Dan-Florin Anghel

Determination of thermodynamic parameters of ethoxylated nonionic surfactants by means of reversed-phase high-performance liquid chromatography

Abstract An approach to evaluate the thermodynamic properties of ethoxylated nonionic surfactants by using the reversed-phase high-performance liquid chromatography (RP-HPLC) is described. The enthalpies (ΔH°) and entropies (ΔS°) of transfer from the mobile to the stationary phase were determined using the change of capacity factor with temperature. The surfactants taken into consideration were homogeneous (C10–16E8) and polydisperse (C12–18EO10) ethoxylated fatty alcohols. The stationary phase was the alkyl-silica (i.e. octyl- and octadecyl-silica). The eluents were various methanol–water mixtures and the detection was done at 280 nm. The obtained ΔH° and ΔS° values were negative and decreased with the alkyl chain length of the surfactant, the water content of the eluent and the length of the ligand bonded to the stationary phase. The methylenic increments of enthalpy, entropy and Gibbs free energy of transfer from pure water as eluent to the nonpolar stationary phase were also determined. Their values were of −8.60 kJ mol−1, −18.71 J mol−1 K−1 and −2.74 kJ mol−1 on the octyl-silica column and, respectively of −17.28 kJ mol−1, −46.40 J mol−1 K−1 and −2.76 kJ mol−1 on the octadecyl-silica column. The enthalpy–entropy compensation behavior of the surfactants was evaluated and the compensation temperatures obtained were within the 455–586 K range. They agreed with those for alkylbenzenes and polycyclic aromatic hydrocarbons in the RPLC systems. The results reveal the likeness of the interactions involved in the retention of hydrophobic compounds on alkyl-bonded stationary phases and hydro-organic eluents.

Determination of the relative impact of chromatographic conditions and solute structures on the retention behaviour of non-ionic surfactants in RP-HPLC

The retention behaviour of non-ionic surfactants containing different hydrophobic moiety and various lengths of polar ethylene oxide chain was studied on RP-8 and RP-18 columns using methanol and acetonitrile as organic modifiers. The relative impact of the chromatographic conditions on the retention strength was determined using stepwise regression analysis. Non-ionic surfactants can be separated according to the character of the hydrophobic moiety, but not according to the length of the polar ethylene oxide chain. Calculations proved that the organic modifier exerted the highest impact on the retention; the effect of the number of carbon atoms in the hydrophobic moiety, column temperature and the length of the hydrophobic ligand on the surface of silica support are of secondary importance.

Thermodynamic properties of polyethoxylated nonylphenols by reversed-phase high-performance liquid chromatography

Abstract The reversed-phase liquid chromatography (RPLC) was used to investigate the thermodynamic properties of homogeneous ethoxylated nonylphenols (NPE 7–15 ) and their retention mechanism from methanol–water eluents. The enthalpies (ΔH°) and entropies (ΔS°) of transfer from the mobile phase to the octyl- and octadecylsilica stationary phase were determined using the change of the capacity factor (k′) with temperature. The ΔH° and ΔS° values were negative and decreased with the water content of the eluent and with the length of ligand bonded to the stationary phase. The ethylene oxide chain length of the surfactants does not significantly influence the thermodynamic functions of retention. The enthalpy–entropy compensation (EEC) behavior was employed to investigate the mechanism of surfactant retention in the reversed-phase systems. The obtained compensation temperatures were within the 430–590 K range. They were very closed to those of ethoxylated fatty alcohols (455–586 K) and fall within the range of hydrophobic organic compounds. The results reveal the similarity of interactions involved in the RPLC with nonpolar stationary phases and aqueous eluents.

Complex interaction of caffeic acid with bovine serum albumin: calorimetric, spectroscopic and molecular docking evidence

Herein, the interaction between caffeic acid (CA), a hydroxycinnamic acid with favorable health impacts through the prevention of degenerative pathologies, and bovine serum albumin (BSA) was investigated. The effect of CA on the conformation and thermal stability of BSA was studied using isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), circular dichroism (CD) spectroscopy, and molecular docking. Thermodynamic parameters (ΔH, ΔS, and ΔG), number of site types (N), and binding constant (K) were determined by ITC. Competitive binding of ITC with warfarin (WAR) and Ibuprofen (IBP) suggested that one of the BSA binding sites for CA was missed in the fluorescence measurements. Molecular docking studies indicate that CA binds to BSA at two sites located in the IIIA and IIA subdomains of the protein native structure. This confirmed the ITC results of competitive binding. Comprehensive DSC and CD experiments revealed a protein stability enhancement in the presence of CA. At higher concentrations, CA induces a slight decrease in the α-helix content of BSA, as observed in the far-UV CD spectra, whereby a partial unfolding of the protein occurs, as proven via both the DSC and CD experiments. This extensive study provides substantial data regarding the ligand binding effect on the protein structure that is significant for understanding the biological activity of BSA in vivo.

Chitosan/anionic surfactant microparticles synthesized by high pressure spraying method for removal of phenolic pollutants

AbstractBiopolymeric microparticles were prepared by rapid expansion of high pressure CO2-chitosan (Chi) solution in sodium bis-(2-ethyl hexyl) sulfosuccinate (AOT) solution. At pressures higher than 2 MPa, ultrafine particles were formed while under this value, wires were obtained. The formation of Chi/AOT complex was confirmed by Fourier Transform Infrared (FTIR) Spectroscopy, whereas scanning electron microscopy was used to characterize the morphology, size and shape of the particles. The FTIR spectrum proved the interaction between the sulfonate groups of AOT and the amino groups of Chi. Microparticles are quasi-spherical in wet conditions and irregular after freeze drying, presenting a rough surface with many pores. Lyophilized hydrophobic microparticles were used to remove phenol and o-cresol from aqueous solution, and the adsorption process showed a maximum efficiency in the 7–8 pH range. The uptake of phenol and o-cresol increased with the amount of particles and decreased with increasing the pollutant concentration. The adsorption occurred rapidly in the first 60–120 minutes, and leveled off thereafter.