Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing
Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing
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DOI：
10.1021/acs.jpclett.1c03576
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发表时间：
2022-01-20
影响因子：
5.7
通讯作者：
Cremer, Paul S.
中科院分区：
文献类型：
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作者：
Judd, Kenneth D.;Gonzalez, Nicole M.;Cremer, Paul S.
采用振动和频光谱（VSFS）和压力-面积朗缪尔波谷法研究了碱金属离子与二十烷基硫酸酯（ESO 4）表面活性剂在气/水界面上的单分子膜结合.通过将阴离子表面活性剂与二十烷醇混合，可以调节硫酸根基团的数密度。K+与脂肪硫酸盐界面的平衡解离常数显示出比Li+大10倍的亲和力，比Na+大约3倍。当ESO 4的摩尔分数为0.33或更低时，所有三种阳离子形成溶剂共享离子对。在此阈值电荷密度以上，Li+与硫酸根头基形成接触离子对，推测是通过桥接结构。相比之下，K+仅结合到硫酸根部分在溶剂共享的离子配对配置。Na+的行为是中间的。这些结果表明，接触离子对的形成和更强的结合亲和力之间不一定存在相关性。
Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO4) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K+ to the fatty sulfate interface showed 10 times greater affinity than for Li+ and approximately 3 times greater than for Na+. All three cations formed solvent shared ion pairs when the mole fraction of ESO4 was 0.33 or lower. Above this threshold charge density, Li+ formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K+ only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na+ was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.