In this work we show that the forced dynamic dewetting of surfactant solutions depends sensitively on the surfactant concentration. To measure this effect, a hydrophobic rotating cylinder was horizontally half immersed in aqueous surfactant solutions. Dynamic contact angles were measured optically by extrapolating the contour of the meniscus to the contact line. Anionic (sodium 1-decanesulfonate, S-1DeS), cationic (cetyl trimethylammonium bromide, CTAB) and nonionic surfactants (CE, CE and CE) with critical micelle concentrations (CMCs) spanning four orders of magnitude were used. The receding contact angle in water decreased with increasing velocity. This decrease was strongly enhanced when adding surfactant, even at surfactant concentrations of 10% of the critical micelle concentration. Plots of the receding contact angle-versus-velocity almost superimpose when being plotted at the same relative concentration (concentration/CMC). Thus the rescaled concentration is the dominating property for dynamic dewetting. The charge of the surfactants did not play a role, thus excluding electrostatic effects. The change in contact angle can be interpreted by local surface tension gradients, i.e. Marangoni stresses, close to the three-phase contact line. The decrease of dynamic contact angles with velocity follows two regimes. Despite the existence of Marangoni stresses close to the contact line, for a dewetting velocity above 1-10 mm s the hydrodynamic theory is able to describe the experimental results for all surfactant concentrations. At slower velocities an additional steep decrease of the contact angle with velocity was observed. Particle tracking velocimetry showed that the flow profiles do not differ with and without surfactant on a scales >100 μm.

Liquid system N-methylpiperidine-water shows a miscibility gap with a lower critical solution temperature of 316.7 K. The phase separation is most likely due to the aggregation of N-methylpiperidine-water complexes, evident in the intensity of the small-angle neutron scattering at temperatures much lower than the LCST. Such complexes arise because the attraction forces between unlike molecules are stronger than the water-water ones, and aggregation is possible through the O-H/O bonds involving the hydration water molecules. The aggregates are dynamic structures with nanoseconds-order relaxation times, as it was estimated by the ultrasonic absorption experiment. While hydrophilic aggregation prevails at relatively high concentrations of the amine, the hydrophobic hydration is possible at low concentrations, likely consisting of the formation of structures resembling those in the sH clathrates observed in the solid state. The hydrophobic hydration of N-methylpiperidine is manifested in the minima of the partial volume isotherms at the amine mole fraction close to 0.01 and in the limiting partial molar compression approximately equal to zero. Essential similarity of the N-methylpiperidine-water system to aqueous solutions of pyridine and its methyl derivatives studied previously, suggests that those amines are potential clathrate hydrate promoters.

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