J. Cosmet. Sci., 64, 411–427 (November/December 2013) 411 Use of statistical modeling to predict the effect of formulation composition on conditioning shampoo performance CAROLE LEPILLEUR, ANN GIOVANNITTI-JENSEN, and CAROL KYER, Lubrizol Advanced Materials Inc., Personal and Home Care, Brecksville, OH 44141. Accepted for publication May 13, 2013 Synopsis Formulation composition has a dramatic infl uence on the performance of conditioning shampoos. The purpose of this study is to determine the factors affecting the performance of various cationic polymers in those systems. An experiment was conducted by varying the levels of three surfactants (sodium lauryl ether sulfate, sodium lauryl sulfate, and cocamidopropyl betaine) in formulations containing various cationic polymers such as cationic cassia derivatives of different cationic charge densities (1.9, 2.3, and 3.0 mEq/g), cationic guar (0.98 mEq/g), and cationic hydroxyethyl cellulose (1.03 mEq/g). The results show the formulation composition dramatically affects silicone and cationic polymer deposition. In particular, three parameters are of importance in determining deposition effi ciency: ionic strength, surfactant (micelle) charge, and total amount of surfac- tant. The cationic polymer composition, molecular weight, and charge density are also important in deter- mining which of the previous three parameters infl uence the performance most. INTRODUCTION Common conditioning shampoos are formulated with cationic polymers such as cationic cellulose or cationic guar derivatives, which are compatible in the shampoo formula but become incompatible on dilution with water. The literature suggests that on shampoo application, foaming, washing, and rinsing, such cationic polymers form a complex with anionic and amphoteric surfactants that phase separate from the bulk solution. This phase separation, or coacervation, is also known as the “Lochhead effect” (1). Phase separation on dilution has been explained in the literature by the coulombic attraction between the anionic function of the surfactant and the cationic groups of the polymer. Goddard (2) described that at low surfactant concentration [below the critical micelle concentration (CMC)] anionic surfactants condense on the polycations. The resulting ion pairs convert Address all correspondence to Carole Lepilleur at Carole.Lepilleur@lubrizol.com. This paper was presented at the fi fth International TRI/Princeton Conference on Applied Hair Science, September 20–21, 2012.
JOURNAL OF COSMETIC SCIENCE 412 the cationic sites into hydrophobe-substituted sites. Hydrophobic interactions within and between the polycations cause phase separation. The phase separation persists if the positive and negative charge equivalent ratio is at stoichiometry. Above the surfactant CMC, co-micellization of the cationic polymer with the surfactant results in a one-phase soluble complex, or clear system. The coacervate is often described as a gel-like phase that contains a high level of cationic charge and is known to deposit the polymer on negatively charged hair, forming a clear fi lm (3–5). In addition, the coacervate aids in the deposition of insoluble actives such as silicone. The coacervation behavior and the type of coacervate formed vary depending on many criteria, such as the cationic polymer characteristics (charge density and molecular weight), the cationic polymer concentration, the surfactant blend used in the formula- tion, the ionic strength, pH, and temperature. For instance, the molecular weights of the cationic polymers were shown to infl uence the amounts of coacervate, where the high- molecular-weight polymers formed more coacervate than the low-molecular-weight poly- mers (6). The objective of this study is to determine the effect of the surfactant formulation com- position on the conditioning performance of various cationic polymers. A previous study (7) showed that three parameters are of importance in determining silicone deposition of cationic cassia polymers: polymer charge, surfactant micelle charge (i.e., the amount of anionic content in the surfactant micelle), and total amount of surfactant. However, the study also showed that the effects of the total amount of surfactant and ionic strength were confounded. A new design of experiments, as illustrated in Figure 1, was developed to unconfound these factors by considering the infl uence of micelle charge, amount of surfactant, and ionic strength independently. The design was also expanded to include different cationic polymers: a cationic cassia polymer of a mid-range cationic charge den- sity (2.3 mEq/g) in addition to the cationic cassia polymers of reference 7, which have Figure 1. New design space for shampoo formulations with various micelle charge (mole fraction), ionic strength (in S/m), and surfactant amount (in mol). The various symbols represent curved surfaces of the over- all design space. Each symbol represents a possible formulation composition for the design.
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