Characterizing the rheological dynamics of sulfate-free surfactant mixtures of cocamidopropyl betaine-sodium methyl cocoyl taurate across composition, pH, and ionic conditions

Introduction

Aqueous binary surfactant systems comprising oppositely charged species are extensively employed across numerous industrial sectors, including cosmetics, pharmaceuticals, agrochemicals, and food processing industries. The widespread adoption of these systems is primarily attributed to their superior interfacial and rheological functionalities, which enable enhanced performance in diverse formulations. The synergistic self-assembly of such surfactants into wormlike, entangled aggregates imparts highly tunable macroscopic properties, including increased viscoelasticity and reduced interfacial tension. In particular, combinations of anionic and zwitterionic surfactants exhibit synergistic enhancements in surface activity, viscosity, and interfacial tension modulation. These behaviors arise from intensified electrostatic and steric interactions between the polar head groups and hydrophobic tails of the surfactants, contrasting with single-surfactant systems, where repulsive electrostatic forces often limit performance optimization.

Cocamidopropyl betaine (CAPB; SMILES: CCCCCCCCCCCC(=O)NCCCN+ (C)CC([O−])=O) is a widely utilized amphoteric surfactant in cosmetic formulations due to its mild cleansing efficacy and hair-conditioning properties. The zwitterionic nature of CAPB enables electrostatic synergy with anionic surfactants, enhancing foam stability and promoting superior formulation performance. Over the past five decades, CAPB mixtures with sulfate-based surfactants, such as CAPB–sodium lauryl ether sulfate (SLES), have become foundational in personal care products. However, despite the effectiveness of sulfate-based surfactants, concerns regarding their dermal irritation potential and the presence of 1,4-dioxane, a byproduct of the ethoxylation process, have driven interest in sulfate-free alternatives. Promising candidates include amino-acid-based surfactants, such as taurates, sarcosinates, and glutamates, which exhibit enhanced biocompatibility and milder properties [9]. Nevertheless, the relatively large polar head groups of these alternatives often impede the formation of highly entangled micellar structures, necessitating the use of rheological modifiers.

Sodium methyl cocoyl taurate (SMCT; SMILES:
CCCCCCCCCCCC(=O)N(C)CCS(=O)(=O)O[Na]) is an anionic surfactant synthesized as a sodium salt via amide coupling of N-methyltaurine (2-methylaminoethanesulfonic acid) with a coconut-derived fatty acid chain. SMCT possesses an amide-linked taurine headgroup alongside a strongly anionic sulfonate group, rendering it biodegradable and compatible with skin pH, which positions it as a promising candidate for sulfate-free formulations . Taurate surfactants are characterized by their potent detergency, hard-water resilience, mildness, and broad pH stability.

Rheological parameters, including shear viscosity, viscoelastic moduli, and yield stress, are critical in determining the stability, texture, and performance of surfactant-based products. For instance, elevated shear viscosity can improve substrate retention, while yield stress governs the formulation's adherence to skin or hair post-application. These macroscopic rheological attributes are modulated by numerous factors, including surfactant concentration, pH, temperature, and the presence of co-solvents or additives. Oppositely charged surfactants can undergo diverse microstructural transitions, ranging from spherical micelles and vesicles to liquid crystalline phases, which, in turn, profoundly affect the bulk rheology. Mixtures of amphoteric and anionic surfactants often form elongated wormlike micelles (WLMs), which significantly enhance viscoelastic properties. Understanding the microstructure–property relationships is, therefore, critical for optimizing product performance.

Numerous experimental studies have investigated analogous binary systems, such as CAPB–SLES, to elucidate the microstructural basis of their properties. For example, Mitrinova et al. [13] correlated micelle size (hydrodynamic radius) with solution viscosity in CAPB–SLES–medium-chain co-surfactant mixtures using rheometry and dynamic light scattering (DLS). Mechanical rheometry provides insight into the microstructural evolution of these mixtures and can be augmented by optical microrheology using diffusing wave spectroscopy (DWS) which extends the accessible frequency domain, capturing short-timescale dynamics particularly pertinent to WLM relaxation processes. In DWS microrheology, the mean square displacement of embedded colloidal probes is tracked over time, enabling the extraction of linear viscoelastic moduli of the surrounding medium via the generalized Stokes–Einstein relation. This technique requires only minimal sample volumes and is thus advantageous for studying complex fluids with limited material availability, e.g. protein-based formulations . Analysis of < Δr²(t)> data across broad frequency spectra facilitates estimation of micellar parameters such as mesh size, entanglement length, persistence length, and contour length. Amin et al demonstrated that CAPB–SLES mixtures conform to predictions from Cates’ theory, showing a pronounced increase in viscosity with salt addition until a critical salt concentration, beyond which viscosity drops precipitously—a typical response in WLM systems Xu and Amin employed mechanical rheometry and DWS to examine SLES–CAPB–CCB mixtures, revealing a Maxwellian rheological response indicative of entangled WLM formation, which was further corroborated by microstructural parameters inferred from the DWS measurements. Building upon these methodologies, the current study integrates mechanical rheometry and DWS microrheology to elucidate how microstructural reorganizations drive the shear behavior of CAPB–SMCT mixtures.

In light of escalating demand for gentler and more sustainable cleansing agents, the exploration of sulfate-free anionic surfactants has gained momentum despite formulation challenges. The distinct molecular architectures of sulfate-free systems often yield divergent rheological profiles, complicating conventional strategies for viscosity enhancement such as via salt or polymeric thickening. For instance, Yorke et al. explored non-sulfate alternatives by systematically investigating the foaming and rheological properties of binary and ternary surfactant mixtures featuring alkyl olefin sulfonate (AOS), alkyl polyglucoside (APG), and lauryl hydroxysultaine. A 1:1 ratio of AOS–sultaine showed shear-thinning and foam characteristics akin to CAPB–SLES, indicating WLM formation. Rajput et al. [26] evaluated another sulfate-free anionic surfactant, sodium cocoyl glycinate (SCGLY), alongside nonionic co-surfactants (cocamide diethanolamine and lauryl glucoside) through DLS, SANS, and rheometry. Although SCGLY alone formed predominantly spherical micelles, co-surfactant addition enabled the construction of more intricate micellar morphologies, amenable to pH-driven modulation.

Despite these advances, comparatively few investigations have targeted the rheological properties of sustainable sulfate-free systems that involve CAPB and taurates. This study aims to fill this gap by providing one of the first systematic rheological characterizations of the CAPB–SMCT binary system. By systematically varying surfactant composition, pH, and ionic strength, we elucidate the factors governing shear viscosity and viscoelasticity. Using mechanical rheometry and DWS microrheology, we quantify the microstructural reorganizations underlying the shear behavior of CAPB–SMCT mixtures. These findings elucidate the interplay between pH, CAPB–SMCT ratio, and ionic levels in promoting or inhibiting WLM formation, thereby offering practical insights into tailoring the rheological profiles of sustainable surfactant-based products for diverse industrial applications.