What is Glycerin? Chemistry, Sources, and Grades

Glycerin, also known as glycerol or 1,2,3-propanetriol, is a simple polyol (sugar alcohol) compound with the chemical formula C₃H₈O₃. This colorless, odorless, viscous liquid is characterized by its sweet taste and remarkable hygroscopic properties—its ability to attract and retain moisture from the environment.

Chemical Structure and Physical Properties

Molecular structure: Glycerin contains three hydroxyl (OH) groups attached to a three-carbon backbone, giving it the chemical structure CH₂OH-CHOH-CH₂OH. These multiple hydroxyl groups are responsible for glycerin's exceptional water solubility, hygroscopic nature, and ability to form hydrogen bonds—properties that make it invaluable in formulations.

Key physical characteristics:

Sources of Glycerin: Natural vs Synthetic

Glycerin can be derived from multiple sources, though the end product is chemically identical regardless of origin.

Vegetable-Derived Glycerin (Most Common)

Production method: Produced as a byproduct of vegetable oil saponification (soap making) or transesterification (biodiesel production). Common source oils include palm, soy, coconut, and rapeseed (canola).

Advantages:

• Natural origin appeals to consumers seeking plant-based ingredients
• Renewable and sustainable when sourced responsibly
• Acceptable for vegan and vegetarian products
• Generally lower cost due to abundant supply from biodiesel industry
• Meets USP, EP, and JP pharmacopeia standards when properly refined

Considerations:

• Quality varies based on source oil and refining process
• Potential for trace impurities from source material
• Sustainability concerns with palm oil-derived glycerin

Animal-Derived Glycerin (Less Common)

Production method: Byproduct of animal fat rendering and soap production (tallow saponification).

Characteristics:

• Chemically identical to vegetable glycerin after purification
• Not suitable for vegan, vegetarian, or halal/kosher products
• Less common in modern formulations due to consumer preferences
• Quality can match USP standards when properly refined

Synthetic Glycerin (Petrochemical-Derived)

Production method: Synthesized from propylene, a petroleum derivative, through chemical reactions.

Characteristics:

• Highest purity potential—fewer impurities from natural sources
• Consistent quality batch-to-batch
• Higher cost than vegetable-derived glycerin
• Less appealing to consumers preferring natural ingredients
• Can meet pharmaceutical grade specifications

Glycerin Grade Classifications

Glycerin serves as a key humectant ingredient in moisturizing creams and lotions

Key Properties: Why Glycerin Works in Formulations

Understanding glycerin's functional properties helps formulators optimize concentrations and predict product performance.

Humectant Properties: Moisture Attraction and Retention

Mechanism of action: Glycerin's three hydroxyl groups create multiple sites for hydrogen bonding with water molecules. This strong affinity for water allows glycerin to:

1. Draw moisture from the environment into the product and onto the skin (at humidity >50%)
2. Bind water within formulations, preventing evaporation and maintaining product hydration
3. Create a moisture-retaining layer on skin, reducing transepidermal water loss (TEWL)

Concentration-dependent effects:

Emollient and Skin Conditioning Properties

Beyond humectancy, glycerin provides direct skin benefits:

• Improves skin barrier function: Enhances the stratum corneum's moisture-holding capacity
• Accelerates wound healing: Maintains moist environment that promotes cell migration and proliferation
• Anti-irritant effects: Can reduce irritation from surfactants and other potentially harsh ingredients
• Softening and smoothing: Improves skin texture and tactile feel
• Enhances penetration: Can improve delivery of other active ingredients through the stratum corneum

Solvent Properties

Glycerin serves as an excellent solvent for many ingredients:

• Water-soluble actives: Dissolves vitamins (C, B-complex), herbal extracts, and hydrophilic compounds
• Preservatives: Helps solubilize and distribute antimicrobial preservatives
• Surfactants: Compatible with most anionic, cationic, and nonionic surfactants
• Fragrance fixation: Helps retain volatile fragrance compounds in formulations

Antimicrobial Properties

At high concentrations (>50%), glycerin exhibits antimicrobial activity:

• Disrupts microbial cell membranes through osmotic stress
• Reduces water activity (aw), inhibiting microbial growth
• Can extend preservation in certain formulation types
• However, typical cosmetic concentrations (3-20%) don't provide significant antimicrobial effect

Viscosity Modification

Glycerin's high viscosity (945 cP) affects formulation texture:

• Thickening effect: Increases product viscosity and body
• Spreadability: Can improve or reduce spreadability depending on concentration and base
• Sensory feel: Creates characteristic smooth, slightly tacky feel
• Stability: Can improve emulsion stability by increasing continuous phase viscosity

Cosmetic Applications: Formulating with Glycerin

Glycerin appears in virtually every category of cosmetic and personal care products. Here's how to optimize its use across different formulation types.

Range of personal care products utilizing USP grade glycerin for moisturization and texture

Moisturizing Lotions and Creams

Typical glycerin concentrations: 3-10%

Formulation considerations:

• Light lotions (oil-in-water emulsions): 3-5% glycerin provides moisturization without heaviness
• Rich creams (water-in-oil or high oil content): 5-8% glycerin balances moisture with emollients
• Intensive treatment creams: 8-10% glycerin for maximum hydration (hands, feet, very dry skin)

Synergistic ingredient combinations:

Phase A (Water Phase):
Deionized Water                     73.0%
Glycerin USP                         5.0%
Propylene Glycol USP                 2.0%
Disodium EDTA                        0.1%

Phase B (Oil Phase):
Emulsifying Wax NF                   6.0%
Cetyl Alcohol                        2.0%
Sweet Almond Oil                     5.0%
Vitamin E Acetate                    0.5%

Phase C (Cool Down):
Preservative System                  1.0%
Fragrance                            0.3%
Citric Acid (pH adjustment)          q.s. to pH 5.5
                                  ________
                                   100.0%

Procedure:
1. Heat Phase A to 75°C with stirring
2. Heat Phase B to 75°C separately
3. Add Phase B to Phase A with high shear mixing
4. Cool to 45°C, add Phase C ingredients
5. Mix until uniform, cool to room temperature
6. pH adjust to 5.5-6.0
7. Fill into containers

Facial Serums and Treatments

Typical glycerin concentrations: 2-8%

Why lower concentrations in serums:

• Serums are designed for layering under moisturizers—too much glycerin creates tackiness
• Active ingredient delivery is primary goal—glycerin supports but shouldn't dominate
• Lightweight texture is desired—excessive glycerin adds unwanted viscosity

Optimal formulation approaches:

• Vitamin C serums: 3-5% glycerin helps solubilize and stabilize ascorbic acid
• Hyaluronic acid serums: 2-4% glycerin complements HA's moisture-binding
• Peptide serums: 3-5% glycerin improves peptide penetration and stability
• Retinol serums: 2-3% glycerin provides moisturization to counteract retinoid dryness

Cleansers and Face Washes

Typical glycerin concentrations: 1-5%

Functions in cleansing products:

• Mildness: Reduces surfactant irritation potential
• Post-cleanse feel: Prevents tight, dry sensation after washing
• Foam stabilization: Can improve foam quality and stability
• Preservation of barrier: Minimizes stripping of natural moisturizing factors

Concentration by cleanser type:

Hair Care Products

Typical glycerin concentrations: 0.5-5%

Benefits in hair formulations:

• Moisture retention: Helps hair retain water, reducing frizz
• Detangling: Improves slip and combability
• Shine enhancement: Creates reflective surface on hair shaft
• Heat protection: Provides some thermal protection during styling

Climate considerations for hair products:

Soap and Bar Products

Typical glycerin concentrations: 5-25%

In traditional saponified soap:

• Glycerin is naturally produced during saponification (fat + lye → soap + glycerin)
• Handcrafted soaps retain natural glycerin (8-15%)
• Commercial soaps often remove glycerin for separate sale
• "Glycerin soap" contains added glycerin (15-25%) for extra mildness and clarity

In synthetic detergent bars (syndet bars):

• 5-15% glycerin added for moisturization and mildness
• Balances cleansing with skin conditioning
• Can improve bar hardness and prevent cracking

Pharmaceutical Applications: Glycerin as an Excipient

In pharmaceutical formulations, glycerin serves as a multifunctional excipient with both functional and therapeutic benefits.

Oral Liquid Formulations

Typical glycerin concentrations: 10-50%

Functions in oral liquids:

• Solvent/co-solvent: Dissolves poorly water-soluble drugs
• Sweetener: Masks bitter taste of active ingredients
• Viscosity modifier: Creates desirable syrup-like consistency
• Preservative enhancement: Reduces water activity, supporting preservation
• Stability: Prevents crystal formation in supersaturated solutions

Common applications:

Topical Pharmaceutical Preparations

Typical glycerin concentrations: 5-20%

Dermatological Preparations

• Eczema and psoriasis treatments: 10-15% glycerin provides moisture to compromised skin barrier
• Wound healing preparations: 15-20% glycerin maintains moist wound environment
• Corticosteroid creams: 5-10% glycerin enhances penetration and reduces irritation
• Antifungal creams: 5-10% glycerin improves spreadability and adherence

Ophthalmic Preparations

• Artificial tears/lubricating eye drops: 0.2-2% glycerin as demulcent and viscosity agent
• Contact lens solutions: 1-5% glycerin as wetting agent and lubricant
• Must meet strict USP sterility and particulate standards

Suppositories

• Glycerin suppositories: 85-95% glycerin as laxative (osmotic effect)
• Base for medicated suppositories: 10-30% glycerin in gelatin-glycerin bases
• Glycerin draws water into rectum, stimulating evacuation

Parenteral Formulations (Intravenous)

Typical glycerin concentrations: 10-50%

Clinical applications:

• Cerebral edema treatment: 10% IV glycerin solution reduces intracranial pressure
• Glaucoma treatment: 50% oral glycerin solution reduces intraocular pressure
• Excipient in injectable formulations: Co-solvent for poorly soluble drugs
• Requires injection-grade glycerin meeting stringent USP standards

Formulation Guidelines: Optimizing Glycerin Use

Concentration Selection Decision Matrix

Incorporation Methods

Stability Considerations

Factors affecting glycerin-containing formulations:

Microbial Stability

• Water activity reduction: Glycerin at 20%+ lowers water activity, supporting preservation
• Doesn't replace preservatives: Typical cosmetic concentrations (3-10%) don't provide antimicrobial effect
• Compatible preservatives: Works with most preservation systems (parabens, phenoxyethanol, organic acids)
• Challenge testing: Always conduct preservative efficacy testing per USP <851> or ISO 11930

Physical Stability

• Emulsion stability: Glycerin can improve or destabilize emulsions depending on concentration and emulsifier system
• Phase separation: Very high glycerin (>20%) may cause phase separation in some emulsions
• Crystallization: Pure glycerin can crystallize at 17.8°C—formulations generally don't have this issue
• Hygroscopicity: Glycerin formulations absorb moisture from air if not properly packaged

Chemical Stability

• Oxidation: Glycerin itself is stable, but can be oxidized under extreme conditions (high temp + oxygen)
• Acrolein formation: At temperatures >150°C, glycerin can degrade to acrolein (irritant)—not an issue at normal processing temps
• pH stability: Glycerin is stable across wide pH range (3-11)
• Active compatibility: Generally compatible with most cosmetic and pharmaceutical actives

Packaging Recommendations

For glycerin-containing products:

Quality Standards and Regulatory Compliance

USP Monograph Requirements

USP Grade glycerin must meet specifications outlined in the United States Pharmacopeia monograph:

Regulatory Considerations by Region

United States (FDA)

• Cosmetics: Glycerin recognized as safe (GRAS) cosmetic ingredient. Listed in FDA's Cosmetic Ingredient Database. No concentration restrictions for topical cosmetic use.
• Pharmaceuticals: Must meet USP standards. Included in FDA's Inactive Ingredient Database for various routes (oral, topical, ophthalmic, parenteral).
• Food: GRAS status, permitted food additive. FCC specifications apply for food use.
• INCI Name: Glycerin

European Union

• Cosmetics: Approved under EU Cosmetics Regulation (EC) No 1223/2009. CosIng database lists glycerin with no restrictions.
• Pharmaceuticals: Must meet European Pharmacopoeia (EP) specifications.
• INCI Name: Glycerin
• E-number (food): E422 (approved food additive)

Other Markets

• Japan: Japanese Pharmacopoeia (JP) standards for pharmaceutical use
• China: Approved cosmetic ingredient, must meet Chinese Pharmacopoeia standards for drugs
• Canada: Approved for use in cosmetics, drugs, and natural health products
• Australia: TGA approved for therapeutic goods

Labeling Requirements

INCI Declaration: Glycerin

Alternative names that may appear:

• Glycerol
• Glycerine
• 1,2,3-Propanetriol
• Glycerolum (Latin pharmaceutical name)

Label claims considerations:

• "Vegetable glycerin" - Can be claimed if glycerin is vegetable-derived (requires verification)
• "Natural" - Acceptable if glycerin is plant-derived and minimally processed
• "Moisturizing" - Substantiated claim based on glycerin's known humectant properties
• "Hydrating" - Similar to moisturizing, well-supported
• "Organic" - Only if glycerin is certified organic (rare, requires specific certification)

Frequently Asked Questions

Alliance Chemical: Your Source for Pharmaceutical-Grade Glycerin