Comment pubs.acs.org/JPCB
Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX
Comment on “The Mayonnaise Effect” Bernhard Wessling* LaoWei Chemical Technology Consulting (Shenzhen), Inc., Neverstaven 2a, D-23843 Travenbrueck, Germany
J. Phys. Chem. B 2018, 122. DOI: 10.1021/acs.jpcb.8b01428 K. Wynne introduces his paper1 (DOI: 10.1021/acs.jpclett.7b03207) 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 behavior 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) is 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 and 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 behavior when looking at their viscosity properties.2 Already an oversaturated solution which is “jamming” is not a solution anymore; in a true solution, every solute molecule or ion is completely surrounded (“solvated”) by solvent molecules. Already such oversaturated liquids cannot be compared anymore with true solutions; one cannot extrapolate the properties of a true solution onto the properties of an oversaturated one which is not a solution anymore but a colloidal system. 3. Emulsions and other dispersions are (in contrast to solutions) far-from-thermodynamic-equilibrium systems (free energy higher than and entropy lower than the sum of those values for the isolated components);3 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 © XXXX American Chemical Society
surfaces); the entropy is strongly decreasing, as shown in ref 3, and such processes are nonlinear and exhibit bifurcation points because of nonlinear interactions between the components. Such systems behave in viscosity aspects as non-Newtonian liquids, either dilatant (shear-thinning)4 or pseudoplastic (shearthickening). One cannot extrapolate viscosity properties of a non-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 to5 ln η(res) = (1 − xn) ln η(matrix) + xnan
(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 independent of its pure material viscosity, and depends only upon its particle/droplet size in the dispersion/emulsion. This means, if you want to understand and predict mayonnaise viscosity, you measure the viscosity of water at various shear rates and 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) × 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, see ref 6): New surfaces are formed both of the dispersed as well 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/ Received: February 1, 2018 Revised: February 7, 2018 Published: February 12, 2018 A
DOI: 10.1021/acs.jpcb.8b01006 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Comment
The Journal of Physical Chemistry B
so that no (or not enough) matrix molecules are available for free flow. For mayonnaise, one can find the confocal microscopy photo shown in Figure 1 on the Web.10 The small bright areas in Figure 1 are showing where the droplets touch each other and are not separated anymore from each other by adsorbed emulsion layers (water + lecithine + ...), but have flocculated which causes a three-dimensional network of “pearl chains” to be formed, visualized by the red lines in Figure 2 (which can only show two dimensions) drawn by me.
Figure 1. Confocal microscopy photo of an oil-in-water emulsion.10 Reproduced with permission from Ian Hopkinson.
droplets are not statistically evenly distributed in the dispersion medium but form “pearl chains” and fine membranes, also in the 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 networks7 responsible for a sudden dramatic increase of conductivity or viscosity. This is the significant difference between equilibrium and nonequilibrium (far from equilibrium) systems: the latter ones exhibit self-organization of dissipative structures, as was first described by Ilya Prigogine.8 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 three-dimensional structures are responsible for the viscosity increase and the nonNewtonian behavior. 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 dependence 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 behavior (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!9 (Mayonnaise is by far not as surprising as this example! It will not bounce back to the spoon when it falls to the floor.) 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 of the matrix fluid has been adsorbed on the surface of the dispersed particles or droplets,
Figure 2. 2D sketch of branched structures in the oil-in-water emulsion of Figure 1.6
What emulsions and dispersions characterize with respect to their viscosity is their ability to form very complex threedimensional structures, “(frozen) dissipative structures”, due to self-organization far from equilibrium. This, 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.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bernhard Wessling: 0000-0002-2583-3047 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS I thank Dr. Helmut Baumert very much for his various valuable suggestions to my comments. REFERENCES
(1) Wynne, K. The Mayonnaise Effect. J. Phys. Chem. Lett. 2017, 8, 6189−6192.
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DOI: 10.1021/acs.jpcb.8b01006 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry B (2) Baumert, H. Universal Equations and Constants of Turbulent Motion - Parameter-free Turbulence Theory of Newtonian Liquids. Phys. Scr. 2013, T155, 014001. (3) Wessling, B. Critical Shear Rate − the Instability Reason for the Creation of Dissipative Structures in Polymers. Z. Phys. Chem. 1995, 191, 119−135. (4) Baumert, H.; Wessling, B. On Turbulence in Dilatant Dispersions - Parameter-free Turbulence Theory of non-Newtonian Liquids. Phys. Scr. 2016, 91, 074003. (5) Wessling, B. On the Structure of Binary Conductive Polymer/ Solvent Systems. Synth. Synth. Met. 1991, 41, 907−910. 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.: New York, 1997. (6) Wessling, B. https://www.researchgate.net/post/Are_there_ any_recent_and_convincing_SEM_photos_showing_the_structure_ of_mayonnaise_or_other_emulsion_structures (link will download jpg) (accessed Jan 29, 2018). (7) 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 ed., revised and expanded; Nalwa, H. S., Ed.; J. Wiley: New York, 1998; Chapter 19, pp 467−530 (here p 489 ff). (8) Prigogine, I. 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). (9) Wessling, B. Dispersion Hypothesis and non-Equilibrum Thermodynamics. Synth. Met. 1991, 45, 119−149 (Figures 10−12). (10) Hopkinson, I. Understanding Mayonnaise. http://www. ianhopkinson.org.uk/2010/05/understanding-mayonnaise/ (accessed Jan 29, 2018).
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DOI: 10.1021/acs.jpcb.8b01006 J. Phys. Chem. B XXXX, XXX, XXX−XXX
