Wetting Agents in Detergent(Teresa Glass)

Title: Detergents as wetting Agents

Principle(s) Investigated: Detergents make water wetter by being wetting agents. wetting agents break the electrical bonds holding water molecules together. alcohol is another wetting agent.

Standards :

NGSS DCI PS1-A: Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it.

NGSS MS-PS1-1: Develop models to describe the atomic composition of simple molecules and extended structures.

NGSS HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

Answer the questions in the Wetting Agents Quickwrite.

Materials: string scissors, 2 plastic cups, liquid detergent, pepper. These can be obtained at a supermarket or department store.

Procedure: 1.Cut the string into several 3 to 4 cm. pieces.

2. Fill both plastic cups nearly to the top with water and add one teaspoon of detergent to one.

3. Place four to six pieces of string in each cup.

4. Observe what happens to the glass with the wetting agents and to the other glass. In which one do the strings sink the fastest?

5. Rinse out both glasses. fill both with water and drop detergent in one sprinkle pepper in both. Observe what happens to the pepper.

6. Make sure that you rinse out the plastic cups throughly between uses.

Student prior knowledge: Students need to know the chemical bonds that form water molecules and the surface tension in them.

Explanation:Water, the liquid commonly used for cleaning, has a property called surface tension. In the body of the water, each molecule is surrounded and attracted by other water molecules. However, at the surface, those molecules are surrounded by other water molecules only on the water side. A tension is created as the water molecules at the surface are pulled into the body of the water. This tension causes water to bead up on surfaces (glass, fabric), which slows wetting of the surface and inhibits the cleaning process. You can see surface tension at work by placing a drop of water onto a counter top. The drop will hold its shape and will not spread.

In the cleaning process, surface tension must be reduced so water can spread and wet surfaces. Chemicals that are able to do this effectively are called surface active agents, or surfactants. They are said to make water "wetter."

Surfactants perform other important functions in cleaning, such as loosening, emulsifying (dispersing in water) and holding soil in suspension until it can be rinsed away. Surfactants can also provide alkalinity, which is useful in removing acidic soils.

Questions & Answers: Does the string sink faster with the detergent or with just plain water?The string sinks faster in the detergent glass because the detergent breaks up the electrical bond in the water molecules.Why is the detergent called a wetting agent? It is called a wetting agent because it breaks up the water molecules. It needs a liquid environment od do this.Do all detergents such as laundry and dishwashing detergents make string sink at the same rate?It would be good to do a comparison of detergent by timnimg how long the string has to sink in each one to get accurate data for a comparison.

Applications to Everyday Life:Detergents perform important function in cleaning. they loosen soil in water. they are called surfacants. surfactants also perform emulsifying in water meaning they disperse in water to act as a cleaning agent Surfactant can also remove acids form soils because they are alkaline.

Wetting Agents

Last updated
Jan 31, 2017

A substance is referred to as a wetting agent if it lowers the surface tension of a liquid and thus allows it to spread more easily.

Introduction

There are two general types of bulk intermolecular forces:

Cohesive Forces: The forces exerted between molecules holding them together. If cohesive forces are strong, a liquid tends to form droplets on a surface.
Adhesive Forces: The forces between liquid molecules and a surface. If adhesive forces are strong, a liquid tends to spread across a surface.

Wetting agents are substances that reduce the surface tension of water to allow it to spread drops onto a surface, increasing the spreading abilities of a liquid. Lowering the surface tension lowers the energy required to spread drops onto a film, thus weakening the cohesive properties of the liquid and strengthening its adhesive properties. One example of how wetting agents work is in the formation of micelles. Micelles consist of hydrophilic heads forming an outer layer around lipophilic tails. When in water, the micelles' tails can surround an oil droplet while the heads are attracted to the water.

Figure 1.1

: Schematic diagram of a micelle of oil in aqueous suspension, such as might occur in an emulsion of oil in water. In this example the surfactant molecules' oil-soluble tails project into the oil, while the water-soluble ends remain in contact with the water phase. Image used with permission from Wikipedia (SuperManu)

Dish soap is a great example of a wetting agent. With all the food oils and such on the plate cohesive forces make it difficult for the water to spread and clean the plate. The soap dissolves all theses unwanted particles, exposing a clean surface. The soap also lowers the surface tension of water, allowing it to spread evenly across the entire surface.

There are four main types of wetting agents: anionic, cationic, amphoteric, and nonionic.

Anionic, cationic, and amphoteric wetting agents ionize when mixed with water.
Anions have a negative charge, while cations have a positive charge.
Amphoteric wetting agents can act as either anions or cations, depending on the acidity of the solution.
Nonionic wetting agents do not ionize in water. A possible advantage for using a nonionic wetting agent is that it does not react with other ions in the water, which could lead to formation of a precipitate.

How to Tell if a Liquid Contains a Wetting Agent

One method of knowing whether or not a liquid has a wetting agent in it is to spread the liquid on a surface that is coated in grease. If the liquid does not contain a wetting agent, the its cohesive forces would overpower adhesive forces, causing the liquid to for droplets on the surface. If the liquid does contain a wetting agent, the grease would be dissolved and the surface tension of the liquid would be lowered, causing the adhesive forces to overpower the cohesive forces. This would result in the liquid spreading evenly along the surface.

Figure 1.1

: (left) A water droplet is sitting on a brass surface while being immersed in oil as an example of poor wetting. (right) A water droplet is sitting on a glass surface while being immersed in oil as an example of better wetting. Images used with permission from Wikipedia (Guro Aspenes, SINTEF Petroleum Research).

Another method is to place the liquid in a test tube and observe the liquid's meniscus (Figure 3). If the liquid contains a wetting agent, its adhesive forces are stronger than cohesive forces, which means the liquid molecules are more inclined to stick to the surface than other liquid molecules. This results in a concave meniscus. If the liquid does not contain a wetting agent and is naturally very cohesive, like mercury, it forms a convex meniscus. This is caused by the fact the the molecules of the liquid have a stronger attraction to each other than to the surface of the test tube.

Figure 1.1

The Phenomenon of Capillary Action. Capillary action of water compared to mercury, in each case with respect to a polar surface such as glass. (a) This drawing illustrates the shape of the meniscus and the relative height of a mercury column when a glass capillary is put into liquid mercury. The meniscus is convex and the surface of the liquid inside the tube is lower than the level of the liquid outside the tube. (b) Because water adheres strongly to the polar surface of glass, it has a concave meniscus, whereas mercury, which does not adhere to the glass, has a convex meniscus. Figure used with permission from Wikipedia.

Intermolecular forces also cause a phenomenon called capillary action, which is the tendency of a polar liquid to rise against gravity into a small-diameter tube (a capillary), as shown in Figure 4. When a glass capillary is put into a dish of water, water is drawn up into the tube. The height to which the water rises depends on the diameter of the tube and the temperature of the water but not on the angle at which the tube enters the water. The smaller the diameter, the higher the liquid rises.

Figure 1.1

: The Phenomenon of Capillary Action. Capillary action seen as water climbs to different levels in glass tubes of different diameters. Credit: Dr. Clay Robinson, PhD, West Texas A&M University.

References

Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Prentice Hall, 2007 .

Problems

Would it be beneficial to use a wetting agent when waxing a car?
An unknown liquid forms a convex meniscus when poured into a test tube. Does the liquid wet the test tube?
Do wetting agents increase or decrease the adhesive properties of a liquid?
Soap can form a precipitate when used as a wetting agent. Is it a nonionic or ionic wetting agent?
A liquid's cohesive forces overwhelm its adhesive forces. Do you think it contains a wetting agent?

Answers

No, when waxing a car, you do not want water to wet the car's surface.
No, if it forms a convex meniscus, its cohesive forces overpower its adhesive forces, causing the liquid's molecules to want to stick to each other as much as possible.
They increase the adhesive properties of a liquid.
It must be an ionic wetting agent, since nonionic wetting agents do not form precipitates.
The liquid most likely does not contain a wetting agent, since it is more inclined to stick to itself than to wet the surface.

Contributors

Ulysses Morazan (UCD), Abheetinder Brar (UCD)

Wetting Agents in Detergent(Teresa Glass)

Wetting Agents

Introduction

References

Problems

Answers

Contributors

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