Resolving the structure of a model hydrophobic surface: DODAB monolayers on mica
Resolving the structure of a model hydrophobic surface: DODAB monolayers on mica
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DOI：
10.1039/c2ra20108a
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发表时间：
2012-01-01
期刊：
影响因子：
3.9
通讯作者：
Perkin, Susan
中科院分区：
文献类型：
--
作者：
Gosvami, Nitya Nand;Parsons, Edward;Perkin, Susan
疏水表面的性质和相互作用是决定广泛工业应用的因素，也是一个远未解决的基本科学问题。Langmuir-Blodgett （LB）脂质单层经常被用作模型疏水表面，但只是亚稳的，这损害了实验的解释。利用调频原子力显微镜（FM-AFM），我们发现lb沉积在云母上的二十八烷基二甲基溴化铵（DODAB）单层在浸入水中后经历了两个转变：(i)分子快速重排，从完全的单层覆盖到更密集的单层，其中有暴露云母衬底的孔；（ii）在许多天的时间尺度上，单层中的脂质逐渐翻转形成双层，比以前报道的要慢几个数量级。单分子层的（元）稳定性与沉积压力（5-25 mN - m(-1)）关系不大，但很大程度上取决于制备的清洁度，在原子力显微镜实验中，当原子力显微镜尖端施加的力没有保持在1 nN以下时，稳定性从几天缩短到几分钟。经过适当的制备和分析，DODAB/云母表面因此产生了一个明确的结构，具有足够的稳定性来研究表面间的力，尽管其非均质性一方面会在裸云母上方，另一方面会在单层和双层区域产生非常不同的力。
The properties and interactions of hydrophobic surfaces in water are determining factors in a wide range of industrial applications, and represent a fundamental scientific problem that is far from solved. Langmuir-Blodgett (LB) lipid monolayers have often been used as model hydrophobic surfaces, but are only metastable, which compromises the interpretation of experiments. Using frequency-modulation atomic force microscopy (FM-AFM), we find that LB-deposited monolayers of dioctadecyldimethylammonium bromide (DODAB) on mica undergo two transitions upon immersion in water: (i) a rapid molecular rearrangement from a complete monolayer coverage to a more densely packed monolayer with holes exposing the mica substrate, followed by; (ii) a gradual flipping of lipids in the monolayer to form bilayers, at a timescale of many days, orders of magnitudes slower than previously reported. The (meta) stability of the monolayer shows little dependence on the deposition pressure (5-25 mN m(-1)), but strongly depends on the cleanliness of the preparation and, in AFM experiments, is reduced from days to minutes when the force applied by the AFM tip is not kept to well below 1 nN. When properly prepared and analysed, the DODAB/mica surface thus yields a well-defined structure of sufficient stability to study intersurface forces, albeit with a heterogeneity that gives rise to very distinct forces above the bare mica on one hand, and on the monolayer and bilayer areas on the other.