J. Cosmet. Sci., 69, 397–405 (September/October 2018) 397 Applying Light Scattering Theory to Measure Rinsability of Hair Conditioners SELENE VELEZ, SARAH HEETHER, JUNG-MEI TIEN, and FARAHDIA EDOUARD, Croda Inc., Edison, NJ 08837 (S.V., S.H., J.-M.T., F.E.) Synopsis As the world is striving to become more sustainable, water consumption is considered an important area of focus, especially in those regions with limited freshwater resources. To address this issue, the personal care industry has identifi ed faster rinsability of hair care products as a way to contribute to water preservation efforts. To understand rinsability, analysis of colloidal systems and an investigation into concentration of whole products in water is critical. However, particle size and particle migration in colloidal systems require the use of specialized optical methods. In previous research, we learned that conditioners form colloidal particles rather than true solutions during the rinsing process, and hence cannot be studied using ultraviolet-visible spectroscopy. Through this study, a Turbiscan instrument was determined to have the capability of measuring multiple light scattering given off by conditioner systems. Therefore, measurements of light scatter from a series of diluted conditioner dispersions can be used to generate a calibration curve to calculate unknown concentrations of conditioner in rinse water at different rinsing time intervals. The newly developed test method was successfully applied to determine the rinsability of various conditioner formulations on both virgin and bleached hair. The fi ndings of our study will be presented here. INTRODUCTION Water scarcity is a paramount concern as it was once considered to be a renewable resource, and is now gradually becoming a non-renewable resource. Globally, there is only 3.5% of available freshwater, relative to salt water, to meet the demand of an ever growing global population (1). Water use has been growing globally at more than twice the rate of popu- lation increase in the last century. As a result, an increasing number of regions are reaching the limit at which water services can be sustainably delivered, especially in arid regions. Currently, 1/5 of the global population, about 1.5 billion people, live in areas where water is physically scarce. By 2025, about 1.8 billion people are expected to live in areas with absolute water scarcity (2). As water scarcity increases across developing regions, manufac- turers of rinse-off beauty and personal care products will face challenges in these regions where most of the potential growth in sales in the future is expected. Address all correspondence to Farahdia Edouard at Farahdia.Edouard@croda.com.

JOURNAL OF COSMETIC SCIENCE 398 To fi ght the global water crisis, we see more and more water conservation efforts being under- taken by consumers, including cutting down on the use of water in personal grooming practices. Consumer awareness of this issue is on the rise, and water conservation is becom- ing a top priority for them. As consumers are cutting back on their water usage, they are expecting personal care brands to do the same (3). Sustainability is becoming more and more integral to the business model of consumer goods companies as it is becoming increas- ingly important in the minds of consumers. Essentially, consumers need products that require less rinsing without compromising the performance of the product. In an effort to reduce the water footprint in the personal care industry, a few companies have started to develop water-smart products with a primary focus on “faster rinsing” claims with the aim of conserving water. However, there is currently no method available to describe the rinsing behavior of products. To live a life of responsible consumption and production, a method is needed to quantify the rinsability of products to prove that they do, in fact, rinse out faster than your conventional shower product. Rinsability is a term coined by the industry to describe the rinsing behavior of personal care products. It is not a new concept. Many companies have developed their own rinsability method by focusing on hair attributes such as wet friction or bending force. However, these methods aim to only focus on the hair itself. Although this is a good starting point, they fail to analyze the product as it washes off the hair. For this reason, studies of the rinsed water have been conducted, using conditioner systems, to describe the rinsing per- formance of such systems and to approximate what ultimately remains on the hair surface. MATERIALS AND METHODS Although conditioners are generally composed of 80–90% water, the remaining percent- age is composed of miscellaneous ingredients that are insoluble in water, thus creating a colloidal system. We can take advantage of the colloidal system to approximate the con- centration of product that is washed off using properties of light scattering. Light scattering is simply the redirection of light it happens when energy waves are forced to deviate from a straight path because of imperfections in a given medium (4). Light scatter can be used to approximate unknown amounts of conditioner material in water by the Beer–Lambert law. The Beer–Lambert law, or A = εmCℓ, is a linear relationship between absorbance and concentration. It is normally used for samples that absorb light at a particular wave- length. However, because conditioner materials mainly scatter light, optical density is used instead of absorbance. Optical density is not a measure of absorbance, but rather a measure of light scattered by particle suspension which manifests itself as absorbance. As visible light passes through a suspension, the light is scattered. Greater scatter indicates more particles present. This method is often used in biological related fi elds to study bacteria that are often colorless and do not scatter light (5). The optical density is represented in terms of transmittance. Transmittance is inversely proportional to absorbance, given by the Beer– Lambert Law. Traditionally, spectrophotometers are used to measure absorbance. However, they are not optimized for light scattering measurements. Spectrophotometers commonly result in differences in measurements between scans and between instruments (6). Because of the nature of the samples, the Turbiscan from Formulaction was used for this study. The Turbiscan uses multiple light scattering theory associated to a vertical scanning head, enabling it to measure particles of varying size and movement. The Turbiscan is used here to obtain transmittance values for our samples (7).