Yaseen M, Wang Y, Su T J, Lu J R
Biological Physics Group, School of Physics and Astronomy, University of Manchester, P.O. Box 88, Sackville Street, Manchester M60 1QD, UK.
J Colloid Interface Sci. 2005 Aug 15;288(2):361-70. doi: 10.1016/j.jcis.2005.03.024.
The surface adsorption of n-dodecyl phosphocholine (C12PC) has been characterised by a combined measurement of surface tension and neutron reflectivity. The critical micellar concentration (CMC) was found to be 0.91 mM at 25 degrees C in pure water. At the CMC, the limiting area per molecule (A(cmc)) was found to be 52+/-3 A2 and the surface tension (gamma(cmc)) to be ca. 40.0+/-0.5 mN/m. The parallel study of chain isomer n-hexadecyl phosphocholine (C16PC) showed a decrease of the CMC to 0.012 mM and a drop of gamma(cmc) to 38.1+/-0.5 mN/m. However, A(cmc) for C16PC was found to be 54+/-3 A2, showing that increase in alkyl chain length by four methylene groups has little effect on A(cmc). The almost constant A(cmc) suggested that the limiting area per molecule was determined by the bulky PC head group. It was further found that the surface tension and related key physical parameters did not vary much with temperature, salt addition, solution pH or any combination of these, thus showing that surface adsorption and solution aggregation from PC surfactants is largely similar to the zwitterionic betaine surfactants and is distinctly different from ionic and non-ionic surfactants. The thickness of the adsorbed monolayers measured from both dC12hPC and dC16hPC was found to be 20-22 A at the CMC from neutron reflectivity. Neither A(cmc) nor layer thickness varied with alkyl chain length, indicating that as the alkyl chain length became longer it was further tilted away from the surface normal direction and the layer packing density increased. It was also observed that the thickness of the layer varied little with surfactant concentration, indicating that the average conformational orientation of the alkyl chain remained unchanged against varying surface coverage.
通过表面张力和中子反射率的联合测量对正十二烷基磷酸胆碱（C12PC）的表面吸附进行了表征。在25℃的纯水中，临界胶束浓度（CMC）为0.91 mM。在CMC时，发现每个分子的极限面积（A(cmc)）为52±3 Ų，表面张力（γ(cmc)）约为40.0±0.5 mN/m。对链异构体正十六烷基磷酸胆碱（C16PC）的平行研究表明，CMC降至0.012 mM，γ(cmc)降至38.1±0.5 mN/m。然而，发现C16PC的A(cmc)为54±3 Ų，表明烷基链长度增加四个亚甲基对A(cmc)影响不大。几乎恒定的A(cmc)表明每个分子的极限面积由庞大的PC头基决定。进一步发现，表面张力和相关关键物理参数随温度、加盐量、溶液pH值或这些因素的任何组合变化不大，因此表明PC表面活性剂的表面吸附和溶液聚集在很大程度上与两性离子甜菜碱表面活性剂相似，与离子型和非离子型表面活性剂明显不同。从中子反射率测得，在CMC时，由dC12hPC和dC16hPC测得的吸附单层厚度为20 - 22 Å。A(cmc)和层厚度均不随烷基链长度变化，表明随着烷基链长度变长，它进一步偏离表面法线方向倾斜，层堆积密度增加。还观察到层厚度随表面活性剂浓度变化不大，表明烷基链的平均构象取向相对于不同的表面覆盖率保持不变。
