J. Cosmet. Sci., 67, 59–70 (March/April 2016) 59 Streaming potential measurements to understand the rheological properties of surfactant formulations containing anionic and zwitterionic surfactant JOCHEN KLEINEN and JOACHIM VENZMER, Evonik Nutrition & Care GmbH, 45127 Essen, Germany. Accepted for publication April 13, 2016. Synopsis Surfactant formulations are often based on an anionic primary surfactant combined with an amphoteric secondary surfactant. One popular option is the combination of lauryl ether sulfate and cocamidopropyl betaine, because such formulations are not only mild but also easy to thicken. Changes in the molecular structure of the betaine in terms of alkyl chain length distribution and headgroup structure do have dramatic effects on the viscosity of these formulations, as can be explained in terms of properties of rod- like micelles and exchange kinetics by oscillatory rheological measurements. The root cause of the effect of the different betaine derivatives on the micellar structure, however, remains unclear when considering rheology only. Although the streaming potential of colloidal objects is typically determined to forecast the stability of dispersions, we have used the streaming potential to characterize micellar solutions of different betaine surfactant structures. It could be shown that (a) the hydrophilicity of the surfactants can be nicely probed by this method and (b) there is a good correlation of these values with the rheological properties of binary mixtures of the betaines with anionic surfactant. Also, the chemical structure of the headgroups has a signifi cant infl uence on both the isoelectric point and the magnitude of the streaming potential of the zwitterionic surfactants. These effects have again a dramatic infl uence on the interaction with anionic surfactants, as becomes obvious when looking at the rheology of such mixtures. Therefore, the fi ndings obtained can be utilized to better understand and design surfactant formulations of a desired viscosity profi le. INTRODUCTION Aqueous surfactant formulations such as shampoos, shower gels, or hand dishwashing liquids are often based on a combination of an anionic primary surfactant with a zwit- terionic secondary surfactant. The goal is to obtain formulations having Brookfi eld vis- cosities in the range of 1000–5000 mPa·s this is typically achieved by the use of rheological additives. The fi rst option is hydrophobic thickeners, i.e., cosurfactants, which cause transition of the spherical micelles of the primary surfactant to mixed Presented at the 70th Annual Scientifi c Meeting and Technology Showcase, New York, December 10–11, 2015. Address all correspondence to Joachim Venzmer at joachim.venzmer@evonik.com.

JOURNAL OF COSMETIC SCIENCE 60 rod-like micelles. The second option is hydrophilic, associative thickeners, which are able to bridge the surfactant assemblies—or a combination of both (1). While Brook- fi eld viscosity is just an indication of the viscosity under shear, oscillatory rheological measurements can provide a lot more information, not only on the behavior at rest (e.g., existence of a yield point) but also on the type and properties of the surfactant micelles present. A network of rod-like micelles can be described by the Maxwell model the plot of the storage modulus G′, the loss modulus G″ and the complex viscosity as a function of frequency yields information on the zero shear viscosity, the network den- sity, and the structural relaxation time, which is a measure of the exchange kinetics of the surfactants (2). The combination of the anionic sodium lauryl ether sulfate (SLES) with the zwitterionic cocamidopropyl betaine (CAPB) is quite popular not only because of its mildness, but also because it is easy to thicken (3,4). In general, the origin of the viscosity behavior is known the pH of personal care formulations is typically set to about 5.5 to match the pH of human skin. At these slightly acidic conditions, the zwitterionic CAPB is par- tially protonated and thus carries some positive charge, leading to a strong interaction with the anionic SLES. While SLES forms spherical micelles of only low viscosity, the combination with the protonated CAPB leads to a sphere-to-rod micelle transition (5–7) and thus to an increase in viscosity of the mixture (8). The resulting viscosity of the surfactant mixtures depends on the mixing ratio (9), the amount of minor components such as sodium chloride (NaCl) and free fatty acids (6), and the chain length of the used betaines (10). The variation of the chain lengths of the betaine has two consequences: on the one hand, longer chains lead to an increase in packing parameter (11), i.e., to a more effi cient sphere-to-rod micelle transition. On the other hand, the exchange kinetics of aggregated surfactants, and hence the viscosity, scales with the chain length of the molecules forming the aggregate (12–15). In general, in the system SLES/CAPB there are only minor adjustments necessary to achieve the desired rheological profi le of the formulation. However, the question remains how the rheological properties of the formulations are related to the molecular structure (chain length distribution, headgroup structure) of the secondary surfactant. Because oscillatory rheological measurements can only show the rheological effects of the different betaine structures, we have used streaming potential measurements to study and understand the root cause of the rheological effects. The streaming potential is a measure of the surface charges of, e.g., colloidal objects (16) it is typically used to determine the isoelectric point (IEP) of proteins or to forecast the stabil- ity of dispersions. We have used the streaming potential of micellar solutions of different betaine surfactant structures as a measure of the hydrophilicity of the surfactants, and to correlate these values with the rheological properties of binary mixtures of the betaines with anionic surfactant. Also, the chemical structure of the betaine headgroup varied, and a signifi cant infl uence on both the IEP and the magnitude of the streaming potential of the zwitterionic surfactants could be shown. These effects have again a dramatic infl uence on the interaction with anionic surfactants, as becomes obvious when looking at the rhe- ology of such mixtures. The alkyl amidopropyl betaines (APBs) used in the fi rst part of the paper for studying the effect of alkyl chain length and its distribution were all based on dimethyl aminopropyl amine (DMAPA), fatty acids or fatty acid esters (triglycerides), and sodium monochloro- acetate. In the second part, the alkyl chain distribution is kept constant, but the structure