Instituto de Investigaciones en Físico-Química de Córdoba (INFIQC), Facultad de Ciencias Químicas, Departamento de Química Orgánica, Universidad Nacional de Córdoba, Ciudad Universitaria (X5000HUA), Córdoba, Argentina.
Chemphyschem. 2012 Jan 16;13(1):124-30. doi: 10.1002/cphc.201100634. Epub 2011 Nov 16.
We found that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP), and p-nitrophenol (p-NPh) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane reverse micelles (RMs) at various W(0) (W(0) = [H(2)O]/[surfactant]) values studied changed with time if (-)OH ions were present in the RM water pool. There is an evolution of ionized phenol (phenolate) bands to nonionized phenol absorption bands with time and this process is faster at low W(0) values and with phenols with higher bulk water pK(a) values. That is, in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, whereas in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert into nonionized phenol species over time. Furthermore, we demonstrate that, independent of the (-)OH concentration used to prepare the AOT RMs, the nonionized phenols are the more stable species in the RM media. We explain our results by considering that strong hydrogen-bonding interactions between phenols and the AOT polar head groups result in the existence of only nonionized phenols at the AOT RM interface. The situation is quite different when the phenols are dissolved in cationic benzyl-n-hexadecyldimethylammonium chloride RMs. Therein, only phenolates species are present at the (-)OH concentrations used. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment, such as in the interior of RMs and challenge the general idea that pH can be determined inside RMs.
我们发现，在不同 W（0）（W（0）=[H2O]/[表面活性剂]）值下，2-乙酰苯酚（2-HAP）、4-乙酰苯酚（4-HAP）和对硝基苯酚（p-NPh）在水/十二烷基磺酸钠（AOT）/正庚烷反胶束（RM）中的吸收光谱会随时间发生变化，如果 RM 水相中存在 -OH 离子。随着时间的推移，酚离子（酚盐）带会向非离子化酚吸收带演变，而在低 W（0）值和具有较高体相水 pK（a）值的酚中，这一过程会更快。也就是说，在体相水中且在使用的氢氧根阴离子浓度下，仅观察到酚盐物种，而在 AOT RM 中，在固定的氢氧根阴离子浓度下，随着时间的推移，离子化酚会转化为非离子化酚物种。此外，我们证明，无论用于制备 AOT RM 的 -OH 浓度如何，非离子化酚在 RM 介质中都是更稳定的物种。我们通过考虑酚与 AOT 极性头基团之间存在强氢键相互作用来解释我们的结果，这导致仅在 AOT RM 界面处存在非离子化酚。当酚溶解在阳离子苄基-十六烷基二甲基氯化铵 RM 中时，情况则完全不同。在那里，仅在使用的 -OH 浓度下存在酚盐物种。这些结果清楚地表明，经典的 pH 定义不适用于受限环境，例如在 RM 内部，并且挑战了 pH 可以在 RM 内部确定的普遍观点。
