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Comment on "The Mayonnaise Effect“ Bernhard Wessling J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01006 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018
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Comment to K. Wynne „The Mayonnaise Effect“
Mayonnaise is an Emulsion, not a Solution - Comment on „The Mayonnaise Effect“ in J. Phys. Chem. Lett. 2017, 8, 6189-6172 Bernhard Wessling* LaoWei Chemical Technology Consulting (Shenzhen), Inc. K. Wynne introduces his paper in the abstract with the phrases: „Structuring caused by the mixing of liquids or the addition of solutes to a solvent causes the viscosity to increase. The classical example is mayonnaise: the mixture of two low-viscosity liquids, water and oil, is structured through the addition of a surfactant creating a dispersed phase ...“ Further below in his text, it is talked about (aqueous) salt solutions, a Jones-Dole equation applicable to solutions containing a coefficient B which is used for ions weakening or strengthening hydrogen bonds. Figures documented in the paper are exclusively describing the behaviour of true solutions. Finally, failure of the Jones-Dole equation above certain critical concentrations (which are far above saturation concentration for the solutes in question) are assigned to a „jamming“ effect which is said to be a „mayonnaise effect“. In essence, the paper claims one could explain „jamming“ and „mayonnaise viscosity“ effects by analyzing and explaining solutions. My comments are addressing this lack of differentiation between solutions (salts in water) and emulsions (like mayonnaise) or other dispersions: The paper is misleading the reader to believe that „emulsions (e.g. mayonnaise) can be understood as solutions and are only gradually different from solutions“ which is not the case. The following facts are relevant: 1. True solutions are (thermodynamically spoken) equilibrium systems, i.e., the free energy of the solution system is (significantly) lower, the entropy is (significantly) higher than that of the sum of the isolated pure components, the two characteristics of an equilibrium systems. 2. In such systems, the (dissolved) components are more or less statistically evenly distributed in the solvent; they all exhibit Newtonian behaviour when looking at their viscosity properties1. Already an oversaturated solution which is „jamming“ is not a solution any more: in a true solution every solute molecule or ion is completely surrounded („solvated“) by solvent molecules. Already such oversaturated liquids can not be compared any more with true solutions; one can not extrapolate the properties of a true solution onto the properties of an over-saturated one which is not a solution any more but a colloidal system; 3. Emulsions and other dispersions are (in contrast to solutions) far-from-thermodynamicequilibrium systems (free energy higher than, and entropy lower than the sum of those values for the isolated components)2; this is the case because the generation of new surfaces of the material to be dispersed / emulsified and of the matrix in which dispersion / emulsion is happening, requires energy (surface tension multiplied with the area of the fresh surfaces), the entropy is strongly decreasing as shown in2; such processes are non-linear and exhibit bifurcation points because of non-linear interactions between the components.Such systems behave in viscosity aspects as non-Newtonian liquids, either dilatant (shear-thinning)3 or pseudo-plastic (shear-thickening). One can not extrapolate viscosity properties of a nonACS Paragon Plus Environment
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Newtonian liquid by looking at Newtonian liquids. 4. The viscosity of such systems can be understood and easily calculated and predicted by using the simple equation containing the natural logarithm of the matrix viscosity and the apparent logarithmic viscosity contribution and the concentration of each single component (dispersion or emulsion medium, e.g. water) according to4: ln η(res) = (1-xn)*ln η(matrix) + xn*an (1) where η is the viscosity resulting(res) from the matrix(matrix) (continuous phase) viscosity plus the contribution a, of component n which has the volume concentration xn, while an, the viscosity contribution of component n also has the form of the natural logarithm of a viscosity, but this is only an apparent viscosity because also solids exhibit a similar contribution factor a, and for liquids n (in emulsions) a is not the viscosity of the pure component n. The contribution of each component (except for the matrix) is independant of its pure material viscosity, and depends only upon its particle / droplet size in the dispersion / emulsion. Which means: if you want to understand and predict mayonnaise viscosity, you measure the viscosity of water at various shear rates, then every component of mayonnaise (oil plus surfactant, egg, …) in water at different concentrations and at different shear rates, take their natural logarithm and make a simple addition of their ln(contribution values) times concentration. Make sure, the particle size is the same in your calibration studies as in the final product. It is important to understand how dispersions and emulsions are formed and structured (for a schematic view see5): new surfaces are formed both of the dispersed as of the continuous phase, the continuous phase (dispersion medium, matrix) will wet the surface of the dispersed / emulsified phase and will be firmly adsorbed there (in contrast to water molecules in the first solvation shell of ions which is, as cited by K. Wynne, only 3 ps); the dispersed / emulsified particles / droplets are not statistically evenly distributed in the dispersion medium, but form „pearl chains“ and fine membranes, also in form of bicontinuous systems; at a critical concentration (the smaller the particle or droplet size, the lower this concentration), the adsorbed matrix molecules partially desorb and allow the particles / droplets to get into close(r) contact, to flocculate and build even stronger chains and networks6 responsible for sudden dramatic increase of conductivity or viscosity. This is the significant difference between equilibrium and non-equilibrium (far from equilibrium) systems: the latter ones exhibit self-organisation of dissipative structures as was first described by Ilya Prigogine7. Such structures can be found in all dispersions and emulsions (most easily by freeze-drying followed by SEM, for mayonnaise, some other technique would probably be more appropriate). These 3-dimensional structures are responsible for the viscosity increase and the nonNewtonian behaviour. We have found many examples in those dispersions and emulsions studied by us, and one of the most striking one was this: We took neutral (compensated) Polyaniline and dispersed it in DMSO; the particle size of the dispersed Polyaniline was below 10 nm; we measured the viscosity in dependance of PAni concentration (which followed the equation shown above) – and were heavily surprised when at only 2.4% the system suddenly not only „jammed“, but became a gel which had elastic behaviour (when forming it into a ball, it was dry and jumped back to the hand when we let it fall down to the floor, like a „bouncy ball“ or „super ball“) – all this with only 2.4% of solids in DMSO!8 (Mayonnaise is by far not as surprising as this example! It will not bounce back to the spoon when it fell to the floor.) ACS Paragon Plus Environment
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Hence, „jamming“ (or, similar to the above example, „gel formation“) is due to the fact that at a certain concentration of the dispersed / emulsified phase all the matrix fluid has been adsorbed on the surface of the dispersed particles or droplets, so that no (or not enough) matrix molecules are available for free flow. For mayonnaise, one can find this confocal microscopy photo in the web9 (Fig 1):
(Fig. 1) Confocal microscopy photo of an oil-in-water emulsion9
(Fig. 2) 2D sketch of branched structures in the oil-in-water emulsion of Fig 15 The small bright areas in the left photo are showing where the droplets touch each other and are not any more separated from each other by adsorbed emulsion layers (water + lecithine + …), but have
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flocculated which causes a 3-dimensional network of „pearl chains“ to be formed, visualized by the red lines in the Fig. 2 (which can only show 2 dimensions) drawn by me. What emulsions and dispersions characterizes with respect to their viscosity is their ability to form very complex 3-dimensional structures, „(frozen) dissipative structures“ due to self-organisation far from equilibrium. Which, by the way, is the basis and the principle of life. We all and the ecosystems we are living in, the earth and our solar system, the milky way and all other stars, planets and galaxies are non-equilibrium systems and the result of dissipative structures which had been more or less quickly „frozen“ to form our observable world. Author Information *E-mail:
[email protected] Acknowledgement I thank Dr. Helmut Baumert very much for his various valuable suggestions to my comments.
References
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Baumert, H. Universal Equations and Constants of Turbulent Motion - Parameter-free Turbulence Theory of Newtonian Liquids. Phys. Scri. 2013, T155 014001 Wessling, B. Critical Shear Rate – the Instability Reason for the Creation of Dissipative Structures in Polymers. Z. Phys. Chem. 1995, 191, 199-135 Baumert, H.; Wessling, B. On Turbulence in Dilatant Dispersions - Parameter-free Turbulence Theory of nonNewtonian Liquids. Phys. Scri. 2016, 91, 1-8 Wessling, B. On the Structure of Binary Conductive Polymer / Solvent Systems. Synth. Metals 1991, 41-43, 907910; and Wessling, B. Rheological Phenomena and Structure Formation. Chapter 11, subchapter 4 in: „Metallic Properties of Conductive Polymers due to Dispersion“, in: Handbook of Conductive Molecules and Polymers, Vol 3, Nalwa, H. S.. Ed., John Wiley & Sons Ltd. 1997 Wessling, B. https://www.researchgate.net/profile/Bernhard_Wessling2/post/Are_there_any_recent_and_convincing_SEM_phot os_showing_the_structure_of_mayonnaise_or_other_emulsion_structures/attachment/5a66535bb53d2f0bba4d0555/ AS:585709568143361@1516655451373/download/dispersion+process+steps.JPG (link will download jpg) (accessed Jan 29, 2018) Wessling, B. Electrical Conductivity in Heterogeneous Polymer Systems – a new Dynamic Interfcial Model. Polym. Eng. Sci. 1991, 31, 1200-1206, cited with more explanations in Wessling, B. Dispersion as the Key to Processing Conductive Polymers“. in: Handbook of Conducting Polymers, 2nd Edn, Revised & Expanded, Nalwa, H. S., Ed., J. Wiley 1998, Chapter 19, p. 467 – 530, here p. 489 ff Prigogine, Y. Time, Structure and Fluctuations. Nobel Lecture 1977. https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1977/prigogine-lecture.pdf with references to original papers therein (accessed Jan 29, 2018) Wessling, B. Dispersion Hypothesis and non-Equilibrum Thermodynamics. Synth. Metals 1991, 45, 119-149, Fig 10-12 Hopkinson, I. Understanding Mayonnaise. http://www.ianhopkinson.org.uk/2010/05/understanding-mayonnaise/ (accessed Jan 29, 2018)
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