The apparent pKa values of the positively charged, hydrophilic indicators 2‐hydroxy‐5‐bromo‐trimethylanilinium chloride (HBTC) and 2‐hydroxy‐5‐nitro‐trimethylanilinium chloride (HNTC) in the presence of water‐in‐oil microemulsion of the surfactants benzylcetyldimethyl ammonium chloride (BCDC) in benzene and polyoxyethylene (4) dodecyl ether (Brij‐30) in heptane were determined spectrophotometrically. The experimental variables were the type of buffer (benzimidazole, phosphate, and tris) and the ratios [water]/[surfactant] and [buffer]/[surfactant]. It is shown that: a) the indicators are confined within the second hydration shell of the nanodroplet, i.e. are not adsorbed at the water/oil interface; b) the use of the starting pH values of the buffer solutions, i.e., those measured outside the micellar domain, to calculate the micellar pKa values of the indicators is inadequate; c) buffer‐independent micellar pKa values can be obtained if the starting pHs of the buffer solutions were corrected for ion exchange with the counter‐ion of the positively charged surfactant, and for the lower polarity of the micelle‐solubilized water (relative to bulk water); d) the micellar ΔpKa (=pKa in the aggregate −pKa in water) ranged from ‐0.45 to ‐0.20 unit for HBTC/BCDC, from ‐0.44 to ‐0.13 unit for HNTC/BCDC, from 0.22 to 0.64 unit for HBTC/Brij‐30, and from 0.27 to 0.59 unit for HNTC/Brij‐30; e) these micelle‐induced pKa shifts can be analyzed in terms of the lower polarity of the micelle‐solubilized water (relative to bulk water) and perturbation of the acid‐base equilibria by the electrostatic effect of the positively charged W/O interface (BCDC); f) the magnitude of latter effect, at the average solubilization site of the indicator was calculated to range from 28 to 67 mV.
Dilute guaraná (Paullinia cupana) powder aqueous extract is a soft
drink that contains three structurally
related methylxanthines (MXs), caffeine (CF), theobromine (TB), and
theophylline (TP); CF is present in a large excess. Literature data
show that values of λmax and εmax of these three MXs are very close. Consequently, quantitative analysis
using a single technique (UV–vis) is difficult, at best. We
asked our students how to determine the three MXs simultaneously.
They suggested use of chromatographic separation followed by mass
spectrometry (MS) detection. Use of electrospray ionization solved
the MX structure identification problem only partially because TB
and TP have the same molar mass. Using LC-MS/MS solved this problem,
as TB and TP have distinct retention times and fragmentation patterns.
This is an important everyday situation (analysis of foods and drinks)
because of the popularity of CF-containing drinks (guarana, coffee,
tea, cacao) and health concerns about abuse of CF consumption.
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