RESOURCES:
What is a Surfactant?
“Surfactant” is just a contraction of the phrase “SURFace ACTive Agent”. What is it that makes a surfactant a surfactant? The answer is surprisingly simple; much simpler than the multitude of applications where surfactants play a critical role. In the presentation linked below, with notes, you will discover that surfactants go by many names. Those names often come from the applications. In emulsion technology surfactants may be called emulsifiers. In foams they may be called foaming agents. In cleaning systems they may be called detergents or even soaps. But in every application, the basic behavior of the surfactant is the same, and it is this basic behavior that makes them useful in such a wide range of modern technologies. And what makes them surfactants is that they are active at interfaces (surfaces).
Adsorption
A molecule is adsorbed when it accumulates at a surface or an interface. Surfactants-being surface active-adsorb on almost every surface and at almost every interface. Surfactants adsorb at the interface between oil and water. Surfactants adsorb at the interface between liquids and gases (the water and air interface, for example). Still, their behavior at a solid-liquid interface is unique. Surfactant adsorption at the solid-liquid interface is especially interesting because adsorbed surfactants form aggregates on the surface of solids that can dramatically change the properties of the interface. This is an important factor in solid-liquid dispersions, in wetting of surfaces, and in cleaning.
Adsorption of a surfactant at a solid-liquid interface can be good or bad. When you want the surfactant to modify the properties of the interface, as you do in wetting applications, you want the surfactant to adsorb. In dispersions, surfactant adsorption is used to control the interaction between solid particles. But in cleaning operations, such as detergency in environmental applications of surfactants, and enhanced oil recovery most of the time surfactant adsorption represents a loss or even degradation in the cleaning efficiency. It is very important, therefore, to be able to choose a surfactant system so that the surfactant adsorption is low when adsorption represents a loss, and is high when it is needed to control the properties of the surface or interface.
Cleaning Technology
Detergency is the process of cleaning without solvents. A detergent removes contaminants from a surface by solubilizing, suspending, or emulsifing them.
The detergency process is surprisingly complex. Good detergency requires the synergism of many simultaneous processes. Surfactants in a detergent or cleaning formulation must cause the cleaning solution to wet both the surface to be cleaned and the contaminant to be removed. The contaminant must then be suspended, solubilized or emulsified by the cleaning formulation, so that it does not become redeposited on the surface. The surfactant must, therefore, not be too water soluble, or it will not have good adsorption on the contaminant. It must also not be too close to a phase boundary, or it may precipitate or form a coacervate.
While surfactants are generally the primary active agents in water based cleaning formulations, modern cleaning systems include numerous other additives to optimize the cleaning. For example, builders are added to reduce the activity of multivalent counterions that otherwise might precipitate the surfactant. Enzymes are often added to assist in solubilizing heavy oil or fats, or even to remove loose fibrils from the fabric that cause it to look grey.
For all of these reasons, a fundamental understanding of detergency requires a good grounding in all aspects of surfactant science, including micelle formation, surfactant precipitation and phase separation, wetting, adsorption, emulsification, microemulsions, dispersion and suspension of particulates, and foaming.
In our short course, detergency is placed at the end of our coverage of these other phenomena, allowing us to cover a great deal of cleaning science in a short time, because of the foundation laid in the first part of the course.
Emulsions & Microemulsions
Emulsions and microemulsions are both stable dispersions of oil-in-water or water-in-oil. Surfactants are the principal agents that enable oil and water to mix. Emulsions are stable dispersions of immiscible liquids, but they are not thermodynamically stable. We say that they are kinetically stable. These type of dispersions are sometimes called macroemulsions. Microemulsions, on the other hand, are thermodynamically stable. While some people insist that microemulsions must be transparent, that is not really a critical part of their description. In the short course we will give a systematic introduction to the mechanisms of kinetic stabilization of macroemulsions, show how macroemulsions are related to microemulsions, and show how to systematically select a surfactant system to make a stable emulsion or microemulsion.
Environmental & Health Factors
Surfactants have an excellent health and environmental profile. Almost all commercial surfactant types have oral toxicity on a par with naturally occurring compounds like table salt. Many surfactants are either naturally occurring or made by simple modifications of natural compounds. The human body contains many surfactants. The best known are the phospholipids that make up the cell membrane. These are essentially low water solubility anionic surfactants. Our lungs produce natural surfactants that make it easier for our lungs to expand and bring in air. Loss of lung surfactant leads to a disease known as sudden adult respiratory distress syndrome.
While our surfactant short course provides a comprehensive introduction to all aspects of surfactant health and environmental factors, as well as a wealth of recommended readings, these papers and PowerPoint presentations illustrate the use of surfactants in the remediation of ground water that has been contaminated by liquid fuels or industrial solvents. If you saw John Travolta in the movie “Civil Action” or Julie Roberts in Erin Brockovich, then you are familiar with the problem of ground water contamination. Here is how we use surfactants to remediate ground water contamination. That’s right, we inject surfactants into ground water in order to make it safer!
The big exception to the generally benign nature of surfactants is their aquatic toxicity. Very low concentrations of surfactants in surface water can be highly toxic to all forms of aquatic life, from insects to fish, reptiles and amphibians. Consequently, a major environmental consideration in the introduction of any new surfactant molecule is determining whether it is sufficiently biodegradable to loose its surface activity before being emitted from municipal or industrial waste treatment plants. Even ppm concentrations of non-degraded surfactants can kill wide ranges of aquatic organisms.
Precipitation, Krafft Points, & Hardness Tolerance
Why do we care about surfactant precipitation? Because precipitated surfactant is surfactant that isn’t changing the properties of a surface or interface. We almost always want to eliminate precipitation from our surfactant systems. The only exception is when we are looking to remove the surfactant from solution, as might occur in waste water treatment or a separation process.
For ionic surfactants there is no property that is more important than the Krafft temperature of the surfactant. The Krafft temperature is the minimum temperature at which the solubility of the surfactant monomer becomes high enough for the surfactant to start to form micelles. In general, surfactants are inactive below their Krafft temperature. Mixtures of surfactants can be used to lower the Krafft point of a surfactant formulation. This is a key approach to improving the performance of a formulation in both accelerated aging chambers and in freeze-thaw cycles. In our short course we will show you how to use this information to avoid a nasty surprise that surfactants have been known to spring on the unwary technologist: Salt shock! It’s not toxic shock, but it’s close!
Solubilization & Supersolubilization
Surfactant micelles are capable of increasing the solubility of most organic molecules in water. The mechanism by which this solubilization occurs is the incorporation of the organic molecule into the micelle. The study of this phenomenon was very important in the history of our understanding of the physical chemistry of surfactants. To a considerable degree of accuracy, the core of the micelle acts like a drop of liquid alkane.
Surface Tension & Wetting
The history of the science of surface tension is even longer and more distinguished that the history of solubilization. Benjamin Franklin published a study of the effect of an insoluble fatty acid oil on the surface of water in the Transactions of the Royal Society of London.
These articles explain the basic science behind the phenomenon of surface tension while discussing the principles behind a simple but powerful technique for measuring the surface tension of liquids.