Polymethylsilsesquioxane/trimethylsiloxysilicate is a compound consisting of:

Polymethylsilsesquioxane, polymer obtained by hydrolysis from methyl trichlorosilane, a type of silicone. 

• Opacifying agent. It is useful into formulations that may be translucent or transparent to make them opaque and less permeable to light.

Trimethylsiloxysilicate (Trimethyl-siloxysilicate) a variable network of polysilicic acid units, blocked by trimethylsilyl groups. It is used in cosmetics as 

• Skin conditioning agent. It is the mainstay of topical skin treatment as it has the function of restoring, increasing or improving skin tolerance to external factors, including melanocyte tolerance. The most important function of the conditioning agent is to prevent skin dehydration, but the subject is rather complex and involves emollients and humectants that can be added in the formulation.
• Skin conditioning agent - Emollient. Emollients have the characteristic of enhancing the skin barrier through a source of exogenous lipids that adhere to the skin, improving barrier properties by filling gaps in intercorneocyte clusters to improve hydration while protecting against inflammation. In practice, they have the ability to create a barrier that prevents transepidermal water loss.  Emollients are described as degreasing or refreshing additives that improve the lipid content of the upper layers of the skin by preventing degreasing and drying of the skin. The problem with emollients is that many have a strong lipophilic character and are identified as occlusive ingredients; they are oily and fatty materials that remain on the skin surface and reduce transepidermal water loss. In cosmetics, emollients and moisturisers are often considered synonymous with humectants and occlusives.
• Antifoaming agent. The constituent factors for foam stabilisation are the concentration of nanoparticles and hydrophobicity. Foam, even when used in separation operations such as fractionation or flotation, can cause a decrease in density and a deterioration in quality in cosmetic products.  The defoaming agent (non-polar oil, silicone oils, hydrophobic solid particles or mixtures of both) is strongly influenced by viscosity and, to an almost directly proportional extent, concentration. However, defoamers can carry an irreversible source of contamination.

It appears in the form of a white powder

What it is used for and where

Cosmetics 

The combination of these two compounds generates Polymethylsilsesquioxane/trimethylsiloxysilicate,  which is able to improve the adhesion of pigments and fillers to skin and hair. It is particularly suitable for long-lasting eye make-up formulations as it is hydrophobic and not particularly sensitive to oils.

Polymethylsilsesquioxane  CAS   68554-70-1

Molecular Formula     C₆₆H₅₆N₄

EC number   232-803-9    614-610-5

Trimethylsiloxysilicate  

Molecular Formula    C₆H₁₈O₅Si₃

EC number    923-739-9

SCHEMBL347192

Polymethylsilsesquioxane studies

Trimethylsiloxysilicate studies