What is necessary to behave hydrophobic

Dr. Wiebke Salzmann

Interfaces

Substances can come in three so-called Phases occur: solid, liquid, gaseous. In liquids and solids there are bonds between the atoms or molecules and their neighbors. In a solid, the bonds are so rigid that the molecules sit firmly in their places; in a liquid the bonds are loose enough that the particles can wander around each other. The liquid as a whole stays together, but can change its shape, for example by adapting to the shape of a vessel or a lake basin. No forces at all act between the molecules or atoms of a gas (if the density is not too high).

The interesting thing now is what happens when two substances adjoin each other in different phases - what shape do the interfaces take on between the two? (This is only about cases in which no chemical reactions occur between the two substances.)
If you put two solids next to each other, nothing happens, both stay as they are. Two gases mix completely over time. If you put a solid in a gas, nothing else happens. That leaves the cases where liquids are involved.

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Interfaces: liquid in gas

Spherical shape of water droplets

Inside a liquid, the molecules are attracted to their neighbors, and to the same degree from all sides, since they have more or less the same number of neighbors on all sides. On the surface, however, the liquid is now adjacent to a gas, so the surface atoms have no bonds to any neighbors on the outside. There are only ties inward, which is why there is an inward pull in the sum. This leads to the fact that liquids want to keep their surface against the gas as small as possible in order to be able to avoid this pull as far as possible. The smallest possible surface is one Bullet. Drops of liquid are therefore always spherical (and have Not such as the well-known teardrop shape).

Fig. 1 ¦ Drops in air
Caption A drop of blue colored water comes off a spout. It takes on a spherical shape. Caption end

This endeavor of liquids to keep their surface area small is called Surface tension. You can see that this is not just an expression, but that a water surface is actually under tension when you watch water striders. These are so light that the surface tension of the water is sufficient to carry them.

Fig. 2 ¦ Water strider
In order to sink the water strider, the surface would have to be deformed - it is also slightly dented on its feet. However, every dent increases the surface of the water, and that is what the water (or its Surface tension) definitely prevent it. In the case of heavier objects, the surface tension is no longer sufficient at some point and they sink. Caption end
Fig. 2a ¦ wave
Caption How well the surface tension holds a liquid together can also be seen in this wave, which overhangs a long way and is not yet atomized. Caption end

The same also applies to the “counterpart” to the drop, the Air bubble in the water. In the bubble, too, the surface takes on a spherical shape in order to keep the interface between air and water as small as possible.

Liquid to solid

Drops of water on cabbage leaves and sponges

You have probably already observed that on some plants the raindrops remain as drops and do not run. Even on oily surfaces, water contracts into drops instead of running into a larger pool (this running is called "Wet"). On greasy surfaces or the leaves, water wants to make its surface as small as possible not only in relation to the air, but also in relation to the respective solid.

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Hydrophobic and hydrophilic

Oil and water don't mix, everyone who has ever touched salad dressings knows that. Substances that dissolve in water are called hydrophilic ("Water-friendly"); those that do not dissolve in water are called hydrophobic ("Water avoiding"). Water runs on surfaces made of hydrophilic materials (as on the untreated wood in Fig. 3a); on hydrophilic surfaces it contracts into drops (Fig. 3b). It would like to occupy as large an interface as possible with hydrophilic surfaces, and as small as possible with hydrophobic surfaces.

Fig. 3a ¦ drops on wood
Caption If you splash some water on these untreated wooden planks, it spreads into a pool - water wets the untreated wood. End of the caption
Fig. 3b ¦ Drops on oiled wood
Caption This wood belongs to a freshly oiled garden bench. The oily surface repels the water, it contracts into drops and does not wet the oiled wood

On hydrophilic materials, it is energetically more favorable for the water to come into contact with this material as much as possible, i.e. to create a large interface with it. That's why it's diverging. This also increases the surface area to the air, but the overall balance is important for the water. If it saves more energy on the one hand than it loses on the other, the extended pool is still cheaper.
With a hydrophobic surface, on the other hand, the water saves energy if it keeps its interface to the hydrophobic material as small as possible and forms droplets.
So the liquid always takes that energetically most favorable surface one - not necessarily the smallest in terms of area, but the one with the lowest energy. And which one it is depends on the adjoining fabrics and their ability to bond with water.

Fig. 3c ¦ Kale under water
Caption The reluctance of the water to wet the cabbage leaves goes so far that air is even retained in places on the leaves when they are immersed under water. Since one looks at another water surface (or water "below") from above, the shimmering impression is created. Caption end
Fig. 3d ¦ plums under water
Caption The shimmer can be seen even more strongly in plums. If they are in the water, they are also not wetted and are surrounded by a thin layer of air. They look downright silvery. Water has a higher index of refraction than air. So when the light from the water enters the thin layer of air on the plums, it moves from an optically denser medium to an optically thinner one and, at suitable angles, total reflection occurs, i.e. the light is completely reflected and does not penetrate into the air layer and thus also not as far as the plum. You then only see the water boundary, but not the plum behind it. Hence the silvery shimmer. (For total reflection, see: Light guide) Caption at the end

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Cohesion and adhesion

The forces that hold a liquid together and want to pull it together into a drop are called Cohesive forces. Forces between the liquid and a solid surface are called Adhesive forces (Adhesive forces). The greater the cohesive force, the more likely the liquid will form droplets (i.e. the greater the surface tension or interfacial tension); the greater the adhesive force, the more the liquid wets the solid. So if cohesion predominates, the liquid forms droplets and does not wet the adjacent solid; if the adhesion predominates, the liquid wets it.

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Interfacial tension and surface tension

Basically, both terms mean the same thing - namely the endeavor of a liquid to keep its surface area small. The Surface tension is a specific case of Interfacial tension, namely the adjoining of a liquid to a gas. In the case of the drop shape of water in air one speaks of the surface tension, in the case of the drop shape in or on oil of the interfacial tension.

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Surface tension and gravity

The question arises as to why (despite the surface tension) the coffee is not spherical in the cup.
Because they still do Gravity acts, which pulls the liquid towards the center of the earth. The liquid must therefore find the most energetically favorable state between surface tension, which wants to reach the energetically most favorable surface, and gravity, which wants to collect as much water as possible at the lowest possible point (i.e. wants to reach the state of the lowest potential energy).
- With small amounts of water, the surface tension predominates and drops form.
- With large amounts of water, gravity gains the upper hand and the water tries to stay as close as possible to the center of the earth - but since this is limited by vessels or ponds, these natural or artificial containers are filled from bottom to top.

You can already see the influence of gravity in this large drop on the kale leaf (Fig. 4). It's flattened by its own gravity.

Fig. 4 ¦ Drops on cabbage leaf
Caption End caption

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Contact angle and wetting

With Contact angle the angle between the surface of the liquid and the surface of the solid is meant (Fig. 5).
A drop that does not wet a solid at all is ideally spherical. It only touches the solid surface at one point and the wetting angle is 180 °. In fact, this ideal case does not occur in nature, a drop is always more or less flattened on its underside and touches the solid with a larger area. The contact angle is therefore less than 180 °.
The other extreme (which also does not occur) would be a contact angle of 0 °, i.e. a drop that completely vanishes. In reality, the water drop remains more or less together even after wetting and the contact angle is greater than 0 °.
The boundary between wetting and non-wetting is a contact angle of 90 °. Wetting liquids form a contact angle of more than 90 °, non-wetting one of less than 90 °.

Fig. 5a ¦ contact angle
Caption Wetting liquids have a contact angle of less than 90 ° (right), non-wetting liquids have a contact angle of more than 90 ° (left). Caption end
Fig. 5b ¦ Contact angle
CaptionWetting liquids have a contact angle of less than 90 °, as with the drop on the branch (right), non-wetting liquids of more than 90 °, as with the drop on the kale leaf (left). Caption end

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Water and soap

Soap

Water has a very high surface tension, which can be annoying. For example, water does not wet greasy surfaces, and therefore also no dirty dishes. In order to wash the fat off the dishes, it must be moistened by the water. So you need an additive that reduces the surface tension of the water and ensures that the dishes are wetted: Soap, in this particular case washing-up liquid.

The fact that the surface tension is reduced is due to the structure of the soap molecules. These have a hydrophilic and a hydrophobic end. The hydrophobic end tries to avoid the water, which is why a layer of detergent is deposited on the surface of the water, the hydrophobic ends of which all protrude outwards. The detergent therefore endeavors to keep the surface area to the air as large as possible - the spherical shape is no longer an option, which is why the detergent water dissolves immediately on the sponge (Fig. 6).

Fig. 6 ¦ Surface tension and detergent
Caption Some water was poured onto a dry sponge - it remains as a drop on the sponge (left) and only begins to seep in after several hours. The water mixed with a little washing-up liquid immediately forms a wider pool (center) and seeps away within a few seconds (right)

The reduction in surface tension ensures that the soapy water wets the objects to be cleaned and can penetrate anywhere, for example into the fibers of textiles. The real one Cleaning effect however, it also comes about through the soap molecules and their different ends. If there is not enough space for all the soap molecules on the surface of the water, they have to be inside the water. In order to still “satisfy” the hydrophobic ends of the soap molecules, the molecules form what are known as Micelles. This means that the soap molecules assemble into spherical structures with the hydrophobic ends pointing inside. Only the outwardly protruding hydrophilic ends come into contact with water.
Substances that do not dissolve in water, however, usually dissolve easily in oil and vice versa. So it is also with the soap molecules - their hydrophobic ends are lipophilic (fat-loving) and dissolve the greasy dirt very easily. The soap molecules therefore form spherical shells around the fat particles, which completely coat them and thus loosen the dirt from the surface. (It is energetically more favorable for the greasy dirt to surround oneself with the hydrophobic soap ends than to stick to the soiled surface.) Since the outside of the dirt coating is formed by hydrophilic soap ends, the coated fat particles are distributed in the water (that means they form one emulsion) and can be washed away.

Fig. 7 ¦ Soap solution
Caption Soap molecules have a hydrophilic and a hydrophobic end. The hydrophobic end does not want to come into contact with water if possible. The molecules are therefore only arranged in a certain way: 1. as a layer on the surface with the hydrophobic end protruding into the air; 2. as a micelle in which the hydrophobic ends all point inwards; 3. As a shell around fat particles, where the hydrophobic (and at the same time lipophilic) ends point into the fat droplets. Caption end

Mains water

Even on dry earth, irrigation water remains for the time being. When watering flowers, you can wait calmly until the water finally seeps in - if you want to put out a fire in a peatland or a grain silo, it should go a little faster before the water seeps in. To do this, an agent is added to the extinguishing water, which ensures that the surface tension drops and the water gives up the drop shape. It is then no longer on the ground, but spreads out as a film and seeps into the ground practically immediately. In other words, it wets the ground. That is why such means are also called Wetting agents (the term has nothing to do with "wetting", but refers to the improvement of wetting) and the water to which the wetting agent has been added is called Mains water.

Lotus effect

Also the well-known one Lotus effect has to do with the surface tension of the water. The lotus effect describes the self-cleaning ability of some surfaces. This ability was named after the lotus flower because it is always clean because of this effect. Self-cleaning surfaces are very hydrophobic and the contact angle is correspondingly large (up to 170 ° for lotus leaves). As a result, water droplets lying on top have an almost ideal spherical shape and can easily roll off. When rolling, the drops then take dirt particles with them. With the lotus effect, it is precisely the non-wettability that leads to cleaning.

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foam

Soap solutions create foam - although the foam itself is not actually needed for washing, it shows that there is soap and that the solution is “washable”, so to speak.
Inside a foam bubble there is air, the wall consists of three layers: detergent, water, detergent. The hydrophobic ends of the detergent protrude into the air (into the outer space or inwards into the bladder), the hydrophilic ends into the water layer.
The fact that bubbles don't last forever is that again Gravity Thanks to: The water flows downwards under their influence, the water layer becomes thinner and the bubble finally bursts.

You do not need the foam itself to wash the dishes, the detergent water is sufficient. If you add a larger amount of wetting agent to the extinguishing water (it is then called Foaming agent) to, one generates Extinguishing foam. Here you really need the foam - it can form a blanket and stifle fire, it prevents outgassing of flammable liquids (what burns is usually the vapors, not the liquid itself), and it also sticks to vertical walls.

Fig. 8 ¦ Scheme of a foam bubble
Caption Imagine an air bubble surrounded by a layer of water all around. Such a structure will not hold without foam concentrate: Since water wants to make its surface as small as possible, everything flows together to form a ball. However, it looks different with foam concentrate: the water bubble has two surfaces, one inside and one outside. There is now a layer of foam compound on both surfaces and the water-shy ends of both protrude into the air space (the inner layer into the inside of the bladder, the outer layer into the outer space). The foam concentrate again tries to make the surface area to the air as large as possible, which is why the bubble (and with it the whole foam) now remains stable

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Capillary forces

In thin tubes or pores ("Capillaries") the surface is very large compared to the internal volume. The influence of surface or interfacial tension can therefore also exceed that of gravity here.
On the one hand, it is energetically favorable for the liquid to give in to gravity and flow downwards. If it is a wetting liquid, on the other hand it is also beneficial to have the largest possible interface with the capillary. The state arises that represents the optimum under both influences, i.e. has the lowest possible energy.
Therefore, adhesive forces allow the liquid to rise a little up the walls against gravity. The cohesive force of the water holds the liquid together and pulls the water column behind it, so that there is a column of liquid in the tube. The water column "hangs" on its upper surface and the meniscus is in the capillary concave. Here again the contact angle is less than 90 °.
The liquid rises until an equilibrium is reached between the force of adhesion and the force of gravity. This means that the liquid in the capillary is higher than in the surrounding liquid body. The tighter the radius of the capillary, the higher the liquid rises.
A non-wetting liquid, the liquid rises accordingly only to a lower level than the surrounding liquid body it has, and has a convex meniscus, which corresponds to a contact angle of more than 90 °.

Fig. 9a ¦ capillary rise
Caption One end of a piece of kitchen paper is immersed in blue colored water. The water rises quickly in the pores of the paper, after a minute the paper is completely damp (which is why it then loses its strength and tips over)
Fig. 9b ¦ capillary rise
Caption If the water wets the capillary wall (e.g. glass), a concave meniscus forms (curved inwards); if it does not wet the capillary wall (e.g. greased glass), a convex (outwardly curved) meniscus forms

Capillary forces help plants absorb water from the soil. The evaporation from the leaves creates a suction, and capillary forces on the roots facilitate the ascent in the plant.
Sometimes the plants also owe it to the capillary forces that they can even get at water: The groundwater also rises a little above the actual groundwater body in the soil pores. The height of the rise is very different depending on the type of soil - while in clay with its fine pores the capillary rise is in the meter range, coarse gravel has almost no capillary fringe.

If you put a thin tube into a liquid and then pull it out again, a remainder of the liquid will remain in it. This also shows that the interfacial tension (the adhesive forces) can be stronger than gravity. The thinner the tube, the higher the column of liquid that remains.

Fig. 10 ¦ Column of liquid
Caption I held a straw and the tube of a cotton swab in red-colored water and pulled them out again.
Straw: diameter: approx. 4 mm, water column: approx. 2 mm
Cotton swab: diameter: approx. 1.5 mm, water column: approx. 1.5 cm, caption end

© Wiebke Salzmann, January 2014

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