Micellar structures and Whisky
Table of content 1 Table of content Table of content...1 Introduction...2 Micellar structures in Whisky...4 Methods...4 Results...5 Discussion... 10 Disclaimer... 11
Introduction 2 Introduction The use of surfactant molecules to create micro-emulsions and dispersions with substances otherwise immiscible has become a huge factor in the chemical industry. Surfactant molecules are used especially for their property to self assemble. Usually a surfactant consists of a long carbon linked chain with two different ends, a head and a tail. One of these is repelled by the solvent, i.e. by water, the other not. Fig.1 surfactant If surfactant molecules are added to water above a critical concentration called critical micelle concentration (CMC) and at a temperature above the critical formation temperature a certain number of surfactants (aggregation number) will aggregate or self assemble to form a micellar structure. Unlike popular belief there is no water-free haven in the center of a micelle but an entanglement of hydrophobic tails which still contains up to 30% of water. Fig.2 micellar structure
Introduction 3 The spontaneous (i.e. without adding energy to the system) formation of micelles can be understood as a result of negative Gibbs free energy for micelle formation as a system spontaneously tries to reach a state of lower Gibbs Energy. G = G after. reaction Gbefore. reaction This means the system has less Gibbs Energy when micelles are formed than it has without micelles. This can be understood as the driving force for creating such highly aligned and complex structures. The difference in Gibbs Energy for micelle formation can be described (simplified) with the known CMC of a surfactant molecule. G = R T ln ( ) micelle c free R = gas constant [ ~ 8.3 J ] T = Temperature [ K ] c free mol K = concentration of free surfactants = c CMC (if CMC is reached) Below the CMC every surfactant occurs to be in ionic form. No micelles are formed. At the CMC micelles will be formed. If the concentration of surfactants should rise above the CMC the fresh surfactants will immediately form new micelles. This means the concentration of free surfactants cannot rise above the CMC. The difference in Gibbs free energy G micelle can be understood as the amount of energy necessary to add one surfactant to a micellar structure. If this energy is above zero (i.e. Positive) one would have to add energy for micelles to form. If the energy is below zero (i.e. Negative) the reaction or formation will consume no energy, but release energy in form of heat and the micelle formation will be spontaneous. Some properties micellar structures show are of high value especially for the chemical industry. One of these properties is the ability to stabilize water-immiscible monomers for emulsion polymerization and to form micro-emulsions or dispersions. In our exapmle waterimmiscible molecules are stabilized (or even trapped) in the low water content center of a micellar structure and can thus be dispersed. This reaction takes place in several steps: Water-immiscible, or slightly water soluble molecules will eventually dissolve in the solution Eventually some of this dissolved small portion will enter a micellar structure and begin to swell it above its usual size until a critical size is reached
Methods 4 Micellar structures in Whisky In oak cask matured distilled beverages ethyl esters derived from in oak casks naturally occurring long-chain fatty acids due to their amphiphilic nature may be responsible for agglomeration and forming micellar structures. Literature indicates that mixtures of Ethanol and Water above a concentration of 20% will form micellar structures without the addition of long chain carbon linked molecules. Immiscible or slightly soluble components like smaller esters, aldheydes or other components known to the sommelier for giving aroma and flavour, will eventually dissolve into the solution and penetrate a micellar structure where it will be stabilized and the micelle will grow until it reaches a critical size. Thus micellar structures may be able to stabilize and over time release aromatic components. If the concentration (i.e. the ABV) of the solution is altered, the solubility of the volatile components is altered, too leading to a release of volatile components detectable for the sommelier. Methods Particle size distribution of two different single malts at different concentration and water ethanol mixtures at different concentrations have been measured with a Malvern Instruments Ltd. Zetasizer Nano using dynamic light scattering (DLS) methods. Particles undergo a Brownian motion which correlates with their hydrodynamic diameter according to the Stokes-Einstein relationship. During a DLS analysis particles are illuminated with a laser. The intensity of the scattered light is detected and fluctuates at a rate that is dependent upon the size of the particles. Smaller particles will move more rapidly in the solution when accelerated. Analysis of these intensity fluctuations yields the velocity of the Brownian motion a particle performs. According to the Einstein-Stokes relationship the stokes-radius or the hydrodynamic radius of the measured particle is R H = K b T 6πηD K b = Boltzmann constant T = = Temperature D = Diffusion constant = η Viscosity of the fluid R Hydrodynamic or Stokes radius H
Results 5 Results Fig.3 Breas of Glenlivet 61.7% Cask Strength
Results 6 Fig.4 Breas of Glenlivet 40%
Results 7 Fig.5 Breas of Glenlivet 25%
Results 8 Fig.6 Auchentoshan 12 40%
Results 9 Fig.7 Water Ethanol mixture 45%
Discussion 10 Fig.8 Water Ethanol mixture 25% Discussion Dynamic light scattering analysis of a Scottish Breas of the Glenlivet 1989 single malt at a cask strength of 61.7% ABV show a high polydispersity index (PDI). This indicates very large and very small particles are present. The author believes this is due to residues from the cask or bottling process. Diluted with DI-Water to a drinking strength of 40% and below the PDI took on a much more stable value of well below 0.05. All three samples show an average particle size of around 470nm. An undiluted sample of an Auchentoshan 12 years old with chill filtration shows particles at around 320nm. Samples of DI-Water and Ethanol mixtures at 45% and 25% show particles at 492nm (45%) and 139nm (25%). It can not be foreclosed the measured peaks show impurities in the samples.
Disclaimer 11 Disclaimer I am a student of chemistry at a German university and currently (2011) working on my diploma thesis about organic dispersions and emulsion polymerization. This little peace of work I put together just for the fun of it. It should not be seen as a true scientific paper. I just like whisky and chemistry. For questions feel free to contact me at www.scotswhisky-community.de/forum. My nickname is LessLemming