Monomolecular films of phenolic esters.
Abstract The properties of three phenolic esters, characterized by gradual changes in their molecular constitution, have been evaluated in pure monomolecular films, as well as in mixed films with stearic acid. The pressure-area isotherms for the pure compounds reflect a similar orientation for compounds I (resorcinol monoundecanoate) and II (4-chlororesorcinol 1-undecanoate) at the air-water interface. At pressures corresponding to the limiting area per molecule, the molecules are apparently rather erect and anchored to the aqueous subphase surface by the phenolic-OH function; at lower pressures, the molecules are less erect, with their aliphatic chains randomly intertwined. The isotherm characteristics for III (4-chlorophenyl undecanoate), on the other hand, are compatible with an orientation in which the molecule is anchored to the aqueous subphase surface by its ester function, with both the aliphatic chain and the substituted phenyl ring projecting upwards. Isotherms for 1:1 mixtures of each of the phenolic esters with stearic acid indicate that the properties of the mixed films were controlled, generally, by the esters (at low pressures) and by stearic acid (at high pressures), after the esters had been forced out of area-determining positions in the films. The experimentally derived isotherms for the mixtures were compared with those calculated from the isotherms of the pure components, and observed deviations from additive behavior discussed.
Monomolecular films of mixtures: I. Stearic acid with isostearic acid and with tri-p-cresyl phosphate. Comparison of components with octadecylphosphonic acid and with tri-o-xenyl phosphate
Abstract Pressure-area isotherms have been obtained for mixed monolayers of pure compounds whose structures are related to those of lubricant additives. Individually, stearic acid and octadecylphosphonic acid with long unbranched chains form strong films. Isostearic acid has a larger molecular area and a much lower collapse pressure than stearic acid. This difference demonstrates the great sensitivity of monolayers to a single methyl side chain. Tri- p -cresyl phosphate and tri- o -xenyl phosphate have much larger areas and lower collapse pressures. An equimolar mixture of two similar compounds, stearic acid and isostearic acid, has monolayer properties close to the calculated average. An equimolar mixture of dissimilar compounds, stearic acid and tri- p -cresyl phosphate, shows film properties of the phosphate at low pressures, expulsion of the phosphate from the monolayer at intermediate pressures, and properties of stearic acid at high pressures.
An Ultrasensitive Analytical Method Based on Chemiluminescence Sensing Electrochemical Parallel Catalytic Reaction Event
An ultrasensitive electrochemiluminescence (ECL) method on the combination of electrochemical parallel catalytic reaction and chemiluminesence signal sensing was proposed for improving ECL analytical characteristics using vanadate(V) as a representative. Vanadate(V) could be electrochemically reduced to generate vanadate(II) which could be chemically oxidized by potassium periodate to regenerate vanadate(V) and give parallel catalytic wave effect. Then, the reduced product of potassium periodate could react with butyl-rhodamine B to emit a sensitive chemiluminescence signal. The chemiluminescence intensity was correlative with vanadate(V) concentration. The investigation on the electrochemical reaction rate constant (k0) confirmed that the speed of electrochemical reaction was faster than that of the subsequent chemiluminescence reaction. The possibility of the combination of electrochemical parallel catalytic reaction with chemiluminescence signal sensing was proved. The similar ECL behaviors could be observed at zirconia nanowires-Nafion modified electrode. Because of the separation and enrichment effect of the modified electrode on vanadate(V), the selectivity and sensitivity was further improved greatly. Based on these findings, a new concept on the combination of electrochemical parallel catalytic reaction and chemiluminesence signal sensing was proposed and an ultrasensitive ECL method for the determination of vanadate(V) was developed at zirconia nanowires-Nafion modified electrode. Under the optimum experimental conditions, the ECL intensity was linear with the concentration of vanadate(V) in the range of 2.0×10−12 mol/L–2.0×10−10 mol/L. The detection limit was 8.0×10−13 mol/L, which was more than 6 orders of magnitude lower than that observed by electrochemical current transduction for electrochemical parallel catalytic reaction at zirconia nanowires-Nafion modified electrode.