RHEOLOGY OF COSMETIC PRODUCTS 485 CTAT solutions evaluated ranged between 6.9 and 7.4, and any increase in the solution pH leads to a progressive decrease in the low-shear rate viscosities. Move toward sustainability: Biosurfactant addition effect. Perso nal care and cosmetic compa- nies are increasingly moving toward enhanced sustainability. This implies substituting synthetic surfactants with more biodegradable alternatives such as biosurfactants. This substitution will require gaining an understanding of how biosurfactants will self-assemble and impact on the traditional surfactants’ self-assembly process. Xu and Amin (21) car- ried out a microrheological study of ternary surfactant–biosurfactant mixtures. This re- port explores the rheological impact of rhamnolipid biosurfactant [mono-/di-rhamnolipid mixture concentrated clarifi ed broth (CCB)] on an SLES and CAPB mixed surfactant system. In this report, the evolution in the wormlike micellar structure in the SLES/ CAPB system is explored when a rhamnolipid biosurfactant is introduced. Techniques used in this study include mechanical rheometry and microrheometry (diffusive wave spectroscopy). The biosurfactant mixture explored CCB has a rhamnolipid concentration of 49.7% in which the mono- to di-rhamnolipid ratio is 9:13. CCB is the target of inves- tigation for all three experiments. It was found that the SLES/CAPB/CCB system had a Maxwellian-type response which indicates the formation of wormlike micelles. The vis- cosity of the SLES/CAPB system was signifi cantly reduced on just substituting 2% of SLES with the biosurfactant CCB. Microrheometry measurements allowed for the extrac- tion of microstructural parameters, and it was observed that the contour length of worm- like micelle formed by SLES/CAPB decreased from 445.8 to 88.37 nm with only 2 wt% addition of CCB. Change in electrostatic interactions obtained through changes in pH allowed for a rebuilding of the viscosity through re-formation of elongated wormlike micelles. This work identifi ed understandings of microstructure–rheology linkages in complex biosurfactant/surfactant mixtures that can be used as a guideline for future ap- plications of rhamnolipids in cosmetics and personal care products. Catio nic wormlike micelles. The m ost common type of wormlike micelle stems from anionic surfactants as discussed previously. Cationic surfactants also form wormlike micelles in solu- tion. Raghavan and Kaler (22) investigated viscoelastic properties of unsaturated cationic surfactants, two C22 surfactants with a cis unsaturation at the 13-carbon position: erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and erucyl trimethylammonium chloride . These two surfactants were studied in the presence of sodium salicylate (NaSal) or NaCl. Previous reports and studies of similar surfactants with long, monounsaturated tails mostly focused on their drag-reducing ability where they effectively reduced drag at ele- vated (higher) temperatures when compared with their saturated counterparts (23,24). Solutions of these C22 surfactants also show complex phase tilized in the presence of salt (22). It is reported that EHAC/NaCl solutions phase separate at high salt content (ca. 2 M), whereas EHAC/NaSal solutions phase separate beyond a certain molar ratio of salt to surfac- tant. These differences are connected to the fact that the Cl- counterion does not penetrate the surfactant aggregate, whereas the hydrophobic salicylate counterions can attach be- tween the charged head groups, thus screening electrostatic repulsions and promoting mi- cellar growth even at low salt concentrations (25). Giant wormlike micelles are reported to be formed by the surfactant EHAC on addition of NaCl. This makes it an ideal surfactant for rheology-control applications. The contour length at elevated temperatures of 60°C was estimated to be similar to that of other micellar systems at room temperature. Thus, the solution viscosity was extremely high, even at high temperatures of 90°C. Furthermore, a nonmonotonic dependence of viscosity on temperature was reported.

JOURNAL OF COSMETIC SCIENCE 486 Polyme r effect. Neutral polymers and polyelectrolytes are used quite extensively in cosmetic formulations for rheology modifi cations and also to form complex coacervates that enable deposition in formulations where the deposition of certain active ingredients to the sub- strate (i.e., hair/skin) surface is necessary. Therefore, understanding the interactions of polymers and surfactant is a necessary and critical step for the formulation of effi cient cosmetic products. Padasala et Al. (26) reported the effect of polymers on wormlike mi- celles of CTAT and discussed how the size of wormlike micelles can easily be controlled by different polymers. The ion tosylate belongs to the family of hydrotropes, and in the pres- ence of hydrotropes, surfactants present pronounced changes in their viscosity as a result of the micellar growth induced by electrostatic screening which leads to the formation of longer, more fl exible and entangled micelles, a change from linear to branched micelles or an increase in fl exibility of wormlike micelles. CTAT is capable of forming wormlike mi- celles at very low concentrations (27–30) and was combined with a triblock copolymer made up of poly(ethylene oxide) 4000 (PEO 4K) and poly(propylene oxide) 1000 (PPO 1K). These homopolymers are nonamphiphilic in nature and do not self-assemble in aque- ous solution, whereas their block copolymers, that is, PEO-PPO-PEO or PPO-PEO-PPO, form thermoresponsive micelles above their critical micelle temperature because they have distinct hydrophilic (PEO) and hydrophobic (PPO) parts which provide amphiphilic char- acteristics to them (31) and these polymers interact differently with wormlike micelles of CTAT. First, the formation of wormlike micelles at concentrations higher than its CMC (~0.26 mM at 25°C) was studied using SANS measurement, and the presence of wormlike micelles was studied using cryogenic transmission electron microscopy. The effect of tem- perature on solution viscosity of 20 mM CTAT solution was studied. It was reported that a slight increase in temperature results in signifi cant loss in viscosity which becomes nom- inal at elevated temperatures. It was then concluded that an increase in temperature leads to an increase in the charge repulsion between charged head groups and favors demicelliza- tion, resulting in drop in viscosity at elevated temperatures. To gain information on the interaction of CTAT with polymers, surface tension and conductance of CTAT solutions over a wide range of concentrations well below and above the CMC in the presence of 0.1% polymers were carried out. It is reported that hydrophobicity of polymers plays a key role in altering the morphology of wormlike micelles. PEO-PPO-PEO–type triblock co- polymers form core–shell micelles with PPO block as core and hydrated PEO blocks as corona. When such block copolymers are added to solutions containing wormlike CTAT micelles, depending on hydrophobicity, they can lead to demicellization. It was concluded in this report that the extent of demicellization largely depends on the hydrophobicity of block copolymers. This study extensively shows how nonionic polymers control the mi- celle forming and micelle size property of the cationic surfactant CTAT. Effect of shear on wormlike micelles. Shear is used during formulation processing as well as during product application. It is important to have an understanding of how wormlike micelles respond to shear and the implications of high or low shear rates on its rheological properties. Developing this understanding is not simple and requires the utilization of advanced characterization techniques such as SANS. This information further helps in understanding certain processing issues that are observed in wormlike systems such as shear banding (32). Arenas-Gómez et Al. (33) used rheology combined with SANS (rheo- SANS) to determine the local structural order in the 3-[dimethyl (tetradecyl)azaniumyl] propane-1-sulfonate and sodium dodecylsulfate micelle solutions under fl ow and quies- cent conditions. This information is critical in understanding the rheological performance