Marieta Balcan

Analysis of alkylphenol-based non-ionic surfactants by high-performance liquid chromatography

Abstract Ethoxylated non-ionic surfactants derived from octyl- and nonylphenols were analyzed by normal-phase HPLC. A silica column (Si-100, 5 μm) with gradient elution and UV detection at 280 nm were used for oligomer determination. Baseline separation of the ethylene oxide adducts of alkylphenols with average ethoxymer numbers as high as 40 was achieved. The elution system and the gradient profile used depend on the average ethoxymer number of the surfactant. The method can provide full information for the quantitative calculation of the oligomer distribution from ethoxylated surfactants. The hydrophobic moiety of the surfactants was characterized by reversed-phase chromatography. An octadecylsilica column (RP-18, 5 μm) and methanol—water (8:2, v/v) as eluent were used. The procedure is useful for the rapid identification of surfactants according to the alkylphenyl chain.

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.

Influence of organic modifiers on the retention characteristics of graphitized carbon column

Abstract The retention time of α-(1,1,3,3-tetramethylbutyl)phenyl ethylene oxide oligomer surfactants was determined on a porous graphitized carbon column (PGC) in eluents containing acetonitrile, methanol, ethanol, 2-propanol, tetrahydrofuran, dioxane and 2-ethoxyethanol as organic modifier. Linear relationships were calculated between the logarithm of the capacity factor and the concentration of organic modifier in the eluent. Ethylene oxide oligomers were well separated on the PGC column in acetonitrile-water eluent mixtures. Oligomers were not separated in eluents containing 2-propanol, tetrahydrofuran. dioxane and 2-ethoxyethanol as organic modifier. This result was explained by the supposition that the bulky organic modifiers occupy the active adsorption centers on the PGC surface decreasing in this manner the separation efficiency of the support.

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.