Long carbon chain quaternary ammonium salts are cationic surfactants. They are used widely as conditioners to keep hair flexible and soft and to improve combability, among other benefits.1, 2 Long hydrocarbon chain quaternary ammonium salts also carry a positive charge on their nitrogen atom, and thus have good affinity to hair. After shampoo and conditioner treatments, they remain deposited on the hair surface, which provides good combing, anti-static and detangling performance.3, 4 However, long carbon chain quaternary ammonium salts are not compatible with anionic surfactants. They form complexes and precipitates in the formulation, which create instabilities. In some cases, modifications to the molecular structure can improve compatibilities—with the addition of ethylene oxide (EO) and propylene oxide (PO) groups, for example.
In contrast to cationics, alkyl phosphate esters are anionic surfactants, widely used in body washes and facial cleansers. Here, the authors combined the former quaternary ammonium structure with this phosphate structure to produce a surfactant with both positive and negative charges (i.e., an amphoteric structure), resulting in behenamidopropyl PG-dimonium chloride phosphate (BMDCP). Such long carbon chain quaternary salt-phosphate esters have desired amphoteric characteristics, in addition to preservative effects.5-7 This article reports the authors’ findings on the benefits of this material for hair care applications.
Materials and Methods
BMDCP (see Figure 1) is a naturally derived phospholipid composed predominantly of diester and triester phosphatides with multiple chain groups. Having an amphoteric character, it is compatible with most types of ingredients, including anionic surfactants. Here, the authors assessed its hair conditioning effects.
Formulas and substrates: A standard shampoo with and without 0.5% BMDCP added was prepared for combability tests. Three rinse-off conditioners containing BMDCP and stearyl alcohol were also prepared for visual characterization, hair softening and combability tests. Asian (Chinese) and bleached hair samples were obtained as the test substrates.
Test methods: Wet and dry combing force measurements of treated hair tresses were assessed using a tensile testera. Subjective evaluations of wet combing, dry combing, wet feel, hair softening and anti-static characteristics were given by 20 panelists. The structures of three test hair conditioners were inspected under an optical microscope having crossed polarizers; finally, the softness of hair bundles was determined by a texture analyzer.
Results and Discussion
Table 1 and Figure 2 show the data and effects of standard and 0.5% BMDCP shampoos on the wet and dry combing forces of hair tresses. The addition of 0.5% BMDCP reduced the combing force required in both cases. In the subjective evaluations of 20 female participants, 90% of panelists found the shampoo with BMDCP to provide improved conditioning performance, with better wet-combing feel during washing and less static when hair was dry.
For visual assessments, the compositions and descriptions of the three rinse-off conditioners prepared are listed in Table 2. The BMDCP incorporated was 67.2% solid, and all formulas were kept at the same fatty alcohol to quat ratio of 3/1. It is clear that the formulation viscosity increased with increasing solid content. Polarized microscopy images of these formulations are presented in Figure 3, Figure 4 and Figure 5, respectively. At the lowest solid concentration, Formula A at 6.0%, the formulation showed few liquid crystals and some undissolved raw materials, likely the quat. At a higher solid concentration, Formula B at 9.0%, the conditioner contained more liquid crystals and fewer undissolved raw materials. At the highest solid concentration, Formula C at 12.0%, the conditioner formed a nice liquid crystal structure; Figure 6 demonstrates the liquid crystal phase at a magnification of 400×.
Formula C was next used to evaluate the material’s softening performance on Asian and bleached hair tresses. As noted, the hair samples were obtained from a commercial supplier, and a texture analyzer was used to determine the bending force of the hair bundles. A modified three-point bending mode was used. The maximum bending distance of the center of the tested bundle was set at 5.0 mm. Data on the average changes in bending force of Asian hair and regular bleached hair bundles, before and after the conditioner treatment, is presented in Figure 7.
It is apparent that both Asian and bleached hair tresses treated with the conditioner showed significant reductions in bending forces, compared with their respective controls. On average, their bending forces were reduced by 25.5% and 35.8%, respectively, demonstrating good hair softening performance.
Finally, Formula C was evaluated for its conditioning performance on Asian and bleached hair tresses. A tensile tester was used to determine the wet and dry combing forces required. Experimental results are summarized in Figure 8 and Figure 9. The conditioner improved wet and dry combing for both hair samples.
Described here is BMDCP, a material combining the functionalities of a quaternary ammonium structure with a phosphate structure for applications in hair shampoos and conditioners. Results from objective measurements and subjective evaluations indicate that standard shampoos containing BMDCP provide good conditioning performance. Further, a simple conditioner system containing BMDCP formed a nice liquid crystal structure, and demonstrated good hair softening performance.
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