

  • @vitalys that is good. 
    Do you know why they emulsion breaks down when applied to skin? 
  • The skin has several barriers. Initially, an emulsion comes in contact with invisible hydro-lipid layer (mantle), which defines pH of the skin surface. This layer contains lipids and components of the natural moisturizing factor including some salts and water. Next, the Stratum Corneum is a highly structured layer of the skin and it has literally "briсk and mortar" configuration where the bricks are corneocytes (contain keratin and NMF) and the mortar that contains lipids. It is important that the lipids are not just a mixture, but they are organized as lipid bilayers and the structure of those bilayers looks very alike as liquid crystal lamellar emulsions. Thus applying the emulsion on the skin equals to mixing of two different emulsions. When you rub your emulsion into the skin, the most of the water content evaporates almost immediately, the rest of the components remain on the surface of the skin while others (mostly lipids) fuse with the lipid bilayers of the Stratum Corneum. Such factors as temperature, skin pH, electrolytes also have a significant impact on the emulsion. 
  • @vitalys what about glycerin? 
    Doea it penetrate the skin or not? 
    If yes then fast or slow?

  • @Abdullah Yes, it does to some extent. The absorption rate as well as with any other compounds will depend on a type of the skin - thin epidermis or thick epidermis (soles and palms), physiochemical properties of the area at the moment of application, concentration of glycerin, time of exposure, consistency of the barrier lipid bilayers, integrity of the Stratum Corneum, temperature, pathway of absorption - intercellular, transcellular or via skin appendages. Glycerin will penetrate faster in upper layers of Stratum Corneum, and the rate of absorption slows down at the level of Stratum Lucidum and Stratum Granulosum. 
  • @vitalys thanks
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