Surface tensionWithout most of us never paying attention to it, surface tension is acting all around us, everywhere and all the time, affecting our daily life in a number of ways. In fact it is surface tension which keeps the billions of cells in our body functional, ensuring the proper organization of their biomolecules, proteins, lipids, and nucleic acids, into membranes and various types of cellular organelles. Surface tension is a truly fundamental property of water, making it an ideal medium allowing for life as we know it to exist. On a more easily accessible scale, familiar to us all, it is surface tension what makes water drops spherical. What causes surface tension?In brief, surface tension arises from the strong interactions between water molecules, called hydrogen bonding. It is this strong interaction which also manifests in the other unusual property of water, its high boiling point.
In the bulk of a liquid, each water molecule can make an optimal number of H-bonds to other water molecules. On the surface, however, the interactions with the neighboring molecules are limited and weaker, resulting in a higher free energy and reduced intermolecular hydrogen bonding of the molecules. In nature, water has one of the highest surface tensions, 72.8mN/m (at 20 degrees Celsius), only exceeded by very few liquids, such as mercury, which has a surface tension of about 480mN/m. Surface tension of water also manifests as the so-called hydrophobic effect, evident as the lack of mixing of oil and water.
The hydrophobic effectThe hydrophobic effect is the driving force for the formation of micelles by amphiphilic molecules and membranes by phospholipids. In these assemblies the polar, water soluble parts of the amphiphiles seek interaction with water, while in contrast, their hydrophobic, water insoluble, oily parts cluster to minimize contacts with water. The hydrophobic effect can be easily studied through changes in the surface tension for an air/water interface. More specifically, measuring changes in the surface tension of the air-water interface, allows to monitor the efficiency of the accumulation of compounds such as drugs, surfactants and lipids into the surface. This gives the academic research and the pharmaceutical industry totally new possibilities and benefits to analyze drugs and conduct physicochemical profiling to support their QSAR and ADME/Tox research. Measuring surface tension provides data reflecting thermodynamics of the compounds tested, and reveals fundamental physicochemical properties associated with processes such as adsorption, hydrogen bonding interactions, and self-assembly. The latter is particularly important for fields such as nanotechnology. Controlling surface tension by surface active materials means control of molecular level behavior and self-assembly - a key to nanotechnology. Why do we use soap?Surface tension plays a big role in many of our daily activities. Soaps and detergents include surfactants, that reduce the surface tension of the liquid. This allows the liquid to have a good contact with the material and to remove the dirt from it efficiently. Measuring changes in surface tension and determining the CMC (critical micelle concentration) values, the chemical industry can optimize the amount of surfactants in their products, bringing more environment friendly, yet efficient goods to the market. Surfactants and oil reduce the surface tension of water. Accordingly, by measuring the surface tension of natural waters the presence of substances such as oils or detergents can easily be verified. Most precise measurement of surface tension/pressureAll Kibron instruments use the best, most sensitive and most precise method for the measurement of surface tension: a combination of the DuNuoy-Padday et al. ‘Maximum Pull Force Technique’ and Kibron's proprietary sensor, a unique ultrasensitive microbalance. This technique is unmatched in its performance and yields accuracy far better than obtained using filter paper or platinum Wilhelmy plates or duNuoy ring, which are used in the tensiometers and Langmuir-troughs made by our competitors. With the aim of being able to provide the users of Kibron equipment the most precise determination of surface tension/pressure, we developed a new microbalance, with a resolution of better than 0.2 micrograms. This sensitivity was then combined with the "Maximum Pull Force" principle. In this technique a probe is first immersed into the interface and then slowly pulled out, while recording the weight of the meniscus (liquid adhering to the probe). At some point the probe detaches from the interface and this is the force used to calculate surface tension. Importantly, because of the specific geometry of the Kibron's DyneProbes, unlike for both the Wilhelmy plate and duNuoy ring method, there is no error due to buoyancy and thus no need for any correction. Further, the easiest way to calibrate the instrument is to use a liquid with a known surface tension. The results speak for themselves. In the following the accuracy and precision of the Kibron sensor is demonstrated by the data for solvents with a broad range of known surfaces tensions, measuring each solvent five consecutive times and analyzing the results by common statistical means (see below). A comparison of the mean surface tension values and the literature values reveals an excellent agreement.
A line has been fitted to the above data (figure below), giving a correlation coefficient of 0,999 and demonstrating the excellent accuracy of the Kibron sensor throughout this surface tension range.
Benefits of the Kibron solutionThe Kibron core technology is applied in all Kibron products. The technique brings the following key benefits into surface chemistry:
The Kibron microbalance is insensitive to vibrations, allowing experiments to be made practically everywhere (no need for a vibration isolation table). The Kibron microbalance allows also for the use of very thin probes, without sacrificing sensitivity. The small size of the measuring probes allows consequently for the use of small sample volumes, beneficial for industries using expensive materials or samples of limited availability. Small sample volumes mean also better sample mixing and thermostation, when required. In the fully automated Kibron Delta-8 a multichannel sensor measures eight samples simultaneously, only 50 microliters each, deposited in a cuvette of the standard 96-well plate format. The measurement of all 96 samples takes 3-10 min, yielding unprecedented productivity and new possibilities to physicochemical profiling and high throughput screening of critical micelle concentrations, for instance. The measurement is completed by fully automated analysis of the recorded data by CMCeeker, our unique, truly powerful tool for your laboratory. Kibron - automated all the way from the measurement to the comprehensive, informative report on your screen, with all data securely stored in the dedicated database. Kibron technology brings also benefits to single channel measurements. With the portable Kibron AquaPi tensiometer one measurement takes less than 30 seconds and requires only a 3 ml sample. Surface tension: looking at the hydrophobic effect in actionSurfactants and amphiphiles are chemical compounds containing both hydrophilic (water soluble) and hydrophobic (water repelling) moieties in the same molecule. In classical surfactants such as in sodium dodecyl sulfate (SDS, common ingredient in e.g. detergents), these parts are well separated, giving rise to the nomenclature used: hydrophobic tail consisting of a hydrocarbon chain, and the hydrophilic and polar sulfate headgroup. A common way of characterizing amphiphilic compounds is to study them at an air/water interface. Amphiphiles adsorb to the interface and orient so as to have their polar headgroups with water, while projecting the hydrophobic tail into the air, thus forming in the interface a single layer of surfactant molecules. The driving force is the hydrophobic effec t as in this way the hydrophobic tails escape from contacting water. The result is decreasing order in water (increase in entropy). Upon reaching the solubility limit for the amphiphile monomers, they start to aggregate, forming structures known as micelles. Now the non-polar, hydrophobic parts are hidden inside the micelle core while the micelle surface consists of the polar parts. Again, this process is driven by the hydrophobic effect.
Interfaces in numbers - Gibbs isothermAdsorption and aggregation of a compound can be monitored through changes in the air/water interfacial tension (surface tension) upon increasing the compound's concentration, thus providing the so-called adsorption isotherm. The data can be described by various adsorption models (we use the Gibbs model) to obtain the following molecular parameters.
In the picture above surface pressure is used instead of surface tension. Surface pressure is related to surface tension through: π = γ0 - γ where γ0 is the surface tension for the clean air/water interface and γ is the surface tension measured in the presence of the surface active substance. The partitioning coefficient, Kaw−1 is determined as the intersection between the extrapolated line and the x-axis, log c. It measures the compound's affinity for the air/water interface, and simplistically put can be understood as the ratio of the species found in the bulk phase and in the interface. For monolayers the difference between surface concentration and surface excess (slope of the isotherm) becomes negligible. Thus, the surface concentration is inversely proportional to the area available per surfactant molecule in the interfacial region. The critical micelle concentration (CMC) or solubility limit results in a sharp transition above which the concentration of the free surfactant/amphiphile molecules remains constant, resulting in a plateau in the surface pressure vs. concentration curve. |
