An experiment investigating the properties of surfactants is an experiment designed to understand how surfactants lower the surface tension of water, disperse oily dirt, and cause foaming and emulsification.
Surfactants are amphiphilic molecules that contain both a hydrophilic group, which has an affinity for water, and a hydrophobic group, which has an affinity for oil, within a single molecule.
Because of this structure, surfactants gather at interfaces such as water-air, water-oil, and solid-water interfaces and greatly change the properties of those interfaces.
In a discussion of surfactants, it is not sufficient simply to write that “foaming occurred,” “the oil was removed,” or “the surface tension decreased.”
It is necessary to explain why surfactants lower surface tension, why water spreads more easily, why oily dirt can be removed, and how micelle formation is related to cleaning action.
It is also possible to discuss why the results change depending on concentration, temperature, hard water, pH, the type of oily dirt, and the type of surfactant.
This article clearly explains, as examples of discussions that can be used in laboratory reports on the properties of surfactants, hydrophilic and hydrophobic groups, reduction of surface tension, wettability, foaming, emulsification, dispersion, micelle formation, cleaning action, the effects of hard water, experimental errors, and points for improvement.
Note:
This article is a reference intended to assist with discussions of experimental results concerning the properties of surfactants obtained in basic chemistry experiments, physical chemistry experiments, colloid chemistry experiments, interfacial chemistry experiments, and food chemistry experiments at universities and similar institutions.
For the actual surfactant, concentration, measurement method, type of oily dirt, temperature, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is a Surfactant?
- Main Items to Include in the Results
- Roles of Hydrophilic and Hydrophobic Groups
- What Is Surface Tension?
- Discussion of Surface-Tension Reduction
- Improvement of Wettability
- Interfacial Tension and Oily Dirt
- Micelle Formation and Cleaning Action
- Process of Cleaning Action
- Discussion of Foaming
- Discussion of Emulsification
- Discussion of Dispersion
- Effect of Surfactant Concentration
- Effect of Temperature
- Effect of Hard Water
- Ionic and Nonionic Surfactants
- Effect of pH
- Methods for Evaluating Cleaning Power
- Causes of Error in Experiments on Surfactant Properties
- When the Results Can Be Considered Good
- Example Discussions When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Surfactant Properties
- Summary
What Is a Surfactant?
A surfactant is a substance that gathers at interfaces such as water-oil, water-air, and water-solid interfaces and changes the properties of those interfaces.
Because it contains both a hydrophilic group and a hydrophobic group within the molecule, it has the property of being partially compatible with both water and oil.
Therefore, surfactants lower surface tension and interfacial tension and cause phenomena such as wetting, foaming, emulsification, dispersion, and cleaning.
Surfactants are used in many products, including detergents, shampoos, foods, cosmetics, pharmaceuticals, pesticides, and paints.
These functions occur not simply because surfactants “remove dirt,” but because they adsorb at interfaces and adjust the properties of water and oil.
In experiments, differences in surface tension and cleaning effect can be observed by changing the concentration or type of surfactant.
Example Discussion:
Surfactants are amphiphilic molecules containing both hydrophilic and hydrophobic groups.
Because of this structure, they adsorb at water-oil interfaces and water-air surfaces and change the properties of the interfaces.
As a result, phenomena such as reduced surface tension, improved wettability, emulsification, dispersion, and cleaning action were considered to have been observed.
Main Items to Include in the Results
In experiments investigating surfactant properties, organize the type of surfactant used, concentration, surface tension, how droplets spread, foaming, presence or absence of emulsification, ease of removing oily dirt, and changes over time.
If pure water or a solution without surfactant is used as a comparison, it is important to clearly describe the differences.
Main Items to Include in the Results
- Type of surfactant used
- Surfactant concentration
- Difference between ionic and nonionic surfactants
- Comparison with pure water
- Measured surface-tension value
- How the droplet spreads
- Change in contact angle
- Degree of foaming
- Foam stability
- How the surfactant mixes with oil
- Presence or absence of emulsification
- Ease of removing oily dirt
- Presence or absence of micelle formation
- Temperature conditions
- Presence or absence of hard water or salts
- Causes of error and points for improvement
Example of How to Write the Results:
In the surfactant aqueous solution, the surface tension was lower than that of pure water, and droplets spread more easily over the solid surface.
In addition, when oil was added and the mixture was stirred, oil-water separation occurred immediately in pure water, whereas a cloudy emulsified state was maintained for a certain period in the surfactant aqueous solution.
Furthermore, in samples with oily dirt attached, the dirt was more easily removed with the surfactant aqueous solution.
Roles of Hydrophilic and Hydrophobic Groups
The hydrophilic group of a surfactant is the part that readily interacts with water, and examples include ionic groups, hydroxyl groups, and polyethylene oxide chains.
The hydrophobic group is the part that has an affinity for oil or air and often consists of a long hydrocarbon chain.
Having these two properties within the same molecule is the basis of surfactant action.
In water, the hydrophilic group comes into contact with water, while the hydrophobic group tends to avoid contact with water.
Therefore, surfactant molecules readily adsorb at the water surface or oil-water interface.
At the water surface, the hydrophobic group points toward the air and the hydrophilic group toward the water, while at the oil-water interface, the hydrophobic group points toward the oil and the hydrophilic group toward the water.
This orientation lowers the energy of the interface.
Example Discussion:
Because surfactants have hydrophilic and hydrophobic groups, they readily orient themselves at interfaces in water.
The hydrophilic group faces the aqueous phase, while the hydrophobic group faces the air or oil phase, allowing stable adsorption at the interface.
This adsorption weakens the strong interactions among water molecules and is considered to reduce surface tension and interfacial tension.
What Is Surface Tension?
Surface tension is the force that tends to make the surface area of a liquid as small as possible.
In water, water molecules strongly attract one another through hydrogen bonding, so water has a relatively high surface tension.
Therefore, pure-water droplets tend to become rounded and may not spread easily over a solid surface.
When a surfactant is added, surfactant molecules adsorb at the water surface and weaken the interactions among water molecules.
As a result, the force that maintains the liquid surface becomes smaller and the surface tension decreases.
Reduction of surface tension is a fundamental phenomenon underlying wetting, foaming, emulsification, and cleaning action.
Example Discussion:
Pure water has a high surface tension because water molecules strongly attract one another.
When a surfactant is added, its molecules adsorb at the water surface and weaken the interactions among water molecules.
Therefore, the surface tension of the surfactant aqueous solution was considered to have become lower than that of pure water, making the droplets spread more easily.
Discussion of Surface-Tension Reduction
At low surfactant concentrations, increasing the concentration greatly reduces the surface tension because more surfactant molecules adsorb at the water surface.
This is because the interface becomes increasingly occupied by surfactant molecules and the strong interactions among water molecules are weakened.
This is why droplets may become flatter or measured surface-tension values may decrease in experiments.
However, above a certain concentration, the water surface becomes almost saturated with surfactant and adding more surfactant causes only a small further decrease in surface tension.
In this concentration region, excess surfactant molecules tend to form micelles in the water.
The concentration dependence of surface tension is also related to understanding micelle formation and the CMC.
Example Discussion:
The surface tension decreased as the surfactant concentration increased because the number of surfactant molecules adsorbed at the water surface increased.
At low concentrations, the added molecules mainly move to the interface, so the decrease in surface tension is large.
On the other hand, once the interface is nearly saturated, the added molecules are increasingly used to form micelles, and the change in surface tension becomes smaller.
Improvement of Wettability
Wettability is a property describing how easily a liquid spreads over a solid surface.
A liquid with high surface tension tends to form a rounded droplet on a solid surface and may spread poorly.
When surface tension is lowered by a surfactant, the droplet spreads more easily over the solid surface and wettability improves.
In cleaning, it is important for the cleaning solution to spread sufficiently over fibers and solid surfaces.
A surfactant aqueous solution spreads more easily into gaps that are difficult for water alone to enter and over surfaces covered with oil.
As a result, the cleaning solution can enter between the dirt and the surface, making it easier to separate the dirt.
Example Discussion:
In the surfactant aqueous solution, the droplet spread more easily over the solid surface than in pure water.
This was because the surfactant reduced the surface tension and weakened the force causing the droplet to remain rounded.
During cleaning, this improvement in wettability allows the cleaning solution to penetrate more easily between dirt and fiber surfaces, making the dirt easier to remove.
Interfacial Tension and Oily Dirt
Water and oil do not mix easily, and interfacial tension acts at the oil-water interface.
Oily dirt is difficult to remove with water alone because water has little affinity for oil and cannot easily enter between the oily dirt and the solid surface.
Surfactants adsorb at the oil-water interface and lower the interfacial tension.
When the interfacial tension decreases, oily dirt becomes easier to disperse as small oil droplets.
In addition, the hydrophobic groups of the surfactant enter the oily dirt while the hydrophilic groups face the water, making the surface of the oily dirt more compatible with water.
As a result, the oily dirt separates from the solid surface and becomes easier to disperse in the water.
Example Discussion:
Oily dirt was easier to remove with the surfactant aqueous solution because the surfactant adsorbed at the oil-water interface and lowered the interfacial tension between oil and water.
The hydrophobic groups entered the oily dirt while the hydrophilic groups faced the water, stabilizing the surface of the oily dirt in the aqueous phase.
As a result, the oily dirt was considered to have dispersed more readily as small oil droplets and been removed from the solid surface.
Micelle Formation and Cleaning Action
When the surfactant concentration reaches a certain level, surfactant molecules aggregate in water and form micelles.
In a micelle, the hydrophobic groups face inward and the hydrophilic groups face outward.
Because the interior is a hydrophobic environment with an affinity for oil, it can incorporate oily dirt and hydrophobic substances.
In cleaning, oily dirt is not only dispersed into fine droplets but is also retained in water by being incorporated into the interior of micelles.
This also helps prevent dirt that has already been removed from reattaching to the surface.
However, below the CMC, the concentration at which micelle formation begins, the ability to incorporate oily dirt is limited.
CMC = Surfactant concentration at which micelle formation begins
Example Discussion:
When the surfactant concentration is sufficiently high, the molecules form micelles in water.
Because the interior of a micelle is hydrophobic, it can incorporate oily dirt.
Therefore, in the surfactant aqueous solution, oily dirt was considered to have become easier to disperse and solubilize in water, increasing the cleaning action.
Process of Cleaning Action
Cleaning by surfactants can be considered as a series of several stages.
First, the surfactant lowers the surface tension of water and allows the cleaning solution to spread more easily over dirt and fiber surfaces.
Next, the surfactant adsorbs at the interface between oily dirt and water, making it easier to separate the oily dirt from the surface.
Furthermore, the separated oily dirt is emulsified and dispersed as small oil droplets and stabilized in water by micelles and surfactant films.
Therefore, the oily dirt becomes less likely to reattach to the solid surface.
Cleaning action is a combined phenomenon involving reduction of surface tension, wetting, emulsification, dispersion, and solubilization.
Example Discussion:
In cleaning with a surfactant, the surface tension first decreases, allowing the cleaning solution to spread more easily over the dirt surface.
Next, the hydrophobic groups of the surfactant enter the oily dirt while the hydrophilic groups face the water, making the oily dirt easier to disperse into the aqueous phase.
Furthermore, the oily dirt is retained in water by being incorporated into micelles, which is considered to suppress redeposition.
Discussion of Foaming
Surfactants also adsorb at the air-water interface.
Therefore, when air is incorporated into water by stirring or shaking, the bubble surfaces become covered with surfactant and the foam becomes more stable.
In pure water, bubbles tend to break quickly, whereas in a surfactant aqueous solution, foam may remain for a longer time.
Foaming is one phenomenon indicating the properties of a surfactant, but strong foaming does not necessarily mean high cleaning power.
Cleaning action involves wettability, adsorption onto oily dirt, emulsification, dispersion, micelle formation, and other factors.
Foam affects appearance and user sensation during cleaning, but it must be considered separately from cleaning power.
Example Discussion:
The foam was stable in the surfactant aqueous solution because surfactant molecules adsorbed at the bubble surfaces and stabilized the air-water interface.
In pure water, the bubble surfaces are less stable and the foam disappears quickly.
However, the strength of foaming does not directly indicate cleaning power, and cleaning action must be considered together with emulsification and micelle formation.
Discussion of Emulsification
Emulsification is the phenomenon in which one of two poorly miscible liquids, such as water and oil, is dispersed as fine droplets within the other.
Surfactants adsorb at the oil-water interface and lower the interfacial tension, making it easier to disperse the oil into small droplets.
In addition, by covering the droplet surfaces, surfactants prevent the droplets from coalescing with one another.
In cleaning, it is important for oily dirt to become emulsified as small oil droplets in water.
Emulsified oil droplets are stabilized by surfactants and become easier to wash away with water.
If a cloudy state persists after oil and water are shaken together in an experiment, emulsification by the surfactant can be considered to have occurred.
Example Discussion:
The cloudy state of the oil and water was maintained under conditions where surfactant was added because the oil was emulsified as small droplets in the water.
The surfactant adsorbed at the oil-water interface, lowering the interfacial tension and stabilizing the surfaces of the oil droplets.
As a result, coalescence of the oil droplets was suppressed and the dispersed state was more easily maintained.
Discussion of Dispersion
Dispersion is the action of distributing solid particles, oil droplets, and similar materials finely throughout a liquid.
Surfactants adsorb onto particle surfaces and oil-droplet surfaces and make those surfaces more compatible with water.
They also prevent particles or droplets from approaching one another and suppress aggregation and coalescence.
During cleaning, it is important to keep dirt that has been removed from the surface dispersed in water so that it does not reattach.
When dispersion is stabilized by surfactants, dirt becomes easier to remove together with the cleaning solution.
Therefore, dispersion is an important component of cleaning action.
Example Discussion:
Surfactants adsorb onto the surfaces of dirt particles and oil droplets and make them more compatible with water, allowing them to disperse more easily in the aqueous phase.
In addition, when particle surfaces are covered with surfactant, aggregation between particles is suppressed.
As a result, removed dirt becomes less likely to reattach to the surface, increasing the cleaning effect.
Effect of Surfactant Concentration
If the surfactant concentration is too low, the surface and oil-water interface cannot be sufficiently covered, and reduction of surface tension, emulsification, and cleaning action are limited.
As the concentration increases, adsorption at interfaces increases, surface tension decreases, and dispersion and emulsification of oily dirt become easier.
Above the CMC, micelle formation also makes solubilization of oily dirt more likely.
However, adding more surfactant does not increase cleaning power without limit.
Above a certain level, the amount of dirt, stirring, temperature, water quality, rinsability, and other factors become limiting.
Excess surfactant may also cause excessive foaming or residue.
Example Discussion:
The cleaning effect increased as the surfactant concentration increased because adsorption at interfaces increased and reduction of surface tension, emulsification, dispersion, and micelle formation became easier.
In particular, above the CMC, micelles form and the ability to incorporate oily dirt increases.
However, even if excessive surfactant is added, the cleaning effect eventually reaches a plateau and problems such as excessive foaming or residue may occur.
Effect of Temperature
As the temperature increases, the viscosity of oily dirt may decrease and the dirt may soften and become easier to remove.
The solubility of surfactants, micelle formation, and the dispersion state of dirt are also affected by temperature.
Therefore, even at the same surfactant concentration, the cleaning effect may change with temperature.
However, higher temperature is not always better.
Depending on the type of surfactant, its properties may change at high temperature or the emulsified state may become unstable.
When comparing cleaning power or surface tension in experiments, it is important to keep the temperature constant.
Example Discussion:
If oily dirt was removed more easily under higher-temperature conditions, this may have resulted from a decrease in the viscosity of the oily dirt, allowing the surfactant to act on the dirt more readily.
Temperature also affects surfactant solubility and micelle formation.
Therefore, to accurately compare surfactant properties, the measurement temperature must be kept constant.
Effect of Hard Water
Hard water contains large amounts of metal ions such as Ca2+ and Mg2+.
Some anionic surfactants may react with these metal ions to form salts that are poorly soluble in water.
In that case, the number of molecules functioning as surfactants decreases and cleaning power is reduced.
If foaming becomes weaker or a white precipitate forms in hard water, reactions with Ca2+ or Mg2+ can be considered.
On the other hand, nonionic surfactants and surfactants resistant to hard water may be relatively less affected by hard water.
Differences in water quality are also important conditions in cleaning experiments.
Example Discussion:
One possible reason why the cleaning effect and foaming decreased in hard water is that Ca2+ and Mg2+ reacted with the surfactant and formed poorly soluble salts.
As a result, the amount of surfactant molecules effectively functioning in the water decreased, and sufficient reduction of surface tension or micelle formation may not have occurred.
Therefore, surfactant performance is also affected by water quality.
Ionic and Nonionic Surfactants
Surfactants are classified as anionic, cationic, amphoteric, nonionic, and other types according to the properties of their hydrophilic groups.
Many anionic surfactants have strong cleaning power and foaming ability and are widely used in detergents.
Cationic surfactants may have antibacterial properties and softening effects.
Because nonionic surfactants do not carry a charge, they may be less affected by hard water and electrolytes.
However, some may show changes in hydrophilicity depending on temperature.
If the experimental results differ depending on the type of surfactant, differences in the charge and structure of the hydrophilic group should be discussed.
| Type | Characteristics | Point for Discussion |
|---|---|---|
| Anionic | Many have strong cleaning power and foaming ability | May be affected by metal ions in hard water |
| Cationic | May exhibit antibacterial properties and adsorption | Consider adsorption onto fibers and particle surfaces |
| Nonionic | May be less affected by electrolytes | Pay attention to changes in hydrophilicity with temperature |
Example Discussion:
Differences in foaming and cleaning effect among surfactant types were considered to result from differences in the charge and structure of the hydrophilic groups.
Anionic surfactants often have strong cleaning power and foaming ability, but may be affected by Ca2+ and Mg2+ in hard water.
Because nonionic surfactants do not carry a charge, they may be less affected by electrolytes.
Effect of pH
pH affects the charge state of surfactants and the properties of dirt.
In surfactants containing weakly acidic or weakly basic functional groups, the ionization state of the hydrophilic group changes with pH, which may change solubility and adsorption at interfaces.
Protein dirt and fatty-acid dirt may also differ in ease of removal depending on pH.
Under alkaline conditions, oily and fatty dirt may become easier to disperse, but the material being washed may also be damaged.
Under acidic conditions, inorganic dirt such as water scale may be removed more effectively, but surfactant performance may change depending on the surfactant type.
If pH is varied in an experiment, the effects on both the surfactant and the dirt should be discussed.
Example Discussion:
Changes in pH affect the charge state of the surfactant hydrophilic groups and the properties of the dirt, thereby influencing the cleaning effect.
For example, oily and fatty dirt may become easier to disperse or remove under alkaline conditions.
Therefore, when comparing surfactant performance, the pH conditions must either be kept constant or changes caused by pH must be considered.
Methods for Evaluating Cleaning Power
Cleaning power can be evaluated not only by visually comparing how much dirt is removed, but also by measuring mass changes, absorbance, reflectance, turbidity, or using photographic analysis.
For oily dirt, the amount of dirt removed can be determined by measuring the sample mass before and after cleaning.
For dirt containing dyes, the amount transferred into the cleaning solution may be estimated from the absorbance of the cleaning solution.
Visual evaluation is simple, but subjective judgment is easily introduced.
Therefore, it is important to compare samples under the same lighting, against the same background, and after the same observation time.
Performing multiple measurements and calculating the average makes it possible to evaluate differences in cleaning power more reliably.
Example Discussion:
When cleaning power is evaluated visually, subjective error may be included in judging how much dirt remains.
For a more objective evaluation, mass changes before and after cleaning, absorbance of the cleaning solution, or reflectance of the sample surface can be measured.
It is also important to compare samples using the same amount of dirt, cleaning time, temperature, and stirring conditions.
Causes of Error in Experiments on Surfactant Properties
Causes of error in experiments investigating surfactant properties include errors in preparing surfactant concentrations, insufficient sample mixing, temperature changes, dirty equipment, contamination with oil, effects of bubbles, insufficient standing time before measurement, variation in the amount of dirt, differences in cleaning time, and differences in stirring intensity.
Because surface tension and emulsified states are sensitive to interfacial conditions, even a small amount of contamination may change the results.
In cleaning experiments, the cleaning power cannot be compared accurately if the amount and state of dirt adhesion are not standardized.
In addition, if temperature, stirring, cleaning time, or liquid volume differ among samples, it becomes difficult to determine whether the differences are caused by surfactant type or concentration or by operating conditions.
Causes of error are easier to organize when divided into solution preparation, measurement operation, cleaning conditions, and evaluation method.
Example Discussion:
Possible causes of variation in the experimental results include errors in surfactant-concentration preparation, temperature changes, contamination of the equipment surface, and adhesion of bubbles.
In particular, during surface-tension measurement, even a small amount of oil or dirt may affect the interface and greatly change the measured value.
In cleaning experiments, it is important to standardize the amount of dirt, cleaning time, and stirring conditions.
When the Results Can Be Considered Good
An experiment investigating surfactant properties can be considered to have produced good results when, compared with pure water, the surfactant aqueous solution shows lower surface tension, droplets spread more easily, and oily dirt or oil droplets disperse more readily.
In addition, if increasing the concentration strengthens the reduction of surface tension and cleaning action and the effect reaches a plateau above a certain concentration, the result corresponds to the concepts of interfacial adsorption and micelle formation.
Furthermore, if the results change depending on hard water, temperature, or surfactant type, and those differences can be explained from the surfactant structure, water quality, or molecular aggregation state, this makes for a good discussion.
Even if the measured values are not ideal, it is important to relate the trends in the results to surface tension, emulsification, micelle formation, and cleaning action.
Example Discussion:
In this experiment, the surface tension decreased in the aqueous solution containing surfactant, and oily dirt was removed more easily than with pure water.
This was because the surfactant adsorbed at the water surface and oil-water interface and lowered the surface tension and interfacial tension.
Furthermore, because the oily dirt was finely dispersed, emulsification and dispersion by the surfactant and incorporation into micelles were considered to have contributed to the cleaning action.
Example Discussions When the Experiment Did Not Go Well
When a surfactant experiment does not go well, possible causes should be considered from results such as little decrease in surface tension, weak foaming or emulsification, no observable cleaning effect, variation in measured values, or only a small difference from pure water.
Organizing the causes according to surfactant concentration, temperature, equipment contamination, hard water, amount of dirt, stirring conditions, and observation method makes the discussion easier.
Example Discussion:
One possible reason why the decrease in surface tension was small in this experiment is that the surfactant concentration was low and an insufficient number of molecules adsorbed at the interface.
In addition, if oil or dirt remained on the measurement equipment, the surface condition may have changed and the measured values may have become unstable.
Furthermore, if the standing time before measurement was too short, adsorption of surfactant at the interface may not have reached equilibrium.
Another Example Discussion:
Possible reasons why the cleaning effect was smaller than expected include an excessively large amount of attached dirt, a short cleaning time, and insufficient stirring, which reduced contact between the dirt and the cleaning solution.
In addition, in hard water, Ca2+ and Mg2+ may have reacted with the surfactant and reduced the amount of effective surfactant.
How to Write Points for Improvement
In a discussion of surfactant properties, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to solution preparation, surface-tension measurement, cleaning tests, and evaluation methods.
Improvements to Solution Preparation
- Prepare the surfactant concentration accurately
- Set the concentration series at finer intervals
- Measure only after complete dissolution
- Keep the temperature constant
- Standardize water quality, such as hard water or soft water
- Keep the pH conditions consistent
Improvements to Measurement Operations
- Thoroughly clean the equipment
- Avoid oil contamination and detergent residue
- Measure surface tension in the absence of bubbles
- Allow the sample to stand for a fixed period before measurement
- Observe droplets and contact angles using the same method
- Perform multiple measurements and calculate the average
Improvements to Cleaning Tests
- Standardize the type and amount of dirt
- Keep the dirt-adhesion time consistent
- Keep the cleaning-solution volume constant
- Keep the cleaning time constant
- Keep the stirring speed constant
- Measure mass or absorbance before and after cleaning
- Record the samples with photographs for comparison
Example of How to Write Points for Improvement:
To accurately compare surfactant properties, the surfactant concentration, temperature, water quality, and pH must be kept constant.
In addition, during surface-tension measurement, it is important to avoid the effects of equipment contamination and bubbles and to allow the sample to stand for a fixed period before measurement so that interfacial adsorption stabilizes.
In cleaning tests, the amount of dirt, cleaning time, and stirring conditions should be standardized, and the results should be evaluated not only visually but also using mass changes or absorbance.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of surfactant properties, simply writing that “foaming occurred” or “the oil was removed” results in a superficial discussion.
A good discussion relates molecular structure, interfacial adsorption, reduction of surface tension, micelle formation, emulsification and dispersion, and cleaning action.
| Superficial Discussion | Good Discussion |
|---|---|
| The surface tension decreased. | Surfactant molecules adsorbed at the water surface and weakened the interactions among water molecules, so the surface tension was considered to have become lower than that of pure water. |
| The droplet spread. | Because the decrease in surface tension weakened the force causing the droplet to remain rounded, it was considered to have spread more easily over the solid surface. |
| The oily dirt was removed. | The hydrophobic groups of the surfactant entered the oily dirt while the hydrophilic groups faced the water, causing the oily dirt to disperse into the aqueous phase and become incorporated into micelles, making it easier to remove. |
| Foaming occurred. | The surfactant adsorbed at the air-water interface and stabilized the bubble surfaces, so the foam was considered to have remained longer than in pure water. |
| The effect was weaker in hard water. | Ca2+ and Mg2+ in hard water may have reacted with some surfactants and reduced the amount of effective surfactant, weakening the reduction of surface tension and cleaning action. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of experiments on surfactant properties.
Adjust the necessary parts according to your own experimental results.
- Surfactants are amphiphilic molecules containing hydrophilic and hydrophobic groups.
- Surfactant molecules adsorb at water-air or water-oil interfaces.
- Adsorption of surfactants weakens interactions among water molecules and lowers surface tension.
- When surface tension decreases, liquids spread more easily over solid surfaces.
- Surfactants adsorb at oil-water interfaces and lower interfacial tension.
- Because the hydrophobic group has an affinity for oily dirt and the hydrophilic group faces the aqueous phase, oily dirt becomes easier to disperse in water.
- Above the CMC, micelles form and oily substances become easier to incorporate.
- Foaming occurs because surfactants stabilize bubble surfaces.
- Cleaning action is a combined phenomenon involving wetting, emulsification, dispersion, and solubilization by micelles.
- In hard water, metal ions may weaken the action of surfactants.
Points to Check When Discussing Surfactant Properties
Checking the following points before writing the report makes the discussion easier to write.
- Is the definition of a surfactant explained?
- Are the roles of hydrophilic and hydrophobic groups described?
- Is adsorption at interfaces explained?
- Is the reason for the reduction in surface tension described?
- Is improved wettability related to surface tension?
- Is the reduction of interfacial tension at the oil-water interface explained?
- Are micelle formation and cleaning action related?
- Are foaming and cleaning power distinguished?
- Are emulsification and dispersion explained?
- Are the effects of hard water, temperature, and pH considered?
- Is the method for evaluating cleaning power described specifically?
- Do the points for improvement correspond to the causes of error?
Summary
Surfactants are amphiphilic molecules containing hydrophilic and hydrophobic groups, and they adsorb at interfaces such as water-air, water-oil, and solid-water interfaces and change the properties of those interfaces.
When surfactants adsorb at the water surface, they weaken the interactions among water molecules and reduce surface tension.
When surface tension decreases, liquids spread more easily and more readily wet solid surfaces and dirt.
The cleaning action of surfactants results not only from reduction of surface tension but also from reduction of interfacial tension at the oil-water interface, adsorption onto oily dirt, emulsification, dispersion, and solubilization through micelle formation.
Above the CMC, micelles form and oily substances become easier to incorporate, making micelle formation important for removal of oily dirt.
However, the cleaning effect is also affected by concentration, temperature, water quality, pH, the type of dirt, and stirring conditions.
In a report, rather than simply writing that “foaming occurred” or “the oil was removed,” organize and discuss the molecular structure of surfactants, interfacial adsorption, reduction of surface tension, wettability, emulsification, dispersion, micelle formation, cleaning action, the effects of hard water, causes of error, and points for improvement.
Experiments on surfactants are important experiments for understanding the relationship between interfacial chemistry and everyday cleaning phenomena.
