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Self-association of medium-chain alcohols in n-decane solutions has been studied by infrared absorption of the fundamental OH stretching vibration. The alcohols investigated were 1-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 1-butanol, 1-pentanol, and 1-hexanol. Infrared spectra were acquired for varying alcohol molalities, the highest concentration being 0.2 mol/kg. The spectra for each alcohol were collected in a data matrix. The bilinear multicomponent data were successfully resolved into spectra and concentration profiles by a multivariate method. The result indicates that monomers dominate the spectral variance in the low-molality region, while cyclic oligomers dominate in the upper concentration range. It further indicates that minor amounts of open-chain aggregates may be present. The monomer and cyclic tetramer appear to be the dominant species, while the amount of open-chain aggregates was negligible even in the low-molality region. The equilibrium constants for the monomer–tetramer association reactions ( K1–4) were calculated by a least-squares method. The calculated values for the equilibrium constants, based on the molality, range from 138 to 106 for the linear alcohol molecules. The result shows that 1-butanol, 1-pentanol, and 1-hexanol have similar constants, while 1-propanol displays a markedly higher value. The equilibrium constants obtained for 2-methyl-1-propanol and 2-methyl-2-propanol were 77 and 39, respectively. The considerably lower values for the branched alcohol molecules indicate that steric interaction between the chain prevents self-association into larger aggregates.

The equilibrium between H2O, D2O, and HOD in the liquid phase has been analyzed for different concentrations of the reactants by using the attenuated total internal reflectance (ATR) technique. The infrared spectroscopic profiles of the equilibrium mixtures were broken down into the contributing components by the alternating least-squares (ALS) curve resolution technique. Furthermore, the concentration profiles of H2O, D2O, and HOD were determined together with the equilibrium constant. Band assignment of the resolved HOD spectrum was further carried out. The equilibrium constant determined in this work agrees excellently with values determined by both theoretical calculations and experiments.

Multivariate calibration models for predicting the initial reactant and an impurity in a synthesis of contrast media have been developed using near-infrared (NIR) spectroscopy. The reaction is presently monitored off-line by HPLC. The calibration models are based on spectra measured online. The process solution is chemically complex and the intention of this study was to enhance knowledge about the predictive ability of the models. Different data-preprocessing and variable selection methods reported in the literature have been tested in order to enhance chemical information and reduce irrelevant variability due to physical influences on measured spectra such as scattering and temperature. In addition to standard preprocessing methods such as normalization, differentiation and multiplicative scatter correction, orthogonal signal correction and optimized scaling were tested. The modelling was performed by partial least squares (PLS), except for optimized scaling which uses principal component regression (PCR). Optimized scaling on differentiated data gave the lowest prediction error. The NIR region from 1100 to 1900 nm gave better models than variable selection and fewer variables. Models with correlation coefficients for best fit equal to 0.998 and 0.961 were obtained for the initial reactant and the impurity, respectively.