The fundamental benefits sought by hair care consumers relate to cleansing and care, and with the exception of cleansing, most of these benefits are delivered by conditioning ingredients deposited onto hair to change surface properties such as friction and surface energy. However, simply adding more ingredients to a formula will result in higher costs and may not even improve product performance, especially for a rinse-off product. To develop a hair care product with a high performance-cost profile, knowledge of the deposition efficiency of ingredients is key. In addition, correlating this deposition profile with consumer feedback will provide important guidance for formulators as they develop and optimize hair care products. In relation, traditional techniques to measure deposition are reviewed here, and a novel approach is introduced.
During washing, deposition and rinse-off are opposing behaviors. The driving force behind an ingredient’s deposition is related to its affinity to the hair surface. This is affected by chemical and physical properties such as hydrophobicity, charge type and density, molecular weight, particle size, conjugation/inert action with other ingredients, etc. Logically, if the affinity of an ingredient to the hair’s surface is stronger than the forces rinsing it away, the ingredient will remain on the hair surface. Many studies have been conducted to increase this deposition onto hair. To study deposition efficiency, a reliable, easy-to-use quantitative method is critical and thus far, two approaches generally are used: direct measurement and indirect measurement with extraction.
Direct measurement: Direct measurement analyzes treated hairs for the presence of ingredients. For example, x-ray fluorescence (XRF) can measure silicone deposits on hair by image analysis; however, XRF is limited to only silicone ingredients, and it cannot differentiate between pre-existing silicones on hair and those applied by test products. Thus, image analysis methods are usually not quantitative or suitable for routine deposition studies.
Indirect measurement with extraction: The indirect approach, which is more commonly used, analyzes extracts taken from a treated hair sample. The test product containing the target ingredient is applied to hair via a predetermined protocol, the treated hair is extracted using organic solvent, then the extract is collected and measured to determine the ingredient(s) and amount(s) present. Depending on the polarity of the target ingredient, different solvents can be used to prepare the test samples. For example, to analyze silicone, toluene and methyl isobutyl ketone (MIBK) often are used; for cationic surfactants, trichloromethane or methanol. Thus, to analyze silicone and quats, the preparation of samples must be done separately, which is time-consuming.