Thermodynamics of aggregation of long chain carboxylic acids in benzene

Abstract

Publication costs assisted by the Practical Engineering CollegeVapor pressure osmometric measurements on benzene solutions of n-hexanoic, n-nonanoic, and n-dodecanoic acids were performed at 31°. The osmometric data in the concentration range 0.5-9.0 X 10-2 M were interpreted in terms of dimer and trimer equilibria and oligomerization constants were calculated. The aggregation tendency increases with increasing chain length and the same trend was observed in the calculated activity coefficients. Enthalpies of aggregation, estimated from the heats of dilution and the distribution of the oligomers, were found to be -7.6 to -9.2 kJ per mole monomer units for the dimer and from -9.2 to -10.9 kJ per mole of monomer units for the trimer, for the three acid studied. The heats of dilution for concentrations up to 0.04 M were the same for the three acids studied and the differences in the free energy were attributed to an entropy contribution. The activity of the solute, calculated from osmometric measurements, was combined with the integral heat of dilution data to calculate the entropy change for the aggregation process.

Structure of Mixed Acid/Decyl Monolayers Grafted on Oxide-Free Si(111) Surfaces

“…The origin of the above phase separation probably results from a structuration of the grafting solution. Molecular or supramolecular assembly of carboxylic acid chains in benzene or in THF at concentration at least down to the millimolar range was indeed reported and similar behavior may be anticipated for our mixed grafting solutions. Therefore, we propose that undecylenic acid molecules self-organize as reverse micelles when the acid chain concentration becomes close to the critical micellar concentration (cmc).…”

“… SOL = 10 -4 corresponds to an acid chain concentration of 0.5 mM which is close to reported cmc for non-ionic amphiphile molecules in non-polar solvents. 33,34,35 The following mechanism is then proposed for the formation of a mixed monolayer in very dilute conditions. The micelle concentration being in the µM range (see estimate below of the number of chains per micelle), the reverse micelles are progressively adsorbing as isolated entities on the H-terminated surface in the dark (Figure 8a).…”

Effects of unsaturation on film structure and friction of fatty acids in a model base oil

“…A lower value of 4 kcal/mol (1.4 × 10 −20 J/dimer) has also been reported for dimer formation of dodecanoic acid in benzene [56], and values in this lower range have been reported [55] for many short-chain alkanoic acids in nonpolar solvents.…”

“…Although the layered structure in our confined film is stabilized by interactions between the exposed diene groups on the monolayer-covered surfaces and the ones on the intervening bilayer, this attraction is overcome by the compression discussed above, which amounted to an applied energy of 1-5 × 10 −22 J/molecule. Many carboxylic acids form hydrogen-bonded dimers in the gas phase [55], in the neat liquid [46], in the crystalline state [41,44,45,47], and when dissolved in nonpolar solvents [55][56][57][58][59]. In the gas phase [55], and in liquid stearic acid [55], each of the two hydrogen bonds between the carboxylic acid groups has a binding energy of ca.…”

On the microstructure of organic solutions of mono-carboxylic acids: Combined study by infrared spectroscopy, small-angle neutron scattering and molecular dynamics simulations

“…carboxylic acids in non-polar media have been investigated by various spectroscopic techniques including infrared (IR) and Raman spectroscopy (see recent examples in [3,4]), X-ray absorption [5] and others; recently, for this purpose the computer simulation techniques have been applied [6,7]. Besides dimerization, long chain-length fatty acids (the number of alkyl groups is above ten) show a tendency to form more complicated associates [8][9][10][11] including a liquid crystal type ordering. The association of the acids in solutions is strongly affected by the solute-solvent interaction.…”