Sheikh Khizar H, Evans Stephen D, Christenson Hugo K
School of Physics and Astronomy, University of Leeds, Leeds, United Kingdom.
Langmuir. 2007 Jun 19;23(13):6893-5. doi: 10.1021/la7007304. Epub 2007 May 19.
We have determined the pKa of surface-bound primary amine groups by determining the surface potential as a function of solution pH from the magnitude of the electric double-layer force. Using colloid-probe atomic force microscopy (AFM), we measured the force as a function of separation between a particle of radius R = 10 microm and a planar surface, each coated with a self-assembled monolayer of HS(CH2)2CONH((CH2)2O)8(CH2)2NH2. The force was measured from pH 3 to 7, and the surface potential was determined by fitting the results to solutions of the nonlinear Poisson-Boltzmann equation. The surface pKa of the primary amine group was found to be 5.0 +/- 0.2, in agreement with the results of contact-angle and chemical-force titrations on similar surfaces with primary amine groups. The surface charge density indicates that less than 1% of the NH2 groups are dissociated at pH 3, suggesting that ionization is very unfavorable in the local environment of the ethylene oxide chains. This noncontact method should be of general applicability to surfaces with ionizable groups and avoids the possible complications of large contact forces on the surfaces under study.
我们通过从电双层力的大小确定表面电势作为溶液pH值的函数，来测定表面结合的伯胺基团的pKa。使用胶体探针原子力显微镜（AFM），我们测量了半径R = 10微米的颗粒与平面表面之间的力随间距的变化，每个表面都涂有HS(CH2)2CONH((CH2)2O)8(CH2)2NH2的自组装单层膜。在pH值为3至7的范围内测量力，并通过将结果拟合到非线性泊松-玻尔兹曼方程的解来确定表面电势。发现伯胺基团的表面pKa为5.0±0.2，这与在具有伯胺基团的类似表面上进行的接触角和化学力滴定结果一致。表面电荷密度表明，在pH值为3时，不到1%的NH2基团解离，这表明在环氧乙烷链的局部环境中电离非常不利。这种非接触方法应普遍适用于具有可电离基团的表面，并避免了在所研究表面上产生大接触力的可能复杂性。
