## 1 Introduction
Upon aggregation, cellulose nanocrystals form rods with a high aspect ratio and rectangular cross-section that can be crucial for explaining foam stability in the mixtures with surfactants. The aspect ratio of nanocellulose can be a key parameter determining the stability of Pickering emulsions [22]. Emulsion stability is also inversely proportional to the surface charge density of cellulose nanocrystals, since less charged CNC are more amphiphilic [23]. Thus, cellulose nanocrystals' surface charge density can affect interfacial properties and foaming properties at the liquid/air interface [24]. The redispersion of sulfated CNC can impact the rheological properties of their dispersions [25]. They adsorb at the liquid/air interface after several hours when the addition of salt screens their charge [26]. CNC forms lyotropic liquid crystals at concentrations above 1% by weight [27]. It was shown that the addition of cellulose nanocrystals enhances the stability of methylcellulose foams after the formation of gels [28]. There were other reports concerning the influence of nanocellulose on foaming properties [29], where size and bulk rheology of nanocellulose played a significant role. However, only a few studies have been related to simple liquid foams without gelled phase, without polymers as additives and with nanocellulose in the form of nanocrystals. Various additives modify the electrostatic properties of nanoparticles, the interaction of nanoparticles and surfactants and the foaming properties of their mixtures. The addition of monovalent electrolyte decreases electrostatic repulsion between particles and enhances surfactant adsorption at the interface, due to lower electrostatic repulsion between ionic molecules [30]. Sodium salicate belongs to organic counterions and hydrotropes solubilizing hydrophobic compounds. It can interact with other surfactants or nanoparticles electrostatically and hydrophobically. It enables the transition of surfactant structure in the solution from spherical micelles to worm-like micelles [31]. Research on interfacial layer viscoelasticity in such systems is rarely reported for surfactant-nanoparticle systems. Some works were devoted to protein interfacial layers that demonstrated that sodium salicate could dramatically reduce interfacial viscoelasticity of the protein films and change the properties of wheat dough. It was hypothesized that sodium salicylate acts as a hydrogen bond breaker [32; 33]. As it is known from the literature, urea increases critical micelle concentration of surfactants [34], increases counterion dissociation [35], displaces the water from the surface of ionic surfactants, helps solvate the hydrophobic micelle cores by localizing at their surfaces, and changes the micelle shape and the number of surfactants associating in the micelle [36]. Recent data show urea orients at the interface within the hydrogen-bonded water network. Its orientation depends on the sign of the charged interface--the net orientation of urea is possible only in the presence of surfactants. Depending on the concentration, urea orients with its C=O group toward a positively charged interface, as shown by characteristic vibrational modes of urea detected by SFG spectroscopy [37]. It was evidenced that urea can improve the solubility and stability of cellulose in alkaline solutions [38]. Urea can compete with water for hydrogen bonding that, together with ionic interactions, contribute the most to the protein tertiary structure. The structure might be entirely destroyed at the urea concentration of 8 mol/L. Guanidine hydrochloride (GuaHCl), is an even stronger protein denaturant than urea. It contains a common cation as a positively charged group of LAE and is well known for its chaotropic properties. In protein solutions, it acts as an unfolding agent [39]. It is also known that urea forms hydrogen bonding with protein backbone amides at high concentrations, while guanidine hydrochloride acts preferentially on hydrophobic residues of the protein [39].