The structure of zwitterionic phosphocholine surfactant monolayers
The structure of zwitterionic phosphocholine surfactant monolayers
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DOI：
10.1021/la053316z
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发表时间：
2006-06-20
期刊：
影响因子：
3.9
通讯作者：
Penfold, J.
中科院分区：
文献类型：
--
作者：
Yaseen, M.;Lu, J. R.;Penfold, J.
利用中子反射率结合 H/D 同位素取代确定了吸附在水表面的两性离子磷酸胆碱 (PC) 表面活性剂单层的结构。这项研究最重要的结果是 PC 头基的水合水平以及随着温度升高和盐添加而没有脱水的情况。研究发现，对于所研究的所有三种链异构体，浸入水中的烷基链分数 (f(c)) 约为 0.15，这表明 PC 头基几何形状不仅影响头基水合，还影响烷基链在水中的浸入程度。在临界胶束浓度 (CMC) 下，CmPC 中与 PC 头基相关的水分子数量 (m = 12、14、16) 约为 15。该值明显大于在 CMC 下每分子具有相似极限面积的非离子和离子表面活性剂获得的值 (A(cmc))。然而，离子型和非离子型表面活性剂的链浸入水中的部分要大得多。这表明PC表面活性剂独特的表面生物相容性源于其对水的强亲和力，并且与烷基链的混合比例相对较低源于PC单层内的较高结构顺序。随着表面覆盖度的降低，与每个 PC 头基相关的水分子数量增加，但所有表面活性剂的 fc 保持不变。这一观察结果与头基区域厚度的微小变化一致，并且整个层随表面活性剂浓度变化很小。这归因于 PC 头基几何形状在堆积密度变化时维持层内构象顺序的作用。基于运动学方法的进一步结构分析表明，随着CMC处链长从C-12增加到C-14再到C-16，烷基链的倾斜角分别从40°增加到48°再到53°，而整个层的厚度和PC头区域的厚度基本恒定。这些单烷基链表面活性剂的 PC 头基几乎垂直的投影与其强烈倾斜的构象形成鲜明对比，如二棕榈酰甘油磷酸胆碱 (DPPC) 等双链磷脂的报道。
The structure of a zwitterionic phosphocholine (PC) surfactant monolayer adsorbed on the surface of water has been determined using neutron reflectivity in combination with H/D isotopic substitution. The most significant results of this study are the level of hydration of the PC headgroup and the lack of dehydration with increasing temperature and salt addition. The fraction of the alkyl chain (f(c)) immersed in water for all three chain isomers studied was found to be around 0.15, suggesting that the PC headgroup geometries influenced not only the headgroup hydration but also the degree of immersion of the alkyl chain in water. At the critical micelle concentration (CMC), the number of water molecules associated with the PC headgroup in CmPC (m = 12, 14, 16) was on order of 15. This value was significantly greater than that obtained for nonionic and ionic surfactants with similar limiting area per molecule at the CMC (A(cmc)). However, the fraction of the chain immersed in water for the ionic and nonionic surfactants was much greater. This suggests that the unique surface biocompatibility of PC surfactants arises from their strong affinity for water, and the relatively low fraction of mixing with the alkyl chain arises from the higher structural order within the PC monolayer. As surface coverage decreased, the number of water molecules associated with each PC headgroup increased, but fc remained constant for all the surfactants. This observation was consistent with the small variation in the thickness of the headgroup region, and the entire layer changed little with surfactant concentration. This is attributed to the role of PC headgroup geometries to maintain the conformational order within the layer as packing density varies. Further structural analysis based on a kinematic approach showed that, as the chain length was increased from C-12 to C-14 to C-16 at the CMC, the angle of tilt for the alkyl chain increased from 40 to 48 to 53, respectively, whereas the thickness of the whole layer and that of the PC head region was largely constant. The almost vertical projection of the PC headgroup from these single alkyl chain surfactants is in sharp contrast to its strongly tilted conformation, as reported for dichain phospholipids such as dipalmitoyl glycerol phosphocholine ( DPPC).