Stretching and Collapse of Neutral Polymer Layers under Association with Ionic Surfactants

We provide experimental evidence for stretching and collapse of neutral polymer layers, already adsorbed at an oil-in-water interface, due to its interaction with surfactants. Upon stretching, the first interaction length ($2L_0$) follows a power law dependence on surfactant concentration ($\propto C_s^x$, where $x\approx 0.5$ for cationic surfactants) and collapses in the presence of salt, as a relatively weak power law ($C_s^{-y}$, where $y=0.17$), in good agreement with brush length decay for polyelectrolyte brushes.

Figure 1

Forces between the ferrofluid droplets, as a function of distance, for various CTAB concentrations. The best fit obtained using Eq. (1) is shown by the solid line.

Figure 2

The first interaction length ($2L_0$) values deduced from the force curves, for different surfactant concentrations. At the intermediate concentrations of CTAB and TTAB, the first interaction length $2L_0$ follows a power law dependence $2L_0\propto C_s^x$, where the exponent $x$ for CTAB and TTAB were, respectively, 0.51 and 0.44. The structures of polymer-surfactant complex on emulsion droplets at two extremes are depicted in the inset.

Figure 3

The force profiles for different salt concentrations for PVA-CTAB. The polymer and surfactant concentration was 0.68 wt % and 13.5 mM, respectively. The concentration of salt for each curves is shown in the inset. The best fit is shown by the solid line.

Figure 4

The first interaction length ($2L_0$) at different concentrations in the presence of salt. For CTAB and TTAB, in the presence of salt, the first interaction length follows a power law scaling $2L_0\propto C_s^{-y}$ with a critical exponent $y=0.179$. In the case of NP10, the exponent was very weak (0.06). The structures of polymer-surfactant complex on emulsion droplets at two extremes are depicted in the inset.