Heat capacity and thermodynamic functions of nano-TiO2 rutile in relation to bulk-TiO2 rutile

Several conflicting reports have suggested that the thermodynamic properties of materials change with respect to particle size. To investigate this, we have measured the constant pressure heat capacities of three 7 nm TiO₂ rutile samples containing varying amounts of surface-adsorbed water using a combination of adiabatic and semi-adiabatic calorimetric methods. These samples have a high degree of chemical, phase, and size purity determined by rigorous characterization. Molar heat capacities were measured from T = (0.5 to 320) K, and data were fitted to a sum of theoretical functions in the low temperature (T < 15 K) range, orthogonal polynomials in the mid temperature range (10 > T/K > 75), and a combination of Debye and Einstein functions in the high temperature range (T > 35 K). These fits were used to generate $\mathit{Cp}\text{,}{\text{m}}^{\circ }$, ${\mathrm{\Delta }}_0^{\text{T}}{S}_{\text{m}}^{\circ }$, ${\mathrm{\Delta }}_0^{\text{T}}{H}_{\text{m}}^{\circ }$, and ${\phi }_{\text{m}}^{\circ }$ values at selected temperatures between (0.5 and 300) K for all samples. Standard molar entropies at T = 298.15 K were calculated to be (62.066, 59.422, and 58.035) J · K⁻¹ · mol⁻¹ all with a standard uncertainty of 0.002·${\mathrm{\Delta }}_0^{\text{T}}{S}_{\text{m}}^{\circ }$ for samples TiO₂·0.361H₂O, TiO₂·0.296H₂O, and TiO₂·0.244H₂O, respectively. These and other thermodynamic values were then corrected for water content to yield bare nano-TiO₂ thermodynamic properties at T = 298.15 K, and we show that the resultant thermodynamic properties of anhydrous TiO₂ rutile nanoparticles equal those of bulk TiO₂ rutile within experimental uncertainty. Thus we show quantitatively that the difference in thermodynamic properties between bulk and nano-TiO₂ must be attributed to surface adsorbed water.     

Photoreactivity of biologically active compounds. XIX: Excited states and free radicals from the antimalarial drug primaquine

The formation and reactivity of excited states and free radicals from primaquine, a drug used in the treatment of malaria, was studied in order to evaluate the primary photochemical reaction mechanisms. The excited primaquine triplet was not detected, but is likely to be formed with a short lifetime (<50 ns) and with a triplet energy <250 kJ/mol as the drug is an efficient quencher of the fenbufen triplet and the biphenyl triplet, and forms ${}^1{\text{O}}_2$ by laser flash photolysis $({}^{\text{PQ}}{\Phi }_{\mathrm{\Delta }}=0.025)$. Primaquine (PQ) exists as the monocation $({\text{PQH}}^{+})$ in aqueous solution at physiological pH. ${\text{PQH}}^{+}$ photoionises by a biphotonic process and also forms the monoprotonated cation radical $({\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}})$ by one electron oxidation by ${\text{HO}}^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ (k_q = 6.6 × 10⁹ M^?1 s^?1) and ${\text{Br}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ (k_q = 4.7 × 10⁹ M^?1 s^?1) at physiological pH, detected as a long-lived transient decaying essentially by a second order process (k₂ = 7.4 × 10⁸ M^?1 s^?1). ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ is scavenged by O₂, although at a limited rate (k_q = 1.0 × 10⁶ M^?1 s^?1). The reduction potential (E°) of ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}/{\text{PQH}}^{+}$ is < +1015 mV, as measured versus tryptophan (${\text{TRP}}^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}/\text{TRPH}$). Primaquine also forms ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ at pH 2.4, by one electron oxidation by ${\text{Br}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ and proton loss (k_q = 2.7 × 10⁹ M^?1 s^?1). The non-protonated cation radical (${\text{PQ}}^{+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$) is formed during one electron oxidation with ${\text{Br}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ at alkaline conditions (k_q = 4.2 × 10⁹ M^?1 s^?1 at pH 10.8). The estimated pK_a-value of ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}/{\text{PQ}}^{+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ is pK_a ～ 7–8. Primaquine is not a scavenger of ${\text{O}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ at physiological pH. Thus self-sensitization by ${\text{O}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ is eliminated as a degradation pathway in the photochemical reactions. Impurities in the raw material and photochemical degradation products initiate photosensitized degradation of primaquine in deuterium oxide, prevented by addition of the ${}^1{\text{O}}_2$ quencher sodium azide. Photosensitized degradation by formation of ${}^1{\text{O}}_2$ is thus important for the initial photochemical decomposition of primaquine, which also proceeds by free radical reactions. Formation of ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ is expected to play an essential part in the photochemical degradation process in a neutral, aqueous medium.     

Thermodynamic study of (heptane + amine) mixtures. III: Excess and partial molar volumes in mixtures with secondary,tertiary, and cyclic amines at 298.15 K

Excess molar volumes V^E at 298.15 K were determined by means of a vibrating tube densimeter for binary mixtures of {heptane + open chain secondary (diethyl to dibutyl) and tertiary (triethyl to tripentyl) amines} as well as for cyclic imines (C₂, C₃, C₄, C₆, and C₇) and primary cycloalkylamines (C₅, C₆, C₇, and C₁₂). The V^E values were found positive for mixtures involving small size amines, with V^E decreasing as the size increases. Negative V^E’s were found for tributyl- and tripentylamine, heptamethylenimine, and cyclododecylamine. Mixtures of heptane with cycloheptylamine showed an s-shaped curve.Partial molar volumes $V^{°}$ of amines at infinite dilution in heptane were obtained from V^E and compared with $V^{°}$ of hydrocarbons and other classes of organic compounds taken from literature. An additivity scheme, based on the intrinsic volume approach, was applied to estimate group (CH₃, CH₂, CH, C, NH₂, NH, N, OH, O, CO, and COO) contributions to $V^{°}$. These contributions, the effect of cyclization on $V^{°}$, and the limiting slope of the apparent excess molar volumes were discussed in terms of solute–solvent and solute–solute interactions.