Surface
surface tension and surface energy
- Surface tension: the cohesive force of molecules at the surface of an element attracting toward one another to take up the least possible surface area (the tendency of liquid surfaces at rest to shrink into the minimum surface area possible).体相液体分子受到各个方向的cohesive forces而合力为0，界面处分子因部分侧缺少邻居分子而合力不为0，受到向内拉的surface tension，增大了内部压力并使液面收缩。此外，还有一个与界面平行（相切）的surface tension抵抗外力。从力的角度，液滴成球有利于降低界面处cohesive force的不均衡，最小化”wall tension” （参见 Laplace’s law）。从能的角度，一个分子周围均为相同分子时处于能量比单个分子时能量更低的状态，所以界面处分子处于能量更高的状态，为降低系统能量，界面处的分子会尽可能地变少，形成球面。
- Surface energy(also interfacial free energy or surface energy): the amount of intermolecular force created at the surface of a material, and it determines the amount of attractive or repulisve force a surface can exert on another material. Quantifies the disruption of intermolecular bonds that occurs when a surface is created.In solid-state physics, surfaces must be intrinsically less energetically favorable than the bulk of the material (the atoms on the surface have more energy compared with the atoms in the bulk), otherwise there would be a driving force for surfaces to be created, removing the bulk of the material (see sublimation). The surface energy may therefore be defined as the excess energy at the surface of a material compared to the bulk, or it is the work required to build an area of a particular surface. Another way to view the surface energy is to relate it to the work required to cut a bulk sample, creating two surfaces. There is “excess energy” as a result of the now-incomplete, unrealized bonding between the two created surfaces.Cutting a solid body into pieces disrupts its bonds and increases the surface area, and therefore increases surface energy. If the cutting is done reversibly, then conservation of energy means that the energy consumed by the cutting process will be equal to the energy inherent in the two new surfaces created. The unit surface energy of a material would therefore be half of its energy of cohesion, all other things being equal; in practice, this is true only for a surface freshly prepared in vacuum. Surfaces often change their form away from the simple “cleaved bond” model just implied above. They are found to be highly dynamic regions, which readily rearrange or react, so that energy is often reduced by such processes as passivation or adsorption.
In general, as surface energy increases, the contact angle decreases because more of the liquid is being “grabbed” by the surface. Conversely, as surface energy decreases, the contact angle increases, because the surface doesn’t want to interact with the liquid.
Types of Surface Tension
- Contact angle analysis is used to measure the quality of a solid surface.
- Surface tension analysis is used to measure the quality of a liquid.
there are two types of surface tension at play:
- the surface tension of the liquid；
- the interfacial tension between the liquid and the solid：the measure of adhesive force between the liquid phase of one substance and the liquid, solid, or gas state of another substance.
Wettability
For wetting, the surface energy is stronger than the surface tension of the liquid’s molecules that would normally keep it in droplet form.
The surface tension of the liquid doesn’t change, but rather the surface energy of the solid is stronger than the liquid’s surface tension and overpowers it, causing the liquid to spread out over the solid surface. Low contact angle = high surface energy and high interfacial tension.
For a low wetting surface, the surface energy is weaker than the surface tension of the liquid, meaning that the liquid can better keep its droplet shape. The interfacial tension between the solid and liquid is low because the interaction between the two is not as strong.
表面活性剂
表面张力（Surface tension）：液体表面分子的向心收缩力，表面张力可使液滴的表面积收缩到最小程度，即表面张力越大，喷雾时形成的雾滴越大；表面张力越小，形成的雾滴越小。表面活性剂分子具有两亲性，其极性基一端虽受到水分子的拉力，但因有非极性基一端和气相亲和，因而它本身的表面张力很小，所以液体表面张力得以降低，即E=δ×S中的δ得以降低。降低液体表面张力是因为液体分子与表面活性剂的亲和力强，相互直接的亲合力就弱下来，所以表面活性剂可以降低液体表面的张力。
65°为亲疏水临界
As a matter of practical convenience, this Leading Opinion defines hydrophilic as all solid surfaces that support advancing water (or buffer) contact angles θ < 65° and hydrophobic as solid surfaces supporting θ > 65°. The basis of this definition is the observation that biological responses such as bioadhesion, protein adsorption, and blood plasma coagulation seem to “pivot” from high-to-low or vice-versa within an approximately ±15° range around θ = 65° (note that these citations include reviews of a broad literature).[引自doi: 10.1016/j.biomaterials.2011.10.059]
纯液体表面张力
参见课程:第二节 纯液体的表面张力
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