How do surfactants reduce surface tension

Find more information on the Altmetric Attention Score and how the score is calculated. We study the influence of high NaCl concentrations on the equilibrium and dynamic surface tensions of ionic CTAB and nonionic Tween 80 surfactant solutions.

Dynamic surface tension measurements allow comparing the surface tension as a function of time for pure surfactant solutions and in the presence of NaCl. For the ionic surfactant, the dynamics agree with the usual diffusion-limited interfacial adsorption kinetics; however, the kinetics become orders of magnitude slower when NaCl is present.

Sum-frequency generation spectroscopy experiments and the equilibrium adsorption measurements show that the presence of NaCl in CTAB solution leads to the formation of ion pairs at the surface, thereby neutralizing the charge of the head group of CTAB.

This change, however, is not able to account for the slowing down of adsorption dynamics; we find that it is rather the decreases in the monomer concentration CMC in the presence of salt which has the major influence. For the nonionic surfactant, the kinetics of interfacial tension is seen to be already very slow, and the addition of salt does not influence it further.

This also correlates very well to the very low CMC of Tween Figure 1. Filled symbols correspond to binary aqueous solutions, and open symbols correspond to ternary solution with 5. Black squares: water; red circles: 0. Figure 2. Figure 3. Figure 4. Surface tension as a function of time for different concentrations of CTAB measured by the maximum bubble pressure method.

Figure 5. Characteristic time determined from DST plots Figure 4 as a function of surfactant concentration. Figure 6. Figure 7. Characteristic time as a function of CTAB concentration with 5. Figure 8. Kinetic SFG spectra of a 0. Figure 9. Figure Characteristic time determined from the DST data of Figure 10 as a function of Tween 80 concentration. This lends one more confirmation to the argument that the concentration of monomers determines the DST trends despite the presence of large quantities of electrolyte impurities.

More by Mohsin J. More by Simon J. More by Ellen H. More by Mischa Bonn. More by Daniel Bonn. More by Noushine Shahidzadeh. Cite this: Langmuir , 36 , 27 , — Article Views Altmetric -. Citations Abstract High Resolution Image. Surface-active agents surfactants are one of the most commonly used compounds in both daily life as well as in diverse industrial processes. The ability of surfactants to partition at the interface makes them ideal candidates for influencing the surface properties even in very small concentrations.

In principle, immediately after the instantaneous creation of a new surface in a surfactant solution, the surface tension has the same value as that of the pure liquid. Subsequently, the adsorption of surfactant in time at the interface leads to the decrease of the surface tension of the liquid until the equilibrium value in the presence of surfactants is reached.

The dynamic surface tension DST is therefore controlled on the one hand by the nature of the surfactant, for example, chain length, size, and charge of the polar head, and on the other hand by the nature of the liquid and the presence of other additives.

In many applications, salts are present or added as an extra additive to enhance the performance of surfactants. Beginning in , Milner first suggested that the kinetics of adsorption of a specific surfactant Sodium oleate to the interface was limited by diffusion in the bulk of the solution.

Reviewing DST data for different surfactants, most studies to date report the adsorption to be controlled by diffusion. In this study, we investigate both the equilibrium and dynamic behavior of the surfactants in the presence of sodium chloride at concentrations ranging from very low to very high, that is, the solubility limit of NaCl.

We find that the kinetics of the ionic surfactant are rather fast compared to the nonionic one and both agree with the above mentioned notion of diffusion-limited adsorption. However, the dynamics of surface tension equilibration slows down dramatically for the ionic surfactant in the presence of a high concentration of salt. Experimental Section. The solutions were stirred and left to stabilize overnight.

Densities of both binary and ternary solutions were measured by weighing a precise volume of the solution. We observed that the density of the surfactant solution remains the same as water 0. For the ternary solutions, that is, surfactant—salt solutions, the density of the solution was the same as the salt solution at this particular concentration [e.

Viscosities of both binary and ternary systems were measured using an Anton Paar MCR rheometer equipped with a 50 mm 1-degree cone-plate geometry. Moreover, the viscosities remained constant over the range of shear rate measured Figure 1 , showing that the addition of salts to these surfactant solutions does not affect the shape of micelles in the solution, as has been reported in studies on other surfactants or under different conditions.

High Resolution Image. When working with ternary surfactant—salt solutions, the dilution was achieved by using salt solutions, at the same salt concentration, as the ternary solution in order to keep the salt concentration constant throughout the measurement. For DST measurements, depending upon the time scale of the surface tension decay, two different instruments were used. For pure surfactant solutions, with fast decay times of the surface tension of the order of tens of milliseconds , the measurements were done by using a Kruss Maximum Bubble pressure BP50 tensiometer.

The maximum bubble pressure method has been widely used for measuring DST of surface tension. Unlike earlier versions, the latest model of BP50 controls the surface age instead of bubble frequency, thereby avoiding hydrodynamic errors and errors arising from dead time. For longer time-scale dynamics, a Kruss Easy Drop tensiometer based on the pendant drop method was used. The latter measures the surface tension by fitting the shape of the droplet to the Laplace equation, balancing interfacial tension and gravity.

The adsorption of surfactants at the interface results in a change of the shape of the droplet in time from which surface tension can be deduced as a function of time. The time interval between each measurement was set to 1 s, and measurements were taken until the surface tension reaches its equilibrium value.

In order to prevent the evaporation of droplet and hence concentration changes, the measurements were carried out in a controlled humidity chamber, in which the relative humidity was maintained at the equilibrium relative humidity above the solution throughout the measurement. Results and Discussion. These observations qualitatively agree with the earlier findings reported in the literature 36,37 and at the same time extends these to much higher salt concentrations. The decrease of the CMC in the presence of electrolytes can be explained by the fact that the salt anions screen the electrostatic repulsion between the cationic head groups of the CTAB molecules, thereby facilitating their aggregation into micelles.

With the addition of salt, fewer monomers will thus be present in the solution in equilibrium with micelles. This suggests that the maximum charge screening by the counterions is already reached, and the addition of more salt to the solution does not change the situation very much. The results are plotted in Figure 2 b. The results are depicted in Figure 2 b in green and red squares.

The data indicate that in the presence of salt, the surface excess concentration decreases by a factor of 2.

This seems unlikely, because as mentioned before, the main action of the salt should be to screen electrostatic interactions between the charged head groups, enabling the surfactants to pack more efficiently at the surface. As such, we conclude a similar packing of molecules at the surface with and without salt is present because of the charge being screened by the counterions that are then bound to the surfactant. To prove that the surfactant behavior at the interface is not changing significantly upon adding salt, we performed sum-frequency generation SFG spectroscopy.

In SFG, a broadband infrared laser beam exciting molecular vibrations and a narrow band near-visible laser beam are overlapped in space and time at the interface. Because of the selection rule of the method, this process is forbidden in centrosymmetric media like bulk water.

If the infrared light is in resonance with a molecular vibration, the sum-frequency signal is strongly enhanced. As such, the vibrational spectrum of only interfacial molecules is obtained. The result is depicted in Figure 3 for a 0. The peaks below cm —1 are C—H stretch vibrations and hence serve as signatures for the presence of CTAB at the interface. The signal above cm —1 originates from the water O—H stretch vibrations near the interface.

The charge of the surfactant aligns the water molecules resulting in a large symmetry breaking and thus a relatively large O—H stretch signal. Upon adding salt, the water signal diminishes roughly by a factor of Please note the different y -axis scale, as shown in Figure 3. This strong reduction clearly demonstrates that the effective surface charge has been reduced because of screening of the charge by Cl — ions. As a result, the water molecules are less aligned.

The C—H signals seem also to reduce. However, because of interference with the water signal, it is impossible to draw conclusions about the C—H signals without analyzing the data quantitatively. The black lines in Figure 3 a,b are fit with the Lorentzian lineshape model. The structure of the surfactant layer is apparently not changing significantly upon adding salt, consistent with the conclusion drawn above assuming ion-pair formation for CTAB.

The data obtained are presented in Figure 4. It can be seen that at higher surfactant concentrations, the equilibrium surface tension is reached faster. In Figure 5 , the characteristic time is plotted against the concentration of CTAB for a range of concentrations, which fall well above and below the CMC.

However, as the concentration reaches above CMC, there is very little change in the characteristic time Figure 5. This confirms that the adsorption is greatly affected by the concentration of surfactant monomers, and that the presence of micelles makes no difference in the adsorption dynamics. This is consistent with the theory of Ward and Tordai, which takes only monomer concentration into consideration while correlating it with the characteristic adsorption time.

Moreover, for concentration further above CMC, the slope does not remain quadratic anymore, implying that the higher concentration of ionic micelles leads to deviation of adsorption mechanism from purely monomer diffusion controlled to adsorption barrier or mixed diffusion—adsorption barrier controlled. Earlier studies by Ritacco et al. We proceed to investigate the influence of high salt concentration on the adsorption mechanism of the ionic surfactant CTAB.

Although keeping the concentration of NaCl constant and changing the surfactant concentration, we again observe that the equilibration time decreases with increasing the surfactant concentration Figure 6. However, the dynamics become very slow compared to what was observed in pure surfactant solutions.

The dynamics changes from the time scale of milliseconds to tens of seconds, as shown in Figure 6. An important point which needs to be noted here is that we could measure only the concentrations above the CMC. The reason being that the salts decreased the CMC to very low concentrations and also increased the equilibration time, which eventually makes DST measurements for pre-CMCs unfeasible.

Other factors could be that the high concentration of salt changes the diffusion constant or changes the properties of the surfactant itself. For the former, the high concentration of salt changes the viscosity by roughly a factor of 2 Figure 1 , which is too small to account for the observed change of more than an order of magnitude.

For the latter, even if the surfactant forms ion pairs with the added salt, this should not affect the dynamics very significantly either. If anything, the charge neutralization by ion-pair formation rules out the possibility of any electrostatic barrier, which would slow down the adsorption.

As far as the change in the surfactant monomers is concerned, the addition of salt leads to 2 orders of magnitude decrease in CMC, which in turn means that the concentration of monomers drops drastically in the presence of salt.

The Ward and Tordai model accurately describes the characteristic time in terms of the monomer concentration, suggesting that the dramatic increase in the characteristic time is simply because of the lowering of the CMC. The linear instead of quadratic dependence of characteristic time, as shown in Figure 7 , is likely due to the fact that all these concentrations are above CMC, and hence, the micelles rather than monomers are playing a central role in determining the adsorption dynamics, similar to the regime change that was seen above CMC in pure CTAB solutions Figure 5.

In a study by Song and Yuan 42 using fluorescence microscopy, the transport of micelles from the bulk to interface and their demicellization in the subsurface was visualized. They suggested a combined influence of micellar diffusion and monomer adsorption on determining the overall adsorption kinetics.

With the addition of salt, we have abundance of micelles, and hence, their diffusion is likely to play a major role.

We will show below, after having discussed the results for Tween 80, that the Ward and Tordai model can quantitatively explain our data. We confirm the transfer of CTAB molecules from the bulk to the interface as a function of time by taking kinetic SFG spectra with a time interval of 1 min. The signals below cm —1 represent the CTAB molecules at the interface. The results for pure 0. However, the results for 0. This confirms that the slower dynamic surface tension is directly correlated to the concentration of CTAB molecules at the interface.

Now in order to isolate the effect of polar heads of the cationic surfactant CTAB on the presence of salt, we investigate the influence of salts on the CMC of the nonionic surfactant Tween In the same way as CTAB, the equilibrium surface tension as a function of Tween 80 concentration is measured in the pure Tween 80 solution as well as with the addition of different concentrations of NaCl.

In literature, it has been reported that, like ionic surfactants, salts have depreciating influence on the CMC of nonionic surfactants also. In view of the slow surface tension decay with time, the pendant drop method was used to measure the time-dependent surface tension. The results, as shown in Figure 10 a, show that the characteristic time decreases with the increase in the concentration of Tween 80, However, the time scale, in which the surface tension reaches to equilibrium, is of the order of tens of seconds in contrast the millisecond time scale as in the case of CTAB.

Moreover, the addition of salt to the solution Figure 10 b does not substantially affect the time scale of reaching the equilibrium value. In order to determine the mechanism of adsorption of the surfactant molecules to the interface, the characteristic time was determined for both binary and ternary solutions, as described previously, and plotted as a function of the concentration of Tween 80 Figure The addition of salt does not affect this behavior.

Although the DST data agree with the Ward and Tordai model and confirm that the adsorption is diffusion controlled, the question remains why the process is so slow. For this, we once again refer to the earlier explanation considering the surfactant monomer concentrations.

As is clear from Figure 9 , the CMC of Tween 80 is very low compared to most common surfactants, which means that the monomer concentration stops to increase at a very low concentration.

Hence, as in case of CTAB with NaCl, the small number of monomers present at a certain time is responsible for the slow adsorption of Tween The applicability of Ward and Tordai equation, even in the micellar concentrations, suggests that Tween 80 micelles have negligible influence on the adsorption kinetics. Because of neutral nature, the micelles do not cause any electrostatic effects, and micelle diffusion and dissociation equilibria do not seem to be playing a significant enough role either.

Moreover, in cases of Tween 80, the addition of salt does not change the CMC and thus also not the concentration of monomers. As a result, no change in the adsorption dynamics upon addition of salt Figure 10 b is observed. Noskov et al. They reported that addition of salt made the adsorption kinetics faster, and the adsorption rate was proportional to the salt concentration.

We do not see any enhancement of adsorption rate by adding salt, and the reason could be that in case of polymer chains, the salts are affecting the chain configuration, which is not the case in Tween In order to get a perspective of the very low monomer concentration and its influence on DST, we plot the characteristic time for the concentrations closest to the CMC along with the characteristic times for CTAB plotted in Figure 5. This confirms that it is actually the reduction in the number of monomer molecules causing the slow dynamics.

Hence, the mechanism, in which the salt slows down the adsorption, is by favoring the micellization, which leads to decrease in the monomer concentration. The effect of salt sodium chloride with concentrations up to 5. In case of equilibrium surface tension, we show that the salt affects the CMC and the equilibrium surface tension only in case of an ionic surfactant.

Consequently, with the addition of salt, much fewer monomers will be present in the solution in equilibrium with micelles. Although from the data, it appears that NaCl decreases the surface excess concentration of CTAB, the precise factor of 2 decrease suggests that it can be the result of ion-pair formation, which makes the ionic surfactant in the presence of salt behave more like a nonionic surfactant.

Our SFG results confirm this hypothesis by showing that the surface concentration of CTAB does not change significantly upon the addition of salt. From the DST data of pure ionic surfactant solutions, we show, by using the Ward and Tordai model, that the adsorption kinetics is controlled by a diffusion mechanism, and that the rate depends specifically on the concentration of monomers in bulk.

We show that the addition of salt slows down the adsorption dynamics very strongly. As soon as the concentration exceeds the CMC, its effect on the DST becomes much smaller, indicating that the dynamics of the micelles do not contribute very much to the DST.

The existence of an adsorption barrier has previously been proposed as a possible rate-limiting step for the adsorption of surfactants; however, its origin remains debated. There have been various propositions, including the electrostatic repulsion between surfactants, or between interfacial water molecules and the surfactant molecules, the orientation of molecules before adsorbing to the interface, and steric repulsion by molecules already adsorbed at the interface.

This rules out the possibility that an electrostatic barrier is playing a role in slowing down the monomer adsorption to the interface. The equilibrium surface tension data rather show that the CMC decreases significantly, which effectively decreases the number of monomer molecules. This depletion in the number of molecules eventually makes the dynamics slow. For nonionic surfactant Tween 80, the rate of adsorption is already very slow, and the addition of salt does not change the time scale.

Author Information. Mohsin J. Simon J. Ellen H. The authors declare no competing financial interest. Surfactants and interfacial phenomena ; Wiley , ; pp 1 — 4. Surfactants: Fundamentals and applications in the petroleum industry. Dynamic surface tension and adsorption mechanisms of surfactants at the air - water interface.

Colloid Interface Sci. Elsevier Science B. A review, with refs. This review article Falk , J Surfactants Deterg discusses surfactants as antimicrobials in much greater detail. Viruses are diverse infectious particles that can only replicate inside of a cell. They fall into one of two broad categories:. Anti-viral capacity depends on the type of surfactant, as well as the type of virus. Surfactants can break down enveloped viruses in much the same way as cell walls are broken down — by attacking and breaking down the lipid membrane that surrounds and protects the virus.

Non-enveloped viruses can be more difficult to inactivate due to the stable protein shell capsid , some surfactants are capable of destroying the protein capsid as well.

Regardless, particularly in the case of hand-washing, viral inactivation is not the only way to rid yourself of viruses — the combination of surfactant activity and mechanical agitation such rubbing hands together helps lift viruses from surfaces so they can be easily removed with water. This video summarizes how soap or more accurately, the surfactants in soap break down viruses such as COVID Surfactants are a fascinating group of molecules that play an important role across many areas of industry and our personal lives.

Though varying widely in chemical properties, safety, and capabilities, the basic principles of how surfactants work remain the same. What are surfactants? Digram of a generalized surfactant molecule. Surfactants align at the water-air interface, reducing surface tension. The critical micelle concentration CMC is the concentration at which a surfactant begins to form micelles. Surfactants form micelles around oil-soluble particles, breaking it down and facilitating either emulsion or cleaning depending on the application.

Typically, polar solvents have higher surface tension than their non-polar counterparts. For example, the high surface tension of water The weaker the intermolecular forces are, the lower the surface tension is e. Surface tension influences properties in a system. The smaller the dimensions of a system are, the more critical surface tension becomes.

Surface tension is relevant at larger scales as well, particularly in wetting, adhesion, and meniscus shape. When the dimensions are in the micro- to millimetre range, dynamics are given by a balance of viscosity, density, and surface tension. Additionally, surface tension is an easily measured parameter for quality control.

Surfactants are molecules that enrich at surfaces or interfaces, i. In general, surfactants contain hydrophilic and hydrophobic moieties, and hence in aqueous solutions water contact with the hydrophobic parts can be avoided by arranging at the surface with the hydrophobic parts towards the air. Surfactant adsorption lowers the surface tension. This phenomenon is frequently used to improve wetting.

Click the image below to read more on adsorption and surface tension. Surfactants display another important property, namely self-assembly into micelles. Micellation resembles phase formation, in that many surfactants form one micelle. Thus, micellation occurs over a fairly narrow concentration range critical micelle concentration, CMC , and further addition of surfactant increases the micelle concentration rather than the concentration of surfactant monomers.

Consequently, surface tension decreases with increasing total surfactant concentration up to CMC, and then remains constant.