Öhrwall Gunnar, Prisle Nønne L, Ottosson Niklas, Werner Josephina, Ekholm Victor, Walz Marie-Madeleine, Björneholm Olle
MAX-lab, Lund University , P.O. Box 118, SE-221 00 Lund, Sweden.
J Phys Chem B. 2015 Mar 12;119(10):4033-40. doi: 10.1021/jp509945g. Epub 2015 Mar 3.
The acid-base speciation of surface-active carboxylate ions in the surface region of aqueous solutions was studied with synchrotron-radiation-based photoelectron spectroscopy. The protonated form was found at an extraordinarily large fraction compared to that expected from the bulk pH. When adding salts containing the weak acid NH4(+) to the solution, the fraction of the acidic form at the surface increases, and to a much greater extent than expected from the bulk pH of the solution. We show that ammonium ions also are overrepresented in the surface region, and propose that the interaction between the surface-active anionic carboxylates and cationic ammonium ions creates a carboxylate-ammonium bilayer close to the surface, which increases the probability of the protonation of the carboxylate ions. By comparing the situation when a salt of the less volatile amine diethanolamine is used, we also show that the observed evaporation of ammonia that occurs after such an event only affects the equilibrium marginally.
利用基于同步辐射的光电子能谱研究了水溶液表面区域中表面活性羧酸根离子的酸碱形态。与根据本体pH预期的比例相比，发现质子化形式的比例异常大。当向溶液中加入含有弱酸NH4(+)的盐时，表面酸性形式的比例增加，且增加程度远大于根据溶液本体pH预期的程度。我们表明铵离子在表面区域也有过量存在，并提出表面活性阴离子羧酸盐与阳离子铵离子之间的相互作用在表面附近形成了羧酸盐 - 铵双层，这增加了羧酸根离子质子化的概率。通过比较使用挥发性较小的胺二乙醇胺的盐时的情况，我们还表明在此类事件后观察到的氨蒸发仅对平衡产生轻微影响。
