Predicting hexadecane–air equilibrium partition coefficients (L) using a group contribution approach constructed from high quality data

A group contribution-based quantitative structure–property relationship (QSPR) for the hexadecane–air equilibrium partition coefficients (L) of organic chemicals is developed using the iterative fragment selection (IFS) approach. This new QSPR includes in its training and external validation data sets L values for a large number of structurally complex chemicals measured by the same group using consistent methods. The resulting QSPR has better predictive power than other prediction methods trained primarily using data for chemicals of simpler structures, and measurements of L values from diverse sources. For a subset of chemicals in which the L values have non-additive effects caused by intramolecular hydrogen bonds, the new QSPR gives much better performance in comparison to the most commonly used prediction method.     

Physical and chemical changes of medicinals in mixtures with adsorbents in the solid state. III. Determination of vapor pressure of solid drugs by steam distillation

The vapor pressures of solid drugs were determined by the steam distillation method. Experiments using a series of benzoic acid derivatives indicated that this method offered consistent and satisfactory values for vapor pressure at about 100 degrees C. The vapor pressures of two nonsteroidal antiinflammatory drugs, flufenamic acid (FFA) and mefenamic acid (MFA), were found to be 3.8 x 10(-3) and 1.8 x 10(-4) mmHg, respectively. These values explain the difference in the rate of change to the amorphous state between mixtures of each with magnesium aluminum silicate (MAS) when they were stored under reduced pressure. It was concluded that the vapor pressures of solid drugs at about 100 degrees C, as determined by steam distillation, may be suitable indices for predicting whether or not these drugs have an inherent propensity to become amorphous readily in mixtures with an adsorbent.     

Statistical rate theory determination of water properties below the triple point

We report a new method for determining the saturation vapor pressure, Ps(T), of water at conditions below its triple point. Ps(T) appears as a parameter in the statistical rate theory (SRT) expression for the net evaporation flux. We use measurements of the interfacial conditions during steady-state evaporation and condensation experiments and SRT to determine the values of Ps(T) from 50 different experiments over a range of interfacial conditions. From these values of Ps(T), we develop an analytical expression and from it calculate the liquid-vapor latent heat, Llv(T), and the constant pressure specific heat, cp(L)(T). The calculated values of these properties are compared with those obtained from independent measurements. This comparison indicates the SRT expressions for Llv(T) and cp(L)(T) are consistent with the measurements over a range of temperatures.     

气相色谱法测定及预测有机磷阻燃剂的蒸气压

以p,p′-二氯二苯三氯乙烷(p,p′-DDT)为参考化合物,采用气相色谱法(GC)测定了14种有机磷阻燃剂(OPFRs)在不同温度(T)条件下的蒸气压(P_(GC))及其蒸发热。根据获得的蒸气压值,发现14种OPFRs的log P_(GC)随着温度的增加而减小。初步探讨了分子结构与log P_(GC)的相关性,结果发现卤代烷基磷酸酯的log P_(GC)低于烷基和芳基磷酸酯,且随着分子摩尔体积的增大,相应的log P_(GC)呈下降趋势。采用逐步回归的方法,建立了OPFRs的相对保留时间(RRT)与log P_(GC)之间的定量构效关系(QSPR)模型,模型的累计交叉有效判别系数Q2cum为0.946。模型对OPFRs的log P_(GC)预测值与实验值有良好的相关性(r=0.980),结果表明所建模型具有良好的稳健性和预测能力。利用建立的方法,通过测定的保留时间参数以及定量构效关系模型,可有效预测目前没有标准品的OPFRs的log P_(GC)值。     

Determination of evaporation rates and vapor pressures of very low volatility compounds: a study of the C4-C10 and C12 dicarboxylic acids

A method for the measurement of evaporation rates and vapor pressures of low volatility compounds was developed and applied to the homologous series of C4-C10 and C12 dicarboxylic acids. Proton-transfer chemical ionization mass spectrometry was used to follow directly the temperature-dependent evaporation rates of aerosol samples collected on a cold plate that could be heated at a known rate. The vapor pressures of the deposited compounds were derived from observed evaporation rates through application of the Hertz-Knudsen equation. Temperature programmed desorption allowed for quantification of the enthalpy (DeltaHsub) and entropy (DeltaSsub) of sublimation of the diacids and is described. A strong odd-even dependence with respect to the total carbon number was observed in the derived diacid vapor pressures, consistent with previous measurements. However, the vapor pressures from this method were systematically lower than previous measurements. Though seen in the vapor pressure, no odd-even carbon chain length dependence was readily discernible in the measured values of DeltaHsub and DeltaSsub. Perhaps most importantly, these experimental results also suggest that residual solvent molecules (from the aerosol generation process) trapped in the diacid samples can have a considerable influence on the measured thermodynamic parameters and, if not properly accounted for, may give erroneous results.     

GEMC和GDI方法计算流体气液相平衡的比较

考察采用TraPPE联合原子和OPLS全原子力场两种分子力场, Gibbs系综蒙特卡罗(GEMC)方法和Gibbs-Duhem积分(GDI)方法计算流体气液相平衡的适用性、计算速度、计算精度等问题. 结果表明, 在采用全原子力场情况下, GDI方法比GEMC方法极大地节省了计算时间. 从计算结果来看, 两种方法各有适用范围, 在使用时可互为补充. 在给定力场的前提下, 两种方法所得到的液相密度、蒸发焓、临界温度和临界密度相差不大, 而当力场中的缺陷导致蒸发焓的计算不够准确时, 两种计算方法得到的气体的压力和密度明显不同,进而导致预测的临界压力也明显不同.