Surface-adsorbed Film Research Articles

Counterions under a Surface-Adsorbed Cationic Surfactant Monolayer: Structure and Thermodynamics.

The surface adsorption of ionic surfactants is fundamental for many widespread phenomena in life sciences and for a wide range of technological applications. However, direct atomic-resolution structural experimental studies of noncrystalline surface-adsorbed films are scarce. Thus, even the most central physical aspects of these films, such as their charge density, remain uncertain. Consequently, theoretical models based on contradicting assumptions as for the surface films' ionization are widely used for the description and prediction of surface thermodynamics. We employ X-ray reflectivity to obtain the Ångström-scale surface-normal structure of surface-adsorbed films of the cationic surfactant cetyltrimethylammonium bromide (CTAB) in aqueous solutions at several different temperatures and concentrations. In conjunction with published neutron reflectivity data, we determine the surface-normal charge distribution due to the dissociated surfactants' headgroups. The distribution appears to be inconsistent with the Gouy-Chapman model yet consistent with a compact Stern layer model of condensed counterions. The experimental surfactant adsorption thermodynamics conforms well to classical, Langmuir and Kralchevsky, adsorption models. Furthermore, the Kralchevsky model correctly reproduces the observed condensation of counterions, allowing the values of the adsorption parameters to be resolved, based on the combination of the present data and the published surface tension measurements.

Electrocatalytic Oxidation of Carbohydrates via Surface-Immobilized Viologen

Electricity produced directly from renewable carbohydrates—Earth’s most abundant biomolecules—offers tremendous potential if it can be done cheaply, efficiently, and practically. Previous work demonstrated that the viologen-mediated process accelerates carbohydrate oxidation but also results in incomplete oxidation products reducing the efficiency of electricity production. Subsequent studies suggested that soluble polymeric viologen compounds improved oxidation efficiency by increasing the local concentration of the viologen mediators. In the present study, the utility of surface-immobilized viologen mediators for the oxidation of simple carbohydrates was investigated. Self-assembled monolayers of thiolated viologen derivatives were prepared on gold anodes to enhance the electrocatalytic oxidation of dihydroxyacetone, fructose, and glucose. The thiolated viologen mediators formed surface-adsorbed films on gold electrodes that were consistent with incomplete monolayers, as characterized by quartz crystal microbalance and cyclic voltammetry. Monolayers formed from dialkylated viologens enhanced the spontaneous electrooxidation of carbohydrates in an unoptimized alkaline electrochemical cell. These results indicate that immobilized mediators can be used to enhance the spontaneous oxidation of carbohydrates to generate electricity from renewable sources. The same tethered-mediator strategy could be used for other mediators to increase the electrochemical cell efficiency and enable long-lasting anodes in carbohydrate fuel cells.

Preferential partitioning of uncharged local anesthetics into the surface-adsorbed film

The surface tension and pH of aqueous solutions of three hydrochloric acid (HCl) – uncharged anesthetic (mepivacaine (MC), bupibacaine (BC) and dibucaine (DC)) mixtures were measured as a function of total molality and composition of local anesthetic in order to investigate the competitive surface-adsorption of uncharged and charged local anesthetics. The behavior of the surface tension versus total molality and pH versus total molality curves remarkably changed at the composition corresponding to an equimolar mixture. The pH measurements showed that uncharged and charged forms coexisted only at compositions more than the equimolar mixture. The partitioning quantities of respective uncharged and charged anesthetics into the surface-adsorbed film were estimated from their surface densities calculated thermodynamically. The greater quantity of uncharged anesthetics existed in the adsorbed film at the coexisting composition, that is, the uncharged anesthetics adsorbed more preferentially than charged ones. The relative ease with which uncharged anesthetics transferred into the surface-adsorbed film was proportional to the hydrophobicities and well correlated the anesthetic potencies. At compositions in the vicinity of physiological pH (ca. 7.4), the bulk solution is more abundant in charged anesthetics than uncharged ones, whereas the uncharged molecules is conversely more abundant in the surface region. The present results clearly imply that the surface-active molecule of local anesthetic in the physiological pH is the uncharged form and the partitioning is greatly dependent on the hydrophobicity among the anesthetics.

Dissociation equilibrium between uncharged and charged local anesthetic lidocaine in a surface-adsorbed film

The dissociation equilibrium between uncharged local anesthetic lidocaine (LC) and charged local anesthetic LC (LC•H+) in a surface-adsorbed film was investigated by measuring the surface tension and pH of aqueous solutions of a mixture of hydrochloric acid and LC. The surface tension values decreased slightly with increasing total molality mt at 0≤X2≤0.5, where X2 is the mole fraction of LC in the mixture, while they decreased rapidly with increasing mt at 0.5<X2≤1. It was shown from the pH measurements that almost all LC molecules were changed into LC•H+ ions by protonation at 0≤X2≤0.5 and both forms coexisted only at 0.5<X2≤1. The quantities of the respective LC and LC•H+ transferred from the aqueous solution to the adsorbed film, i.e., their surface densities, were calculated by applying the thermodynamic equations derived to the surface tension and pH data. A greater quantity of LC than LC•H+ existed in the adsorbed film at the coexisting composition. The partitioning behavior of LC and LC•H+ in the adsorbed film was characterized by three composition regions: (1) slight partitioning of low surface-active LC•H+ in the region at 0≤X2≤0.5, (2) preferential partitioning of LC at 0.5<X2<around 0.7, and (3) negative partitioning of LC•H+ at around 0.7≤X2≤1. The present results clearly indicate that uncharged local anesthetics transfer into hydrophobic environments such as cell membranes more than charged ones.

Partitioning of uncharged local anesthetic benzocaine into model biomembranes

The partitioning of uncharged local anesthetic benzocaine (BzC) into molecular aggregates formed by cationic surfactant decylammonium chloride (DeAC) and phospholipid dipalmitoylphosphatidylcholine (DPPC) was studied from the surface tension and light transmittance measurements. The quantities concerning the partitioning of BzC, the compositions of BzC in the surface-adsorbed film and micelle and three kinds of differential partition coefficients corresponding to phase transitions of the DPPC bilayer membrane were evaluated from thermodynamic analysis of the experimental data. The surface-adsorbed film and micelle were more abundant in BzC than the aqueous solution and significantly large differential partition coefficients for the DPPC membranes were observed. The results clearly showed that the BzC molecules greatly partitioned into hydrophobic environments produced by surfactant-monolayer and phospholipid-bilayer membranes. The partitioning behavior of BzC was also compared with that of charged local anesthetic procaine hydrochloride (PC·HCl). It was shown that the PC·HCl molecule did not or hardly partition into such hydrophobic environments. The contrasting results of the partitioning between BzC and PC·HCl are attributable to the drastic decrease of hydrophilicity of BzC due to the lacking of ionic polar head group in comparison with PC·HCl.

Surface tension study on the molecular-aggregate formation of local anesthetic dibucaine hydrochloride

The formation of molecular aggregates of local anesthetic dibucaine hydrochloride (DC·HCl) was investigated by measuring the surface tension of aqueous DC·HCl solutions as a function of temperature at various concentrations around the critical micelle concentration (CMC). The thermodynamic quantities concerning the aggregate formation of DC·HCl, surface density, entropy and energy changes of adsorption from the monomeric and micellar states, and entropy change of micelle formation, were calculated. They were compared with the corresponding results of straight-chain cationic surfactants, decylammonium chloride (DeAC) and decyltrimethylammonium chloride (DeTAC). From the surface pressure versus area per adsorbed molecule curves, the surface-adsorbed film became more expanded in the order of the geometry of their molecular structures, DeAC<DeTAC<DC·HCl, at a high surface pressure. The entropy and energy changes of adsorption showed that the surface adsorption of DC·HCl roughly resembles DeTAC rather than DeAC. On the other hand, the behavior of entropy change of micelle formation versus temperature curves suggested that the micelle formation of DC·HCl may be different from those of DeAC and DeTAC.

Partitioning of Charged Local Anesthetics into Model Membranes Formed by Cationic Surfactant: Effect of Hydrophobicity of Local Anesthetic Molecules

The partitioning of five hydrochloride salts of local anesthetics, dibucaine (DC·HCl), tetracaine (TC·HCl), bupivacaine (BC·HCl), lidocaine (LC·HCl), and procaine (PC·HCl), into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was studied by the surface tension of aqueous solutions of DeAC−local anesthetic mixtures. Thermodynamic quantities of the partitioning of the anesthetics, total surface density, and compositions of the anesthetics in the surface-adsorbed film and micelle were evaluated by applying thermodynamic equations to the surface tension data. The quantities of the anesthetics partitioned into the molecular aggregates of DeAC were determined from the phase diagrams of surface adsorption and micelle formation. The phase diagrams of surface adsorption and micelle formation showed that the local anesthetic partitioned into the surface-adsorbed film and micelle of DeAC decreases in the order of DC·HCl, TC·HCl, BC·HCl, LC·HCl, and PC·HCl. A good correlation was seen betw...

Exclusion of the Local Anesthetic Procaine Hydrochloride from a Surface-Adsorbed Film and Micelle of Decylammonium Chloride

Incorporation of the local anesthetic procaine hydrochloride (PC·HCl) into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was investigated by measuring the surface tensions of aqueous solutions of a DeAC−PC·HCl mixture. The thermodynamic quantities of the miscibility in these molecular aggregates, the total surface density, and the compositions of PC·HCl in the surface-adsorbed film and micelle were calculated by applying thermodynamic equations to the surface tension data. The phase diagrams of surface adsorption and micelle formation showed a peculiar feature; namely, the compositions of PC·HCl in the adsorbed film and micelle showed negative values over the whole composition range measured. The negative values were caused by the exclusion of the PC·HCl molecules from the adsorbed film and the micelle of DeAC. PC·HCl was not incorporated into such hydrophobic environments. In contrast, tetracaine hydrochloride (TC·HCl) was incorporated into the adsorbed film and micelle of DeAC at all DeAC−TC·HCl compositions. The difference in incorporation between TC·HCl and PC·HCl is attributable to the hydrophobicity of their molecules.

Incorporation of Micelle-Forming Local Anesthetics into Surface-Adsorbed Films and Micelles of Decylammonium Chloride

The incorporation of two micelle-forming local anesthetics, dibucaine hydrochloride (DC-HCl) and tetracaine hydrochloride (TC-HCl), into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was studied by measuring the surface tension of aqueous solutions of DeAC-DC.HCl and DeAC-TC.HCl binary mixtures. The quantities of the anesthetics incorporated into the adsorbed film and micelle of DeAC were estimated from the compositions of the anesthetics in the adsorbed film and micelle, which were obtained by thermodynamic analysis of surface tension data. The phase diagrams of surface adsorption and micelle formation constructed on the basis of composition in the molecular aggregates indicated that although significant amounts of both anesthetics were incorporated into the adsorbed film and micelle, larger quantities of DC.HCl than of TC.HCl were present in the hydrophobic environment of the aggregates. The relative ease with which these two anesthetics were incorporated into the model membranes correlates with their observed clinical potency. Interactions between DeAC and the anesthetic molecules in the adsorbed and micellar states were also analyzed by calculating the ideal mixing lines in both states. It was shown that the bulky polar head groups of the anesthetics were more favorable for micelle formation than for surface adsorption by comparing the incorporation of the anesthetics into the adsorbed film with that into the micelle at the critical micelle concentration.

Effect of Counterions on the Surface Adsorption and Volume Behavior of Micelle-Forming Local Anesthetics

Abstract The surface tension and densities of aqueous solutions of micelle-forming local anesthetic dibucaine and tetracaine salts having different halide ions as counterions from each other (dibucaine hydrochloride (DC·HCl), hydrobromide (DC·HBr) and hydroiodide (DC·HI), and tetracaine hydrochloride (TC·HCl) and hydrobromide (TC·HBr)) were measured as a function of the molality at 298.15 K under atmospheric pressure. The thermodynamic quantities of adsorption and volume for the anesthetics were evaluated numerically based on the experimental results. The surface densities of the anesthetics increased in the order of chloride, bromide, and iodide salts. This behavior may result from differences in the degree of three factors, that is, a hydration interaction between water molecules and counterions, the counterion distribution in the surface-adsorbed film, and ion-pair formation in the adsorbed film between anesthetic cations and counterions. It was further shown from the surface pressure vs. area per adsorbed molecule curves that the adsorbed films of the anesthetics undergo a phase transition from a gaseous state to an expanded state at low concentrations. On the other hand, the volumes of micelle formation for DC·HCl and DC·HBr have been found to be almost equal to each other. This fact means that the effect of these halide counterions on the volume of micelle formation is considerably small in contrast to the results of surface adsorption.

Miscibility of dodecyltrimethylammonium bromide and dodecyltrimethylammonium chloride in surface-adsorbed film and micelle

Surface tension of aqueous solutions of mixtures of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium chloride (DTAC) has been measured and analyzed by using thermodynamic relations. The adsorbed film has been found to contain more DTAB molecules than the solution. The shape formed by the curves of the total molality at constant surface tension against the solution and surface compositions indicates the ideal mixing of the DTAB and DTAC molecules in the adsorbed film. Micellar composition has been estimated at the critical micelle concentration (CMC). The micelles have been found to be richer in DTAB than the solution, but poorer in DTAB than the adsorbed film at the CMC. The DTAB and DTAC molecules have been shown to mix ideally in the micelles. From the comparison with the results on the system of decylammonium bromide and decylammonium chloride, it has been concluded that, on the mixing of surfactants differing only in counter ions, the adsorbed film is influenced more significantly by the ionic head group of the surfactant than the micelle.