Surface Tension Tutorial

Dynamic Surface Tension Measurement Tutorial
 DYNAMIC FINGERPRINTS OF FORMULATIONS USING SURFACE TENSION MEASUREMENT
 
INTRODUCTION 
A unique dynamic fingerprint of any formulation can be generated in terms of surface tension versus surface age, from zero time to several minutes, using a differential maximum bubble pressure technique. This allows formulators and users to characterize formulation performance under all possible conditions before it is used. When multiple surfactants are used, the unique surface tension fingerprint may not be a “classical” dynamic curve, as different surfactants may diffuse to the interface at different rates, resulting in a complex dynamic curve with several peaks and valleys. In some cases, the lowest surface tension formulation may not be the best one to use because it can cause other problems. 
 
THE MOLECULAR DYNAMIC SURFACE TENSION FINGERPRINT
Each fluid or formulation has a unique molecular fingerprint that is the tangential intermolecular force that keeps the fluid or formulation together, expressed in Dynes/Centimeter (or milliNewtons/Meter). This parameter is the Fluid Surface Tension; the force that keeps the fluid together at any air/fluid interface. It is the intermolecular force of attraction between adjacent molecules, expressed in force per unit width. Water, at ambient temperature, has a high surface tension in the range of 72 Dynes/Cm. while alcohols are at a much lower range of 20 to 22 Dynes/Cm. Solvents, typically, are in the 20 to 30 Dynes/Cm. range. If a simple pure fluid, or very complex formulation, changes at the molecular level then the fluid surface tension will change. We just have to measure it to see how the fingerprint has changed.

Pure fluids and pure solvents have a single surface tension value called “static” or “equilibrium” surface tension. If the formulation contains surfactants, then it will have a “dynamic” surface tension. This dynamic surface tension varies as the surface age varies; essentially a dynamic “fingerprint” of surface tension versus surface age; surface age being the amount of time the process allows surfactant molecules in solution to migrate to any newly created air/fluid interface, where their purpose is to lower the surface tension.  

If anything contaminates the fluid in question then the molecular fingerprint will change, since the intermolecular force at the air/fluid interface will change. Since we are able to measure this molecular fingerprint, we can therefore use it to measure, in real time, whether formulation changes are taking place and to what degree.

THE IMPORTANCE OF DYNAMIC SURFACE TENSION IN FORMULATIONS
Surface tension dictates whether a formulation will wet and spread over, or retract from, a solid substrate. Any formulation exhibits both an adhesive force that is a measurement of the degree of association of the formulation for the substrate, and a cohesive force that is a measure of the degree of self-adhesion of the formulation. Spreading coefficient is the difference between work of adhesion and work of cohesion. If work of adhesion is greater than work of cohesion then spontaneous spreading occurs. If work of cohesion is greater than work of adhesion, then retraction occurs as the formulation will preferentially associate with itself.

It is quite useful, and often very essential, that formulators have a good understanding of the principles and relationships of surface tension, and surface energy or “wetting tension”. They must control fluid transfer and adhesion by developing formulations that perform well under widely varying application speeds and operating conditions. There are factors that can be controlled, and others that cannot be controlled. A review of these important principals and relationships, with a brief introduction to formulation variables, will reinforce the knowledge that it takes to mitigate or entirely avoid problems.

Solvent-based formulations, with their naturally lower surface tensions, wet readily and transfer well onto most substrates, even though they may have higher coefficients of spreading on select substrates. The increasing shift to water-based formulations, due to environmental concerns, has inherent problems of surface wetting, foaming, flow and leveling, common to all water-based systems. Water-based formulations require alcohols and surfactants to lower their surface tensions to acceptable levels for transfer, spreading and adhesion.  

Alcohols have low surface tensions and are “non-active”, but most common surfactants used in water-based systems generally have high surface activity that varies with changes in concentration, molecular weight, and molecular structure. Spraying, spreading, dipping or applying in any other manner a fluid on a substrate is a dynamic process, and active surfactants cause surface tensions to change as speeds and formulations change. Surfactant activity directly impacts coating, spreading, and adhesion. Surface tension of formulations must be lower than the wetting tension of substrates to attain good wetting, printability, adhesive bondability, or formulation lay down.  

FORMULATION CHANGES
Formulation and application of water-based solutions are influenced somewhat by controllable, external factors, such as shipping and storage, freezing and thawing, both before and after each process takes place. The physical properties of handling and mixing systems, to a large degree, are fixed and uncontrollable. Many facilities suffer the uncontrollable effects of seasonal changes; hot summer versus cold winter environments, for example. Processed solutions can and often do change at the formulation site, and change even more just prior to, and during, application. What is often formulated and tested in the laboratory is not what is ultimately used during application. Alcohols and reagents are lost to evaporation, and these losses can go undetected unless testing can be done at formulation and process sites. 

When any formulation is applied, surfactant molecules in the bulk solution react by diffusing to the interface. It is at this interface that the surfactant molecules try to line up with their hydrophobic ends pointed at the interface and the surface tension is lowered. The concentration and nature of the surfactant determines by how much the surface tension can be reduced, but the speed at which diffusion takes place influences how fast wetting will occur. Wettability, and ultimately “coatability”, is influenced by the combined physical and chemical properties of the constituents. It is this area of controllable factors that we briefly examine, with emphasis on techniques used to measure the dynamic surface tension of formulations. 

SURFACTANTS
Surfactants are utilized and found in many fluid formulations because of their ability to reduce the surface tension in water-based and solvent-based systems. In some industrial applications they are even added for emissions reduction purposes, as defoaming agents to mitigate hazardous degassing. In many industrial cleaning formulations, for example, surfactants are added since they boost cleaning effectiveness and aid in wetting and defoaming. In inks, fountain solutions, adhesives, and coatings, surfactants are added to lower the surface tension below the surface energy of the substrate to be coated in order to obtain a positive spreading coefficient that enhances wetting, spreading and adhesion.  

At the instant a formulation is deposited on a substrate, at zero time of new surface generation, the concentration of surface active molecules at the interface will be the same as in the bulk solution; equal to the surface tension of the pure fluid or fluid combination. The surfactant molecules then begin to diffuse to, and adsorb at, the newly created fluid/ substrate interface and the fluid/air interface. It takes a finite amount of time for the surface tension to reach equilibrium, anywhere from several seconds to several minutes. This is why the relevant parameter in designing formulations is dynamic surface tension rather than static (also know as equilibrium) surface tension.  

Surfactants are classified by the ionic charge of the surface acting part of the molecule. Anionic surfactants have a negative molecular charge, cationics positive, and nonionics no charge. In amthoterics there are both positive and negative charges. Anionics and nonionic surfactants provide most of industrial surfactant requirements and are the most common. Selection of surfactants is based on specific needs and often mixed surfactants are used which result in the afore-mentioned “complex” dynamic surface tension fingerprints.   

In general, surfactants with a smaller (lighter) molecule mass (short hydrophobic tail) diffuse more rapidly to any air/fluid interface, where they are vertically adsorbed at the interface, causing a compressive force to act on the surface, thereby reducing surface tension. Most surfactants at higher concentrations exert strong molecular attractions between adjacent molecules causing strong surface films, the strength of which determines the surface properties of the surfactant solutions. Nonionic surfactants with ethylene oxide groups usually diffuse very rapidly to the surface while fluorinated surfactants are slower and more effective at equilibrium. 

For example, surfactants that diffuse slowly may not lower surface tensions sufficiently, within dynamic coating time constraints to acceptable levels and may be partially responsible for defects such as: bénard cells (hexagonal cells with well marked centers produced by circulation patterns in thin films); craters (bowl-shaped depressions) or pin holes; crawling or retraction (de-wetting of the applied coating); floating (mottled, blotchy, or streaked appearance); orange peel (surface bumpiness); and picture framing (edge buildup). Both rheology and surface tension modifications are used to reduce or eliminate surface defects. Increasing the viscosity can eliminate retraction, for example, but if a viscosity increase is not viable, then the formulator may have to address defect reduction through surfactant modification. 

A rapidly diffusing surfactant can mitigate surface defects, by eliminating surface tension gradients. This occurs through rapid surfactant migration from high concentrations (low surface tension) to low concentrations (high surface tension). Formulators will sometimes mistakenly increase surfactant concentration in order to reduce gradients, rather than use a better surfactant. This can result in higher surfactant costs and other problems. A surfactant having both low equilibrium and dynamic surface tension in water may not necessarily have the same characteristics in a highly formulated system. Extra surfactant can become “tightly bound” with polymeric binders, or can be solubalized with pigment micelles, becoming ineffective in producing further surface tension reduction. 

Some classes of surfactants have rapid diffusion characteristics because of  unique polar molecular structures. This type of surfactant is capable of reducing surface tension with the extra benefit of reducing or preventing foam. With a hydrophobe-hydrophile-hydrophobe structure which displaces materials that form solid structural films at the interface, they make excellent wetting agents or surface tension reducers. These acetylenic diol-based surfactants have highly-branched alkyl groups and are horizontally, rather than vertically, adsorbed at the air/liquid interface. At low concentrations, these molecules cover large areas, but can be squeezed together as the concentration increases, exhibiting a unique compressible nature. The centrally located hydrophile gives the molecule a flat surface orientation, while the hydrophobic chain groups minimize intermolecular attraction which contributes to low-foaming/defoaming capability. Disadvantage of a surfactant that only lowers surface tension is that separate defoamers may have to be added, which can themselves contribute to surface defects. A formulator should try to use a suitable surfactant that ideally has both low equilibrium and low dynamic surface tension values; low enough so that the coating is applied to the substrate at process speeds with a desirable viscosity. It is ideal if the surfactant used can perform more than a single function.  

DYNAMIC SURFACE TENSION MEASUREMENT
Independent studies[1] have shown that surface tension measurements made using classical du Noüy Ring, Wilhelmy Plate or Capillary Drop equipment are not very precise, subject to considerable operator measurement technique and error, and require tedious mechanical adjustments and calibrations. These methods are also subject to error from particles and oily impurities in solution. Since surfactants facilitate wetting of substrates, they also facilitate final surface cleaning by emulsifying residual oil and grease on substrates or the walls of fluid containing vessels, which usually end up at the fluid surface. Surface oil and grease make surface tension measurements with static tensiometers very difficult, if not impossible. The patented[2,3] Maximum Differential Bubble Pressure method, on the other hand, is not affected by surface foam or surface contamination because measurements are made in the body of the fluid.  

Two probes of dissimilar orifice sizes (most commonly 0.5 mm and 4.0 mm) bubble into a fluid where the differential pressure value of the formed bubbles is measured. This value is directly proportional to the fluid surface tension. Since the method allows continuous bubbling, it also allows continuous in-process measurement. While classical methods measure only equilibrium (static) surface tension, Maximum Bubble Pressure Tensiometers can measure both equilibrium and dynamic surface tension, since the user can choose the rate at which the bubble forms. This determines the aforementioned Surface Age; the amount of time during which surfactant molecules can migrate to the gas/fluid interface. Additionally, by varying the bubble rate and therefore the surface age in a pre-selected sequence, a complete dynamic curve can be generated either manually using the QC-Series Tensiometers or automatically and automatically repeated at user-programmed time intervals if using the PC500-Series. 

Differential Bubble Pressure Tensiometers are calibrated using two fluid standards of known surface tension values, such as deionized water and alcohol. Immersing the probes in the middle of a sample is the extent of expertise demanded of the user. This method complies with ASTM test method D 3825-90[4]. These tensiometers can also be used to measure surface tension under pressures up to 250 PSI (1700 kPa). 

CLASSIC DYNAMIC SURFACE TENSION CURVES
Classic dynamic surface tension are “classic” from the standpoint that the dynamic zone for each fluid is a relatively smooth and decreasing curve; that if extrapolated to “zero time” (zero surface age), would give the highest surface tension possible for this formulation, as if it contained no surfactant. It decreases steadily into the equilibrium zone as the surface age is increased, allowing more and more surfactant molecules to migrate to the interface where the surface tension is lowered. When graphed, the surface tension changes as the bubble interval and (more significantly) the surface age changes for each fluid within it’s respective dynamic zone. Equilibrium is reached when the surface tension reaches its lowest attainable value for each fluid. This occurs when either all of the available surfactant molecules have reached the air/fluid interface, or no more surfactant molecules will fit at the interface. Equilibrium surface tension values depend on the type of surfactant used and the surfactant concentration.  
Figure 1
Fluids and formulations can also be fingerprinted in three dimensions as a function of Dynamic Surface Tension, Surfactant or Additive Concentration, and Surface Age (surfactant diffusion time). This is done by combining a dynamic tensiometer with an automatic dispensing system and software with three-dimensional graphing capabilities, and performing a series of surfactant concentration tests as varying surface age rates[5]. Surfactant users and formulators now can easily determine surfactant operating and efficiency limits, and improve wetting, transfer, spreading, and adhesion properties by judiciously choosing more effective surfactant and additive combinations.
 
DYNAMIC CRITICAL MICELLE CONCENTRATION (CMC)
Many surface tension related properties such as detergency, foaming, emulsification, dispersion, and wetting are believed to either maximize or minimize at the surfactant Critical Micelle Concentration (CMC); the surfactant concentration beyond which surfactant molecules in solution self-assemble into aggregates called micelles. Figure 2 shows the result of plotting the surface tension of increasing concentrations of a surfactant, sodium lauryl sulfate (SLS), with the concentration plotted on a logarithmic axis. This results in a curve that essentially has a “knee” (discontinuity) in it, and it is at this point where the Critical Micelle Concentration (CMC) occurs.  
Figure 2
While CMCs have been most commonly measured using classical DuNoüy Ring or Wilhelmy Plate instruments, these are limited to equilibrium measurements, which result in equilibrium CMCs. Dynamic surface tension measurements of these same surfactants, with the aid of automatic dispensing systems, can reveal the level at which surfactant effectiveness is at its highest, which is not necessarily at the equilibrium CMC.[6,7] Optimum surfactant effectiveness, which does not necessarily occur at the equilibrium CMC, can be determined by correlation to actual process conditions. 

The shift in the dynamic CMC is toward increasing surfactant concentration, as surface age is reduced. Common sense will tell us that if something limits the diffusion of surfactant molecules to the interface, one needs more surfactant to reach the same CMC point. Dynamic CMCs more accurately reflect practical, in-process operation and surfactant utilization. Judicious selection of correct surfactant concentration levels should be based on dynamic CMCs rather than equilibrium CMCs.

Dynamic measurements more accurately reflect actual, in-process, surfactant and formulation performance. In effect, if you limit surfactant migration time (by using a faster application process, (increased press speed on a printing press, for example), you require more surfactant to perform the same job as in the slower process. Figure 3, illustrates a series of CMC curves for a surfactant, run at different surface ages. While the “knees” in these curves are not as pronounced as the previous SLS curve, plotting them (intersection of solid lines – upper and lower curves) would show a distinct shift of the CMC points from lower left to upper right, verifying the need for added surfactant concentration for faster process application (faster surface age) conditions.
Figure 3
COMPLEX FINGERPRINTS
When multiple surfactants are present in a solution the dynamic surface tension curve can become quite complex, as illustrated in Figure 4. Experience and many repeatability tests have verified that these complex characteristics do exist, even though this appears as a departure from the normally expected (classical) dynamic surface tension curves of Figure 1. Usually, these complex curves occur when certain combinations of surfactants are used to lower the surface tension of a formulation. The dynamic surface tension undergoes several transitions. First one surfactant migrates quickly to the air/fluid interface to lower surface tension and then another one “kicks in” at the same developing interface, and supplements or displaces the faster migrating surfactant molecules.

As alluded to earlier, in general, surfactants with a smaller (lighter) molecule mass (short hydrophobic tail) diffuse more rapidly to the interface, and are vertically adsorbed at the interface, causing a compressive force to act on the surface thereby reducing surface energy or surface tension. Nonionic surfactants with ethylene oxide groups usually diffuse very rapidly to the surface while fluorinated surfactants are slower and more effective at equilibrium. Most surfactants at higher concentrations exert strong molecular attractions between adjacent molecules causing strong surface films, the strength of which determines the surface properties of the surfactant solutions.

 These more complex dynamic characteristics explain why sometimes the expected performance of a multiple surfactant-containing formulation may have some process-time-related zones of performance that greatly contradict initial expecta-tions, as we may be fluctuating between positive and negative spreading coefficients.

                                                                                                              Figure 4
INK JET CASE STUDY
Similar complex dynamic fingerprints of solvent-borne ink jet formulations were tested for a customer that had a specific problem. In this application, a certain formulation that printed very well caused other operational problems for the user. In this particular case, the test sample with the lowest surface tension fingerprint was the poorer formulation of the two, despite it’s better “printability” because it caused problems at the ink jet nozzle. Instead of the ink jet atomizing into ideal droplets that exited the nozzle cleanly, the ink was “back splashing”, as part of the droplet was separating and splashing back over the external nozzle face.

 Tests indicated that all of the samples tested had complex dynamic curves. When the customer indicated that these formulations had only one surfactant, we were puzzled... until we tested the solvent used in these formulations and found that the solvent exhibited both dynamic and complex characteristics. So this was a case of a complex dynamic curve due to multiple surfactants used by the solvent manufacturer and not the ink formulator. 

CORRELATION OF SURFACE TENSION TO OTHER PROCESS PARAMETERS
In certain industries, surface tension is used for quality control of processes. In semiconductor plating baths, for example, the percent of additive concentration in the bath can be directly correlated to the fluid surface tension of the formulation. In cases such as this, a correlation curve is easily generated between surface tension of the bath formulation and additive concentration. This is used to predict when the additive level starts to deplete to marginal levels, but well before the bath reaches unacceptable performance. Samples are taken each hour, or half hour, and the additive concentration monitored so that the bath can be re-formulated back to its optimal operating range.  
 
SUMMARY
By generating formulation fingerprints, problems can readily be mitigated by comparing “good” and “bad” formulations. Formulators can improve transfer, spreading, and adhesion of formulations by choosing surfactant and additive combinations that provide the best surface tension profiles for their applications.
 
BIBLIOGRAPHY
1.         Theresa Souza, “Analysis of 72BC Additive using the SensaDyne 6000 Tensiometer”, Technic Inc. (1989).
2.         Klus J.P. at al, U.S. Patent 4,416,148 (1983)
3.         Tanya C. Christensen et al, U. S. Patent 6,085,577 (2000).
4.         Test Method D 3825-90, ASTM, Philadelphia, PA 19103 (1990)
5.         Janule V.P. “Characterizing Active Surfactants in Three Dimensions”, Pigment & Resin Technology, 23, 3-8 (1994),
6.         Christensen T.C., Janule V.P., Teichmann A.F., “Automatic Determination of Dynamic CMCs”, Chem-Dyne Research Corporation, Mesa, AZ 85275-0430 (1997)
7.         Janule V.P., “Solving Formulation Problems using Dynamic Surface Tension and Dynamic CMC Measurements”, American Laboratory, 29, 12 October (1997
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