Surface charge | EPFL Graph Search

A surface charge is an electric charge present on a two-dimensional surface. These electric charges are constrained on this 2-D surface, and surface charge density, measured in coulombs per square meter (C•m−2), is used to describe the charge distribution on the surface. The electric potential is continuous across a surface charge and the electric field is discontinuous, but not infinite; this is unless the surface charge consists of a dipole layer. In comparison, the potential and electric field both diverge at any point charge or linear charge. In physics, at equilibrium, an ideal conductor has no charge on its interior; instead, the entirety of the charge of the conductor resides on the surface. However, this only applies to the ideal case of infinite electrical conductivity; the majority of the charge of an actual conductor resides within the skin depth of the conductor's surface. For dielectric materials, upon the application of an external electric field, the positive charges

Organisation and electrochemical reactivity at molecular interfaces

Organisation and heterogeneous photoreactivity of water-soluble zinc meso-tetrakis(4-carboxyphenyl) porphyrin (ZnTPPC) were investigated at polarisable liquid / liquid interfaces by systematically varying the organic phase solvent. The remarkable affinity of ZnTPPC for the molecular interfacial boundary manifested itself by modulations in the surface excess charge. Potential dependence of interfacial coverage of the porphyrin appeared to be affected by the nature of the organic phase solvent. Heterogeneous photoinduced ET between the specifically adsorbed porphyrin molecules and the redox species present in the organic solvent manifests itself as photocurrent responses. Photocurrent responses are also found to be strongly dependent on the nature of the organic phase solvent. Variations in the photocurrent magnitude were rationalised in terms of the interfacial organisation of the porphyrin and the phenomenological electron transfer rate constant using a modified Marcus model. Formation of a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) on gold electrode was investigated by Kelvin probe technique and electrochemical impedance spectroscopy as a function of immersion time in thiol solution. Kelvin probe measurements exhibited a monotonic increase in surface potential with immersion time. The increase in surface potential was attributed to the formation of gold-thiolate bonding. Maximum coverage of thiol molecules were obtained after an immersion time of 10 h. Double-layer capacitance (Cdl) values of thiol modified electrodes were extracted from the impedance measurements as a function of immersion time. Furthermore, Cdl values dropped significantly and reached a constant value within a short immersion time in thiol. Multilayer film of pLys and pGlu was fabricated on MUA modified gold electrodes. Polyelectrolyte modified electrodes displayed characteristics electrochemical features of diffusion through pinholes. Photocurrent responses originating from the porphyrin sensitised ultrathin polyelectrolyte multilayer assembly is investigated as a function of bias potential and illumination light intensity. Redox couples incorporated within the multilayer assembly act as carriers of the photogenerated charges across the film. The incident photon to current conversion efficiency (IPCE) of porphyrin sensitised multilayer assemblies is estimated to be 0.02%. Finally, electrochemical and optical properties of metal nanoparticle architectures on pLys modified gold electrodes were investigated. Monodispersed randomly distributed arrays of Au nanoparticles were formed upon electrostatic deposition from solution. Kelvin probe measurements exhibited a steady increase in compensation potential difference (CPD) with deposition time which reached a plateau after 4 h. From the scanning electron micrographs, we estimated maximum particle coverage of 8.6 × l010cm-2 separated by an average edge-to-edge separation of 25 nm from the nearest neighbour. The plasmon resonance band of Au nanoparticles in the ensemble featured a shift to longer wavelengths and were broader compared to that of colloidal particles in aqueous solution. Cyclic voltammetry and impedance measurements indicated strong electronic coupling between the nanoparticles in the ensemble and the electrode. Furthermore, fabrication of a three-dimensional assembly of metal nanoparticles and pLys using electrostatic layer-by-layer method is also illustrated.

EPFL2004

Second harmonic scattering of water in a biological context

Tereza Schönfeldová

Water is one of the most abundant molecules in the universe. It forms a hydrogen bond network with unique structure and dynamics. Hydrogen bonding is highly relevant to many biological processes including membrane formation, protein folding, adsorption, and lubrication. However, the underlying molecular interactions within water and between water and membranes are difficult to study and hence not fully understood. In this thesis, we utilize second harmonic scattering (SHS) to investigate inhomogeneities in bulk water, probe structural changes in lipid bilayer membranes, and detect and characterize protein-membrane interactions. Firstly, we explore the structure of bulk water in comparison to a common model liquid, carbon tetrachloride. Combining SHS measurements and molecular dynamics simulations, we report on the nanoscale inhomogeneities in bulk water that occur on femtosecond timescales. We attribute the rise in the coherent second harmonic (SH) intensity to charge density fluctuations with enhanced nanoscale polarizabilities around transient voids. Even though the transient voids also exist in carbon tetrachloride, they do not show polarizability fluctuations and the SH response of carbon tetrachloride thus corresponds to that of a model isotropic liquid. Then, we study the structure of water around large unilamellar vesicles (LUVs), which serve as a model for cell membranes. We focus on the role of water during the lipid melting phase transition. Using SHS, differential scanning calorimetry, and temperature-dependent zeta-potential measurements, we investigate the structural changes of hydration shells around the vesicle membrane. We use lipids with phosphoserine, phosphocholine, and phosphate headgroups and observe different hydration behavior upon the lipid melting transition depending on the membrane composition. Next, we characterize the interface of membranes composed of lipids containing phosphocholine and phosphate headgroups in solutions with different CaCl2 concentrations. We observe that the SH intensity is decreasing with the addition of CaCl2, which we attribute to the formation of a condensed layer of Na+ counterions on the inner leaflet of the LUVs. Additionally, we extract the vesicle surface potential and second-order surface susceptibility from SHS data for membranes with different content of lipids with phosphate headgroups, and observe composition dependent behaviour which suggests structural membrane reorganization. We continue by exploring protein-membrane interactions using water as a contrast agent. We study a pore-forming toxin, perfringolysin O (PFO), known for selective insertion into cholesterol-rich membranes. Combining SHS measurements and cryo-electron microscopy, we compare the interaction between PFO and LUV membranes with a high and low cholesterol content. By obtaining a sub-picomolar insertion constant of PFO, we show that SHS can be used to determine protein-membrane binding constants in concentration ranges not accessible by conventional methods.Finally, we investigate the interaction of protein aerolysin and its mutants with lipid membranes. We observe changes in surface charge in the cap region of membrane-bound aerolysin at different pH of the surrounding solution. By combining SHS with single-channel measurements, we obtain the membrane binding constant of wild-type aerolysin and its mutants, and we dissect their pore-forming mechanism.

EPFL2022

Molecular insights into the electrical double layer of colloidal oxide particles obtained through second harmonic scattering

Marie Bischoff

Photocatalytic applications play an essential role in the search for alternative energy sources and environmental decontamination techniques. It is of fundamental interest to understand on a molecular level the aqueous solid/liquid interface, where photocatalytic reactions occur, to improve the reaction efficiency.Surface charge, ion diffusion and adsorption are all elements that can have a significant impact on reaction rates and, in the case of nanoparticles, colloidal stability. However, not many experimental methods are able to probe the electrical double layer (EDL) and its peculiar properties, particularly when it comes to colloidal nanoparticles directly dispersed in solution. In this thesis we apply polarimetric angle-resolved second harmonic scattering (AR-SHS) to study the EDL of colloidal oxide particles in aqueous environments. The insulating oxide silica (SiO2) and the semiconducting metal oxide titanium dioxide (TiO2) are investigated and further compared. We show that from AR-SHS measurements, we can extract the surface potential of the particles with respect to bulk liquid and the surface susceptibility, as a measure of interfacial molecular orientation. This can be achieved without assuming a specific charge distribution in the EDL, and hence without employing a model such as the Gouy-Chapman (GC) or the Gouy-Chapman-Stern (GCS). Through electrophoretic mobility measurements, we additionally obtain the zeta potential. Knowing the surface potential, the surface susceptibility and the zeta potential enables us to establish a molecular-level picture of the oxide particle/liquid interface as a function of ionic strength and pH. First, we provide evidence that a compact layer of hydrated ions is formed at the 300 nm amorphous SiO2/aqueous interface at high pH and high NaCl concentrations (> 1mM). Computation of surface charge density values based on our measured surface potential values and using the GC and GCS models are in good agreement with literature values.Next, we study the EDL of 100 nm amorphous TiO2 particles as a function of NaCl concentration and of basic pH. Three regions, reflecting three different phenomena occurring as a function of ionic strength, can be identified. First, inner-sphere adsorption at the lowest concentrations, then formation of a diffuse layer of counterions and finally, accumulation of hydrated counterions near the interface. Similar regions are observed for 100 nm SiO2 particles as a function of NaCl concentration. The TiO2 surface is found to have a stronger affinity for Na+ ions than SiO2.We further evaluate ion-specific effects at the SiO2 and amorphous TiO2 colloidal aqueous interface. The addition of NaCl, RbCl or CaCl2 to the solution leads to relativedifferences in surface potential and in the evolution of the interfacial H-bonding network as a function of ionic strength, which reveal surface and cation- specific preferences for inner- and outer-sphere adsorption.Finally, we demonstrate that AR-SHS measurements as a function of pH can be used to determine the pKas of 100 nm anatase TiO2 particles. Our data reveals a correlation between the change in orientation of interfacial water molecules and the pKas of surface TiO2 groups. This work shows that AR-SHS is a powerful tool to uncover the EDL structure of colloidal oxide particles, thus contributing to a better understanding of interfacial processes important to a variety of (photo-)catalytic applications.

EPFL2022