Topical and Transdermal Part II (Emulsifiers)

Anionic Soaps & Surfactants

*Soft Soaps* These are salts of fatty acids in which the positive ion is univalent (such as Na+ and K+) *Anionic Surfactants* Detergents - Sodium Laureth Sulfate - Sodium Lauryl Sulfate

Total Amount of Surfactant Recommended

2-5% w/v of the total emulsion of 10-20% of the internal phase. Note: Often a blend of emulsifiers produces a more stable emulsion than does the use of a single emulsifier with a correctly calculated HLB.

Emulsion

An emulsion is a dispersion of two immiscible liquids: Water-in-oil (W/O) emulsion, where oil is the continuous phase containing water droplets Oil-in-water (O/W) emulsion, in which oil droplets are dispersed in a continuous phase of water

Emulsifiers & Surfactants

Anionic Soaps and Surfactants (Sodium lauryl sulfate) Cationic Surfactants (Benzalkonium Chloride) Non-ionic surfactants: - Natural (Fatty alcohols: Stearyl Alcohol) - Synthetic (Spans and Tweens)

Emulsifying Agents (Emulsifiers, Surfactants, Surface Active Agents)

Compounds that lower the interfacial tension and form a film at the oil/water interface. They adsorb onto the surfaces or interfaces of the system and lower the surface or interface free energies.

An Emulsion Consist of...

Dispersed phased (internal phase or discontinous phase) Dispersion medium (external phase or continuous phase) Emulsifying Agent

Required HLB of the Surfactant Blend

Each lipophilic ingredient used in O/W or W/O emulsion has what we call a "required" HLB. This the HLB value of the surfactant (or surfactant blend) that provides the lowest interfacial tension between the oil and water.

HLB Value of a Surfactant Blend

Example: A 50/50 blend of Span 60 (HLB 4.7) and Tween 40 (HLB 15.6) HLB of the blend: Span 60 (0.5 x 4.7) + Tween 40 (0.5 x 15.6) = 2.35 + 7.8 = 10.15

Natural Non-Ionic Surfactants

Fatty alcohols: Stearyl and Cetyl Alchohol

Surfactants

Have two regions in their chemical structure, one hydrophilic and the other hydrophobic.

Tweens (Polysorbates)

Poly: Polyoxyethylene (POE) or polyethelyneglycol (PEG)

Required HLB of the Surfactant Blend

Required HLB values for various types of formulations with common ingredients (determined experimentally).

Synthetic Non-Ionic Surfactants: Spans and Tweens

Spans (sorbitan esters) are lipophilic surfactants that tent to form W/O emulsions (Uniqema), Crill® (Croda) Tweens (polysorbates) are hydrophilic surfactants that tend to form O/W emulsions (Uniqema), Crillet® (Croda)

Synthetic Non-Ionic Surfactants

Synthetic surfactants predominantly are either: - Lipophilic (i.e. They would prefer to from W/O emulsions) - Hydrophilic (i.e. They would prefer to form O/W emulsions) The relationship (or balance) between the hydrophilic portion of the surfactant to the lipophilic portion is what we call HLB.

Hydrophile-Lipophile-Balance (HLB) System/Scale

The HLB value indicates the polarity of the molecules in an arbitrary range of 1-40, with the most commonly used emulsifiers having a value between 1 and 20. - The lower the HLB value, the more lipophilic or oil soluble the surfactant is (would like to form W/O emulsions). - The higher the HLB value, the more water soluble or hydrophilic the surfactant (would like to form O/W emulsions).

Tweens (Polysorbates)

The hydrophilic portion consists of free oxyethylene groups. The lipophilic part consists of long-chain hydrocarbons of fatty acids.

Spans (Sorbitans Esters)

The lipophilic part consists of long-chain hydrocarbons of fatty acids. The hydrophilic portion consists of free hydroxyl groups. Sorbitan monoesters Sorbitan diesters Sorbitan triesters

Tween 20 Polysorbate 20 POE (20) Sorbitan Monolaurate

The number (i.e., 20) following the polyoxyethylene part refers to the total number of oxyethylene groups found in the molecule. The number following the Tween part is related to the type of fatty acid: Monolaurate is indicated by 20, monopalmitate is indicated by 40, monostearate by 60, and monooleate by 80.

Cationic Surfactants

They do not make good emulsifiers but are useful as antimicrobial agents. These include quaternary ammonium compounds such as Benzalkonium Chloride.

Spans (Sorbitans Esters)

Water soluble sugar