Advances in shampoo formulation : synergism of anionic / cationic / zwitterionic surfactant ternary system

For any information: ask.author@journalbinet.com There is a rule among cosmetic formulators of not mixing an anionic surfactant with a cationic surfactant in the same formulation, especially if it’s a clear product, since this interaction will result in the appearance of turbidity (precipitation). Nevertheless, vast reports show not only the possible but also synergistic combination of surfactants with opposed charge, conferring unique properties to the final product. The aim of this study is to evaluate the use of a co-surfactant as a novel tool for maintaining high levels of both cationic and anionic surfactants mixed in the same product, without precipitation. Not only we found this possible at certain rates, but also attributes of the system such as viscosity, detergency and foam stability, were improved.


I. Introduction
The formulation of a 2-in-1 shampoo that efficiently works as a cleaner as well as a conditioner has been one of the holy grails in the cosmetic industry.Initially, conditioning agents in shampoos were oils that deposited randomly over the hair surface, in order to reduce the system entropy.Cationic polymers were introduced later to aid in the deposition of oils, through several mechanisms, like coacervation and ionic interactions with hydrophilic groups present in hair proteins.In the general practice, the Published with open access at www.journalbinet.comintroduction of a cationic surfactant in a shampoo is thermodynamically restricted due to the formation of neutral complexes with anionic surfactants in the form of precipitates.Kume et al. (2008) reviewed the extensive scientific literature that exists about the interaction of oppositely charged surfactants.In one of the reports reviewed, Sun et al. (2011) showed that equimolar mixtures of sodium dodecyl sulfate (SDS) and dodecylammonium chloride (DDAC) form stoichiometric complexes that may or not coprecipitate, while the excess of one or the other component may lead to the formation of mixed micelles.It has been shown that the phase behaviors of cationic/anionic surfactant mixtures strongly depend on the molar ratio, the actual concentrations of the individual surfactants, the relative number of alkyl chains per surfactant, and the temperature, resulting in a rich array of aggregates (Sohrabi et al., 2008).While most of the studies focus only in a cationic/anionic binary system, Shiau et al. (1994) studied the influence of the addition of a nonionic surfactant to this system.The addition of a nonionic surfactant affects the precipitation behavior of the mixture, by lowering its critical micelle concentration, enhancing mixed micellization.Moreover, the presence of both cationic and anionic surfactants is of great interest in the cosmetic field (like in 2-in-1 shampoos) to produce a product that efficiently cleans and conditions.Recent attempts include the inclusion of a quaternized polymer (Chan et al., 2017), the use of long chain esters of fatty acids (Dierker et al., 2014), and the in-situ production of a gel network (Simone et al., 2015).Our study aims to investigate the effect of the addition of a zwitterionic surfactant to the cationic/anionic surfactant binary system, which at the isoelectric point bares both a positive and a negative charge simultaneously.
Preparation of mixed surfactant solutions: Tested samples were prepared by combining variable amounts of Sodium laureth sulfate (SLES), Cocamidopropyl betaine (CAPB) and Cetrimonium chloride (CTC).All samples were prepared in deionized double distilled water, and equilibrated at 25°C in a water thermostat.Recordings were performed within the first day of sample preparation.
Methods: Viscosity measurements were carried out on an Ubbelohde suspended level capillary viscometer with appropriate thermostating (±0.1°C).The cloud point is determined visually by noting the temperature at which the continuously heated solution suddenly became turbid.Heating was regulated to about 1 °C per min around the cloud point.Oil solubilization capacity was determined by a turbidimetric method in which samples of hexane plus surfactant solution were tumbled for 72 hours at 21°C, and then measured visually with a Brinkmann PC 800 probe colorimeter.Foam height was evaluated by the Ross-Miles method at 50 °C, by measuring foam volumes at 30 s and 10 min.Foam stability was determined by comparing the foam volumes after 10 min and 30 s in the Ross-Miles apparatus.

Viscosity
When SLES and CTC are mixed at a similar concentration (1 wt%), precipitation occurs as a result of the formation of a SLES-CTC complex, due to the attraction between oppositely charged head groups and the hydrophobic tails of the two surfactants.The interaction between both ionic groups starts as a binding process, followed by a charge neutralization which usually leads to the formation of a highly hydrophobic mixed surfactant, that behaves as a non-ionic surfactant.The viscosity of this solution is similar to that of water, correlating with the loss of interfacial activity of the mixture.On the contrary, when CAPB is introduced in the mixture before the CTC, instead of precipitation, there is an increase in Published with open access at www.journalbinet.comviscosity which continues as the level of CAPB increases, while maintaining a 1 wt% fixed concentration of SLES and CTC (Figure 01).This increase is explained by the expansion of micelles due to partial neutralization of the micelle charge by the added surfactant (Koehler et al., 2000).But after a peak in viscosity at 1.5 wt% of CAPB, a gradual descent is followed with every new addition of surfactant.At this point, micelles become less polar with every CAPB increase, resulting in chain contraction and relative viscosity decrease.

Cloud point
The solution of SLES-CTC was turbid when both surfactants were mixed at a 1:1 ratio, but became clear after additions of CAPB (Figure 02).