## 3 Results and Discussion
### Surface Tension  
Figure 4 illustrates surface tension kinetics in LAE-cCNC mixtures at LAE concentration 0.006 wt.% and cCNC 0.3 wt.%, in the presence of added 5 mM NaCl, NaSal, GuaHCl and 6 mol/L urea. In general, the addition of cCNC to the surfactant solution decreased the equilibrium surface tension from 48 mN/m for pure 0.006 wt.% LAE (cf. Figure 5) to 40 mN/m for the dispersion with 0.3 wt.% cCNC. Similar results were obtained previously for cCNC [21]. In the case of sodium chloride and guanidinium chloride addition, compared with \"no salt\" conditions, some decrease in the surface tension was observed at short adsorption times. That effect is similar to one observed for ionic surfactants resulting from the screening of the electrostatic interactions. On the other hand, at longer adsorption times, a small surface tension increase could be noted, which may be explained by aggregation of nanoparticles and more extensive surfactant binding to those aggregates in the suspension. That can be correlated with some reduction in foam stability. Sodium salicate had the strongest effect on the surface tension, due to the surface activity of the salicate anion that can more effectively neutralize the positive charge of LAE cations at the interface. Characteristic differences in surface tension kinetics could be observed by adding 6 M urea. The surface tension value first seemed to reach a plateau and then decreased. The addition of urea caused a faster decrease in the surface tension compared to the pure LAE-cCNC mixture; however, after the 2000 s of adsorption, similar surface tension values (c.a. 40 mN/m) could be observed. A similar effect of urea was observed for the surfactant solutions without cCNC, as demonstrated in Figure 5.  
Figure 4: Surface tension of the dispersion of LAE 0.006 wt.% and cCNC 0.3 wt.% with the addition of NaCl, NaSal, GuaHCl and urea. Figure 5: The effect of the addition of urea (6 mol/L) on the surface tension of 0.006 wt.% LAE—Mirenat solution (85% purity). The frequency dependence of the dilational elasticity modulus measured by the oscillating drop technique was illustrated in Figure 6. For the LAE-cCNC dispersion without additives, the modulus values are similar to 0.006 wt.% LAE solution and c.a. are three times lower than LAE-sCNC at the same concentrations. That can be the effect of higher charge and bigger size of sCNC aggregates or their different arrangement at the interface. On the other hand, the imaginary part of the dilational elasticity for the surfactant solution without CNC was constant in that frequency rate and equal to 3 mN/m. Thus, the addition of 0.3 wt.% cCNC increases the dilational viscosity of an interface. The addition of 5 mM NaCl resulted in the highest dilational elasticity. That can be attributed to the decrease in the electrostatic repulsion between charged cellulose nanocrystals that can pack closely at the interface. Data presented in Figure 6 indicate that adding 5 mM of guanidine hydrochloride also increased the elastic modulus (although less than 5 mmol/L NaCl); however, the surface layer became more viscous (c.f. Figure 7). The origin of that effect is unclear, but it demonstrates that both counterions and coions can affect the dilatational moduli at liquid interfaces with ionic surfactants [45]. Specific ion effects relevant for foaming were described in the literature [46]. In our case, both NaCl and GuaHCl show an almost equal effect on foaming properties of LAE-cCNC at the concentration of 5 mmol/L, despite different surface dilational viscoelastic properties. Adding 5 mmol/L NaSal or 6 mol/L urea to the LAE-cCNC dispersion caused a slight decrease in elasticity modulus compared to pure LAE-cCNC. The qualitatively different behavior of the imaginary part of the dilational modulus was observed for those dispersions. Upon addition of GauHCL, NaSal or urea, its values were increased compared to ones for LAE-cCNC, while 5 mM NaCl caused their decrease.