## 3 Results and Discussion  
### Foaming  
Figure 1 illustrates foam half-life (the time after which 50% of the foam breaks) as the function of surfactant concentration for foams generated after 10 th cycle of mixing the air and the 20 mL of LAE-CNC dispersion in connected syringes. Foam and liquid volume could be measured after approximately 30 s and observed for up to 280 min. As shown in Figure 1, for all concentrations above 0.004%, foam half-life was at least 280 min. In contrast, for sCNC foam half-life was at maximum for 0.006 wt.% of LAE and then decreased for higher surfactant concentrations (Figure 1b) [21]. The aggregation can be the reason for the limited foam stability in the case of sCNC. The results presented in Figure 1 show a threshold value of surfactant concentration (0.003%, 0.07 mM) in the LAE-cCNC mixture, at which foam stability significantly increased. That threshold value was slightly higher for sCNC (0.004 wt.%). Since the zeta potential of nanoparticles in the LAE-cCNC dispersion (\(-35\) mV) was lower than in LAE-sCNC (\(-40\) mV), they can be more amphiphilic, as suggested in [23]. The LAE concentration above the foam stability threshold (0.006 wt.%) was selected to study the effect of various additives on foaming and interfacial properties of the LAE-cCNC suspension. It should be noted that no stable foam can be formed at that surfactant concentration without nanoparticles. The results collected in Table 1 illustrate that at the surfactant concentration corresponding to the LAE-sCNC maximum foam lifetime, cCNC are much more efficient at foaming. The foam volume was almost doubled for carboxylated CNC compared to sulfated ones, while its stability was also prolonged. To elucidate the possible reason for the differences in foam stability between two types of CNC for LAE concentration above 0.008 wt.%, we measured the dependence of cCNC hydrodynamic size on the surfactant concentration. The results are listed in Table 2. They show that carboxylated cellulose nanocrystals are resistant to aggregation up to 0.02 wt.% LAE. Moreover, they indicate that cCNC were less aggregated than sCNC. For the surfactant concentration 0.015 wt.% the hydrodynamic size of cCNC was 112 nm (PDI 0.44), significantly smaller and less polydisperse than for sCNC--187 nm (PDI 0.82) [21]. That seems to support the hypothesis that the defoaming effect of large aggregates can cause lower foam stability of sCNC-LAE dispersion. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **LAE-sCNC** & **LAE-cCNC** \\ \hline Foam volume [mL] & \(19\pm 1\) & \(37\pm 1\) \\ Foam half-life [min] & 260 \(\pm\) 20 & \textgreater{}280 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Initial foam volume and foam half-life in LAE 0.006 wt.%—CNC 0.3 wt.% dispersions with respect to CNC hydrophilic groups. Figure 1: Foam half-life of dispersion of the 0.3 wt.% cellulose nanocrystals with various LAE concentrations; (**a**) cCNC; (**b**) sCNC, (see ref. [21]). Maximum observation time 280 min. Figure 2 illustrates the effect of additives--ionic and non-ionic--on the initial foam volume, generated with the double syringe technique. The addition of monovalent electrolytes, sodium chloride or guanidinium hydrochloride at a concentration of 5 mmol/L to LAE-cCNC mixtures did not the change foam volume significantly. The difference was seen for 5 mM sodium salicate, which contains a bulky surface-active anion that can effectively penetrate the interfacial layer formed by cationic LAE and cCNC. The foam volume decreased to about 12 mL, more than three times with respect to LAE-cCNC without additives, or more than two times with respect to one with NaCl. The most significant effect of reducing the initial foam volume was seen for dispersions containing 6 mol/L urea. Right after the foam formation, it filled the whole syringe; however, it collapsed very quickly.