Cationic surfactants are important personal care ingredients for hair conditioning applications. They improve hair combability in both the wet and dry state and can mitigate the effects of hair damage.

The majority of cationic surfactants currently used in personal care formulations are quaternary ammonium species (quats). They have a permanent cationic charge independent of formulation pH, are not 100% biobased, and have potential for environmental and aquatic toxicity over their life cycle.

Amino Lipids, including Brassicyl Valinate Esylate and Brassicyl Isoleucinate Esylate are 100% biobased cationic surfactants derived from the esterification of brassica alcohol with amino acids. They function as hair and skin conditioning agents, emulsifiers, and structuring agents.^1,2

They form liquid crystalline (LC) systems when combined with nonionic amphiphiles, such as fatty alcohols or glyceryl esters. In formulation, lamellar LC structures are desired for their ability to create long-range order and structure which improves formulation stability and builds viscosity. Lamellar LC structures also give greater substantivity to skin and hair.

To further understand the science behind conditioning performance differences between BIE and BVE, characterization of lamellar LC formation was done at the bulk phase, with the amino lipids as aqueous solutions, and in a blend with fatty alcohols as a chassis formula.

Differential Scanning Calorimetry (DSC) assesses thermal transitions to determine the degree of crystallinity of a material. DSC was run on BVE and BIE neat (≥ 94% purity) to assess differences in thermal transitions. BVE displayed higher temperature thermal transitions than BIE (Tm = 48.8°C vs. 41.8°C), suggesting BVE molecules in the crystalline state are more closely packed than BIE molecules in the crystalline state. The smaller head group in BVE allows for greater van der Waals attraction as carbon chains pack closer together, leading to this greater intermolecular cohesion that manifests as a higher melting temperature.⁴

Above critical concentrations, disordered surfactant solutions turn to an ordered state. Depending on head group size and/or polarity, the LC phases formed will change and affect bulk properties and stability.⁵ To evaluate these properties, aqueous solutions of BVE or BIE with concentrations ranging from 5-50% of the BVE or BIE were prepared.

Visual assessments of the solutions over a temperature range from 0-80°C showed the greatest change in appearance and texture of BVE and BIE occurred at 60°C. Below this temperature, both were viscous and flowable. Above 60°C, the solutions became more elastic in appearance and behavior. BIE turned from an opaque solution to clear, suggesting a transition into an isotropic cubic phase, whereas BVE stayed stable in the lamellar phase.

Viscosity measurements were performed at room temperature as a function of increasing BVE or BIE concentration. Viscosities of the aqueous solutions remained similar up to 30% amino lipid. At concentrations ≥30%, the viscosity of BVE increased dramatically versus BIE. This difference in concentration dependent viscosity is attributed to BVE’s propensity to form more ordered structures.

To further evaluate the differences between BIE and BVE, a dynamic frequency sweep over a range of angular frequencies was performed. The BVE solution had a greater storage modulus and more elastic character than BIE, as well as having a higher complex viscosity, displaying BVE’s greater resistance to deformation.⁴

Lamellar LC structures are known to impact conditioning performance. To have maximum benefits, these structures must persist under typical formulation conditions including homogenization and elevated temperatures. Chassis formulations were used to evaluate BIE and BVE structure under typical formulation conditions. Solutions of BVE or BIE with pH adjuster and brassica alcohol (“HC Blends”) were prepared.

Elevated temperature effects on structure were assessed on 15% HC Blends via small angle x-ray scattering (SAXS) over a temperature range from 20-70°C. Structures that are crystalline in nature present “Braggs peaks” and were seen for both BVE and BIE HC Blends. The BVE HC Blend maintained structure peak intensity at 60°C, compared to the BIE HC Blend which started to lose structure at this temperature.⁴

In summary, the difference of one methylene (CH₂) group in the amino lipid head group leads to pronounced differences in the LC phase behavior and corresponding hair conditioning performance of cationic amino lipids. BVE is observed to form lamellar LC phases over a broader range of concentrations and temperatures compared to BIE. This is attributed to the more compact head group of BVE and correspondingly greater critical packing parameter value compared to BIE. The enhanced ability of BVE to form lamellar LC phases translates to improved formulation stability and hair conditioning performance over BIE.

References

1. R. Burgo, Non-petrochemically derived cationic emulsifiers that are neutralized amino acid esters and related compositions and methods, US 8,287,844 B2, 2012.

2. R. Burgo, Non-petrochemically derived cationic emulsifiers and related compositions and methods, US 20180193233 A1, 2018.

3. https://www.triprinceton.org/hair-claims-testing
4. Pease, B. (2019, December). Understanding Biobased Cationic Amino Lipids: Effects of amino acid structure on liquid crystalline phase behavior in conditioning formulations. Inolex, Inc., presented at Society of Cosmetic Chemists Scientific Meeting & Technology Showcase.
5. Holmberg, K, et al. Phase behaviour of concentrated surfactant systems. Ch. 3 in Surfactants and Polymers in Aqueous Solution, John Wiley & Sons Inc.: Hoboken, NJ, 2003, pp 67-95.