Why local and non-local terms are essential for second harmonic generation simulation?

Second Harmonic Generation (SHG) today represents one of the most powerful techniques to selectively probe all types of interfaces. However, the origin of the SHG signal at a molecular level is still debated since the local dipole contribution, which is strongly correlated to the molecular orientation can be counterbalanced by non-local quadrupole contributions. Here, we propose a method to simulate the SHG signal of a model water/air interface from the molecular response of each contribution. This method includes both local and non-local terms, which are represented, respectively, by the dependency of the polarisability and hyperpolarisability upon the chemical environment of the molecule and by the bulk quadrupole response. The importance of both terms for the sound simulation of the SHG signals and their interpretation is assessed. We demonstrate that the sole dipole term is unable to simulate a SHG signal, even if the dependency of the hyperpolarisability on the local environment is considered. The inclusion of the bulk quadrupole contribution, which largely dominates the dipole contribution, is essential to predict the SHG response, although the accuracy of the prediction is increased when the dependency upon the local environment is considered.

Deciphering second harmonic generation signals.

Second harmonic generation (SHG) has emerged as one of the most powerful techniques used to selectively monitor surface dynamics and reactions for all types of interfaces as well as for imaging non-centrosymmetric structures, although the molecular origin of the SHG signal is still poorly understood. Here, we present a breakthrough approach to predict and interpret the SHG signal at the atomic level, which is freed from the hyperpolarisability concept and self-consistently considers the non-locality and the coupling with the environment. The direct ab initio method developed here shows that a bulk quadrupole contribution significantly overwhelms the interface dipole term in the purely interfacial induced second-order polarisation for water/air interfaces. The obtained simulated SHG responses are in unprecedented agreement with the experimental signal. This work not only paves the road for the prediction of SHG response from more complex interfaces of all types, but also suggests new insights in the interpretation of the SHG signal at a molecular level. In particular, it highlights the modest influence of the molecular orientation and the high significance of the bulk quadrupole contribution, which does not depend on the interface, in the total experimental response.

Superchaotropic nano-ions as foam stabilizers.

HYPOTHESIS: Weakly hydrated nanometric ions, called superchaotropes, were recently shown to adsorb strongly to non-ionic surfaces affecting drastically the surface's physical-chemical properties due to a charging effect. Superchaotropic ions could serve as stabilizing agents for non-ionic colloidal systems, such as non-ionic surfactant foams. EXPERIMENTS: We study foams of the non-ionic surfactant BrijO10 (C18:1E10) without and in presence of the superchaotropic Keggin-ion SiW12O404- (SiW). The foams are investigated under free drainage conditions by image analysis and conductimetry to reveal the effect of SiW on the foam stability, liquid drainage, and bubble size. Additionally, small angle neutron scattering on the same foams, but in a dry quasi-stationary state, provides insight into effects of SiW on the foam films. FINDINGS: SiW strongly stabilizes non-ionic surfactant foams at millimolar concentrations by inducing electrostatic repulsions between foam film interfaces resulting in thicker and monodisperse foam films. A similar effect is observed with the ionic surfactant sodium dodecylsulfate (SDS) but to a lesser extent and with a different mechanism. At the foam films' interface, SiW adsorbs to the polar non-ionic surfactant heads driven by the superchaotropic effect whereas DS- anchors between non-ionic surfactant alkyl chains by the hydrophobic effect. The potential of superchaotropic ions as foam stabilizers is herein demonstrated.

Highlights on the Binding of Cobalta-Bis-(Dicarbollide) with Glucose Units.

Metalla-bis-dicarbollides, such as the cobalta-bis-dicarbollide (COSAN) anion [Co(C2 B9 H11 )2 ]- , have attracted much attention in biology but a deep understanding of their interactions with cell components is still missing. For this purpose, we studied the interactions of COSAN with the glucose moiety, which is ubiquitous at biological interfaces. Octyl-glucopyranoside surfactant (C8G1) was chosen as a model as it self-assembles in water and creates a hydrated glucose-covered interface. At low COSAN content and below the critical micellar concentration (CMC) of C8G1, COSAN binds to C8G1 monomers through the hydrophobic effect. Above the CMC of C8G1, COSAN adsorbs onto C8G1 micelles through the superchaotropic effect. At high COSAN concentrations, COSAN disrupts C8G1 micelles and the assemblies become similar to COSAN micelles but with a small amount of solubilized C8G1. Therefore, COSAN binds in a versatile way to C8G1 through either the hydrophobic or superchaotropic effect depending on their relative concentrations.

Are Keggin's POMs Charged Nanocolloids or Multicharged Anions?

Owing to their multiple charges and their nanometric size, polyoxometalates (POMs) are at the frontier between ions and charged colloids. We investigated here the effect of POM-POM electrostatics repulsions on their self-diffusion in water by varying POM and supporting salt concentrations. The self-diffusion coefficients of two Keggin's POMs [silicotungstate (SiW12O404-) and phosphotungstate (PW12O403-)] were determined by dynamic light scattering (DLS) and 1H/31P DOSY NMR, whereas POM-POM electrostatic repulsions were investigated by the determination of the static structure factors using small-angle X-ray scattering (SAXS). The self-diffusion coefficients for the two POMs and for different POM/background salt concentrations were collected in a master curve by comparing the averaged POM-POM distance in solution to the Debye length. As for classical charged colloids, we show that the POM's counterions should not be considered in the calculation of the ionic strength that governs POM-POM electrostatic repulsions. This result was confirmed by fitting the POM-POM structure factor by considering a pair potential of spherical charged particles using the well-known Hayter mean spherical approximation (MSA). These Keggin POMs also behave as (super)chaotropic anions (i.e., they have a strong propensity to adsorb on (neutral polar) surfaces, which was also investigated) here on the surface of octyl-ß-glucoside (C8G1) micelles. The variations of (i) the chemical shift of 1H/31P NMR signals and (ii) the self-diffusion coefficients obtained by DOSY 1H/31P NMR of PW3- and of C8G1 were in good agreement, confirming the strong adsorption of POMs on the micelle polar surface from static and dynamic points of view. We concluded that Keggin's POMs behave (i) as anions because they adsorb on surfaces as chaotropic anions and (ii) as colloids because they can be described by a classical colloidal approach by dynamic and static scattering techniques (i.e., by the investigation of their interparticle electrostatic structure factor and self-diffusion without considering the POM's counterions in the ionic strength calculation). This work highlights the dynamic properties of POMs at soft interfaces compared to bulk aqueous solution, which is essential in the understanding of functional properties of POMs, such as (photo)catalysis and the rational design of POM-based hybrid nanomaterials from soft templating routes (i.e., in aqueous solutions at room temperature).

Analysis of the second harmonic generation signal from a liquid/air and liquid/liquid interface.

Three different liquid interfaces, water/air, thiophene/air, and water/thiophene, were probed using the second harmonic generation (SHG) technique. Thiophene and water have been chosen because the hyperpolarizability of these molecules has already been measured or calculated and the different values can be found in literature. We have studied the microscopic structure of these interfaces by comparing the components of the second order susceptibility tensor determined from the SHG polarization curve analysis with those determined via a molecular dynamics (MD) simulation of these interfaces. We have indeed computed the structure and orientation of water and thiophene molecules at the liquid/air and liquid/liquid (L/L) interfaces as a function of the distance from the interface. The integrated susceptibility values calculated by MD simulations agree well with SHG results and validate the choice of force fields that should permit to quantify more complex L/L interfaces.

Surface activity and molecular organization of metallacarboranes at the air-water interface revealed by nonlinear optics.

Because of their amphiphilic structure, surfactants adsorb at the water-air interface with their hydrophobic tails pointing out of the water and their polar heads plunging into the liquid phase. Unlike classical surfactants, metallabisdicarbollides (MCs) do not have a well-defined amphiphilic structure. They are nanometer-sized inorganic anions with an ellipsoidal shape composed of two carborane semicages sandwiching a metal ion. However, MCs have been shown to share many properties with surfactants, such as self-assembly in water (formation of micelles and vesicles), formation of lamellar lyotropic phases, and surface activity. By combining second harmonic generation and surface tension measurement, we show here that cobaltabis(dicarbollide) anion {[(C2B9H11)2Co](-) also named [COSAN](-)} with H(+) as a counterion, the most representative metallacarborane, adsorbs vertically at the water surface with its long axis normal to the surface. This vertical molecular orientation facilitates the formation of intermolecular and nonconventional dihydrogen bonds such as the B-H(δ-)···(δ+)H-C bond that has recently been proven to be at the origin of the self-assembly of MCs in water. Therefore, it appears here that lateral dihydrogen bonds are also involved in the surface activity of MCs.