Surface Tension Measurement Discussion Examples | Effects of Concentration, Temperature, and Surfactants

What Is Surface Tension?

Surface tension is a physical quantity representing the tendency of a liquid surface to become as small as possible.
Molecules inside a liquid are pulled almost equally by surrounding molecules, but molecules at the surface have fewer molecules outside the liquid and therefore experience a force pulling them inward.
As a result, the liquid surface tends to contract, producing surface tension.

The greater the surface tension of a liquid, the more energy is required to increase its surface area.
The tendency of water to form rounded droplets and the ability of small insects to stand on the water surface can be explained by surface tension.

Main Items to Include in the Results

In the results of surface tension measurements, organize the sample name, concentration, temperature, measurement method, measured values, mean values, calculated surface tension, and comparison with literature values.
When concentration or temperature is changed, organizing the results in tables or graphs makes the trends easier to explain.

Main Items to Include in the Results

• Type of liquid or solution measured
• Solution concentration
• Measurement temperature
• Measurement method
• Measured values such as drop count, rise height, and ring pull force
• Mean value when multiple measurements were performed
• Equation used to calculate surface tension
• Calculated surface tension
• Concentration-surface tension graph
• Temperature-surface tension graph
• Relationship with surfactant concentration
• Comparison with literature values
• Error rate
• Presence or absence of bubbles, contamination, and temperature changes

Reference Experimental Values and Analysis Examples for Surface Tension Measurement

Here, reference experimental values are organized for measuring the surface tension of liquids and discussing the effects of concentration, temperature, and surfactants.
The drop-count method, capillary-rise method, ring method, contact angle, estimation of CMC, and measurement errors are summarized in a form that is easy to use in reports.

Surface tension represents the tendency of a liquid surface to become as small as possible.
Water has high surface tension because hydrogen bonding between molecules is strong, while adding ethanol or surfactants lowers the surface tension.
In addition, as temperature increases, molecular thermal motion increases and surface tension generally decreases.

Reference Experimental Conditions

Comparison of Surface Tension of Representative Liquids

Water has higher surface tension than many other liquids because of strong hydrogen bonding.
In contrast, ethanol and acetone have lower surface tension than water and tend to spread more readily as droplets.

Example of Surface Tension Measurement by the Drop-Count Method

In the drop-count method, the number of drops formed from a fixed volume of liquid is counted, and the surface tension is determined by comparison with a reference liquid.
When the same apparatus is used, surface tension can be compared from density and drop count using the following equation.

γx = γw × (ρx / ρw) × (nw / nx)

When the same volume is dispensed, the greater the number of drops, the smaller the volume and mass of each individual drop.
Therefore, a sample with a larger drop count can be considered to have lower surface tension.

Calculation Example for the Drop-Count Method

Let the surface tension of water be 72.0 mN/m, the density of water be 0.997 g/mL, and the drop count of water be 25 drops.
If the density of a 20% aqueous ethanol solution is 0.970 g/mL and the drop count is 38 drops,

γx = 72.0 × (0.970 / 0.997) × (25 / 38)

γx = 72.0 × 0.973 × 0.658 = 46.1 mN/m

Therefore, the surface tension of the 20% aqueous ethanol solution is calculated to be approximately 46 mN/m.

Changes in Surface Tension With Ethanol Concentration

Surface tension decreases as the ethanol concentration increases.
This is considered to occur because ethanol disrupts the surface structure of water and weakens the strong interactions between water molecules.

Changes in Surface Tension With Surfactant Concentration

In the low-concentration range of a surfactant, molecules adsorb at the air-water interface and greatly reduce the surface tension.
However, once the interface becomes nearly saturated, additional surfactant is more readily used for micelle formation and the decrease in surface tension becomes smaller.

Example of Reading the CMC

In a graph of surfactant concentration versus surface tension, surface tension decreases sharply on the low-concentration side and becomes nearly constant on the high-concentration side, producing a bend in the graph.
The concentration near this bend is regarded as the critical micelle concentration, or CMC.

In this reference example, the region around 0.5–1.0 mmol/L, where the decrease in surface tension begins to become small, can be considered a rough estimate of the CMC.

Changes in the Surface Tension of Water With Temperature

As temperature rises, molecular thermal motion increases and the effect that draws molecules together at the liquid surface becomes relatively weaker, so surface tension decreases.

Changes in Surface Tension With Salt Concentration

Surfactants greatly reduce surface tension, whereas inorganic salts such as NaCl may instead show a tendency to increase it slightly.

Calculation Example for the Capillary-Rise Method

In the capillary-rise method, surface tension is determined from the height to which the liquid rises in a narrow tube.

γ = ρghr / 2cosθ

Here, ρ is the liquid density, g is gravitational acceleration, h is the rise height, r is the capillary radius, and θ is the contact angle.
When water wets glass well, θ can be approximated as nearly 0° and cosθ as 1.

Liquids with lower surface tension show a smaller rise height even when the capillary radius is the same.

Example Measurement by the Ring Method

In the ring method, ring contamination, fluctuations of the liquid surface, pull-up speed, and treatment of correction factors affect the results.

Relationship Between Wettability and Contact Angle

Contact angle is affected not only by the surface tension of the liquid but also by the properties of the solid surface.
Therefore, when discussing surface tension from contact angle, both the liquid side and the solid side must be considered.

Example of Variation in Measured Values

If only the fourth measurement has a smaller drop count and the calculated surface tension is higher, possible causes include an excessively high drop rate, liquid remaining on the nozzle, or a counting error.

Main Sources of Measurement Error

Time Dependence After Adding Surfactant

In surfactant solutions, if the waiting time before measurement is not standardized, differences in adsorption time rather than differences in concentration may be measured.

Example of How to Write the Results

The surface tensions of water, aqueous ethanol solutions, and aqueous surfactant solutions were compared.
The surface tension of water was approximately 72 mN/m, while that of a 20% aqueous ethanol solution was approximately 46 mN/m and that of an aqueous surfactant solution was approximately 35 mN/m.
These results confirmed that adding ethanol or a surfactant greatly reduces the surface tension of water.

As the ethanol concentration increased, the surface tension decreased continuously.
This is considered to have occurred because ethanol molecules disrupted the hydrogen-bonding network between water molecules and weakened the intermolecular interactions that maintain the liquid surface.
In contrast, aqueous NaCl solution showed a slight increase in surface tension, exhibiting a different trend from surfactants.

When the surfactant concentration was increased, the surface tension decreased sharply in the low-concentration range.
This occurred because surfactant molecules adsorbed at the air-water interface and reduced the surface free energy of water.
However, as the concentration became higher, the decrease in surface tension became smaller and the value became almost constant around 0.5–1.0 mmol/L.
This bend is considered to be a rough indication of the CMC.

Points for Connecting the Results to the Discussion

In a discussion of surface tension measurements, it is important to explain not only the magnitude of the measured values but also intermolecular forces, adsorption at the interface, temperature, concentration, and the characteristics of the measurement method in relation to one another.

• Can you explain why water has high surface tension in relation to hydrogen bonding and intermolecular forces?
• Can you explain why surface tension decreases as the ethanol concentration increases?
• Can you explain that surfactants adsorb at the surface and reduce surface tension?
• Can you discuss why the decrease in surface tension becomes smaller near the CMC in relation to micelle formation?
• Can you explain why surface tension decreases as temperature rises in relation to thermal motion?
• Can you explain why inorganic salts and surfactants affect surface tension differently?
• Do you understand the measurement principles of the drop-count method, capillary-rise method, and ring method?
• Can you discuss how contamination, oil, and detergent residue on the apparatus greatly affect surface tension?
• Can you explain that waiting time before measurement affects the results for surfactant solutions?
• If an outlier appears, can you examine drop rate, counting errors, temperature, and concentration-preparation errors?

Example Discussion

In this experiment, the drop-count method was used to compare the surface tension of water, aqueous ethanol solutions, and aqueous surfactant solutions.
The surface tension of water was approximately 72 mN/m and was higher than that of the other samples.
This is because strong hydrogen bonding acts between water molecules, producing a strong tendency to keep the liquid surface small.

In the aqueous ethanol solutions, surface tension decreased as the ethanol concentration increased.
Because ethanol has lower surface tension than water and weakens the hydrogen-bonding network between water molecules, the surface tension of the mixed solution as a whole is considered to have decreased.
At 100% ethanol, the surface tension was approximately 22 mN/m, which was considerably lower than that of water.

In the aqueous surfactant solution, surface tension decreased sharply in the low-concentration range.
This occurred because surfactant molecules adsorbed at the interface with their hydrophilic groups facing the water and their hydrophobic groups facing the air, thereby lowering the free energy of the water surface.
In contrast, in the high-concentration range, surface tension became almost constant.
This is considered to have occurred because the interface became nearly saturated with surfactant and additional molecules were used mainly for micelle formation.

Regarding the effect of temperature, the surface tension of water decreased as the temperature increased.
This is because the thermal motion of the molecules increased with rising temperature and the effect of attracting molecules at the liquid surface became weaker.
Therefore, when comparing the concentration dependence of surface tension, it is necessary to keep the temperature constant.

Possible sources of error include oil or detergent residue on the surface of the apparatus, differences in drop rate, temperature changes, errors in counting drops, and differences in the waiting time for surfactant adsorption.
Surface tension is particularly sensitive to trace amounts of surface-active substances, so insufficient cleaning of the apparatus may also cause the measured value for water to become low.
In addition, because the surface tension of a surfactant solution may not stabilize immediately after addition, it is important to allow the solution to stand for a fixed period before measurement.

Summary

Surface tension is related to the attractive forces between molecules at a liquid surface and is large for liquids such as water that have strong intermolecular forces.
Adding ethanol or surfactants lowers surface tension, and increasing the temperature also generally lowers surface tension.

This reference example covered the drop-count method, capillary-rise method, ring method, ethanol concentration, surfactant concentration, CMC, temperature dependence, salt concentration, contact angle, variation in measured values, contamination of apparatus, and the effect of adsorption time.
In a report, it is useful to relate the measured values to intermolecular forces and interfacial adsorption and also discuss the sources of error characteristic of each measurement method.

Relationship Between Surface Tension and Intermolecular Interactions

Surface tension is closely related to the attractive forces between liquid molecules, that is, intermolecular interactions.
The stronger the intermolecular interactions in a liquid, the greater the force pulling surface molecules into the interior, and the higher the surface tension tends to be.
Water shows relatively high surface tension because it forms hydrogen bonds.

In contrast, liquids such as alcohols and organic solvents, which have weaker intermolecular interactions than water, may have lower surface tension.
However, surface tension is affected not only by intermolecular interactions but also by molecular size, shape, polarity, and temperature.

Effect of Temperature on Surface Tension

In general, the surface tension of a liquid decreases as temperature rises.
When molecular thermal motion becomes more active because of rising temperature, the effect of intermolecular interactions pulling surface molecules inward becomes relatively weaker.
As a result, the force maintaining the liquid surface becomes smaller and the surface tension decreases.

Because surface tension is a physical quantity that is sensitive to temperature, it is important to keep the measurement temperature constant.
When comparing with literature values, it is also necessary to confirm that the values were measured under the same temperature conditions.

Discussion of a Temperature-Surface Tension Graph

When surface tension is measured at different temperatures, a graph is prepared with temperature on the horizontal axis and surface tension on the vertical axis.
For many liquids, surface tension tends to decrease as temperature rises.
If the measurement points decrease smoothly, the temperature dependence can be considered to have been observed consistently.

If some measurement points deviate from the trend, possible causes include insufficient temperature stabilization, contamination of the measuring apparatus, effects of bubbles or vibration, and reading errors.

Effect of Concentration on Surface Tension

The surface tension of a solution changes depending on the type and concentration of the solute.
Some solutes lower the surface tension of water, while others do not greatly lower it or may even increase it.
This depends on whether the solute tends to accumulate at the liquid surface or remain in the interior of the liquid.

Substances such as surfactants that readily accumulate at the surface greatly reduce surface tension as their concentration increases.
In contrast, substances such as inorganic salts that do not readily accumulate at the surface may cause only small changes in surface tension.

Discussion of a Concentration-Surface Tension Graph

When surface tension is measured at different concentrations, a graph is prepared with concentration on the horizontal axis and surface tension on the vertical axis.
In aqueous surfactant solutions, surface tension may decrease greatly in the low-concentration range and show only a small change in the high-concentration range.

In such a graph, surfactant molecules are considered to adsorb at the surface as the concentration increases, gradually filling the surface with surfactant molecules.
Once the surface is almost saturated, further increases in concentration produce only a small additional decrease in surface tension.

What Is a Surfactant?

A surfactant is a molecule that contains both a hydrophilic region and a hydrophobic region.
In an aqueous solution, the hydrophilic region readily interacts with water, while the hydrophobic region tends to avoid water.
Therefore, surfactant molecules readily accumulate at the water surface or oil-water interface and greatly change the properties of surfaces and interfaces.

When surfactants adsorb at the water surface, the strong interactions between water molecules at the surface are weakened and the surface tension decreases.
For this reason, surfactants are used in detergents, emulsifiers, dispersants, and similar applications.

Why Surfactants Lower Surface Tension

Surfactants lower surface tension because they adsorb at the liquid surface and change the state of the surface molecules.
In pure water, strong hydrogen bonding acts between water molecules at the surface, producing relatively high surface tension.
When a surfactant is added, surfactant molecules arrange themselves at the surface and weaken interactions between water molecules.

As a result, the energy required to expand the liquid surface becomes smaller and the surface tension decreases.
At low concentrations, surfactants successively adsorb at the surface, so surface tension decreases greatly as the concentration increases.

Discussion Near the Critical Micelle Concentration

In an aqueous surfactant solution, once a certain concentration is exceeded, the decrease in surface tension may become smaller and approach an almost constant value.
This is considered to occur because the liquid surface becomes nearly filled with surfactant molecules and additional surfactant molecules mainly form micelles in the interior of the solution.
This concentration is called the critical micelle concentration, or CMC.

In student experiments, a rough estimate of the CMC may be discussed from the bend in the concentration-surface tension graph or the region where the change becomes small.
However, to determine the CMC accurately, the concentration range, number of measurement points, and temperature control are important.

Effect of Alcohol Concentration on Surface Tension

Adding alcohol to water may reduce surface tension.
Alcohol molecules may preferentially accumulate at the surface compared with water and weaken the hydrogen-bonding network between water molecules.
As the alcohol concentration increases, the condition of the water surface changes and surface tension tends to become smaller.

However, the effect on surface tension differs depending on the type of alcohol.
Alcohols with longer carbon chains have greater hydrophobicity and may accumulate more readily at the surface.

Effect of Salt Concentration on Surface Tension

When an inorganic salt is dissolved in water, it may not greatly reduce surface tension, unlike a surfactant.
Ions are hydrated in water and may tend to remain in the interior of the aqueous phase rather than at the liquid surface.
As a result, they may not change the properties of the water surface as greatly as surfactants do.

Depending on the type and concentration of salt, surface tension may even increase slightly.
This is considered to occur because ion hydration and changes in the structure of water molecules alter the energy required to create a surface.

Discussion of the Capillary-Rise Method

In the capillary-rise method, surface tension is determined by measuring how high a liquid rises in a narrow tube.
A liquid rises in a capillary because of the balance among interactions between the liquid and the tube wall, the surface tension of the liquid itself, and gravity.
The greater the rise height, the higher the surface tension of the liquid is considered to be.

In the capillary-rise method, errors in capillary radius, rise height, contact angle, and liquid density affect the results.
If the tube is contaminated or the capillary is not vertical, the rise height cannot be measured correctly.

Discussion of the Drop-Count and Drop-Weight Methods

In the drop-count and drop-weight methods, surface tension is evaluated from the size or weight of a droplet immediately before it falls.
A liquid with greater surface tension does not fall until the droplet becomes larger.
Therefore, surface tension is compared by measuring the number of drops formed from a fixed volume or the mass of a single drop.

In the drop-count method, sources of error include drop rate, the way the droplet detaches, contamination at the tip of the apparatus, temperature, evaporation of the liquid, and errors in counting drops.
It is important to dispense at as constant a rate as possible and perform multiple measurements to obtain an average.

Discussion of the Ring and Plate Methods

In the ring and plate methods, surface tension is determined by measuring the force required to pull a ring or plate away from the liquid surface.
A liquid with greater surface tension requires a greater force to stretch the surface.
Therefore, surface tension can be evaluated from the pull force.

In these methods, the cleanliness of the ring or plate, the state of contact with the liquid surface, pull-up speed, vibration, temperature, and correction factors are important.
If oil or contamination adheres to the apparatus, the surface tension may change greatly.

Error Caused by Contamination of the Apparatus

Contamination of the apparatus is a major source of error in surface tension measurements.
In particular, oil, detergent, residue from the previous sample, and fingerprints can change the properties of the liquid surface and cause the surface tension to deviate from its true value.
Because even a small amount of surfactant can greatly reduce surface tension, particular care must be taken to avoid detergent residue.

Error Caused by Bubbles and Vibration

If bubbles enter during measurement, the shape of the liquid surface and the measured values may be disturbed.
In the capillary-rise method, bubbles interfere with the rise of liquid in the tube, while in the drop-count method they make droplet formation unstable.
In the ring and plate methods, vibration of the liquid surface makes force measurements unstable.

In surface tension measurements, it is important to measure under quiet and stable surface conditions.
If the sample is measured immediately after vigorous stirring or while bubbles remain, reproducibility becomes poor.

Error Caused by Temperature Control

Because surface tension changes with temperature, inadequate control of the measurement temperature causes errors.
If the temperature rises during measurement, the surface tension tends to become smaller.
Conversely, if measurement is performed at a temperature lower than that of the literature value, the surface tension may become larger.

Even when differences caused by concentration are being compared, if the temperature differs among samples, the effects of concentration and temperature become mixed.
Therefore, concentration comparisons must be performed at the same temperature.

Error Caused by Concentration Preparation

In experiments examining the relationship between concentration and surface tension, errors in preparing solution concentrations directly affect the results.
In aqueous surfactant solutions, surface tension may change greatly in the low-concentration range, so even a small concentration error may strongly affect the measured value.

Dilution operations, pipetting, alignment with the calibration mark of a volumetric flask, insufficient dissolution, and insufficient mixing can all cause concentration errors.

When the Measured Value Is Lower Than the Literature Value

If the experimentally determined surface tension is lower than the literature value, possible causes include a higher measurement temperature, residual surfactant or oil on the apparatus, contamination of the sample with impurities, and an unstable liquid surface.
In particular, even a small amount of surfactant or detergent contamination may greatly reduce surface tension.

When the Measured Value Is Higher Than the Literature Value

If the experimentally determined surface tension is higher than the literature value, possible causes include a lower measurement temperature, overestimation when reading the measuring apparatus, errors in values such as capillary radius or density, and inappropriate treatment of correction factors.
In the capillary-rise method, if the rise height is read too high, the surface tension may be overestimated.

Calculation of Error Rate

When comparing experimentally determined surface tension with a literature value, the error rate can be calculated to express the difference quantitatively.
The error rate is obtained by dividing the difference between the experimental and literature values by the literature value and expressing it as a percentage.

Error rate (%) = |Experimental value − Literature value| ÷ Literature value × 100

Simply showing the error rate is not sufficient as a discussion.
Explain which factors, such as temperature control, contamination of the apparatus, concentration preparation, reading errors, bubbles, vibration, and insufficient correction, shifted the surface tension and in which direction.

When the Results Can Be Considered Good

Surface tension measurement results can be considered good when values measured repeatedly under the same conditions agree well and theoretically reasonable trends are obtained with respect to temperature and concentration.
Examples include a decrease in surface tension as temperature rises, a decrease in surface tension as surfactant concentration increases, and no major contradiction with literature values.

Example Discussion When the Experiment Did Not Go Well

If surface tension measurement does not go well, possible causes are considered from results such as large variation in measured values, large deviations from literature values, unnatural concentration dependence, reversed temperature dependence, or an unstable liquid surface.
Organizing the causes separately into temperature control, contamination of apparatus, bubbles, concentration preparation, reading errors, and corrections specific to the measurement method makes the discussion easier.

How to Write Points for Improvement

In a discussion of surface tension measurements, including points for improvement as well as sources of error makes the report easier to organize.
Improvements are easier to write when divided into temperature control, cleaning of apparatus, sample preparation, and measurement operations.

Improvements to Temperature Control

• Use a thermostatic bath or similar equipment to keep the measurement temperature constant
• Measure only after the sample reaches the measurement temperature
• Record the temperature during measurement
• Match the temperature conditions when comparing with literature values
• Perform measurements in an environment where temperature changes are small

Improvements to Apparatus and Samples

• Clean the apparatus thoroughly
• Do not leave detergent or oil on the apparatus
• Ensure the apparatus is clean before measurement
• Avoid introducing bubbles into the sample
• Prepare solution concentrations accurately
• Mix the sample thoroughly to make the concentration uniform

Improvements to Measurement Operations

• Measure only after the liquid surface has stabilized
• Keep the drop rate or pull-up speed constant
• Keep the capillary vertical
• Perform multiple measurements and use the mean value
• Check the corrections required for the measurement method
• If an outlier appears, record the possible cause

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of surface tension measurement, simply writing that “it decreased with surfactant” or “it decreased with temperature” results in a superficial discussion.
Relating intermolecular interactions, surface adsorption, thermal motion, concentration dependence, and sources of error produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of surface tension measurements.
Adjust the necessary parts according to your own experimental results.

• Surface tension is generated because molecules at the liquid surface are pulled toward the interior of the liquid.
• Liquids with stronger intermolecular interactions tend to have greater surface tension.
• Water shows relatively high surface tension because it forms hydrogen bonds.
• As the temperature increased, molecular motion became more active and the surface tension is considered to have decreased.
• Surfactant molecules adsorb at the liquid surface and weaken interactions between water molecules, thereby reducing surface tension.
• In the low-concentration range, surface adsorption of surfactant molecules progresses, so surface tension decreased greatly.
• The reason the change in surface tension became smaller in the high-concentration range is considered to be that the surface became almost saturated with surfactant molecules.
• Because surface tension is sensitive to temperature, the measurement temperature must be kept constant.
• Even a small amount of detergent or oil remaining on the apparatus can reduce surface tension.
• Bubbles and vibrations at the liquid surface cause variation in measured values.

Points to Check When Discussing Surface Tension Measurements

Checking the following points before writing the report makes the discussion easier to write.

• Have you stated the measurement method?
• Have you recorded the measurement temperature?
• Have you written the concentration conditions correctly?
• Have you performed multiple measurements and calculated the mean?
• Have you explained the decrease in surface tension with increasing temperature?
• Have you related intermolecular interactions to surface tension?
• Have you explained surface adsorption of surfactants?
• Have you discussed changes in the concentration-surface tension graph?
• Have you explained the changes near the CMC without overinterpretation?
• Have you considered contamination of the apparatus and detergent residue as sources of error?
• Have you considered bubbles, vibration, and reading errors?
• Do the points for improvement correspond to the sources of error?

Summary

Surface tension is a property that arises because molecules at the liquid surface are pulled toward the interior of the liquid.
The stronger the intermolecular interactions, the more energy is required to expand the surface and the greater the surface tension tends to be.
Water shows relatively high surface tension because it forms hydrogen bonds.

As temperature rises, the surface tension of many liquids decreases.
This is because thermal motion of the molecules becomes more active and the relative effect of intermolecular interactions pulling surface molecules inward becomes weaker.
In addition, surfactants adsorb at the liquid surface and lower the surface tension by weakening interactions between water molecules.

In a report, do not simply write that “surface tension decreased.”
Discuss concentration, temperature, surface adsorption of surfactants, intermolecular interactions, and measurement errors in relation to one another.
Because surface tension is sensitive to contamination of the apparatus, detergent residue, bubbles, vibration of the liquid surface, and temperature changes, it is important to keep the measurement conditions consistent and confirm reproducibility by performing multiple measurements.