Lamellar gel networks (LGNs) are combinations of low- and high-HLB (hydrophilic-lipophilic balance) crystalline surfactants that form colloidal structures that can swell and thicken water. The gel network is stabilized by lamellar bilayers of surfactant, which bind water. A lamellar gel network can be defined as a network formed by bilayer sheets where the alkyl chains in the bilayers are essentially in a frozen, non-melted state. This type of lamellar phase is also called alpha phase or alpha gel. These systems have viscoelastic properties and shear thin when applied to skin. LGNs are important because most oil-in-water skin-care emulsions sold globally are based on LGNs stabilized with polymers. Most hair conditioners sold globally are also based on LGNs. The peroxide phase of many two-part hair-dye formulations and many cream depilatories and hair relaxers are also LGN-based.
The advantages of LGNs include:
- LGNs can formulate pH-, salt- and peroxide-stable systems.
- LGNs can suspend and stabilize powders or pigments.
- LGNs have good moisturizing properties.
- LGNs can prolong the release of oil-soluble actives.
- LGNs can stabilize some types of actives.
- LGNs are very stable, if phase transition temperature is >50°C
- LGNs can make waterproof and wash-resistant products
- LGNs are very low in skin-irritation potential
- LGNs are very cost-effective
The disadvantages of LGNs include processing sensitivity and the need to use high levels of low-HLB surfactant (typically 3 to 10 percent), which can negatively impact skin-feel.
The purpose of the low-HLB crystalline surfactant is to thicken.
Key properties include:
- The melting point typically should be greater than 50°C.
- Fatty alcohols are the most efficient at thickening.
- Cetyl, stearyl alcohols, eicosanol (C20), Behenyl alcohol (C22), glyceryl stearate and sorbitan stearate are the most commonly used materials.
- It is very difficult to swell fatty alcohols longer than C18 to 20 without using a more polar low-HLB co-surfactant like glyceryl stearate or sorbitan monostearate, which are easier to swell than fatty alcohol.
- Using high shear to reduce the particle size can also sometimes help.
The purpose of the high-HLB crystalline surfactant is to swell the low-HLB surfactant to promote lamellar bilayer formation. The surfactant can be nonionic, anionic or cationic; generally C16 to C22 saturated derivatives work the best. The high-HLB surfactant also should be water soluble or dispersible.
The viscosity of LGNs is determined mostly by the crystallinity of the system, the total surfactant concentration and the ratio of the high-HLB surfactant to the total surfactant concentration (high- over high-plus-low HLB). Other factors include the presence of electrolytes, which reduces the bound water in the LGN and polar solvents, such as henoxyethanol or ethylhexylglycerin, which reduce the crystallinity.
The crystallinity of the system is generally referred to as the phase transition temperature (PTT), or the temperature at which the emulsion will lose most of its viscosity when heated. Generally, higher PTTs also correlate with higher viscosity. As long as the PTT is above your oven stability temperature, the LGN will be heat-stable. The ratio of surfactants is important because only certain ratios permit the lamellar phase to form. This is due to the packing parameter or geometry of the surfactants. The optimum ratio when using cetearyl alcohol and a cationic or anionic surfactant is around 15 to 25 percent (high- over high-plus-low HLB).
- Salt-stable LGNs normally need a PTT greater than 60°C.
- Always add salts, fragrances and solvent preservatives below the PTT, during cool-down.
- Low-HLB surfactants like behenyl alcohol require milling above the PTT to promote optimal swelling.
- Adding LGN surfactants to the water phase normally produces smaller particle emulsions.
- Use blends of cetyl and stearyl alcohol when they are the only low-HLB swelling surfactant used (40:60 to 60:40).
- The best LGNs are normally the most efficient—those that generate high viscosity at the lowest surfactant concentration (5 percent and 50 to 100K cps)
- Large viscosity increases over time are usually the result of more lamellar phases being formed, due to unoptimized processing or surfactant ratios (excessive fatty alcohol hydrates).
- LGN graininess after freeze/thaw stability testing can be caused by poor solubility of the high-HLB surfactant, precipitating or crystallizing from the LGN. LGNs formed using behenyl alcohol can also cause this graininess.
- Low levels of hydrophobically modified polymers can dramatically increase LGN viscosity (.05 to .15 percent).
- Always try different ratios of high- to high-plus-low HLB surfactant ratios (10 to 30 percent).
Recommended low-HLB crystalline surfactants:
- Cetyl, stearyl, cetearyl, eicosanol or behenyl alcohols
- Glyceryl stearate, sorbitan stearate, methyl glucose sesquistearate or polyglyceryl-3 methyl glucose distearate
Recommended high-HLB surfactants:
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