Li Haiyan, Tripp Carl P
Laboratory for Surface Science and Technology, Department of Chemistry, University of Maine, Orono, Maine 04469, USA.
Langmuir. 2004 Nov 23;20(24):10526-33. doi: 10.1021/la048898j.
Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was used to identify the structures formed during the adsorption of sodium polyacrylate (NaPA) on charged TiO2 particles and to determine the subsequent interaction of the adsorbed polymer structure with cationic and anionic surfactants. The nature of the polymer structure was deduced from the adsorbed amount in tandem with the information obtained from monitoring the change in the relative intensity of the COO- and COOH infrared bands. In particular, it is found that the relative number of COO- and COOH groups on the polymer backbone for the adsorbed state differs from that of the same polymer in solution. This difference is due to a shift in the population of COO-/COOH groups on the polymer backbone that arises when the COO- groups bind to positively charged sites on the surface. A change in the number COO-/COOH groups on the polymer is thus related to a change in the bound fraction of polymer. It is shown that the initial NaPA approaching the bare surface adopts a flat conformation with high bound fraction. Once the bare sites on the surface are covered, the accommodation of additional polymer on the surface requires the existing adsorbed layer to adopt a conformation with a lower bound fraction. When the adsorbed NaPA is probed with a solution containing the anionic surfactant sodium dodecyl sulfate (SDS), the SDS competes for surface sites and displaces some of the bound NaPA segments from the surface, giving rise to an polymer layer adsorbed with an even lower bound fraction. In contrast, addition of a solution containing the cationic surfactant cetyltrimethylammonium bromide (CTAB) results in the binding of the surfactant directly to the free COO- sites on the adsorbed polymer backbone. Confirmation of a direct interaction of the CTAB headgroup with the free COO- groups of the polymer is provided by intensity changes in the headgroup IR bands of the CTAB.
衰减全反射傅里叶变换红外光谱（ATR-FTIR）用于识别聚丙烯酸钠（NaPA）在带电TiO2颗粒上吸附过程中形成的结构，并确定吸附的聚合物结构与阳离子和阴离子表面活性剂随后的相互作用。聚合物结构的性质是根据吸附量以及从监测COO-和COOH红外波段相对强度变化获得的信息推导出来的。特别地，发现吸附状态下聚合物主链上COO-和COOH基团的相对数量与溶液中相同聚合物的不同。这种差异是由于聚合物主链上COO-/COOH基团的分布发生了变化，这是当COO-基团与表面带正电的位点结合时产生的。因此，聚合物上COO-/COOH基团数量的变化与聚合物的结合分数变化有关。结果表明，最初接近裸露表面的NaPA采取具有高结合分数的扁平构象。一旦表面的裸露位点被覆盖，表面上额外聚合物的容纳需要现有的吸附层采取具有较低结合分数的构象。当用含有阴离子表面活性剂十二烷基硫酸钠（SDS）的溶液探测吸附的NaPA时，SDS竞争表面位点并将一些结合的NaPA链段从表面置换出来，从而产生具有更低结合分数的吸附聚合物层。相反，加入含有阳离子表面活性剂十六烷基三甲基溴化铵（CTAB）的溶液会导致表面活性剂直接与吸附的聚合物主链上的游离COO-位点结合。CTAB头基红外波段的强度变化证实了CTAB头基与聚合物游离COO-基团之间的直接相互作用。
