RHEOLOGY OF COSMETIC PRODUCTS 483 Wormlike micelles: shampoos, body, and face washes . Micelles are either discrete or continuous aggregates with overall circular cross section. Discrete micelles include spherical, prolate- shaped, and disk-shaped aggregates, whereas continuous micelles can include long un- connected rods with variable rigidity or branched rods. Although it is the amphiphilic character of surfactants that promotes self-assembly into the aforementioned aggregates, it is actually the surfactant packing parameter that mainly determines which kind of micelle will be formed (14). These aggregates may occur in water-rich or oil-rich solu- tions (15). Figure 1 highlights some of surfactant micellar microstructures that may be formed. One of the most prevalent micellar structures used in shampoos and body washes is wormlike micelles. The nature, behavior, and performance of products having wormlike micelle structures depend heavily on the self-assembled nanostructure. Wormlike micelles can be formed by anionic, cationic, or nonionic surfactants (15). The structural character- istics of wormlike micelles are signifi cantly impacted by specifi c primary surfactants’ molecular structure and also by surfactant/cosurfactant choice and are additionally infl u- enced signifi cantly by various formulation conditions such as addition of electrolyte and changes in pH. In these systems, it is important to additionally control the dynamic equilibrium between the self-assembled state and dispersed surfactant molecules (16). Cosolvents/cosurfactants have also been long used to alter the dynamic balance of micellar systems (17). The following highlights some of the studies carried out on different worm- like micellar systems to understand the effect of surfactant type, charge, cosurfactant, cosolvents, electrolyte, etc. Cosurfactant effect. In an attempt to understand the impact of cosolvents on micellar sys- tems, Jiang et al. (18) investigated different combinations of non-ionic surfactant, ionic surfactant, and mixed surfactants. The two major aspects that were studied are the CMC and the size of micelles. The combination of sodium lauryl ether sulfate (SLES) and Cocamidopropyl betaine (CAPB) is one of the most frequently used surfactant base mix- tures for cleansing cosmetic products, and the study and analysis of the structure and rheology of these binary mixtures are very common practice (19). In this report, a series of salt concentrations, sodium chloride (NaCl) at 3.01, 3.56, 4.01, 4.5, and 5.00 wt% (0.515, 0.609, 0.686, 0.770, and 0.856 M), with and without the cosolvent diol dipro- pylene glycol (DPG) was investigated at 25°C to understand the effect of DPG on the struc- ture of wormlike micelles in the context of variable counterion concentration. Small-angle Figure 1. Diagrammatic presentation of spherical, rod, and wormlike micelles.

JOURNAL OF COSMETIC SCIENCE 484 neutron scattering (SANS) was used to understand microstructural changes. The viscosity of wormlike micelle solutions is strongly dependent on the contour length, and the re- sults show that the addition of DPG has a dramatic impact on the viscoelastic properties of wormlike micelles solutions because of a dramatic reduction in the contour length as- sociated with the lower dielectric constant media. The decreased dielectric constant of the solvent with the addition of DPG may increase the electrostatic interaction between head groups and lead to the observed smaller micelle radii and dramatic reduction in contour length of the micelles. Electrolyte effect. NaCl induces an electrostatic screening effect on surfactant mixtures, which imparts the rheology of the solution by elongating the micelles formed. Amin et al. (19) revealed structural properties of micelles using dynamic light scattering (DLS), microrheology, and Raman spectroscopy. In this study, Raman spectroscopy was com- bined with DLS–optical microrheology to investigate the impact of ionic strength on the molecular structure and associated changes in the microstructure and rheology of a mix- ture of anionic SLES and zwitterionic CAPB surfactants in solution. It is reported that DLS measurements and Raman spectroscopy were performed on surfactant mixtures in water containing 14% w/w sodium lauryl monoether sulfate (SLES), 2% w/w CAPB, and varying concentrations of NaCl, ranging from 73 to 500 mM. To perform the microrheo- logical measurements, 900-nm polystyrene probe particles were added to obtain a fi nal concentration of 0.15% w/w in the surfactant mixtures and 73 mM NaCl because of commercial preparation of CAPB. Sample aliquots (~20 μL) for Raman work were placed into a titanium cuvette with 120-μm-thick quartz windows and positioned in a temper- ature-stabilized sample holder. For the microrheological work, 1-mL aliquots were placed into a disposable polystyrene cuvette and placed in the same sample holder. It was also reported that all data were collected at 25°C. The effect of electrostatic screening on the rheological properties of anionic SLES and zwitterionic CAPB surfactant mixtures was studied by modulating the NaCl concentration. It was reported that the viscosity of the surfactant mixture increased with NaCl concentration to a maximum level and then de- creased. This phenomenon can be attributed to either a conformational change to a branched micellar network or to a reversal to short cylindrical micelles. To further pro- vide insights into this effect and the exact microstructure differences, Raman spectros- copy was carried out which highlighted that the Raman band at 170 cm-1 correlates well with the NaCl-induced viscosity changes however, the values are different at low and high NaCl values. This indicated that although both low and high NaCl values exhibit low viscosity, the microstructures at these two extreme levels are different. State of ionization or pH change. Under standing how pH and pH change impact wormlike micelle properties and ultimately affect and determine the fi nal rheology of a cosmetic formulation is highly important. Silva et al. (20) did show how to tailor the structure and rheology of aqueous solutions of the cationic surfactant cetyltrimethylammonium tosyl- ate (CTAT) by adding potassium phthalic acid (PPA) or by changing pH. It was reported that the addition of PPA to CTAT solutions greatly increases the viscosity of the solution. It is important to state that the addition of PPA to the mixture promotes a transition from spherical micelles to rod-like micelles and, eventually, to elongated and fl exible wormlike micelles at much lower surfactant concentration. The interaction between both species is maximum at a determined PPA concentration, after which a gradual reduction in viscosity and shear thinning is produced this is a phenomenon also observed in surfac- tant solutions where NaCl is added to build viscosity (21). The initial pH values for all