ABSTRACT
The purpose of this study was to investigate a rapid method for the evaluation of vaporization characteristics for selected benzoic acid derivatives. The compounds studied in this context were the ortho-, meta- and para-derivatives of hydroxy and amino benzoic acids. Calculations for the order of reaction were first carried out for each of the compounds using methyl paraben as the calibration standard. Those compounds undergoing zero order, non-activated evaporation processes, were analyzed by the Antoine and Langmuir equations, conjointly. The coefficient of vaporization was obtained as 1.2 x 10(5)+/-0.8 Pakg (0.5)mol(0.5)s(-1)m(-2)K(-0.5). The vapor pressure values were used to determine the Antoine constants using the SPSS 10.0 software. This study attempts to outline a comprehensive thermogravimetric technique for vapor pressure characterization of single-component systems.
Subject(s)
Benzoates/analysis , Atmospheric Pressure , Chemical Phenomena , Chemistry, Physical , Kinetics , Models, Chemical , Technology, Pharmaceutical/methods , Thermogravimetry/instrumentation , Thermogravimetry/methods , VolatilizationABSTRACT
The objective of this study was to develop and analyze an analytical method in order to evaluate preformulation candidates by their thermodynamic parameters and evaporation characteristics. Ortho, meta and tere-phthalic acids were chosen as model compounds. The relative advantages and disadvantages of a rapid thermogravimetric method have been studied in detail, which would aid in the preformulation characterization for pharmaceuticals. Methyl paraben was taken as the model compound for calibration, as its evaporation characteristics are well known. Using the Antoine and the Langmuir equation for evaporation conjointly, the parameter k, known as the coefficient of evaporation was determined. The value for this constant was validated by three methods simultaneously. Previously the use of such methods for compounds having uninhibited zero order evaporation has been documented. In the present study, phthalic acid was chosen as the model compound since its evaporation is a two-step overlapping phenomenon. In this study we have shown the use of Pressure Differential Scanning Calorimetry in separating such simultaneous endothermic processes. The Clausius-Clapeyron equation seemingly has anomalous behavior for vapor pressure over high temperature ranges. In this study a modification of the equation has been suggested to take into account the changes in the heat capacities that result due to high temperature effects. This study aims at documenting a concise method for rapid pharmaceutical characterization and suggests modifications for some basic thermodynamic parameters over higher temperature ranges.
Subject(s)
Phthalic Acids/pharmacokinetics , Technology, Pharmaceutical/methods , Calorimetry, Differential Scanning/methods , Kinetics , Least-Squares Analysis , Models, Chemical , Phthalic Acids/chemistry , Pressure , Technology, Pharmaceutical/statistics & numerical data , Thermodynamics , VolatilizationABSTRACT
Four non-polymeric plasticizers, propylene glycol, diethyl phthalate, triacetin, and glycerin have been subjected to rising temperature thermogravimetry for kinetic analysis and vaporization-based thermal stability evaluation. Since volatile loss of a substance is a function of its vapor pressure, the thermal stability of these plasticizers has been analyzed by generating vapor pressure curves using the Antoine and Langmuir equations. Unknown Antoine constants for the sample compounds, triacetin and glycerin have been derived by subjecting the vapor pressure curves to nonlinear regression. For the first time, the entire process of obtaining the unknown Antoine constants through thermogravimetry has been validated by developing an approach called the 'double reference method.' Based on this method, it has been possible to show that this technique is accurate even for structurally diverse compounds. Kinetic analysis on the volatilization of compounds revealed a predominant zero order process. The activation energy values for vaporization of propylene glycol, diethyl phthalate, triacetin, and glycerin, as deduced from the Arrhenius plots, have been determined to be 55.80, 66.45, 65.12, and 67.54 kJ/mol, respectively. The enthalpies of vaporization of the compounds have been determined from the Clausius-Clapeyron plots. Rising temperature thermogravimetry coupled with nonlinear regression analysis has been shown to be an effective and rapid technique for accurately predicting the vapor pressure behavior and thermal stability evaluation of volatile compounds.
