A micelle formation experiment is an experiment that investigates how surface tension, electrical conductivity, absorbance, solubilization capacity, and other properties change when the concentration of a surfactant is varied.
Surfactants are molecules that have both hydrophilic and hydrophobic groups, and in water, they mainly adsorb at interfaces at low concentrations, while at higher concentrations they aggregate with one another to form micelles.
The concentration at which these micelles begin to form is called the critical micelle concentration, or CMC.
In a discussion of micelle formation, it is not sufficient simply to write that “the surface tension decreased” or “the change became smaller above a certain concentration.”
It is necessary to explain why adding a surfactant lowers surface tension, why the slope of the graph changes near the CMC, and why the surface tension decreases very little above the CMC.
In addition, when conductivity or solubilization capacity is measured, the reason why the properties change before and after the CMC should be discussed from the viewpoint of molecular aggregation.
This article clearly explains, as examples of discussions that can be used in laboratory reports on micelle formation experiments, surfactant structure, reduction of surface tension, critical micelle concentration, how to determine the CMC, how to read graphs, differences between ionic and nonionic surfactants, effects of temperature and salt concentration, solubilization, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of micelle formation and surface-tension measurement results obtained in physical chemistry experiments, colloid chemistry experiments, interfacial chemistry experiments, and basic chemistry experiments at universities and similar institutions.
For the actual surfactant, concentration range, surface-tension measurement method, temperature, salt concentration, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Micelle Formation?
- Main Items to Include in the Results
- Structure and Properties of Surfactants
- Why Surface Tension Decreases
- What Is the Critical Micelle Concentration?
- How to Determine the CMC
- Discussion of the Surface-Tension Graph
- State Below the CMC
- State Above the CMC
- CMC Viewed from Conductivity
- CMC Viewed from Solubilization
- Relationship Between the CMC and Cleaning Action
- Length of the Hydrophobic Group and the CMC
- Type of Hydrophilic Group and the CMC
- Effect of Salt Concentration
- Effect of Temperature
- Precautions Regarding Concentration Units and Graph Preparation
- Causes of Error in Surface-Tension Measurement
- When the CMC Differs from the Literature Value
- 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 Micelle Formation
- Summary
What Is Micelle Formation?
Micelle formation is the phenomenon in which surfactant molecules, when present above a certain concentration in water, aggregate with their hydrophobic groups facing inward and their hydrophilic groups facing outward.
In water, hydrophobic groups tend to avoid contact with water, so gathering the hydrophobic groups together reduces the area of contact with water.
On the other hand, the hydrophilic groups face outward toward the water, allowing the aggregate as a whole to remain dispersed in water.
Micelles are an important property of surfactants and are related to cleaning, emulsification, solubilization, dispersion stabilization, and other functions.
For example, oily dirt becomes easier to incorporate into a surfactant aqueous solution because oily substances can enter the hydrophobic region inside the micelle.
Therefore, understanding micelle formation is important for understanding how surfactants function.
Example Discussion:
When the surfactant concentration exceeds a certain level, the molecules aggregate with their hydrophobic groups facing inward and their hydrophilic groups facing outward, forming micelles.
This occurs because the hydrophobic groups avoid contact with water, and gathering the hydrophobic groups together stabilizes the overall system.
Through micelle formation, surfactant aqueous solutions become more capable of incorporating oily substances and exhibit properties related to cleaning and solubilization.
Main Items to Include in the Results
In a micelle formation experiment, organize the type of surfactant, concentration, temperature, measured surface tension, conductivity, absorbance, solubilization amount, and other information.
In particular, when determining the CMC, it is important to graph the changes in the measured values against concentration and identify the concentration at which the slope changes.
The concentration axis may also be shown on a logarithmic scale.
Main Items to Include in the Results
- Type of surfactant used
- Whether the surfactant is ionic or nonionic
- Concentration range
- Measurement temperature
- Presence or absence of added salt
- Measured surface-tension values
- Measured conductivity values
- Absorbance or solubilization amount
- Graph of concentration versus measured values
- Concentration at which the graph slope changed
- Determined CMC
- Comparison with literature or theoretical values
- Changes in properties below and above the CMC
- Causes of error and points for improvement
Example of How to Write the Results:
When the surface tension was measured while varying the surfactant concentration, the surface tension decreased greatly as the concentration increased in the low-concentration region.
However, above a certain concentration, the decrease in surface tension became smaller and the slope of the graph changed.
This change in slope was read as the critical micelle concentration, and the concentration at which micelle formation began was evaluated.
Structure and Properties of Surfactants
Surfactants are amphiphilic molecules that have both hydrophilic and hydrophobic groups within a single molecule.
The hydrophilic group interacts readily with water, and examples include ionic groups, hydroxyl groups, and polyethylene oxide chains.
The hydrophobic group has little affinity for water and is often composed of a hydrocarbon chain.
Because of this structure combining hydrophilic and hydrophobic properties, surfactants readily gather at interfaces such as water-air and water-oil interfaces.
At low concentrations, they mainly adsorb at interfaces, while at higher concentrations they form micelles in water.
This molecular structure is the basis of properties such as reduction of surface tension, emulsification, cleaning, and solubilization.
Example Discussion:
Because surfactants have hydrophilic and hydrophobic groups, they show characteristic orientations in water.
At low concentrations, they adsorb at interfaces so that the hydrophobic groups avoid contact with water, while the hydrophilic groups face the aqueous phase.
As a result, interactions among water molecules at the surface are weakened and the surface tension is considered to decrease.
Why Surface Tension Decreases
At the surface of water, water molecules strongly attract one another, creating a force that tends to minimize the surface area.
This is surface tension.
When a surfactant is added, surfactant molecules adsorb at the water surface and weaken the strong interactions among water molecules.
Therefore, the surface tension decreases.
In the low-concentration region, many of the added surfactant molecules adsorb at the interface, so surface tension decreases greatly as the concentration increases.
However, once the interface becomes nearly saturated with surfactant, further adsorption at the interface becomes difficult.
After that, excess surfactant molecules begin to form micelles in the water.
Example Discussion:
The surface tension decreased as the surfactant concentration increased because surfactant molecules adsorbed at the water surface and weakened the interactions among water molecules.
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 becomes nearly saturated, the decrease in surface tension becomes smaller and the excess molecules are considered to form micelles in the water.
What Is the Critical Micelle Concentration?
The critical micelle concentration is the concentration at which surfactant molecules begin to form micelles in water.
It is called the CMC, from the initials of Critical Micelle Concentration.
Below the CMC, surfactant molecules mainly exist as individual molecules in water or are adsorbed at interfaces.
Above the CMC, many of the additional surfactant molecules exist as micelles.
The CMC changes depending on the type of surfactant, the length of the hydrophobic group, the properties of the hydrophilic group, temperature, salt concentration, solvent conditions, and other factors.
A surfactant with a lower CMC can be said to form micelles more readily at lower concentrations.
In cleaning and solubilization, micelle formation becomes important at concentrations above the CMC.
CMC = Surfactant concentration at which micelle formation begins
Example Discussion:
The critical micelle concentration is the concentration at which surfactant molecules begin to change from existing as individual molecules to aggregating and forming micelles.
Below the CMC, surfactants mainly adsorb at interfaces and reduce surface tension.
Above the CMC, the interface is nearly saturated and additional surfactant molecules form micelles in the water, so the change in surface tension becomes small.
How to Determine the CMC
The CMC is determined by measuring changes in physical properties against surfactant concentration and identifying the point where the slope of the graph changes.
When surface tension is used, the surface tension decreases greatly at low concentrations and decreases more gradually above the CMC.
A commonly used method is to extrapolate the two linear regions and read their intersection as the CMC.
When conductivity is used, ionic surfactants show a change in the slope of the conductivity increase before and after the CMC.
When solubilization or absorbance is used, hydrophobic substances become more readily incorporated into micelles above the CMC, causing the measured values to change.
In any measurement method, the CMC is read as the concentration at which the property changes sharply.
Example Discussion:
The CMC was determined from the point where the slope changed greatly in the graph of surface tension against concentration.
In the low-concentration region, the surface tension decreased rapidly because the surfactant adsorbed at the interface.
On the other hand, above the CMC, the interface was nearly saturated and additional surfactant was used for micelle formation, so the decrease in surface tension became more gradual.
Discussion of the Surface-Tension Graph
When surface tension is plotted on the vertical axis and surfactant concentration on the horizontal axis, the surface tension decreases as the concentration increases.
In particular, in the low-concentration region, the surface tension decreases greatly because the number of surfactant molecules adsorbed at the interface increases.
In this region, surfactant molecules are considered to gather at the water surface and change the state of the water surface.
Once the concentration exceeds the vicinity of the CMC, the decrease in surface tension becomes smaller and the graph approaches a nearly horizontal line.
This is because the interface is almost saturated with surfactant and additional molecules form micelles in the water instead of adsorbing at the interface.
Therefore, the bend in the surface-tension graph is important information indicating the onset of micelle formation.
Example Discussion:
Surface tension decreased greatly in the low surfactant-concentration region.
This was because surfactant molecules adsorbed at the water surface and weakened the interactions among water molecules at the surface.
However, because the decrease in surface tension became small above a certain concentration, the interface was considered to have become nearly saturated and excess surfactant molecules to have been used for micelle formation.
State Below the CMC
Below the CMC, surfactant molecules mainly exist as individual molecules in water and also adsorb at interfaces.
As the concentration increases, the number of molecules adsorbed at the interface increases and the surface tension decreases greatly.
However, almost no micelles are formed at this stage.
Below the CMC, the ability to solubilize hydrophobic substances is small.
This is because there are not yet enough hydrophobic regions inside micelles to incorporate oily substances.
Therefore, even though a decrease in surface tension is observed, solubilization by micelles is limited.
Example Discussion:
Below the CMC, surfactant molecules mainly exist as individual molecules and adsorb at the water surface and oil-water interface.
Therefore, surface tension decreases greatly as the concentration increases.
However, because almost no micelles are formed in this concentration region, the ability to incorporate hydrophobic substances is considered to remain small.
State Above the CMC
Above the CMC, the interface is almost saturated with surfactant molecules and additional surfactant molecules form micelles in the water.
Therefore, even if the surfactant concentration is increased further, the surface tension does not decrease greatly.
On the other hand, the number of micelles increases and the ability to incorporate hydrophobic substances becomes greater.
In solutions above the CMC, cleaning and solubilization effects tend to become stronger.
Because the interior of a micelle is a hydrophobic environment, it can incorporate oily dirt and hydrophobic molecules.
Therefore, the CMC is important as a boundary at which the practical functions of surfactants change greatly.
Example Discussion:
Above the CMC, surfactant molecules form micelles in water.
Because the interface is already almost saturated with surfactant, increasing the concentration produces only a small further decrease in surface tension.
On the other hand, the increase in the number of micelles increases the ability to incorporate hydrophobic substances, strengthening cleaning and solubilization effects.
CMC Viewed from Conductivity
In ionic surfactants, increasing the concentration increases the number of ions in solution, so the conductivity increases.
Below the CMC, a large proportion of the surfactant molecules exist as individual ions, so conductivity increases relatively strongly with concentration.
Above the CMC, the surfactant ions aggregate into micelles, causing the slope of the conductivity increase to change.
Micelles have mobility different from that of individual ions and carry some counterions with them, so the increase in conductivity changes before and after the CMC.
Therefore, the CMC can also be determined from the point where the slope changes in a conductivity-versus-concentration graph.
However, because nonionic surfactants show little change in conductivity, this method may not be suitable for them.
Example Discussion:
In ionic surfactants, conductivity increases as the concentration increases.
However, above the CMC, the surfactant ions form micelles and the proportion moving freely as individual ions changes, so the slope of the conductivity changes.
Therefore, the CMC can be estimated from the bend in the conductivity-concentration graph.
CMC Viewed from Solubilization
Because micelles have a structure in which hydrophobic groups face inward, they can incorporate hydrophobic substances into their interior.
This is called solubilization.
Below the CMC, almost no micelles are present, so little solubilization of hydrophobic substances occurs.
Above the CMC, micelles form and the apparent solubility of hydrophobic substances increases.
In experiments using hydrophobic dyes or oily substances, absorbance or the amount solubilized may increase above the CMC.
This is because the dyes or oily substances are incorporated into the interior of micelles.
Changes in solubilization capacity provide another clue for confirming micelle formation.
Example Discussion:
If the absorbance of a hydrophobic dye increased above the CMC, the dye was considered to have been incorporated into the interior of micelles and its apparent solubility increased.
Below the CMC, almost no micelles are present, so there are few regions that can incorporate hydrophobic substances.
Therefore, a sudden increase in solubilization amount indicates that micelle formation has begun.
Relationship Between the CMC and Cleaning Action
The cleaning action of surfactants involves reduction of surface tension, adsorption onto dirt surfaces, emulsification, dispersion, and solubilization by micelles.
Even below the CMC, surface tension decreases and adsorption at interfaces occurs, but because the number of micelles is small, the ability to incorporate oily dirt is limited.
Above the CMC, micelles form and oily dirt becomes easier to incorporate and disperse in water.
However, increasing the surfactant concentration above the CMC does not increase cleaning power without limit.
Above a certain level, the amount of dirt, stirring, temperature, water hardness, and other factors become limiting factors.
The CMC can be considered a guideline for the concentration at which cleaning action begins to appear strongly.
Example Discussion:
Above the CMC, micelles are formed, making it easier to incorporate oily substances into the micelle interior.
Therefore, the cleaning action of surfactants tends to become stronger above the CMC.
On the other hand, below the CMC, although surface tension decreases, solubilization by micelles is insufficient, so the ability to retain oily dirt in water is considered small.
Length of the Hydrophobic Group and the CMC
As the hydrophobic group of a surfactant becomes longer, the tendency of the hydrophobic group to avoid contact with water becomes stronger.
Therefore, the molecules become more likely to aggregate and form micelles, and the CMC tends to decrease.
In other words, surfactants with longer alkyl chains are considered to form micelles more readily at lower concentrations.
However, if the hydrophobic group is too long, the solubility in water may decrease.
The CMC also changes depending on the type of hydrophilic group, temperature, and salt concentration.
The length of the hydrophobic group is one important structural factor affecting the CMC.
Example Discussion:
Surfactants with longer hydrophobic groups have a stronger tendency for the hydrophobic groups to avoid contact with water, making the molecules more likely to aggregate and form micelles.
Therefore, surfactants with longer hydrophobic groups tend to have lower CMC values.
When comparing differences in CMC, it is necessary to consider not only the hydrophilic group but also the length of the hydrophobic group as an important factor.
Type of Hydrophilic Group and the CMC
The type of hydrophilic group also affects the CMC.
In ionic surfactants, the hydrophilic groups carry charges, so hydrophilic groups with the same sign electrostatically repel one another during micelle formation.
This repulsion acts to hinder micelle formation, making the CMC more sensitive to counterions and salt concentration.
In nonionic surfactants, hydration of the hydrophilic groups is important.
As the temperature increases, the hydration state of the hydrophilic groups changes and may affect micelle formation and solubility.
The properties of the hydrophilic group are related not only to the CMC but also to micelle shape and size, emulsifying properties, and solubilization capacity.
Example Discussion:
When the hydrophilic group is ionic, charges of the same sign are arranged on the micelle surface and electrostatic repulsion occurs.
Because this repulsion hinders micelle formation, the CMC of ionic surfactants is strongly affected by salt concentration and counterions.
On the other hand, in nonionic surfactants, hydration of the hydrophilic groups is important, and changes in temperature may alter the CMC and solubility.
Effect of Salt Concentration
In ionic surfactants, adding salt may lower the CMC.
This is because ions in the solution weaken the electrostatic repulsion between the hydrophilic groups of the surfactant, making micelle formation easier.
For example, in anionic surfactants, counterions such as Na+ shield the negative charges of the hydrophilic groups.
When electrostatic repulsion becomes weaker, surfactant molecules aggregate more easily and can form micelles even at lower concentrations.
However, if the salt concentration is too high, it may affect solubility or phase separation.
Changes in the CMC caused by salt addition are a characteristic point of discussion for ionic surfactants.
Example Discussion:
If the CMC decreased under conditions where salt was added, this was considered to result from the counterions weakening the electrostatic repulsion between the hydrophilic groups of the surfactant.
When the repulsion becomes smaller, surfactant molecules aggregate more easily and can form micelles at lower concentrations.
Therefore, the CMC of ionic surfactants is strongly affected by salt concentration.
Effect of Temperature
Temperature affects surfactant solubility, hydration, hydrophobic interactions, and micelle formation.
As temperature increases, molecular motion becomes more active and the state of surface tension and micelle formation may change.
In nonionic surfactants, increasing temperature may weaken hydration of the hydrophilic groups and cause major changes in their properties.
If the CMC is compared under different temperature conditions, it becomes difficult to determine whether changes are caused by surfactant concentration or by temperature.
Therefore, it is important to keep the temperature constant during CMC measurements.
Recording the temperature conditions also makes comparison with literature values easier.
Example Discussion:
Changes in temperature alter the hydration state of surfactants and hydrophobic interactions, thereby affecting the CMC and surface tension.
In particular, in nonionic surfactants, increasing temperature may weaken hydration of the hydrophilic groups and change micelle formation and solubility.
Therefore, the measurement temperature must be kept constant in order to determine the CMC accurately.
Precautions Regarding Concentration Units and Graph Preparation
In experiments for determining the CMC, handling concentration units correctly is important.
It should be clearly stated whether mol/L, mmol/L, g/L, mass%, or another unit is being used.
When comparing with literature values, the units must be made consistent.
Errors in unit conversion can cause large errors in the CMC value.
In addition, if the concentration range is too narrow, it becomes difficult to see the change in slope before and after the CMC.
To read the CMC accurately, the concentration series must sufficiently include both concentrations below and above the CMC.
In the graph, the linear regions on the low-concentration and high-concentration sides should be selected appropriately.
Example Discussion:
In reading the CMC, concentration units and the method of graph preparation greatly affect the result.
If sufficient concentration points are not set around the CMC, the bend in the surface-tension graph cannot be determined accurately.
In addition, when comparing with literature values, units such as mol/L and g/L must be unified and errors in unit conversion must be avoided.
Causes of Error in Surface-Tension Measurement
Causes of error in surface-tension measurement include dirty equipment, bubbles on the sample surface, temperature changes, errors in concentration preparation, insufficient standing time before measurement, incomplete adsorption equilibrium of the surfactant, and poor calibration of the measuring instrument.
Because surface tension is sensitive to the state of the interface, even slight contamination or oil may greatly change the measured value.
In surfactant solutions in particular, adsorption at the interface may require time.
If the solution is measured immediately after preparation, the interface may not yet have reached equilibrium and the measured surface tension may be higher.
It is important to standardize the standing time before measurement and the measurement conditions.
Example Discussion:
Possible causes of error in surface-tension measurement include contamination of the equipment surface, adhesion of bubbles, temperature changes, and incomplete adsorption equilibrium of the surfactant.
Surface tension is sensitive to surface conditions, and even small amounts of oil or dirt may change the measured value.
In addition, if the measurement was performed immediately after solution preparation, the surfactant may not have adsorbed sufficiently at the interface and the equilibrium value may not have been measured.
When the CMC Differs from the Literature Value
Causes of differences between the experimentally determined CMC and literature values include temperature, salt concentration, surfactant purity, errors in concentration preparation, differences in measurement method, graph reading, and impurities in the solution.
Because the CMC strongly depends on conditions, it is necessary to check whether the measurement conditions are the same when comparing with literature values.
In addition, the CMC is not necessarily obtained as one exact point and may differ slightly depending on the measurement method.
Surface tension, conductivity, fluorescent probes, solubilization amount, and other methods measure different properties, so the point at which the change is read may differ slightly.
Therefore, differences should not simply be treated as experimental failure but should be discussed in terms of differences in measurement conditions and methods.
Example Discussion:
Possible reasons why the experimentally determined CMC differed from the literature value include measurement temperature, salt concentration, surfactant purity, and errors in concentration preparation.
Because the CMC strongly depends on solution conditions, it is necessary to confirm that the conditions are the same when comparing with literature values.
In addition, because the method for identifying the change point differs between surface-tension and conductivity methods, differences in measurement methods may also affect the CMC value.
When the Results Can Be Considered Good
A micelle formation experiment can be considered to have produced good results when a clear tendency is observed in which the surface tension decreases as the surfactant concentration increases and the decrease becomes more gradual above a certain concentration.
If the CMC can be determined from this bend, the onset of micelle formation can be considered to have been experimentally confirmed.
In addition, when conductivity or solubilization amount is measured, if changes in slope or measured values are observed before and after the CMC, the changes in physical properties associated with micelle formation can be considered confirmed.
Even if the measured values do not lie perfectly on ideal straight lines, the discussion is sufficient if the overall trend can be explained in terms of molecular adsorption and micelle formation.
Example Discussion:
In this experiment, the surface tension decreased as the surfactant concentration increased, and above a certain concentration the change in surface tension became small.
This tendency is consistent with the idea that surfactants adsorb at interfaces at low concentrations and micelle formation begins above the CMC.
Therefore, the bend in the graph could be determined as the CMC and micelle formation was considered to have been confirmed.
Example Discussions When the Experiment Did Not Go Well
When a micelle formation experiment does not go well, possible causes should be considered from results such as an unclear decrease in surface tension, no visible bend corresponding to the CMC, large variation in measured values, large differences from literature values, or no change in the slope of conductivity.
Organizing the causes according to the concentration series, temperature, cleanliness of the equipment, measurement method, surfactant purity, and standing time makes the discussion easier.
Example Discussion:
In this experiment, no clear bend was observed in the graph of surface tension versus concentration.
One possible reason is that the concentration series was not set sufficiently around the CMC, preventing changes before and after the CMC from being captured.
In addition, if bubbles or contamination remained on the surface, the surface-tension values may have varied and made it difficult to read the CMC.
Another Example Discussion:
Possible reasons why the determined CMC was higher than the literature value include underestimating the actual surfactant concentration, differences in temperature conditions from those in the literature, and the presence of impurities in the solution.
In addition, if measurement was performed before the surfactant had adsorbed at the interface and reached equilibrium, the surface tension may have been measured as higher, affecting the reading of the CMC.
How to Write Points for Improvement
In a discussion of micelle formation, 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, graph preparation, and CMC analysis.
Improvements to Solution Preparation
- Prepare the surfactant concentration accurately
- Set concentration points finely around the CMC
- Use consistent concentration units
- Dissolve the sample sufficiently
- Keep the temperature constant
- Keep the salt concentration and pH constant
Improvements to Measurement Operations
- Thoroughly clean the equipment
- Avoid oil and contamination
- Measure in the absence of bubbles
- Allow the sample to stand for a fixed period before measurement
- Measure at the same temperature
- Perform multiple measurements and calculate the average
Improvements to Analysis
- Create a graph of concentration versus surface tension
- Use a logarithmic concentration axis when necessary
- Approximate the linear regions before and after the CMC separately
- Determine the bend objectively
- Confirm the measurement conditions when comparing with literature values
- Confirm the result using other indicators such as conductivity or solubilization amount
Example of How to Write Points for Improvement:
To determine the CMC accurately, it is necessary to set fine concentration intervals around the CMC and measure the relationship between concentration and surface tension in detail.
In addition, because surface tension is easily affected by contamination, bubbles, and temperature, it is important to thoroughly clean the equipment and standardize the measurement temperature and standing time.
In the analysis, the linear regions before and after the CMC should be approximated separately and the bend should be determined objectively.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of micelle formation, simply writing that “the surface tension decreased” or “the CMC was determined” results in a superficial discussion.
A good discussion relates interfacial adsorption, reduction of surface tension, the CMC, micelle formation, solubilization, and measurement conditions.
| 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 decrease as the concentration increased. |
| The change became smaller after a certain point. | After the interface became nearly saturated with surfactant, additional molecules formed micelles in the water rather than adsorbing at the interface, so the decrease in surface tension became smaller. |
| The CMC was obtained. | The bend in the surface-tension versus concentration graph indicates the transition from a state dominated by interfacial adsorption of individual molecules to a state in which micelle formation begins. |
| It differed from the literature value. | Because the CMC is affected by temperature, salt concentration, surfactant purity, measurement method, concentration series, and adsorption equilibrium, differences from literature values can be discussed in terms of differences in measurement conditions. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of micelle formation.
Adjust the necessary parts according to your own experimental results.
- Surfactants are amphiphilic molecules containing hydrophilic and hydrophobic groups.
- At low concentrations, surfactant molecules mainly adsorb at interfaces.
- When surfactants adsorb at the water surface, the surface tension of water decreases.
- Once the interface becomes nearly saturated, the decrease in surface tension becomes smaller.
- The CMC is the concentration at which surfactant molecules begin to form micelles.
- Above the CMC, additional surfactant molecules are mainly used for micelle formation.
- The interior of a micelle is hydrophobic and can incorporate oily substances.
- Surfactants with longer hydrophobic groups tend to have lower CMC values.
- In ionic surfactants, adding salt may lower the CMC.
- The measured CMC is affected by temperature, salt concentration, measurement method, and concentration preparation.
Points to Check When Discussing Micelle Formation
Checking the following points before writing the report makes the discussion easier to write.
- Is the structure of the surfactant explained?
- Are the roles of the hydrophilic and hydrophobic groups described?
- Is the reason for the decrease in surface tension explained?
- Is the definition of the CMC stated?
- Are the states below and above the CMC distinguished?
- Is the change in graph slope related to micelle formation?
- Is the method for determining the CMC explained?
- Are changes in conductivity and solubilization amount discussed?
- Are the effects of temperature and salt concentration considered?
- Are differences from literature values explained in terms of measurement conditions?
- Are measurement errors related to surface-tension measurement and concentration preparation?
- Do the points for improvement correspond to the causes of error?
Summary
Micelle formation is the phenomenon in which surfactant molecules aggregate above a certain concentration to form assemblies with their hydrophobic groups facing inward and their hydrophilic groups facing outward.
At low concentrations, surfactant molecules mainly adsorb at interfaces and reduce surface tension.
However, once the interface becomes nearly saturated, additional molecules begin to form micelles in the water.
The concentration that forms this boundary is the critical micelle concentration, or CMC.
The CMC can be determined from the concentration dependence of surface tension, conductivity, solubilization amount, absorbance, and other properties.
In the surface-tension method, surface tension decreases greatly at low concentrations and the decrease becomes more gradual above the CMC, so the bend in the graph is read as the CMC.
The CMC changes depending on surfactant structure, temperature, salt concentration, and solution conditions.
In a report, rather than simply writing that “the surface tension decreased,” organize and discuss interfacial adsorption of surfactants, reduction of surface tension, the CMC, micelle formation, changes in state below and above the CMC, changes in conductivity and solubilization, effects of temperature and salt concentration, causes of error, and points for improvement.
Micelle formation experiments are important experiments for understanding the properties of surfactants and the fundamentals of colloid chemistry.
