7.1 Cosmetic Chemistry, pH Scale, Emulsions & Surfactants

Cosmetic Chemistry, pH Scale, Emulsions & Surfactants

Introduction to Cosmetic Chemistry

Chemistry is the foundational science of the esthetics profession. Understanding how products interact with the skin at a molecular level allows professionals to treat conditions effectively and safely. In cosmetic chemistry, we categorize the field into two main branches: organic and inorganic chemistry.

Organic vs. Inorganic Chemistry

Organic chemistry is the study of substances that contain carbon. In the realm of cosmetics, this includes all living or formerly living things, such as plant extracts, synthetic materials derived from petrochemicals, and plastics. Because these substances contain carbon, they can burn. Most skin care products, particularly those with botanical ingredients or active compounds like retinoids, fall into this category.

Inorganic chemistry deals with compounds that do not contain carbon. These substances have never been alive and will not burn. Examples critical to esthetics include water, minerals, and metals. Physical sunscreens like zinc oxide and titanium dioxide are classic examples of inorganic compounds used daily in skincare.

Matter and States of Matter

Matter is anything that occupies space and has mass. It exists in three primary states: solid, liquid, and gas. The state of matter depends on temperature and pressure.

• Solids have a definite shape and volume. Examples include ice or solid waxes in lip balms.
• Liquids have a definite volume but take the shape of their container. Cleansers and toners are typically liquids.
• Gases have no definite shape or volume. Steam used during facials is water in its gaseous state. Cosmetic formulations manipulate these states to create products that are stable, spreadable, and effective.

The pH Scale and the Acid Mantle

The term "pH" stands for potential hydrogen, measuring the acidity or alkalinity of a substance. It is crucial to note that only aqueous (water-based) solutions have a pH. Oils and lipid-based products do not have a pH.

Understanding the Logarithmic Scale

The pH scale ranges from 0 to 14, with 7 being neutral (pure water).

• Acidic solutions fall below 7.
• Alkaline (basic) solutions range above 7. The scale is logarithmic, meaning a change of one whole number represents a tenfold change in acidity or alkalinity. For example, a pH of 5 is 10 times more acidic than a pH of 6, and 100 times more acidic than a pH of 7.

The Acid Mantle

The acid mantle is a protective film on the skin's surface, composed of sebum, sweat, and cellular lipids. The optimal pH of the skin's acid mantle is between 4.5 and 5.5, making it mildly acidic. This acidity is essential for inhibiting the growth of harmful bacteria, viruses, and fungi, while supporting the skin's barrier function.

When the skin's pH is disrupted by highly alkaline cleansers, the barrier is compromised, leading to transepidermal water loss (TEWL), dryness, and increased susceptibility to infection and irritation. Formulating and selecting products that respect and restore the acid mantle is a core competency for any esthetician.

Solutions, Suspensions, and Emulsions

In product formulation, ingredients are combined in various ways to create the final product. The three most common mixtures in cosmetics are solutions, suspensions, and emulsions.

Solutions

A solution is a uniform mixture of two or more mutually miscible substances (meaning they mix easily without separating). It consists of a solute (the substance dissolved) and a solvent (the substance that dissolves the solute). Water is the universal solvent. Toners and micellar waters are typical cosmetic solutions.

Suspensions

Suspensions contain solid particles distributed throughout a liquid medium. These particles are larger than those in a solution and tend to separate over time. Calamine lotion and many liquid foundations are suspensions, which is why they often include instructions to "shake well before use."

Emulsions

An emulsion is an unstable physical mixture of two or more immiscible substances (like oil and water) united with the aid of an emulsifier. Emulsions are the foundation of most creams and lotions.

There are two primary types of emulsions used in skincare:

• Oil-in-Water (O/W) Emulsions: Oil droplets are dispersed in a water base. These are typically lighter, fast-absorbing, and preferred for normal to oily skin types. Most moisturizing lotions are O/W emulsions.
• Water-in-Oil (W/O) Emulsions: Water droplets are dispersed in an oil base. These are heavier, richer, and provide a strong occlusive barrier, making them ideal for dry, compromised skin, or cold climates. Night creams and cold creams are often W/O emulsions.

Surfactants and Emulsifiers

Surfactants (surface-active agents) are the unsung heroes of cosmetic chemistry. They reduce surface tension between the skin and the product, allowing cosmetics to spread and penetrate more easily.

Molecular Structure of Surfactants

A surfactant molecule has two distinct parts:

• Hydrophilic Head: Water-loving, meaning it is attracted to water.
• Lipophilic Tail: Oil-loving, meaning it is attracted to oil and dirt.

In a cleanser, the lipophilic tails bind to sebum and makeup, while the hydrophilic heads attach to water. When rinsed, the water pulls the surfactant molecules away, taking the trapped oil and dirt with them.

Emulsifiers as Surfactants

Emulsifiers are a specific type of surfactant used to bind oil and water together in an emulsion. By positioning their hydrophilic heads in the water phase and their lipophilic tails in the oil phase, emulsifiers stabilize the mixture and prevent the two phases from separating.

Advanced Delivery Systems

For active ingredients to be effective, they must reach the appropriate layer of the skin. Advanced delivery systems encapsulate these ingredients, protecting them from degradation and enhancing penetration.

• Liposomes: These are microscopic, hollow, fluid-like spheres composed of lipid bilayers (similar to cell membranes). They can encapsulate water-soluble ingredients, facilitating their delivery through the lipid-rich stratum corneum. Liposomes slowly release their contents, providing a time-released effect.
• Polymersomes: Similar to liposomes but made from synthetic polymers, polymersomes offer greater stability and can carry both hydrophilic and lipophilic active ingredients. They are highly customizable, allowing formulators to design specific release profiles for potent anti-aging compounds.

Understanding these chemical principles is essential. Estheticians must continuously evaluate product formulations to ensure they are applying the correct chemistry to safely treat their clients' specific skin conditions.