Surfactants, or surface-active agents, are generally organic compounds that are amphiphilic, meaning that they contain both hydrophilic (water soluble) and hydrophobic (water insoluble) parts. Due to their amphiphilic nature, surfactants adsorb at interfaces thus lowering the surface and interfacial tension between two phases.
In many industrial processes, surfactants are added to improve the properties of the products. For example, surfactants are used in detergents to improve the efficiency of the cleaning products, or they can act as wetting agents in paints or emulsifiers in food products. Another aspect of the surface-active agents is naturally occurring compounds that can act as surfactants. These include for example asphaltenes that are found in heavy crude oils and that can give rise to a variety of issues in crude oil production.
The effectiveness of a surfactant is determined by measuring its ability to lower the surface or interfacial tensions and stabilize emulsions as well as by studying its hydrophilic-lipophilic balance (HLB).
By measuring the surface and interfacial tensions as a function of concentration, it is possible to determine the maximum decrease in the ST or IFT value which the given surfactant or surfactant mixture is able to produce. This is also important from economical point of view, as the amount of surfactant used has a direct impact on to the cost of the products. Environmental aspects are also considered, in terms of both the surfactant concentrations needed as and the toxicity of the surfactant. Critical micelle concentration measurements are routinely used to determine the optimum quantity of surfactants in formulations.
Surfactant adsorption on solid surfaces
Understanding the adsorption of surfactants on solid surfaces is important in processes like paint manufacturing, water filtration or crude oil production. Take paint, for example, which is a complex water-based mixture of different components including pigment particles, polymers and surfactants. The pigment particles are there to give the paint the desired color, polymers to increase the viscosity and surfactants to increase the stability and wettability of the paint. Problems can arise if there is preferential adsorption of polymers on the surface of the pigment instead of the surfactant. This can lead to a loss in color appearance and inferior adhesion properties in the dried film.
Asphaltenes are of increasing interest as many of the recently discovered reserves contain asphaltenes (oil sands, heavy oil). Asphaltenes are high molecular weight components of crude oil. While their exact molecular structure is unknown, they are commonly classified as insoluble in n-alkanes, such as n-pentane or n-heptane, but soluble in toluene. Asphaltenes tend to adsorb on interfaces thus increasing the stability of oil-water emulsions (link to emulsion stability) and changing the wetting behavior of the reservoirs. Asphaltene adsorption also causes issues like pipeline fouling during crude oil processing.
To understand the mechanisms of asphaltene deposition, a fundamental understanding of asphaltene-solid interactions is required. QSense® QCM-D can be used to characterize asphaltene adsorption and fouling on various surfaces under different solvent conditions.
Precipitation and deposition of asphaltenes can result in wettability alteration and permeability reductions of the reservoir rock, which consequently leads to a decrease in oil recovery. Wettability and interfacial tension measurements between oil, fluid and rock can be studied through contact angle measurements. Contact angle measurements can also be made at high pressures and temperatures mimicking the reservoir conditions (pdf).
Surfactants and their behavior at the solid-liquid interface are, as we have seen, central to many areas, ranging from applications in biology to soil removal and cleaning to enhanced oil recovery. QSense QCM-D technology, enables the characterization of both surfactants, surfactant systems and surfactant mixtures in terms of their dynamics and behavior at interfaces. With this surface sensitive technology, it is possible to study both surfactant adsorption and desorption kinetics to different surfaces and to monitor the dynamics of adsorption in real time. The surfaces can easily be varied, which makes it possible to map out the influence of, for example, surface material, surface functionalization and degree of hydrophilicity or hydrophobicity. It is also possible to determine the layer thickness of the adsorbed surfactants and to follow morphological changes to the adsorbed film as a function of various surfactant concentration, pH, salt concentration and temperatures. The technology also enables the exploration and characterization of surfactant interaction with, for example, lipid vesicles or polymer and polyelectrolyte layers.