Why do surfactants reduce surface tension

An important tool in our toolboxes for controlling surface tension is the class of chemical compounds known as surfactants. They are pretty much universally found in cleaning agents, coatings, inks, lubricants, adhesives, and cosmetics: one important characteristic of surfactants is that they can help dissimilar substances mix like oil and water.

Surfactants are incredibly useful and beneficial Creating and maintaining chemically clean surfaces is a vital part of building reliable products in many instances. For example, automobile engine blocks are assembled with silicone sealants. If the surfaces are not chemically clean before applying the sealant, the engine will leak oil onto your driveway.

Chemical cleanliness of sealing surfaces can be achieved through carefully monitored and controlled aqueous cleaning processes. These processes depend on surfactants to make contaminants soluble in the cleaning solution.

However, if the surfactants aren't completely removed via a rinsing step, the end effect is that we have just replaced the original soils and contaminants with a new one that will also inhibit sealant adhesion: residual surfactant.

Sometimes a chemically clean surface is all we need for manufacturing success. However, with many applications, we need to further engineer the surface through surface treatment to obtain desirable properties like corrosion resistance or strong and durable adhesion of coatings or adhesives.

The presence of residual surfactants on these surfaces can prevent successful surface treatment. For inks, paints and adhesives to work properly they have to spread over the surface that they're applied to.

This requires that the surface tension of the ink, paint or adhesive is lower than the surface energy of the substrate. This is just another way of saying that the molecules in the paint, adhesive or ink need to be attracted more strongly to the surface than they are to each other. We can control the surface tension of a coating or adhesive by the addition of surfactants. Surfactants are directly related to surface tension and play a very important role in adhesion processes.

But, how do they work and how can manufacturers make sure surfactants are doing their job without disrupting the work of creating chemically clean surfaces? By looking at what surfactants are intended for, their effect on surface tension and how to recognize when they are on our material surfaces, we can make use of these powerful manufacturing tools to create clean surfaces that can guarantee high-performing products.

A surfactant, at its most basic, is a substance that is designed to reduce the surface tension of a liquid. For many operations in manufacturing processes, it is necessary for a liquid to spread out and wet a surface.

Adding a surfactant to a coating or detergent lowers the surface tension of the liquid so it will flow more, covering the entirety of the surface.

For instance, surfactants are often added to insecticides to ensure the substance fully spreads out over the entire surface of leaves instead of just a small portion of the plant. This allows the insecticide to be maximally effective. As a reminder, surface tension is the attractive force of the molecules present at the surface of a liquid towards each other. Modifications to the surface tension of a liquid is one factor that helps determine the performance of a bond between a solid surface and a liquid.

To ensure a strong bond, the liquid and the solid surface must be chemically compatible. 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. Recent advances in understanding dynamic surface tensions DSTs of surfactant solns. For pre-CMC solns. For micellar solns. The dynamic surface tension of atmospheric aerosol surfactants reveals new aspects of cloud activation.

Nature communications , 5 , ISSN:. The activation of aerosol particles into cloud droplets in the Earth's atmosphere is both a key process for the climate budget and a main source of uncertainty. In addition, the surfactant fraction of atmospheric aerosols could not be isolated until recently. Here we present the first dynamic investigation of the total surfactant fraction of atmospheric aerosols, evidencing adsorption barriers that limit their gradient partitioning in particles and should enhance their cloud-forming efficiency compared with current models.

The results also show that the equilibration time of surfactants in sub-micron atmospheric particles should be beyond the detection of most on-line instruments. Such instrumental and theoretical shortcomings would be consistent with atmospheric and laboratory observations and could have limited the understanding of cloud activation until now.

Surfactant adsorption onto interfaces: Measuring the surface excess in time. Langmuir , 28 , — , DOI: American Chemical Society. We propose a direct method to measure the equil. CMC using a pendant drop tensiometer. We studied solns. The variation of the surface tension as a function of surface concn. The time-dependent surface concn. The diffusion coeffs. Dynamic surface tension of aqueous surfactant solutions.

Experiments and simulations of ion-enhanced interfacial chemistry on aqueous NaCl aerosols. Science , , — , DOI: Knipping, E. Science Washington, D. American Association for the Advancement of Science. A combination of exptl. NaCl particles suspended in air at room temp. Measurements of the obsd. Model extrapolation to the marine boundary layer yields daytime chlorine atom concns. Thus, ion-enhanced interactions with gases at aq. Science , , , DOI: Ghosal, Sutapa; Hemminger, John C. Frank; Requejo, Felix G.

It has been suggested that enhanced anion concns. The authors report ionic concns. Using XPS operating at near ambient pressure, the compn. In both cases, the surface compn.

The enhancement of anion concn. By varying photoelectron kinetic energies, depth profiles of the liq. Results are in good qual. Salt effects on intramicellar interactions and micellization of nonionic surfactants in aqueous solutions. Langmuir , 10 , — , DOI: The theor. The exptl. This, in turn, results in both a lowering of cmcs and surface tensions of the CiEj aq. The effect of NaCl on the Krafft temperature and related behavior of cetyltrimethylammonium bromide in aqueous solution.

Surfactants Deterg. This paper presents the effect of NaCl on the Krafft temp. The crit. Thus, the CMC-temp. The micellar dissocn. The processes were found to be both enthalpy and entropy controlled and appeared to be more and more enthalpy driven with increasing temp.

An enthalpy-entropy compensation rule was obsd. The TK of the surfactant decreased significantly in the presence of NaCl, which is a sharp contrast to the usual behavior of the effect of electrolytes on the TK of classical ionic surfactants. The surface excess concns. However, the values were much higher in the presence of NaCl compared to the corresponding values in pure water.

The solubilization behavior of a water-insol. The molar solubilization ratio in the presence of NaCl was found to be about three times higher than that in pure water, indicating that the solubilization of SRB in the CTAB micelles significantly increases in the presence of NaCl.

All the salts used produced shifts of the crit. CMC to lower concns. No appreciable difference was observed when air was substituted for liq. It appeared that shifts of CMC were related to the valency of the gegenion, a divalent gegenion would produce a shift much greater than a monovalent gegenion. Cationic gegenions were more effective in lowering the CMC of Na lauryl sulfate than similar anions while anionic gegenions were effective with cetrimide and cetylpyridinium chloride.

The CMC of cetomacrogol was practically unaffected by the addn. The effect of sodium chloride on the dynamic surface tension of sodium dodecyl sulfate solutions. The dynamic surface tension of SDS solns. The surface tension of various concns. CMC detd. In the presence of 0. The surface tension vs. For the salt-contg. SDS concn.

In the salinized SDS concns. Langmuir , 13 , — , DOI: DSC was employed for the detn. 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.

Surfactants in hand soap can break down the lipid membranes surrounding bacteria and enveloped viruses like those causing COVID and the flu, simultaneously destroying the microbes while facilitating their removal from your hands. Sign up for our newsletter Receive monthly updates on all the latest from Dispersa and clean tech.