Micelle

Micelles [mi-sel] (singular "micelle"), or micellae (singular "micella"), are spherical clusters of hydrocarbon molecules that act as emulsifying agents. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre. This type of micelle is known as a normal-phase micelle (oil-in-water micelle). An Inverse micelle has a hyprophobic and hydrophilic side, with the hyrodphilic side at the center and the hydrophobic side facing the solvent. Micelles are approximately spherical in shape. Other phases, including shapes such as ellipsoids, cylinders, and bilayers, are also possible. The shape and size of a micelle are a function of the molecular geometry of its surfactant molecules and solution conditions such as surfactant concentration, temperature, pH, and ionic strength. The process of forming micelles is known as micellisation and forms part of the phase behaviour of many lipids according to their polymorphism.

Formation

Micelles form when the polar head and the non polar tails arrange in a special way. They are usually driven to arrange either with the polar heads out (oil in water) or with the polar head in (water in oil). Micelles only form when the concentration of surfactant is greater than the critical micelle concentration. The surfactant is any surface active material that can part the surface upon entering. The higher the critical micelle concentration, the more micelles there are. Micelle formation also depend on the Krafft temperature. If the temperature is below the Krafft temperature, then there is no spontaneous formation of micelles. As the temperature increases, the surfactant will turn into a soluble form and be able to form micelles from a crystalline state. The hydrophobic effect is also a driving force that needs to be taken into account. This effect is characterized by the fact that like to form intermolecular aggregates in aqueous substances and in intramolecular molecules. Micelle formation can be summed up by thermodynamics, driven by entropy and enthalpy.

The micelle packing parameter equation is utilized to help "predict molecular self-assembly in surfactant solutions":

where  is the surfactant tail volume,  is the tail length, and  is the equilibrium area per molecule at the aggregate surface.

Role in oil emulsion

Gale (1987) reported the combined solvent power of supercritical fluids with the solvent power of micellar solutions which appears promising in connection with enhanced oil recovery. Likewise, recognition of the possible formation and destruction of reversed micelles by naturally-occurring amphiphilic substances, such as those associated with asphaltenes, may explain problems experienced in some EOR projects. Lipophilic supercritical components of reservoir fluids (e.g. light hydrocarbons and CO2) interact, in a controllable manner, with hydrophilic micellar complexes to achieve selective extraction of desirable components of crude oil. Fundamentals of supercritical fluids and of micellar systems are reviewed in terms of their mutual interaction and their potential applicability in EOR processes.

When surfactants are present above the critical micelle concentration , they can act as emulsifiers that will allow a compound that is normally insoluble (in the solvent being used) to dissolve. This occurs because the insoluble species can be incorporated into the micelle core, which is itself solubilized in the bulk solvent by virtue of the head groups' favorable interactions with solvent species. The most common example of this phenomenon is detergents, which clean poorly soluble lipophilic material (such as oils and waxes) that cannot be removed by water alone. Detergents clean also by lowering the surface tension of water, making it easier to remove material from a surface. The emulsifying property of surfactants is also the basis for emulsion polymerization.

Detection

Micelles are neutral buoyancy droplets with dimensions typically ranging from 10 to 300 nm and so they are very difficult to detect using conventional direct methods. The most promising techniques use indirect micelle detection from fluorescent reporter additives which can be added to test solutions in low concentrations to locate within micelles and produce a measurable and proportional fluorescent response. Such techniques are used widely in research and diagnostic testing in the life sciences. It has been shown that many oilfield corrosion inhibitors behave as typical surfactants, with defined CMC’s and a proportional relationship between numbers of micelles and fluorescence intensity and peak emission wavelength . This has previously been shown to allow the use of a portable handheld fluorescence reader to allow for simple measurements in the field . For such a device to function, visible light must be able to pass in to and out of the analyte solution without being impeded and, knowing the turbid nature of many oilfield samples, alternatives instruments have been investigated. .

Noteworthy papers in OnePetro

Awang, M. B., Japper, A., Kumar, S., & Dzulkarnain, I. (2012, January 1). Wormlike Micelles for Mobility Control in EOR. Society of Petroleum Engineers.

Mackenzie, C., Rowley-Williams, C., Mackay, F. S., Lane, C., Blumer, D., & Achour, M. (2013, March 17). Application of Micelle Detection Method: Field Case Studies. NACE International. https://www.onepetro.org/conference-paper/NACE-2013-2478. http://dx.doi.org/10.2118/155059-MS.

Noll, L. A. (1991, January 1). The Effect of Temperature, Salinity, and Alcohol on the Critical Micelle Concentration of Surfactants. Society of Petroleum Engineers. http://dx.doi.org/10.2118/21032-MS.

External links

Evotherm Chemistry Episode 1 - Introduction

Evotherm Chemistry Episode 2 - Surfactant Chemistry

Evotherm Chemistry Episode 3 - Evotherm Chemistry 101

Micelles

Surfactants

See also

PEH:Polymers,_Gels,_Foams,_and_Resins

PEH:Asphaltenes_and_Waxes

PEH:Crude_Oil_Emulsions

Glossary:Sequestration

Glossary:Resin

Stability of oil emulsions

Foams as mobility control agents

Thermodynamic models for asphaltene precipitation