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有史以来最纯净的一滴水

亲手刷过碗的人都知道,想要真正地擦干净光洁表面并不是轻而易举的事情——对于需要处理微观层面的科学家来说就更是如此。

无论材料本身是多么纯净,它总是会被一层薄薄的分子所覆盖。

现在,为了弄清楚为什么创造完美的自洁表面是是那么的困难,研究人员在实验室环境中制取出了有史以来最纯净的一滴水。

因为,有种说法是,材料表面上的一层分子污垢就是来自于水。

为了检验这一假设,研究人员在真空室中通过超低温凝结的超纯冰来产生完全没有杂志和缺陷的水滴,然后将它滴到由二氧化钛制成的原始表面上。这是因为这种材质具有神奇的自清洁的特性,当表面暴露在紫外线下时,二氧化钛发生反应,分解掉附着在它们表面上的任何有机化合物。所以我们将二氧化钛镀在汽车后视镜和建筑瓷砖表面上,当阳光照射的时候,按道理附着在上面的分子就会消失。但实际上,最后依然会被一层神秘的薄膜所覆盖。

他们发现,如前所述,被超洁净表面吸附的污垢薄膜实际上并不是水分子。实际上结果更出人意料。

这种“污垢”由两种有机酸组成,乙酸(增加了醋的酸味)和甲酸,它们密切相关。

考虑到这些酸在空气中那极低的含量,这是一个令人惊讶的发现,

它促使人们重新思考表面如何吸引和排斥污垢,甚至未来的研究要如何确保表面完全一尘不染。

创造出超纯净水本身也是一项了不起的挑战。

有史以来最纯净的一滴水
左边是真空舱内超纯水蒸气超在极低温度下凝结成的冰,右边是用冰融化成的一滴水|Vienna University of Technology

“为了避免杂质,像这样的实验必须在真空中进行。”奥地利维也纳技术大学的Ulrike Diebold团队解释说。“因此,我们不得不创造出从未与空气接触过的水滴,然后将它滴落在二氧化钛表面上,该表面经过严格的清洁,纯净度达到原子级别。”

污垢层——只有单层分子的厚度——仅在二氧化钛表面从真空中取出并暴露在空气中时才会出现。所以不是水弄脏了材料。

“不知何故,表面上的这些分子正在帮助实现非常有趣的化学过程,自清洁和氧化性能,”纽约康奈尔大学的Melissa Hines研究人员说,“我们刚刚开始明白那里到底发生了什么。”

科学家认为,在化学分子水平上发生的特殊配位双齿结合有助于酸粘附在二氧化钛上,即使这些颗粒在空气中只有几十亿分之一。

空气中更常见的分子在表面上一滑而过,因为它们没有相同类型的结合机制。

为了确认他们的结果,研究人员在美国和奥地利重复了实验过程。

“在多个地点进行实验是至关重要的程序。”Hines说,“如果我们只在维也纳做实验,就会有人说,‘出于某种原因,那里的酸性颗粒物过多。’”

研究人员承认,还有很多有待解决的问题和现象,但这是一个重要的发现,帮助我们理解为什么具有自我清洁表面功能的材料不能尽善尽美。

同时,它也解释了二氧化钛为何排斥几乎所有其他分子——因为它吸引了酸。

“这一结果向我们显示了在进行此类实验时我们需要多么高的谨慎,”Ulrike Diebold说,“即使是空气中极度微痕剂量的分子,实际上有时也能造成决定性的影响。”

原文如下:

As you may know from scrubbing the kitchen, getting a surface well and truly clean is a real challenge – and even more so for scientists working at microscopic levels.

No matter how pristine a material is engineered to be, it always ends up covered in a thin layer of molecules.

Now new research has produced the cleanest water droplets ever created in an attempt to figure out why it’s so hard to create perfect self-cleaning surfaces.

One hypothesis was that this layer of molecular dirt comes from water.

To investigate, researchers used pure ice frozen in a vacuum chamber to produce water droplets completely free of dirt and defects, and then dropped the liquid onto pristine surfaces made from titanium dioxide, known for its self-cleaning properties.

What they found was that the layer of molecular dirt that’s attracted to ultra-clean surfaces isn’t actually water, as previously believed. It’s actually something much more surprising.

This ‘dirt’ is made up of two organic acids, acetic acid (which adds to the sourness of vinegar) and formic acid, which is closely related.

Graphic simulation of molecular layer. (Cornell University)

That’s a surprising finding, considering these acids are only found in very low quantities in normal air.

It could prompt a rethink on how surfaces attract and repel dirt, and even how future studies could ensure surfaces that are completely, spotlessly clean.

Even creating clean water was a challenge.

“In order to avoid impurities, experiments like these have to be carried out in a vacuum,” says one of the team, Ulrike Diebold from the Vienna University of Technology in Austria.

“Therefore, we had to create a water drop that never came into contact with the air, then place the drop on a titanium dioxide surface that had been scrupulously cleaned down to the atomic scale.”

The extra dirt layer – a single layer of molecules thick – only appeared when the titanium dioxide surface was taken out of the vacuum and exposed to the air. So it wasn’t the water making it dirty.

This explains why titanium dioxide, which is used in everything from car mirrors to building tiles, always attracts this extra layer, even after using the Sun’s energy to burn off most of the material that collects on top of it.

“Somehow these molecules on the surface are helping with this really interesting chemistry, the self-cleaning and oxidising properties,” says one of the researchers, Melissa Hines from Cornell University in New York.

“And we’re just beginning to understand what’s going on there.”

The scientists think that a special bidentate (or “two teeth”) binding happening at the chemical level helps the acids stick to the titanium dioxide, even though there are only a few parts-per-billion of these particles in air.

Other molecules that are much more common in air slip or wash right off the surface because they don’t have the same kind of binding mechanism.

To make sure they had their results right, the researchers tested the process of applying ultra-clean water droplets to titanium oxide in both the United States and Austria.

“It was crucial that we do the experiment in more than one place,” says Hines. “If we had just done it in Vienna, everyone would say, ‘For some reason, your building is full of vinegar.'”

There’s still a lot to unpack and investigate here, the researchers admit, but it’s a significant finding in helping us understand why these surfaces can’t get completely clean when they’re exposed to air.

At the same time, it explains how titanium dioxide can repel almost all other molecules – because of the acid it attracts.

“This result shows us how careful we need to be when conducting experiments of this kind,” says Ulrike Diebold. “Even tiny traces in the air, which could actually be considered insignificant, are sometimes decisive.”

The research has been published in Science.

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