## 4 Conclusions  
Foam stability in ethyl lauroyl arginate (LAE) mixtures has a complex origin, including the adsorption of surfactant at the interface, its dynamics at the expanded/contracted interface and the formation of bulk and interfacial aggregates with carboxylated cellulose nanocrystals (cCNC), which change at the time of the foam drainage. Cellulose nanocrystals decorated with oppositely charged surfactants adsorb at the water/air interface, lower the surface tension and modify the surface viscoelastic properties. They reduce drainage and prevent coalescence. Large cellulose nanocrystals aggregates have a significant effect on foam stability, as they accumulate in the Plateau borders and prevent drainage. On the other hand, large aggregates with randomly oriented nanocrystals in the foam films can act as defoamers. The presence of electrolytes such as sodium chloride and guanidine hydrochloride at a small concentration of 5 mM did not change foamability and foam stability, despite a two-fold increase in the surface dilational elasticity. The hydrodynamic diameter of cellulose nanocrystals also increased twice in size, due to electrostatic screening of CNC repulsion. The surface-active anion of sodium salicylate adsorbs at the interface and effectively screen electrostatic interactions of the LAE hydrophilic group. The foamability of LAE-cCNC mixtures in the presence of 5 mmol/L NaSal was much lower, but the foam stability did not change. Such an effect can be explained by the significant influence of cellulose nanocrystals on overall foam stability. A dramatic decrease in the quantity of obtained foam volume and lack of its stability were observed for LAE-cCNC mixtures containing 6 mol/L urea. Minor changes of the equilibrium and dynamic surface tension upon the addition of urea cannot explicate that decrease. The plausible explanation can be its orientation at the interface in the presence of ionic surfactants and charged nanocrystals and the disruption of the water structure. Urea can also destroy large lamellar cCNC aggregates that reduce foam drainage. Despite the  
Figure 7: The frequency dependence of the imaginary part of dilational modulus in LAE-cCNC dispersions with 5 mM electrolytes: guanidine hydrochloride, sodium chloride, sodium salicylate and urea at concentrations 3 mol/L or 6 mol/L. chaotropic nature of both guanidine hydrochloride and urea, their effect on ethyl lauroy1 arginate--carboxylated CNC dispersions bulk and interfacial properties--are different. The differences in the effect of those denaturants on the bulk and interfacial properties of protein solutions have been described before [48; 49]. Our work is the first demonstration of the urea effect on surfactant--nanoparticles interfacial properties and foaming. Future experiments can reveal the complexity of molecular interactions in this system. Importantly, urea affects the interfacial properties of ethyl lauroy1 arginate. Further experiments will reveal if it is connected to surfactant hydrolysis, formation of some interfacial structures or other effects. Detailed analysis of different urea concentrations and hydrodynamic size of cellulose nanoparticles can determine the extent of large nano-aggregates solubility. Data presenting the influence of urea on hydrodynamic size are very consistent for the LAE concentration studied. Thin film balance experiments can assess the direct influence of aggregate sizes on film stability. It should be noted that nanoparticle aggregation and their size distribution can affect the thin film stability in a complex way [50]. Investigations of thin film stability are very interesting for the explanation of the influence of some factors controlling foam stability, but other ones such as water drainage or gas permeabilities of liquid films can also be influenced by the aggregation of nanoparticles. Conceptualization, A.C. and P.W. ; methodology, P.W., A.C. and M.K. ; validation, A.C., P.W. and M.K. ; formal analysis, P.W. ; investigation, A.C.; resources, M.K.