j. Soc. Cosmet. Chem., 43, 259-273 (September/October 1992) Assessment of the substantivity of cationic quaternary compounds to hair by potentiometric titration using the surfactant electrode NGHI VAN NGUYEN, DAVID W. CANNELL, ROGER A. MATHEWS, and HANS H. Y. OEI, Redken Laboratories Inc., Research and Development Department, 6625 Variel Avenue, Canoga Park, CA 91303. Received February 7, 1992. Synopsis A new method to measure the sorption of cationic quaternary compounds by hair is reported. The method employs the potentiometric titration of cationic compounds, using a surfactant electrode as an endpoint indicator. Cationic uptake was determined from the difference of the cationic content in the solutions before and after the treatment of hair. By this method, the substantivity to bleached hair of mono- and poly- functional quaternary ammonium conditioning agents, as well as animal and botanical quaternary ammo- nium proteins, was measured as laurdimonium hydrolyzed wheat protein quaternium-26 stearalko- nium chloride cocodimonium hydrolyzed keratin protein. The results are consistent with previously published accounts: cationic uptake of quaternary compounds increases with increased solution concentra- tion, treatment time, temperature, and hair damage. INTRODUCTION Quaternary ammonium compounds have been widely used as hair conditioning agents in cosmetics. These compounds encompass cationic surfactants, cationic polyelectrolytes (polyquaternary polymers, copolymers), and cationic quaternary derivatives of hydro- lyzed proteins (animal, botanical). Deposition (substantivity) of these cationic com- pounds on hair can produce effects on fiber friction (i.e., wet and dry combing), stiffness, gloss, anti-static qualities, and strength (1). The substantivity of these cationic species to hair has been measured by various methods. These methods can be compli- cated, time-consuming, expensive, and non-quantitative. Light and electron microscopy have been used to visually evaluate the cosmetic effect of substantive proteins (2). Another non-quantitative method is the Rubine dye test in which the cationic uptake by hair is proportional to the intensity of the color developed after the reaction of an anionic dye and the adsorbed cationic compound (3). Some sophisticated means for the qualitative evaluation of the deposition of polymers and surfactants on the surface of hair include the use of the wetting force measurement 259

260 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of individual fibers (4), wettability scanning and microfluorometry'(5), and electron spectroscopy for chemical analysis (6). These methods require expensive instrumenta- tions and expert acquisition and interpretation of the data. One of the most powerful methods to measure the substantivity of quaternary com- pounds to hair is the application of radiotracer techniques (7-9). Being quantitative and accurate, this method is time-consuming and requires synthesis of radio-tagged starting materials. Spectrophotometric methods have also been widely used to analyze the substantivity of conditioning agents to hair. One method involves the absorbance measurement of the quaternary ammonium surfactant-Orange II dye complex (10,11). This procedure re- quires elaborate sample preparations. Recently, colloid titration (12) was shown to be a useful method to quantify the sorption of cationic polymer to bleached hair (13). The technique is based on a direct neutral- ization reaction between cationic species and potassium polyvinylsulfate. The color change of the o- toluidine blue indicator indicates the end point of the neutralization process. Another way to follow the neutralization process is by means of potentiometric titration. In this method, the positive potential reading of the cationic solution decreases as the cationic compound is allowed to react with the anionic standard solution. The complete 1:1 ion pair interaction is determined with the surfactant electrode as an endpoint indicator (14, 15). This technique is simple, fast, and reproducible. Application of the potentiometric titration with a surfactant electrode as an analytical tool in the cosmetic industry has not been reported in the literature. Only recently, Oei et al. applied this method to quantitatively analyze simple cationic surfactants in a commercial shampoo (16). The present investigation demonstrates the feasibility of the potentiometric titration of various mono- and polyfunctional quaternary ammonium conditioning agents as well as animal and botanical quaternary ammonium proteins. It also demonstrates the appli- cation of this simple method in quantitative measurement of the substantivity of these cationic quaternary compounds to hair under various experimental conditions. EXPERIMENTAL MATERIALS The cationic quaternary ammonium compounds tested in this investigation include cationic conditioners, cationic cellulosic polymer, cationic wheat protein, and cationic keratin protein (Table I). All compounds are commercial grade and used as received. Only stearalkonium chloride is not soluble in water at room temperature however, upon warming (40øC), it remains water-soluble under the conditions of the test. METHOD FOR POTENTIOMETRIC TITRATION The titration procedure was adopted from Oei et al. (16): a solution containing a measured amount of cationic compound, 0.6 ml of 0.01 N HCI, 2 ml of 1% solution