Effect of Bivalent Ba, Cu, Ni, and Zn Cations on the Structure of Octadecanoic Acid Monolayers at the Air−Water Interface As Determined by External Infrared Reflection−Absorption Spectroscopy

Abstract

Infrared reflection−absorption spectroscopic (IRRAS) measurements of octadecanoic acid monolayers on aqueous subphases containing 1 mM BaCl2, CuCl2, NiCl2, and ZnCl2, respectively, supplied insight into the molecular order and the coordination complexes of the system alkanoic acid/bivalent cation. The formation of highly ordered crystalline domains already at large areas per molecule is reflected by antisymmetric methylene stretching vibrations which are independent of the compression status, however, cation-dependent (Ba2+, Ni2+, ∼2917 cm-1; Cu2+, 2916.1 cm-1; Zn2+, unusually low, 2914.3 cm-1). The analysis of the antisymmetric and symmetric carboxylate stretching vibrations allowed the determination of coordination types of the respective alkanoic acid/cation system, while from the methylene scissoring vibration the subcell structure could be inferred in most cases. Ba2+ cations form an ionic alkanoate, the chain packing of which appears to be mainly orthorhombic. For Cu2+ cations both bridging bidentate and chelating bidentate complexes were found, and the chain packing is hexagonal. Ni2+ cations appear to show both monodentate and ionic complex formation, while for Zn2+ cations a single species was observed which is assigned to an unsymmetric chelating bidentate complex and a triclinic chain packing. Furthermore, the carboxylate and methylene scissoring bands allowed elucidation of the transformation mechanism that is encountered upon successive in situ acidification of the octadecanoic acid monolayer/Zn2+ system: in addition to the unsymmetric chelating bidentate complex (triclinic) encountered at pH 6.7, an ionic interaction becomes increasingly evident between pH 6.1 and 5.55, and eventually at pH 5.0 the free alkanoic acid (orthorhombic?) is the dominant species.

Effect of humidity on electrical conductivity of zinc stearate nanofilms

“…Kutscher et al reported that the bonding type of anhydrous carboxylates and in acetates in aqueous solution can be classified by the difference (Δ) between the antisymmetric and symmetric COO − stretching frequencies [19]. Therefore, Δ = 142 cm −1 value of the present study can be safely assigned to the bridging bidentate structure [20] where two different metal atoms bind covalently to each of the oxygen atoms of the carboxylate group.…”

“…Parallel arrangement of the CH 2 groups and vertical hydrocarbon chains have been observed in the ZnSt 2 LB films because the symmetric methylene stretching band (ν s CH 2 ) at 2852 cm −1 are comparable with those of the antisymmetric (2956 cm −1 ) and symmetric (2872 cm −1 ) methyl stretching bands (ν a CH 3 and ν s CH 3 ). Kutscher et al indicated that wavenumbers of the antisymmetric CH 2 vibration are conformation sensitive and can be empirically correlated with the conformational order [19]. Lower wavenumbers are characteristic of highly ordered conformations with preferential all-trans characteristics.…”

Influence of Poly(ethylenimine) on the Monolayer of Oleic Acid at the Air/Water Interface

“…The present observation contrasts with the effect of divalent metal ions on the isotherm of unsaturated fatty acid monolayers at an air/water interface. In the latter case, the onset of the first pressure increase generally occurs at a reduced surface area compared with that occurring on pure water (3,12).…”

Surface Rheology Parameters of Source-Specific Surfactant Films as Indicators of Organic Matter Dynamics

“…The effect is caused by more than just the pH. Clearly, the nature of the counter-ion is very important (Simon-Kutscher et al, 1996); this is especially important if there is a tendency to form an insoluble salt, with the film ion. Thus, the presence of quite low concentrations (10 )4 M) of divalent ions leads to the formation of the metal soap of a fatty acid film, unless the pH is quite low.…”