表面是涉及我们生活各个方面的非常重要的科学分支。由于荷叶在自然界中展现出了著名的自洁效果，最近它在学术界和工业界都引起了极大的关注。（1，2）具有特殊润湿性，优异的排斥性和可控制的附着力的表面研究活动已经开展。活跃度很高，近年来每年发表数千篇论文。（3）仅在2013年，已经发表了五篇评论文章。（4-8）静态和动态接触角测量通常用于研究润湿，反润湿，和这些表面的粘合特性（9）图1示出了用于从这些测量确定的不同角度的示意图：静态接触角θ，前进角θ甲，后退角θ - [R ，和滑动角α。接触角滞后是简单地θ之间的差甲和θ [R 。图1原理图的各种接触角：（1）静态的（表观）接触角θ，（B）滑动角α，（c）中前进角θ甲，以及（d）后退角θ ř。表面科学中最可识别的定义是疏水性和亲水性。用希腊语来说，水电表示水，友善的意思是亲和力，而憎恶的意思是缺乏亲和力。在科学界，我们已经接受了这样的定义：当表面的静态水接触角θ> 90°时，表面是疏水的；当θ<90°时，表面是亲水的。实际上几乎没有什么合理化的理由，因为当θ从89°变为91°仅2°时，为什么表面会从亲水性变为疏水性。支持该定义的分子起源或驱动力是什么？实际上，Gao和McCarthy曾在2008年报道观察到水与氟代烷基单层与铁氟龙表面之间有很强的粘附相互作用，甚至在他们的文章中指出“聚四氟乙烯是亲水性的”。（10）因此，绝对需要在表面科学中得到技术数据支持的良好定义。2011年，我们（11）报告了系统研究水在20种不同表面上的润湿和粘附相互作用的研究（1 – 20）在张力计中使用微量天平。根据常见的可接受定义，这20个表面代表所有特性的表面，从亲水性到疏水性再到超疏水性。从物理上讲，它们从原子光滑的薄膜到普通的聚合物薄膜，再到光刻产生的纹理表面，再到玫瑰花瓣，都在变化。当水滴和表面首次接触时，通过微天平将润湿（吸引）相互作用记录为吸附力。粘合力是在水滴与表面接触后分离时的拉脱力。测量的示意图在图2中给出。图2.测量水与各种表面之间的润湿和粘附相互作用的设备和程序的示意图。发现咬合力和拉脱力相互关联仅向前进接触角（θ甲）和后退接触角（θ - [R ），分别，而不是静态接触角θ，滑移角，或接触角滞后。我们从相关性的结论是，θ阿为表面润湿性（或排斥性）和θ的测量值ř是衡量表面附着力的方法。在本来宾评论中，对这两幅图进行了线性回归分析，并获得了合理的良好相关性（图3a和b）。图3.对于20个不同表面的水滴，（a）咬合力与前进接触角的关系图；（b）拔出力与后退接触角的关系图。修改自参考文献11。美国化学会版权所有。结果清楚地表明，在润湿和粘附相互作用的变化是逐步的，并且没有魔法截止在90℃为θ，θ甲，或θ ř。在另一方面，在图3B中的曲线图的仔细检查，一个可能会注意到数据中的θ明确划分ř ≈90°。左侧数据，表面1 –7，9，12，和20，由实心菱形表示，并且在右手侧的数据，表面8，10 - 11，和13 - 19，由空的正方形表示。正如之前的报道，（11），我们实际上表面后，观察到小的残留水滴和水分离与θ表面- [R <90°。该表面是在表面上的水滴缩回与θ后干净ř> 90°。显示这两种情况的照片和示意图在图4中给出。水和表面分离后观察到的微小残留水滴表明存在一定的亲和力。在θ表面之间的明显区别- [R <90°和θ [R > 90°引线我们提议的表面是亲水性的，当θ [R <90°，并且是疏水性的，当θ [R > 90°（图4）。图4.照片和示意图显示了亲水性（顶部）和疏水性（底部）表面的水表面分离。其他人也注意到在亲水性和疏水性方面通用定义的不足。van Oss（12）建议使用水合作用的自由能（ΔG sl）作为亲水性和疏水性的量度。在对许多化合物的水合自由能进行分析的基础上，他发现当ΔG sl > -113 mJ / m 2时，疏水性化合物在水中会相互吸引，而当ΔG sl < −113 mJ / m 2。值-113 mJ / m 2被认为是疏水性和亲水性之间的界限。另一方面，Vogler（13）根据长距离疏水性相互作用的出现和消失提出了一个θ≈65°的临界值。这些定义的主要缺点是在截止角处缺乏明显的表面特性变化。无论是氢键结构的变化还是疏水相互作用，贯穿临界点的变化都是连续的，渐进的。另一方面，本宾客评论中的拟议定义是基于实际测得的水与表面之间的亲和力。为亲水性表面，其θ ř小于90°。这些表面表现出强亲和力，通过在拉脱试验中残留的水滴所指示，而被示出疏水性的表面具有与水的亲和性小，和它们的θ [R “s为> 90°。剩下的问题是为什么从亲水到疏水性在θ表面变化ř ≈90°。重要的是要指出，图3a中的曲线表明，即使吸引力随着θA的增加而减弱，水与疏水性表面之间也始终存在吸引作用。没有观察到残留的水滴时θ的事实ř> 90°可归因于水滴的高内聚力。由于较小的润湿能量，水滴更倾向于处于液滴状态而不是润湿表面。换句话说，润湿和液滴内聚力之间的竞争使表面从亲水性变为疏水性。实际上在Young的原始论文中详细讨论了液体凝聚在润湿中的重要性。（14）表面文献中另一个常用的定义是当表面θ> 150°时的超疏水性。正如Roach及其同事之前指出的（15），该定义仅通过普遍的共识出现，并且对该定义没有技术上的合理化。另一方面，根据图3a的结果，我们注意到表面15 –19是不可测量的。这些超疏水表面的测得的咬合力变为零，表明它们的超疏水状态。从曲线的截距，我们建议，一个表面是超疏水性时，其θ阿是≥145°。有趣的是，神奇的90°截止值也已用于定义表面文献中其他液体的亲和力和疏忽感。如果憎恶性的确来自表面张力，那么人们会期望对另一种液体使用不同的截止值。事实上，在一个类似的研究用十六烷，它的表面张力是很多比水低，72.3与27.5 mN / m的，我们发现预先使亲油性-疏油性截止是在十六烷θ ř≈125°，而当表面十六烷θ发生超疏油性甲是≥165°。十六烷亲油性，疏油性和超疏油性的较大截止角与其较低的表面张力相一致。总之，所测量的润湿和水和各种表面之间的粘附相互作用的基础上，我们建议，一个表面是亲水性的时，它的水θ - [R是<90°，它是疏水性的，当θ [R为> 90°。从亲水性转变为疏水性的驱动力是水的高表面张力。我们进一步定义的表面是超疏水性时，其θ甲≥145°，几乎不与水亲和。作者感谢Hong Zhao博士（施乐公司）为本宾客评论中描述的其他分析提供了原始数据，并感谢Robin A Ras博士（阿尔托大学）指出了对更好的超疏水性定义的需求。本文引用了其他15个出版物。
"点击查看英文标题和摘要"
Definitions for Hydrophilicity, Hydrophobicity, and Superhydrophobicity: Getting the Basics Right
Surface is a very important branch of science that touches all facets of our lives. It has recently attracted a tremendous amount of attention in both academia and industry due to the famous self-cleaning effect displayed by the Lotus leaves in nature.(1, 2) Research activities on surfaces with special wettability, superior repellency, and controllable adhesion have been extremely active, and thousands of papers have been published annually in recent years.(3) Just in 2013, five review articles have already appeared.(4-8) Static and dynamic contact angle measurements are commonly employed to study the wetting, dewetting, and adhesion characteristics of these surfaces.(9) Figure 1 depicts the schematics for the various angles determined from these measurements: static contact angle θ, advancing angle θA, receding angle θR, and sliding angle α. Contact angle hysteresis is simply the difference between θA and θR. Figure 1. Schematics for the various contact angles: (a) static (apparent) contact angle θ, (b) sliding angle α, (c) advancing angle θA, and (d) receding angle θR. The most recognizable definitions in surface science are hydrophobicity and hydrophilicity. In the Greek words, hydro means water, philicity means affinity, and phobicity means lack of affinity. In the scientific community, we have come to accept the definition that a surface is hydrophobic when its static water contact angle θ is >90° and is hydrophilic when θ is <90°. There is actually very little rationalization as why the surface switches from being hydrophilic to hydrophobic when θ changes only 2° from 89 to 91°. What is the molecular origin or driving force to support this definition? In fact, Gao and McCarthy reported observing strong adhesion interactions between water and fluoroalkyl monolayers and Teflon surfaces in 2008, and they even stated that “Teflon is hydrophilic” in their article.(10) Therefore, there is a definite need for a good definition in surface science that is supported by technical data. In 2011, we(11) reported a systematic study on the wetting and adhesion interactions of water with 20 different surfaces (1–20) using the microbalance in a tensiometer. These 20 surfaces represent surfaces of all traits, from hydrophilic to hydrophobic to superhydrophobic based on common acceptable definitions. Physically, they vary from atomic smooth to ordinary polymer films to photolithographic created textured surfaces to rose petals. The wetting (attractive) interaction was recorded as the snap-in force by the microbalance when the water droplet and the surface first make contact. The adhesion force was measured as the pull-off force when the water droplet and the surface separate after making contact. A schematic for the measurement is given in Figure 2. Figure 2. Schematic of the apparatus and procedures for measuring the wetting and adhesion interactions between water and various surfaces. The snap-in force and the pull-off force were found to correlate only to the advancing contact angle (θA) and the receding contact angle (θR), respectively, not to the static contact angle θ, sliding angle, or contact angle hysteresis. We concluded from the correlations that θA is a measure for surface wettability (or repellency) and θR is a measure for surface adhesion. In this Guest Commentary, linear regression analyses were performed on these two plots, and reasonably good correlations were obtained (Figure 3a and b). Figure 3. Plots of the (a) snap-in force versus advancing contact angle and (b) pull-off force versus receding contact angle for water drop with 20 different surfaces. Modified from ref 11. Copyright American Chemical Society. The results clearly show that the changes in wetting and adhesion interactions are gradual and that there is no magic cutoff at 90° for θ, θA, or θR. On the other hand, upon closer examination of the plot in Figure 3b, one may notice a clear divide of the data at θR ≈ 90°. The data on the left-hand side, surfaces 1–7, 9, 12, and 20, are represented by solid diamonds, and the data on the right-hand side, surfaces 8, 10–11, and 13–19, are represented by empty squares. As reported earlier,(11) we actually observed a small residual water droplet after the surface and the water separate for surfaces with θR < 90°. The surface is clean after the water drop retracts from surfaces with θR > 90°. Photographs and the schematic showing these two cases are given in Figure 4. The observation of the tiny residual water droplet after the water and surface separate is indicative of definite affinity. The clear distinction between surfaces at θR < 90° and θR > 90° leads us to propose that a surface is hydrophilic when θR is < 90° and is hydrophobic when θR is > 90° (Figure 4). Figure 4. Photographs and schematics showing water–surface separation from a hydrophilic (top) and hydrophobic (bottom) surface. Others have also noticed the shortfall of the general definition in hydrophilicity and hydrophobicity. van Oss(12) proposed to use the free energy of hydration (ΔGsl) as the measure of hydrophilicity and hydrophobicity. On the basis of the analysis of the free energy of hydration for a number of compounds, he found that hydrophobic compounds attract each other in water when ΔGsl > −113 mJ/m2 and that they repel each other when ΔGsl < −113 mJ/m2. The value −113 mJ/m2 was proposed to be the cutoff between hydrophobicity and hydrophilicity. On the other hand, Vogler(13) proposed a cutoff of θ ≈ 65° based on the appearance and disappearance of long-range hydrophobic interactions. The main shortcoming in these definitions is the lack of explicit change in surface property at the cutoff angle. Whether it is the change in the H-bonding structure or hydrophobic interaction, the change through the cutoff point is continuous and gradual. On the other hand, the proposed definition in this Guest Commentary is based on the actual measured affinity between water and the surface. For hydrophilic surfaces, their θR’s are <90°. These surfaces exhibit strong affinities, as indicated by the residual water droplets in the pull-off experiments, whereas hydrophobic surfaces are shown to have little affinity with water, and their θR’s are >90°. The remaining question is why does the surface change from hydrophilic to hydrophobic at θR ≈ 90°. It is important to point out that the plot in Figure 3a indicates that there is always an attractive interaction between water and the hydrophobic surfaces even though the attraction is weakening as θA increases. The fact that no residual water droplet was observed when θR > 90° can be attributed to the high cohesion of the water droplet. The water droplet prefers to be in the droplet state rather than wetting the surface due to the small wetting energy. In other words, it is the competition between wetting and droplet cohesion that changes the surface from hydrophilic to hydrophobic. The importance of liquid cohesion in wetting was actually discussed in length in Young’s original essay.(14) Another commonly used definition in the surface literature is superhydrophobicity when the surface θ is >150°. As pointed out by Roach and co-workers earlier,(15) this definition appears only by popular consensus, and there is no technical rationalization for this definition. On the other hand, from the results in Figure 3a, we notice that the wetting interaction for surfaces 15–19 is not measurable. The measured snap-in forces for these super water-repelling surfaces become zero, indicative of their super phobic status. From the intercept of the plot, we propose that a surface is superhydrophobic when its θA is ≥145°. It is interesting to note that the magic 90° cutoff has also been used to define philicity and phobicity for other liquids in the surface literature. If phobicity does in fact originate from surface tension, one would expect a different cutoff for a different liquid. Indeed, in a similar study with hexadecane, whose surface tension is a lot lower than that of water, 72.3 versus 27.5 mN/m, we found preliminarily that the oleophilicity–oleophobicity cutoff is at hexadecane θR ≈ 125°, and superoleophobicity occurs when the surface hexadecane θA is ≥165°. The larger cutoff angles for hexadecane oleophilicity, oleophobicity, and superoleophobicity are consistent with its lower surface tension. In summary, on the basis of the measured wetting and adhesion interactions between water and a variety of surfaces, we propose that a surface is hydrophilic when its water θR is <90° and that it is hydrophobic when θR is >90°. The driving force for switching from hydrophilicity to hydrophobicity is the high surface tension of water. We further define that a surface is superhydrophobic when its θA is ≥145°, where it has practically no affinity with water. The author thanks Dr. Hong Zhao (Xerox) for providing the raw data for the additional analysis described in this Guest Commentary and Dr. Robin H. A. Ras (Aalto University) for pointing out the need for a better definition for superhydrophobicity. This article references 15 other publications.
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