Cationic and anionic contributions to the crystal lattice energy and the heat of solution of salts

Up to now values of H, S, and G have been determined for individual ions in the gaseous state and in aqueous solution at infinite dilution. Values of S have also been determined for individual ions entering into a crystal lattice. The Born-Mayer-Huggins crystal lattice theory has been used to calculate the enthalpy of individual ions in the crystal lattice of alkali metal halides. A formula has been derived on the basis of this theory for the difference between the enthalpies of the cation and anion in the crystal lattice. This difference depends on the van der Waals forces resulting from dipole-dipole and dipole-quadrupole interactions and on the quantum mechanical repulsive forces between the electron clouds of the ions. The enthalpy difference between ions in the lattice varies from +3.1 (LiCl) to -10.1 (KCl) kJ/g-ion. Values of H and G for ions in the crystal lattices of twenty alkali metal halides are presented in tabular form. It is found that the lattice energy is almost equally divided between the cations and anions.The heats of hydration and entropies of solution of ions were calculated from the data obtained and from the known heats of solution of electrolytes. They are presented in tables of H and G of solution, i.e., of the change from a crystal lattice to the aquated state, for the individual ions of twenty alkali metal halides.     

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.

Second harmonic generation is one of the most powerful techniques used to selectively probe interfaces of all types. The direct ab initio method developed here allows predicting the signal and highlights the importance of local and non-local effects.     

Theoretical and experimental study of optical second harmonic generation in new chiral thiolates salts

The second harmonic generation ability of new chiral thiolate salts is investigated. Aromatic thiolate anions ArS⁻ are expected from semi-empirical calculations with PM3 parameterization of the MNDO Hamiltonian to possess higher intrinsic polarizabilities than the parent neutral thiols ArSH.

Salts associating (hyper)polarizable aromatic thiolate anions with chiral cations, which ensure the noncentrosymmetry necessary to have second-order nonlinear optical effects have been synthesized. The first hyperpolarizabilities, β, of the salts are measured using hyper-Rayleigh scattering technique. Second harmonic generation powder tests (Kurtz and Perry method) carried out on various salts studied, were positive indicating that these materials crystallize in noncentrosymmetric space groups.     

Study on second harmonic generation of 10-hydrocarbylacridones

Values of second-order polarizabilities (beta(xxx)), the composite magnitude (beta(CT microg)) of molecular hypersusceptibilities and the second harmonic generation (SHG) of microcrystal were used to characterize and evaluate the second-order nonlinear optical performance of 10-hydrocarbylacridones. Using Nd:YAG as a laser source, the SHG values of these compounds in powder as compared with urea powder were obtained, compared with the calculated values of beta(CT microg) and beta(xxx) found by solvatochromic method. The results showed that the SHG values of compounds 1, 2, and 5 are larger than that of urea. Compound 5 has a highest beta(CT microg) value among the 12 compounds. Although the different electronegativities of hydrocarbyl (R) attached to nitrogen atom should have an effect on the extent of intralmolecular charge-transfer and further influence SHG, beta(CT microg) or beta(xxx) values, the relationships between electronegativity of R and SHG, beta(CT microg) or beta(xxx) could not be found. But it is found that the bigger the donating-electron ability of R, the longer the maximum absorption wavelength of 10-hydrocarbylacridone molecules.     

Temperature resolved second harmonic generation to probe the structural purity of m-hydroxybenzoic acid

We report the use of second harmonic generation (SHG) and temperature resolved second harmonic generation (TR-SHG) for in situ probing and monitoring the structural purification of m-hydroxybenzoic acid (MHBA). Pure and mixtures of the two polymorphic forms of MHBA (metastable Pna2₁ and stable P2₁/c) and commercial MHBA were analysed by: DSC, XRPD and SHG. Results obtained with these three techniques are compared and demonstrate for that particular component, the high sensitivity and accuracy of SHG in assessing the polymorphic purity of MHBA. The SHG detection threshold for the metastable polymorph is circa 2 ppm and is several orders of magnitude better than the sensitivity of XRPD (1 wt%) and DSC (only 17 wt%). The proportion of the metastable phase in the commercial MHBA was evaluated to 1 wt% by SHG. Results of TR-SHG measurements performed on commercial MHBA at 5 K min^?1 heating rate show that TR-SHG is a good technique for an in situ monitoring of the structural purity. The present study clearly demonstrates that SHG and TR-SHG are relevant and accurate techniques for probing the structural purity and for the monitoring of solid–solid phase transitions provided one of the two varieties crystallizes in a non-centrosymmetric space group.     

Imaging chirality with surface second harmonic generation microscopy

Chirality is a fundamental construct in nature which arises from an antisymmetric arrangement of atoms, molecules, or larger structures, resulting in the formation of nonsuperimposable mirror images. Bulk chiral effects can easily be measured using circular dichroism (CD) or optical rotary dispersion (ORD). However, the imaging of chirality originating from molecular surface films cannot be obtained with these linear optical methods. By using chiral second harmonic generation (C-SHG), with its inherent surface sensitivity and ability to discriminate between the symmetry of surface adsorbed species in combination with a counter-propagating optical geometry, we have developed the first nonlinear chiral microscope. In the study presented here, the intrinsic chirality of R- and S-(+)-1,1'-bi-2-naphthol (RBN, SBN) has been used to image a patterned planar supported lipid bilayer (PSLB) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using C-SHG. Spatial resolution of the patterned PSLB is visible when either RBN or SBN is intercalated into the membrane. No image is observed when a racemic mixture of RBN and SBN is present. The C-SHG images are compared with those obtained from fluorescence microscopy to verify the C-SHG imaging technique. The results presented here demonstrate that C-SHG possesses the requisite surface selectivity and sensitivity to detect interfacial chirality and provides a direct route for the visualization of chirality originating from molecular surface films.     

Synthesis and second harmonic generation studies of nlo-functionalized polyimides

Several NLO-functionalized polyimides were synthesized through novel synthetic pathways and their NLO properties were characterized. For preventing the disadvantage in poly(amic acid) precursor route, the Mitsunobu reaction and alternative synthetic route for NLO polyimides were chosen. The second-order NLO coefficients (χ⁽²⁾) of the synthesized polyimides were between 48pm/V and 122pm/V. The thermal stability of nonlinearity characterized showed that all polyimide films retain their NLO activities at temperatures below 100°C.     

Organic crystals for second harmonic generation switching based on anil photochromes

In photochromic materials, we demonstrated that non-linear optical (NLO) properties such as second harmonic generation (SHG) can be switched reversibly by alternate UV and visible irradiations. We report here on the design of materials fulfilling the prerequisites for SHG activity (push-pull and acentric space group) and photochromic properties. Photochromic and NLO properties of a series of anils are presented, along with the ability of some of them to switch SHG.     

A second harmonic generation study of the oxidation of copper electrodes

The oxidation of copper in basic media has been studied by in situ second harmonic generation (SHG), where the SHG signal was recorded alongside the cyclic voltammogram. The SHG signal changes markedly as the copper surface is oxidised to first Cu₂O and then CuO in a duplex structure. The development of Cu₂O gives rise to a resonant SHG signal because of the band-gap of the material then the upper CuO layer produces an electric-field induced second harmonic (EFISH) response. A correlation of charge with the SHG signal is informative with regard to the mechanism of reduction of CuO and SHG is shown to be a useful method for the examination of oxidation of electrode surfaces.     

Crystallization and second harmonic generation in potassium-sodium niobiosilicate glasses

Transparent glasses having molar composition (23−x)K₂O·xNa₂O·27Nb₂O₅·50SiO₂ (x=0, 5, 10, 15 and 23) have been synthesized by the melt-quenching technique and their devitrification behaviour has been investigated by DTA and XRD. Depending on the composition, the glasses showed a glass transition temperature in the range 660-680 °C and devitrified in several steps. XRD measurements showed that the replacement of K₂O by Na₂O strongly affects the crystallization behaviour. Particularly, in the glasses with only potassium or low sodium content the first devitrification step is related to the crystallization of an unidentified phase, while in the glass containing only sodium, NaNbO₃ crystallizes. For an intermediate sodium content (x=10 and 15) a potassium sodium niobate crystalline phase, belonging to the tungsten-bronze family, is formed by bulk nucleation. This system looks promising to produce active nanostructured glasses as the tungsten-bronze type crystals have ferroelectric, electro-optical and non-linear optical properties. Preliminary measurements evidenced SHG activity in the crystallized glasses containing this phase.     

In-situ poling and synthesis of NLO chromophore-bearing polyurethanes for second harmonic generation

The preparation of six diol and one triol monomers bearing donor–acceptor chromophores is described. The monomers contain the N,N′-bis(2-hydroxyethyl)aniline unit with various acceptor groups attached in some cases via azo or olefin linkages, at the para position. Acceptors studied include nitro, tricyanovinyl, cyclobutene-1,2-dione, and imidazolidine-2,4-dione. Poled polymer films were prepared by thermal polymerization of these with 2,4-toluenediisocyanate (TDI) in the presence of an electric field generated by a corona tip. The resulting thin (ca. 1 μm), glassy polymer films were shown to exhibit reasonably good second harmonic generation efficiencies. Preliminary results show that the polymers with higher T_g's have the best temporal stability. The polymer derived from the diol bearing an imidazolidine-2,4-dione acceptor shows only a ca. 30% decrease in its second harmonic generation (SHG) signal over 200 days at room temperature, even though it is not crosslinked. We attribute this to hydrogen bonding interactions from the urethane and imidazolidine-2,4-dione groups. A crosslinked film derived from the triol and TDI has an exceedingly stable SHG response at room temperature, and no decrease in the SHG response is observed at 100°C for a few hours. © 1993 John Wiley & Sons, Inc.     

Hydrogen-bonding network in the organic salts of 4-nitrobenzoic acid

The crystal structures of six novel salts of 4-nitrobenzoic acid — namely, 2-hydroxyethylammonium 4-nitrobenzoate (I), 2-hydroxypropylammonium 4-nitrobenzoate (II), 1-(hydroxymethyl)propylammonium 4-nitrobenzoate (III), 3-hydroxypropylammonium 4-nitrobenzoate (IV), bis-(2-hydroxyethylammonium) 4-nitrobenzoate (V), morpholinium 4-nitrobenzoate (VI) — containing the same anion but different cations have been studied. The ionic forms of I-VI serve as building blocks of the supramolecular architecture, and in crystals they are held together via ionic N-H···O and O-H···O hydrogen bonds. In the crystal packing the building blocks of I-III are self-assembled via N-H...O, O-H···O and C-H...O hydrogen bonds to form the chains which are further consolidated into two-dimensional layers by the same type of interactions. In IV-VI the chain-like structures have been generated by building blocks.     

Ellipsometric approach for the real-time detection of label-free protein adsorption by second harmonic generation

Second harmonic generation (SHG) was performed using a novel ellipsometric detection approach to selectively probe the real-time surface binding kinetics of an unlabeled protein. The coherence of nonlinear optical processes introduces new possibilities for exploiting polarization that are unavailable with incoherent methods, such as absorbance and fluorescence. Adsorption of bovine serum albumin (BSA) at silica/aqueous solution interfaces resulted in changes in the polarization state of the frequency-doubled light through weak, dynamic interactions with a coadsorbed nonlinear optical probe molecule (rhodamine 6G). Using a remarkably simple instrumental approach, signals arising exclusively from surface interactions with BSA were spatially isolated and selectively detected with high signal-to-noise. The relative intensities acquired during the kinetics experiments using both circularly and linearly polarized incident beams were in excellent agreement with the responses predicted from SHG ellipsometry polarization measurements. Analysis of the polarization-dependent SHG generated during BSA adsorption at glass/aqueous solution interfaces provided direct evidence for slow conformational changes within the protein layer after adsorption, consistent with protein denaturation. This polarization selection approach is sufficiently general to be easily extended to virtually all coherent nonlinear optical processes and a variety of different surface interactions and architectures.     

Phase transition of lipid-like monolayer characterized by second harmonic generation

Phase transition of a lipid-like hemicyanine compound characterized by second harmonic generation is studied carefully. The phase transition is assigned as the first order transition between solid state and liquid state. The transition temperature increases with an increase in the surface molecular concentration. A monolayer structure parameter a which is very sensitive to the phase transition is introduced.     

Enhancement of second harmonic generation in helical Sc liquid crystals

In a ferroelectric liquid crystal, a special type of phase-matching for optical second harmonic generation (SHG) is possible, where two counter-propagating fundamental waves create second harmonic waves at the edge of the selective reflection band. We compute the SHG intensity in such a situation and show that, at slight detuning from exact phase-matching, useful resonance enhancement can be obtained. A considerable amount of SHG also appears when the second harmonic frequency is in the reflection band, where the SHG wave is non-propagating.     

Aligned inclusion of hemicyanine dyes into silica zeolite films for second harmonic generation

Silicalite-1 films (thickness = 400 nm) supported on both sides of glass plates (SL/G) were prepared, and hemicyanine dyes (HC-n) with different alkyl chain lengths (n, n = 3, 6, 9, 12, 15, 18, 22, and 24) were included into the silicalite-1 films by dipping SL/Gs into each methanol solution of HC-n (1 mM) for 1 d. The included numbers of HC-n per channel (N(C)) generally decreased with increasing n; that is, they were 6.4, 23.1, 15.4, 8.2, 5.7, 3.5, 0.9, and 1.2 molecules per channel, respectively. The d(33) value gradually increased with increasing n but decreased when n > 18; that is, they were 1.12, 0.50, 2.25, 3.59, 4.99, 5.30, 1.71, and 2.57 pm V(-1), respectively. However, d(33)/N(C) progressively increased with increasing n. The d(31) values were approximately 100 times smaller than the corresponding d(33) values, and the average d(33)/d(31) ratio was 109, which is higher than those of Langmuir-Blodgett (LB) films and poled polymers of nonlinear optical (NLO) dyes, by approximately 2-5 and approximately 30-50 times, respectively. The estimated average tilted angle of the dyes with respect to the channel direction was 7.7 degrees, and the calculated average order parameter was 0.97, which is approximately 480 times higher than the values observed from poled polymers. The degree of uniform alignment (DUA) generally increased with increasing n. The progressive increase of both DUA and d(33)/N(C) with n is attributed to the increase in the tendency of HC-n to enter hydrophobic silicalite-1 channels with the hydrophobic alkyl chain first. A more than 134-fold increase in DUA was observed upon increasing n from 6 to 24. The DUA of HC-24 in the silicalite-1 film reached close to 1. Although the observed d(33) values were lower than those of the LB films of NLO dyes due to very small dye densities of the silicalite films, this methodology bears a great potential to be developed into the methods for preparing practically viable NLO films.     

Polymer films on metals investigated by optical second harmonic generation

In two different types of experiments, polymer films on gold substrates were investigated by optical second harmonic generation (SHG). Thickness dependent ex situ measurements on Langmuir-Blodgett films of the octadecylammonium salt of polyamic acid (PACS) show a preferential orientation of the polymer molecules. In situ SHG experiments were performed to monitor the growth of polyamic acid films deposited from the gas phase. Pyromellitic dianhydride (PMDA), 4,4-oxydianiline (ODA), and 4,4-diaminodiphenyl disulfide (DAPS) served as monomers. Similar to the Langmuir-Blodgett films, an oriented growth is observed. The thickness dependence of the SHG signal is strongly dependent on the interfacial chemistry which is very different for the two amines used as monomers. Based on a comprehensive three layer model, the relation between the structure of the films and the SHG signal is discussed.     

Tracking oxytetracyline mobility across environmental interfaces by second harmonic generation

This work examines the binding behavior of the antibiotic oxytetracycline (OTC) to mineral oxide/water interfaces in the presence and absence of organic functional groups using the interface-specific technique second harmonic generation (SHG). Studies show that OTC binding to fused quartz, methyl ester, carboxylic acid, and alkyl interfaces is fully reversible and highly dependent on solution pH, with appreciable adsorption occurring only at pH 8. Relative surface coverage at pH 8 is highest for the polar organic-functionalized surfaces, and surface saturation occurs for the methyl ester-functionalized fused quartz/water interface at 2 x 10(-5) M. Adsorption isotherm measurements indicate that the binding process is controlled by hydrogen bonding and hydrophobic interactions, with free energies of adsorption on the order of -40 kJ/mol for all interfaces studied. The results indicate that OTC transport in the environment will depend heavily on soil pH and composition and have implications for the development of bacterial antibiotic resistance.