Influence of temperature and n-alkane pair on the methylene increments to the thermodynamic functions of solution on SE-30 and Carbowax-20M capillary columns

Difference equation model for isothermal gas chromatography expresses retention behavior of homologues of n-alkanes excluding the influence of holdup time

A difference equation (DE) model is developed using the methylene retention increment (Δtz) of n-alkanes to avoid the influence of gas holdup time (tM). The effects of the equation orders (1st-5th) on the accuracy of a curve fitting show that a linear equation (LE) is less satisfactory and it is not necessary to use a complicated cubic or higher order equation. The relationship between the logarithm of Δtz and the carbon number (z) of the n-alkanes under isothermal conditions closely follows the quadratic equation for C3-C30n-alkanes at column temperatures of 24-260 °C. The first and second order forward differences of the expression (Δlog Δtz and Δ2log Δtz, respectively) are linear and constant, respectively, which validates the DE model. This DE model lays a necessary foundation for further developing a retention model to accurately describe the relationship between the adjusted retention time and z of n-alkanes.

Daunomycin interaction with DNA: microcalorimetric studies of the thermodynamics and binding mechanism

Nucleic acids are an important target for many therapeutics. Small molecules that bind to nucleic acids are important in many aspects of medicines, particularly in cancer chemotherapy. In recent years, many studies have utilized polynucleic acids with various sequences to demonstrate the binding mechanism of daunomycin, a potent anticancer drug. This study describes that isothermal titration calorimetry is a useful tool for studying the fundamental binding mechanism systemically. The results suggest that the binding free energy is more favorable when the temperature is increased. The binding entropy contributes to this effect. Furthermore, the amine group on daunomycin contributes electrostatic interaction that induces the binding process. In addition, enthalpy-entropy compensation is also exhibited in the daunomycin-DNA binding mechanism. This study used an easy, convenient method of performing a systemic study in a recognition system. The results from this study provide additional information about microscopic mechanisms for molecular design and molecular recognition.

Classification and comparison of capillary columns by determination of the solution enthalpy of polar and non polar probes

The behaviour and separation characteristics of capillary columns containing different stationary phases (bonded methyl- and methylphenylsiloxanes and polyglycols, and carbon layer and porous polymer) were classified and compared by measuring the values of the solution enthalpy DeltaHs of n-alkanes and 1-alcohols at various temperatures. The difference in DeltaHs values between straight-chain 1-alcohols and n-alkanes with the same number of carbon atom (DeltaDeltaHs) does not change with changing temperature when gas-liquid partition stationary phases are used, but depends on the column composition and polarity and therefore permits to evaluate the influence of polarity on the solute-solvent interaction. The trend of the DeltaDeltaHs values is correlated with that of the DeltaC parameter and with the McReynolds' polarity constants.

Application of capillary gas chromatography to studies on solvation thermodynamics

The potentiality of capillary gas chromatography (GC) as a means for research on solubility phenomena is focused. Basic thermodynamic information can be obtained in a simple and direct way from this technique relying on few parameters with their associated errors tightly controlled. An unexplored field of solvation phenomenology inaccessible to other techniques is revealed by the accuracy of capillary GC, provided that relevant chromatographic variables are utilized and an adequate treatment of the experimental information performed. The present article reviews different approaches for the attainment of basic thermodynamic information through capillary GC. Some traditional concepts on the treatment of chromatographic data for physicochemical measurement are questioned. Applications of the technique to research on solubility phenomena are depicted.

Considerations on the temperature dependence of the gas-liquid chromatographic retention

A discussion on the temperature dependence of the partition coefficient K is developed. This discussion embraces topics such as the limitations of conventional thermodynamic approaches followed in the chromatographic literature, qualitative theoretical notions arising from molecular thermodynamics and the experimental information that is accessible through modern capillary gas chromatography. It is shown that the heat capacity difference of solute transfer for flexible molecules has at least one maximum in the chromatographic range of temperature. As a consequence, a great amount of experimental data is required for a correct thermodynamic interpretation of the chromatographic retention.

Interpreting the gas chromatographic retention of n-alkanes

Non-linear regressions were applied to n-alkanes retention data for the determination of gas hold-up in a preceding paper. It was found that at temperatures over 100 degrees C the reduced partial molar free energy of solution, deltaG/RT, tends to be negligible for the solute methane in poly(dimethylsiloxane) stationary phases. A consequence of interest can be inferred from this fact. The C-H bonds from terminal methyl groups of n-alkane solute molecules should not contribute significantly to deltaG/RT in these conditions. The analysis of data confirms that, within the chromatographic experimental error, the contributions of n-alkane end C-H bonds are also negligible in this temperature range. Consequently, the regression parameter that contains the phase ratio of the column only includes the gas hold-up as the accompanying factor.

Analysis of the equations for the temperature dependence of the retention index. II. Physico-chemical meaning of the parameters

A general expression for the meaning of various parameters from the retention index-temperature equations was found. This is a linear combination of two solute dependent factors consisting of differences of enthalpies and respective entropies of solution, between the solute and the reference inferior n-alkane. The particular coefficients for each parameter are stationary phase dependent factors. They were calculated from methylene contributions to the thermodynamic functions of solution in SE-30 and Carbowax-20M, revealing the influence of the used temperature range. A short review of structural influences on dI/dT is included.

Analysis of the equations for the temperature dependence of the retention index. I. Relation between equations

The relation between the parameters of several equations for the retention index temperature dependence was established, taking the hyperbola deduced from the retention theory as starting point. The transformation factors depend only on methylene contributions to the thermodynamic functions of solution and temperature. Their evolution with the mean temperature of the range was illustrated for SE-30 and Carbowax-20M glass capillary columns. On this basis the post-run standardisation of dI/dT values at a reference mean temperature is possible. Examples and statistical correlation between series of parameters from different equations for perfumery solutes were shown.