## 1 Introduction  
Cellulose present in plant cell walls is the most abundant polysaccharide and sustainable biopolymer on the Earth. It consists of glucose molecules linked with \(\beta\)-1.4-glycoside bonds. As a raw material, cellulose has been used in the industry for 150 years [1]. Nanocellulose particles, with at least one dimension in the nanoscale, have been increasingly applied in the newest technologies [2, 3]: piezoelectricity and wearable electronics [4], pigments [5], flocculants [6], wound healing materials, drug carriers, implants and tissue engineering [7], coatings, adhesives, antibacterial packaging materials [8], thickeners and rheology modifiers [9], nano-templates, [10] reinforcing agent for composites, foams and aerogels [11], Pickering foams and emulsions [12]. Cellulose nanocrystals released by chemical hydrolysis or oxidation were first produced in 1947. They possess three essential properties: colloidal stability in polar solvents, nano-size and high crystallinity. Since 1990, they have been manufactured in tonne-per-day quantities. They are suitable for commercial applications, including the preparation of foams. In the process of sulfuric acid hydrolysis, glycosidic bonds of cellulose chains are broken, especially in less crystalline regions, and some hydroxy groups on cellulose surfaces are esterified. Released nano-sized crystalline particles, partially esterified, are called sulfated cellulose nanocrystals (sCNC) (containing carbon-oxygen-sulfur bond) [13]. Carboxylated cellulose nanocrystals (cCNC) can be produced in a batch process with dilute hydrogen peroxide oxidation [14]. The cCNC has a relatively low surface charge density compared to sulfated cellulose nanocrystals. They are also less crystalline and have a relatively high surface area. Cellulose is chiral in many length scales from molecules up to mesophases. Importantly, nanocellulose crystals and fibers are twisted [15], which affects the exposure of their more hydrophobic crystallographic planes at the air/water interface. Depending on their concentration, cellulose nanocrystals may form liquid crystals in water [16]. The liquid crystalline behavior of cellulose nanocrystals (CNC) is solvent-dependent, which can be explained with more efficient hydrogen bonding between CNC at lower dielectric screening. Cellulose nanocrystals form needle-like elongated micro-aggregates with broad size distribution in low dielectric solvents [17]. Ethyl lauroyl arginate is a cationic surfactant manufactured from biodegradable compounds. It hydrolyses to other surface-active substances in water: N\(\alpha\)-lauroyl-L-arginine (LAS) or dodecanoic (lauric) acid. In our previous work, we described its interfacial properties [18] and determined critical micelle concentration (CMC) at about 1 mmol/L. Foaming properties are interesting from many technological perspectives, including flotation and detergency. There are several methods of foam formation characterization, including the double syringe technique [19], which provides foams consisting of tiny bubbles of relatively uniform size [20]. Foaming experiments carried out with the double syringe for a commercial brand of LAE (85% purity) showed that at concentrations below 0.35 mM, its solution did not form foams surviving more than several minutes. On the contrary, when mixed with hydrophilic sulfated cellulose nanocrystals, foams could be stable for up to 4 h [21]. Nanometric sCNC with sulfate hydrophilic groups moderately interact with LAE at low surfactant concentrations, and particle zeta potential and hydrodynamic diameter do not change significantly. The explanation of enhanced foam stability in LAE-sCNC mixtures is far from trivial. A slight increase in dispersion viscosity, high interfacial shear elasticity and flow-induced plugs in the Plateau borders may contribute to the enhanced foam stability. On the other hand, sCNC aggregates in the foam films may have an opposite antifoaming effect.