How Interfaces Affect the Acidity of the Anilinium Ion

Abstract

The acidity of a compound is a fundamental property that dictates molecular speciation and reactivity in solution. Measurements of acidity of simple molecules in interfacial environments are rarely carried out but assumptions often are made that the difference is sufficiently small that the change can be ignored. The effect of oil-surfactant-water interfaces in reverse micellar systems on the pKa value of the anilinium ion was measured using titrations by NMR spectroscopy as the size of the bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane reverse micelles decreased. The pKa was observed to drop from 4.85±0.02 to 4.62±0.02 in water as the reverse micelle decreased from w(0) 10 to 4 (that is down to a reverse micellar radius of about 2 nm). NOSEY experiments demonstrated that the aniline moiety resides within the surfactant interface with the amine/ammonium moiety protruding into the waterpool bridging the interface. The presence of the aniline was found to have modest and variable effect on the size of the reverse micelles as observed using dynamic light scattering. Our experimental results provide information important to theoretical studies, which explore interface phenomena and provide a framework for information on such simple molecules. These studies quantitate the small but significant effect on the pKa values upon placement of an aromatic amine molecule at a hydrophilic-hydrophobic interface.

The Interfacial Interactions of Glycine and Short Glycine Peptides in Model Membrane Systems

“…The resulting pK a values calculated in this work are summarized in Table 2 for all the systems investigated in this work and detailed in the descriptions below. Our hypothesis that G is likely positioned such that the N-terminus is near or in the interstitial water region of the RM either facing the negatively charged interface or actually associated with the interface is in line with previous observations and predictions with other charged molecules [51,52]. This pattern was observed for all the G peptides to different degrees, with the largest change for G. The larger difference for G can be explained because this is a smaller amphiphilic molecule, and penetration of the interface by the N-terminus will impact the amphiphilic molecule more than with peptides.…”

“…The smallest G, which is a zwitterion at neutral pH, is likely to interact more strongly with the interface based on the large changes in the pK a of the free amine part of the peptide. As we demonstrated with aniline, the observed differences are likely to be caused by changes in location and not due to an inherent difference in pK a values in the new environment [52]. This may also be due to the disruption of the favorable five-membered ring that is formed by the positively charged protons on the amine and the negatively charged oxygen on the carboxylate group in water by the presence of Na + at the RM interface, as mentioned previously.…”

Structure Dependence of Pyridine and Benzene Derivatives on Interactions with Model Membranes

“…Molecular information on the specific interaction and placement of the compounds with respect to a membrane interface was sought using AOT RMs and NMR spectroscopy. ,− , To this end, the 1 H NMR spectrum of each N-containing compound was acquired in D 2 O and varying sizes of RM. By varying the sizes of the RM ( w 0 ), small changes in the RM microenvironment occur.…”

“…The first model membrane, Langmuir monolayer, has been used to obtain free energies of mixing of hydrophobic molecules, − explore phenomena within the membrane, − and to test how molecules affect a lipid interface. − While compressing a phospholipid Langmuir monolayer in the presence of the molecules shown in Figure , it is possible to determine if the molecules expand, reorganize, or affect the compressibility of the phospholipid monolayer . A commonly used phospholipid, dipalmitoylphosphatidylcholine (DPPC), can comprise up to 40% of lung phospholipid content because of its superior ability to expand and spread at the lung alveolar air–liquid interface. − Similarly, dipalmitoylphosphatidylethanolamine (DPPE) is utilized to model the inner leaflet of bacterial and eukaryotic membranes for drug and lipid interactions. , These two lipids have become a very appealing model interface to study drug and lipid interactions and therefore were used in this study as a model membrane interface to study the interactions of the N-containing compounds in Figure . − To determine the molecular details of interactions of small aromatic N-containing compounds with an interface, their interactions with aerosol-OT (AOT) reverse micelles (RMs) − were examined. ,− RMs are self-assembled microemulsions that form at low water concentration. In these systems, the placement and orientation could be identified by monitoring interactions with the AOT headgroup and/or lipid tails using one-dimensional (1D) and two-dimensional (2D) NMR spectroscopic techniques. ,− With the combination of these two model membrane systems (Figure ), information about how the compounds in Figure interact with membrane-like interfaces was obtained in this study.…”

Confinement Effects on Chemical Equilibria: Pentacyano(Pyrazine)Ferrate(II) Stability Changes within Nanosized Droplets of Water

“…In conclusion, these types of studies are important for investigation of reactions in RMs. Specifically, protonation reactions [ 56 , 57 , 58 , 59 ], nucleophilic substitution reactions [ 60 , 61 , 62 , 63 , 64 ], electron transfer reactions [ 26 , 27 , 28 , 29 ], and enzymatic reactions [ 65 , 66 , 67 ] have been investigated and are known to be affected by the inhomogeneous environment such as those found in microemulsions.…”