j. Cosmet. Sci., 54, 579-588 (November/December 2003) Effects of conditioners on surface hardness of hair fibers: An investigation using atomic force microscopy S. B. RUETSCH, Y. K. KAMATH, L. KINTRUP, and H.-J. SCHWARK, TRI/Princeton, Princeton, NJ 08542 (S.B.R., Y. K.K.), and Henkel KGaA, Diisseldorf, Germany (L.K., H.-J.S.). Accepted •br publication July 29, 2003. Synopsis Conditioners are known to have a prophylactic effect on hair damage caused by cosmetic chemical treatments or mechanical grooming procedures (1). They are known to impart softness and smoothness to hair by moisturizing the fiber (2). Since the amount of conditioners deposited on the fiber is very small in quantity, it is conceivable that mainly the surface is moisturized. This is especially true of polymeric conditioners, which deposit preferentially on the surface of the fiber, rather than penetrate into the cortex. Therefore, this study strictly investigates whether cationic polymeric conditioners impart softness to the surface cuticle cell as a result of their hydrophilicity, with no regard to its applicability to cosmetic effects. Such softening can be detected by indentation of the surface and can be quantified by measuring the depth of the indent in real time. Atomic force microscopy (AFM), equipped with nano-indentation capability, is ideally suited for this purpose. In this work it was used to determine changes in the microhardness (micromechanical properties) of the hair fiber surface as a result of fiber/conditioner/moisture interactions. In a preliminary study, we observed that the scale faces of hair treated with Polyquaternium 10 (PQ-10) conditioner gave deeper indents, while scale edges yielded shallower ones in comparison to cuticle cells of untreated hair. This suggests that the conditioner softens the scale face and hardens the scale edges. However, because of significant amounts of conditioner residues left on the scale face, this conclusion was rather ambiguous. Therefore, the study was repeated in which multiple indentations were made on the surface cuticle cells of a larger number of the same hair fibers before and after multiple applications of the conditioner. This reduces errors due to fiber-to-fiber variation in pre-existing microhardness differences in surface cuticle cells. Also, the larger number of fibers investigated in the current work allowed for a statistical outcome. This latter study has led to a rather definite conclusion that the scale face is indeed softened by polymeric conditioners such as Polyquaternium-10 (PQ-10). These studies will ultimately help in the development of conditioners with suitable moisturizing and softening effect on hair. INTRODUCTION Conditioners are applied to hair fibers to protect them from abrasion/ablation during everyday grooming (1), to improve general manageability, combability, and luster, and to reduce static charging (flyaway). In this study we strictly want to show whether a cationic conditioning compound present on the fiber surface imparts softness to the surface cuticle cell as a result of its hydrophilicity, with no regard to its applicability to 579

580 JOURNAL OF COSMETIC SCIENCE cosmetic properties. Most often, the active component of the conditioning formulation is a polymeric (such as PQ-10) or monomeric (such as cetyl trimethyl ammonium bromide CETAB) cationic quaternary compound, which has great affinity to the nega- tively charged hair fiber surface (pH = 3.67, isoelectric point) at pH values close to neutral. These conditioners adsorb strongly by electrovalent interaction with sulfonic acid groups on the hair fiber surface. Acid/base interaction between the conditioner and the keratin fiber can lead to different degrees of interaction. We hypothesize that penetration of at least the low-molecular-weight components of the cationic condition- ing compounds into the intercuticular regions leads to plasticization of the cuticular sheath, which, in turn, may lead to its softening. On the other hand, in the case of conditioning compounds dominated by hydrocarbon structures, the conditioner/keratin interaction may also occur by hydrophobic bonding between the hydrocarbon chains, which may reduce the moisturization and lead to hardening of the protein-conditioner complex (3) (as observed in our studies involving synthetic fibers). Using AFM, work of this nature has been done at TRI on synthetic fibers, providing useful information on the effect of topical finishes on the hardness of fibers (3). We extended this study to hair fibers. Atomic force microscopy (AFM) techniques have become quite unique for high-resolution examination of various materials on a nanome- ter scale, including keratin fibers. AFM provides information not only on the topogra- phy, but also on the adhesive, attractive, repulsive, viscoelastic, and micromechanical (microhardness) properties of the fibers. Although some work has been done using AFM to study the topography of untreated and conditioner-treated hair, no attempt has been made to study the hardness of the hair surface. In this work, we have used AFM to study the hardness of the hair fiber surface, which has been modified by the deposition of a cationic conditioning compound. EXPERIMENTAL MATERIALS Hair fibers. Root sections of individual hair fibers were from 14-inch-long, dark brown European hair from DeMeo. To avoid problems stemming from fiber-to-fiber variation, great care was taken that the study was carried out on adjacent regions of the same hair fiber root sections before and after multiple applications of a conditioner. Conditioner. The conditioner employed was Polyquaternium- 10 (PQ- 10). TREATMENTS/PROCEDURES 1. Pretreatment/cleaningo An appropriate number of untreated hair fibers with diameters of-90 l•m were selected. The fibers were cleaned while under constant stirring for ca. 30 minutes at 40øC in a surfactant solution (12.5% Texapon ASV 50, pH = 5.0). The cleaned fibers were air-dried overnight under controlled conditions (22øC, 45% RH). 2, Nano-indents before conditioner applications. Six nano-indents were carefully placed onto flat and "clean" regions of surface cuticle cells of seven hair fibers, totaling 42 nano- indentations. AFM tests were carried out on these fibers under controlled conditions (22øC, 45% RH).