I. The Scientific Essence of Silicone Oil
Silicone oil, scientifically known as polysiloxane, is a class of artificially synthesized substances based on silicon-oxygen bonds, featuring high chemical stability and physiological inertness. In skin care products, its molecular structure forms a unique "network film" characteristic, which can fill skin textures without clogging pores, making it a core ingredient in sunscreens, foundations, and repair products. 
II. Fourfold Skin-Care Functions of Silicone Oil
1. Instant Blurring Effect
Silicone oil fills pores and fine lines to form an optical blurring layer, immediately enhancing the smoothness of the skin's texture. For instance, phenyl trimethicone in primer, with its high refractive index, enhances luster and achieves a "skin-smoothing" visual effect. 
2. Long-lasting Moisture-locking Barrier
The silicone oil film is breathable, reducing water evaporation while blocking external contaminants. Clinical studies have shown that when a ceramide-containing moisturizer is used in combination with silicone oil, the transepidermal water loss (TEWL) of the skin is reduced by 98.8% after 14 days of treatment, and the proportion of orthorhombic arrangement of the stratum corneum lipid structure is restored to 73%. Repair products (such as B5 cream) utilize its occlusive property to accelerate wound healing, but attention should be paid to the synergistic use with active ingredients. 
3. Foundation of Formula Stability
Silicone oil can prevent the crystallization of the cream, enhance the spreadability of the emulsion, and reduce the irritation of high-concentration ingredients. Experimental data shows that sunscreens containing 0.5% to 2% silicone oil can increase the stability of SPF values by up to 30%. 
4. Skin Feel Adjustment Expert
Volatile silicone oils (such as cyclopentasiloxane) can neutralize the greasy feeling and are friendly to oily skin; high-viscosity silicone oils (such as 10,000 cst dimethicone) increase the thickness of the moisturizing film and are suitable for dry skin in winter. 
III. Types of Silicone Oil and Suitable Scenarios
Dimethicone: A basic type of silicone oil that provides immediate smoothness and is suitable for daily sunscreens and foundation liquids.
Amodimethicone: Positively charged, it repairs damaged stratum corneum and is commonly found in anti-allergy and repair creams.
PEG12 Dimethicone: Water-soluble formula that avoids residue and is recommended for oily and acne-prone skin.
Quaternary Ammonium Salt Modified Silicone Oil: Used in high-end products, it combines antioxidant and anti-static functions. 
IV. Dispute Focus and Scientific Response
1. Acne Risk
Silicone oil with a molecular weight greater than 500 Daltons cannot penetrate the stratum corneum. Clinical tests show that silicone oil products with a concentration lower than 15% have no significant irritation to Propionibacterium acnes. However, those with oily and acne-prone skin should avoid the combination of high-viscosity silicone oil and mineral oil. 
2. Obstruction of Component Absorption
Silicone oil film may delay the penetration of water-soluble components. It is recommended to adopt a time-separated care routine: use silicone oil-based sunscreen and isolation products during the day, and apply retinol and other active essence at night. 
3. Environmental Degradation Issues
Cyclic siloxanes (such as D4/D5) are subject to usage restrictions in the European Union. It is recommended to choose linear structured silicone oil with a molecular weight of 5000-8000 Daltons (such as polydimethylsiloxane alcohol). The environmentally friendly product formed by the condensation reaction with amino silane coupling agent at 60-80℃ and 0.3-0.5MPa has a biodegradation rate that meets the EU Ecolabel certification standards. 
V. Product Selection Strategy Based on Skin Types
Oily Skin: Choose light and refreshing sunscreens containing cyclopentasiloxane and avoid products with dimethicone ranking among the top three ingredients.
Dry and Sensitive Skin: Prioritize repair creams containing hydroxyl silicone oil and use amino acid surfactants to minimize cleansing residue.
Combination Skin: For a zoned care routine, apply a volatile silicone oil primer with 1.5% salicylic acid (such as a cyclopentasiloxane-based formula) to the T-zone and a moisturizing cream with amino silicone oil and 3% ceramide complex to the U-zone. At night, use a 0.5% glucoside cleanser on the T-zone and an antioxidant essence with lipopeptides on the U-zone. 
Rational Use Suggestions
Keep the daily contact amount of silicone oil within 20% of the total amount of skin care products. Use glucose-glycoside-based cleansers when removing makeup to gently remove the silicone oil film. Check the product filing information to avoid the accumulation of false slip caused by the simultaneous use of more than three types of silicone oil components. 
Silicone oil, as a "double-edged sword" in modern skincare industry, its value lies in the precise blending by formula developers and the scientific understanding by users. Only by grasping the essence of the ingredients can we truly reconcile technology with the skin.