众所周知,在海平面上,水的沸腾温度是212华氏度,或100摄氏度。而且科学家们早就发现,当水被存储于非常小的空间内时,它的沸点和冰点温度会发生一点改变,通常会下降10摄氏度左右。

但现在,麻省理工学院的一个科研团队发现了一系列意料之外的变化:在微小的空间里——在碳纳米管中,它的内部尺寸和几个水分子的大小相当——水在非常高的温度下(通常能够让它沸腾的温度)却冻结成了固体。

这个发现说明,当被限制在纳米结构中,即使是非常熟悉的材料也可能完全改变它们的行为。该发现可能会导致新的应用,如冰填充的电线,可以利用冰电线独特的电和热性能,因为这种‘冰’在室温下能够保持稳定。

研究成果发表在今天出版的《自然纳米技术》期刊上,论文作者是迈克尔 斯特拉诺,麻省理工学院化学工程教授卡本 P 杜布斯;博士后库马尔 阿格拉沃尔;和其他三位研究人员。

“如果你将流体限制在纳米空间内,你可以完全改变它的相变行为,”斯特拉诺说,指的是如何及何时,物质在固态、液态和气态之间进行改变。这种变化是可预计的,但是这种巨大的改变和变化的方向(提高而不是降低冰点温度)完全出乎意料:在该团队的一次测试中,水的冰点温度是105摄氏度或者更高(准确的温度很难确定,但105摄氏度被认为是本次测试中的最低值;实际温度可能高达151摄氏度。)

“这个结果出乎所有人的预料”斯特拉诺说。

事实证明,在微小的碳纳米管中(完全由碳原子组成但直径只有几纳米的苏打水吸管的形状),水的行为发生了改变——关键取决于纳米管的直径。“这是你能想到的最小管直径,”斯特拉诺说。在实验中,纳米管的两端是开放的,每一端都与储水池连接。

研究人员发现,1.05纳米和1.06纳米这点尺寸差别的纳米管,都会出现几十度的、明显的冰点差异。“只是微小的改变,就让一切变得不同,”斯特拉诺说。“这真是一个未知的空间。”

在之前的研究中,当被限制在如此小的空间内,了解水和其它流体的行为如何改变时,“一些模拟会出现矛盾的结果,”他说。其中一部分原因是,许多团队没有精确地测量碳纳米管的尺寸,没有意识到这种微小的尺寸差异可能会产生如此不同的结果。

实际上,水也会进入纳米管中,这很令人惊讶,斯特拉诺说:碳纳米管被认为是疏水性,因此水分子应当很难进入其内部。但水确实能够进入,这仍然是一个未解之谜,他说。

斯特拉诺和他的团队利用高敏感度的成像系统,利用振动光谱技术,跟踪水在碳纳米管中的运动,从而首次精准测量到了水的行为。

研究团队不仅检测到了水在碳纳米管中的存在,而且也发现了它的相态变化,他说:“我们能够判断出它是气态或液态,我们也可以判断它是否处于固态。”水进入固态的时候,该团队没有称之为“冰”,因为这个词意味着一种特定的晶体结构,水在纳米管中还没有呈现出这种状态。“它不是真正的冰,而是与冰类似的相态,”斯特拉诺说,在超过水的正常沸点温度下,这种固态水也没有融化,因此,在室温条件下,它应该能够保持稳定。这种特性使得它具有广阔的应用前景,他说。（张微/编译）

以下为英文原文：

Researchers discover astonishing behavior of water confined in carbon nanotubes

It''s a well-known fact that water, at sea level, starts to boil at a temperature of 212 degrees Fahrenheit, or 100 degrees Celsius. And scientists have long observed that when water is confined in very small spaces, its boiling and freezing points can change a bit, usually dropping by around 10 C or so.

But now, a team at MIT has found a completely unexpected set of changes: Inside the tiniest of spaces—in carbon nanotubes whose inner dimensions are not much bigger than a few water molecules—water can freeze solid even at high temperatures that would normally set it boiling.

The discovery illustrates how even very familiar materials can drastically change their behavior when trapped inside structures measured in nanometers, or billionths of a meter. And the finding might lead to new applications—such as, essentially, ice-filled wires—that take advantage of the unique electrical and thermal properties of ice while remaining stable at room temperature.

The results are being reported today in the journal Nature Nanotechnology, in a paper by Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering at MIT; postdoc Kumar Agrawal; and three others.

"If you confine a fluid to a nanocavity, you can actually distort its phase behavior," Strano says, referring to how and when the substance changes between solid, liquid, and gas phases. Such effects were expected, but the enormous magnitude of the change, and its direction (raising rather than lowering the freezing point), were a complete surprise: In one of the team''s tests, the water solidified at a temperature of 105 C or more. (The exact temperature is hard to determine, but 105 C was considered the minimum value in this test; the actual temperature could have been as high as 151 C.)

"The effect is much greater than anyone had anticipated," Strano says.

It turns out that the way water''s behavior changes inside the tiny carbon nanotubes—structures the shape of a soda straw, made entirely of carbon atoms but only a few nanometers in diameter—depends crucially on the exact diameter of the tubes. "These are really the smallest pipes you could think of," Strano says. In the experiments, the nanotubes were left open at both ends, with reservoirs of water at each opening.

Even the difference between nanotubes 1.05 nanometers and 1.06 nanometers across made a difference of tens of degrees in the apparent freezing point, the researchers found. Such extreme differences were completely unexpected. "All bets are off when you get really small," Strano says. "It''s really an unexplored space."

In earlier efforts to understand how water and other fluids would behave when confined to such small spaces, "there were some simulations that showed really contradictory results," he says. Part of the reason for that is many teams weren''t able to measure the exact sizes of their carbon nanotubes so precisely, not realizing that such small differences could produce such different outcomes.

In fact, it''s surprising that water even enters into these tiny tubes in the first place, Strano says: Carbon nanotubes are thought to be hydrophobic, or water-repelling, so water molecules should have a hard time getting inside. The fact that they do gain entry remains a bit of a mystery, he says.

Strano and his team used highly sensitive imaging systems, using a technique called vibrational spectroscopy, that could track the movement of water inside the nanotubes, thus making its behavior subject to detailed measurement for the first time.

The team can detect not only the presence of water in the tube, but also its phase, he says: "We can tell if it''s vapor or liquid, and we can tell if it''s in a stiff phase." While the water definitely goes into a solid phase, the team avoids calling it "ice" because that term implies a certain kind of crystalline structure, which they haven''t yet been able to show conclusively exists in these confined spaces. "It''s not necessarily ice, but it''s an ice-like phase," Strano says.

Because this solid water doesn''t melt until well above the normal boiling point of water, it should remain perfectly stable indefinitely under room-temperature conditions. That makes it potentially a useful material for a variety of possible applications, he says.