From Micro to Macro: How Nanocelluloses Influence the Structure and Rheology of Surfactant Lamellar Systems

Cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), commonly referred to as nanocelluloses, have been extensively investigated as potential rheological modifiers in various formulations but have been rarely studied in liquid crystalline systems, such as those formed by aqueous surfactant systems. In this study, negatively charged CNC and CNF were compared with their molecular counterpart, carboxymethylcellulose (CMC), to assess their rheological effects when incorporated into a surfactant lamellar phase. This phase, widely used in cosmetic applications, is formed by the cationic surfactant dioctadecyldimethylammonium chloride (DODAC). Rheological changes were correlated with the system's internal organization using a multiscale approach, including scattering, calorimetric, and microscopy techniques. For samples containing CNC, the results showed that the yield value increased with rising CNC concentration up to a maximum, after which the yield value decreased. In contrast, samples prepared with CNF and CMC within the same surfactant and additive concentration range exhibited a decrease in yield value with increasing additive content. Small-angle X-ray scattering (SAXS) followed the same rheological trends: in systems with CNC, the bilayer repeat distance (d-spacing) decreased with CNC concentration until a minimum value was reached, followed by an increase beyond this point. Conversely, the d-spacing in samples with CNF and CMC consistently decreased with additive concentration. Small-Angle Neutron Scattering (SANS) with contrast matching indicated that cellulose could be confined within the aqueous spaces between bilayers. Cryogenic Transmission Electron Microscopy (Cryo-TEM) and Fourier-Transform Infrared Microscopy (FTIR) confirmed the integration of nanocelluloses into the lamellar matrix, revealing micro- and nano-scale cellulose aggregates. Furthermore, tribology measurements demonstrated improved lubrication properties in formulations with CNC and CMC. Our findings suggest that the rigidity and structural parameters of the lamellar phase depend on the surfactant/additive mass ratio and the type of additive, whether CNC, CNF, or CMC. These results highlight the potential of cellulose additives to effectively tailor the rheology of cosmetic formulations, offering a sustainable alternative to conventional polymers.