Stern Layer Formation Induced by Hydrophobic Interactions: A Molecular Level Study

Abstract

The molecular ionic surface structure and charge of the electric double layer around a nanodroplet and its structural change induced by hydrophobic effects are measured using vibrational coherent surface scattering spectroscopy, second harmonic scattering, and electrokinetic mobility measurements. Tetraalkylammonium chloride salts were added to negatively charged nanoscopic oil droplets in water. When we vary the alkyl chain length of the cation from CH3 to C4H10, both the size of the cation and its hydrophobic interaction are increased. We find that tetramethylammonium ions change the electrokinetic potential and the water structure but do not detectably adsorb to the interface. Tetrapropylammonium and tetrabutylammonium ions clearly adsorb to the interface. The corresponding (Stern) layer appears to be a mixed monolayer of anions and cations. An estimate of the amount of cations in the Stern layer is also made.

Direct and quantitative AFM measurements of the concentration and temperature dependence of the hydrophobic force law at nanoscopic contacts

“…if, a very low charge density) within any of the used NaCl concentrations. Similarly, NaCl did not show any significant effects in vibrational coherent surface scattering spectroscopy up to 100 mM solutions46 .…”

“…46,48 Also, it seems that an artificial AFM cantilever motion (rolling and/or sliding due to the arc motion of a typical AFM cantilever) during separation can equally likely lead to such small plateau features. In any case, whether there is dissolved gas in solution or not, the conclusions presented above are equivalent for nanoscopic contacts at D ≥ 2 nm.…”

Characterization of the interface of binary mixed DOPC:DOPS liposomes in water: The impact of charge condensation

“…1 In order to understand the membrane structure, hydration, and the changes therein, it is crucial to characterize the interfacial properties of lipid membranes and their aqueous environment. Non-resonant second harmonic scattering 2,3 (SHS) is an optical process used to measure the net orientational order of water molecules along the surface normal of particles, 4,5 droplets, [6][7][8] or liposomes. 9,10 Polarization-and angle-resolved (AR) SHS were recently demonstrated as a method to obtain a unique value for the surface potential of a particle in aqueous solution.…”

Sum frequency and second harmonic generation from the surface of a liquid microjet

“…30. An 800 nm regeneratively amplified Ti:sapphire system (Spitfire Pro, Spectra physics) seeded with an 80 MHz 800 nm oscillator (Integral 50, Femtolasers) was operated at a 1 kHz repetition rate to pump a commercial OPG/OPA/DFG system (HE-TOPAS-C, Light Conversion), which was used to generate IR pulses.…”

“…28,29 Such a geometry, aside from allowing access to the interface of small particles in buried liquid or solid media, also has advantages in decreasing the inherent challenges that are caused by chemical contaminations, which are present in every sample. 30 Scattering experiments thus form an attractive alternative for studying liquid/liquid interfaces. 31 Liquid jets are another alternative for reducing impurity induced artifacts 32,33 (as the interface is continuously refreshed) or avoiding the use of windows [34][35][36] when the optical excitation demands such applications.…”