Do surfactant solutions and mixtures, e.g., association colloids and emulsions, have fundamentally different, dual properties, when probed by light or by a chemical reaction?
Surfactants of many kinds, simple soaps, phospho- glycol- and proteo- lipids spontaneously self-assemble to form transparent thermodynamically stable solutions of association colloids, e.g., micelles, vesicles and microemulsions. Traditionally (last 40 years), two approaches were taken to model the rates of thermal bimolecular reactions and the distributions of reactants in these solution: (a) by trying to correlate observed rate constants with aggregate size and shape; and (b) by treating the totality of the aggregates in solution as a separate pseudophase composed of oil and interfacial regions in a sea of water. The pseudophase model eventually became the consensus approach with homogenous association colloids because rate constants within them depended upon the medium polarity of the interfacial region and they were insensitive to solution composition except at very high concentrations of added salts. The components of bimolecular reactions in both homogeneous and heterogeneous association colloids move through solution near the diffusion controlled limit (half-lives ca. 10-9 s ± 10±2 s), whereas thermal reactions have half-lives on time scales typically on minutes to hours (ca. 102 to 105 s), many orders of magnitude longer. However, treatment of chemical reactivity and determination of reactant distributions within emulsions are still focused on potential effects of droplet size, methods for measure reaction rates, and models for reactivity and reactant distributions were not developed—until now.
The development of pseudophase models for emulsions was kindled by the two decade long search in food chemistry for determining the most efficient antioxidant for protecting against oxidation of polyunsaturated oils in a particular emulsified food application and understanding the Polar Paradox stated by Porter about 20 years ago. This problem was solved by the joint effort of two research groups at Rutgers and the University of Vigo. What follows is a brief introduction to the logic. The details are in our recent Langmuir Feature article.1
- Emulsions are like Microemulsions at the Molecular and Macromolecular Level. Grumpy and Dopey (see Disney’s Snow White if you forgot) are showing flasks containing an opaque, well-mixed, two phase, heterogeneous emulsion and a transparent, well-mixed, single phase, homogeneous microemulsion. The two mixtures contain exactly the same components of water, oil and a surfactant, but in different amounts (more oil and less water in the emulsion).
- The microscopic image (top row, right) shows discrete oil droplets (within circles) in a continuous aqueous phase. An invisible layer of surfactant molecules (too small to see 10 nm in length) create an interfacial region between the oil and water and provide stability—thermodynamic for microemulsions and kinetic for emulsions. Without a dimension scale, droplet images in microemulsions (< 100 nm radius) and emulsions (> 100 nm radius) are indistinguishable. Grumpy says: “Everyone knows emulsions scatter visible light.” “Well microemulsions don’t” says Dopey. “We can see through them, but we can treat the kinetic data in microemulsions and emulsions the same way.” Humph!” says Grumpy.
- Dynamic Equilibrium? What is required for droplet sizes to affect rates of bimolecular reactions? The entrance and exit rates of reactants must be similar to or slower than the rate of their reaction (middle row, left for any two reactants S and N). Does this occur? Yes, for extremely fast photochemical reactions and excited state reactions that may occur on the nanosecond time scale or faster. The upper limit for molecular diffusion is also on the nanosecond time scale. However, most thermal reactions commonly studied in solution are on the seconds to megaseconds time scale, many orders of magnitude slower than molecular diffusion. Thus, once bulk mixing is complete, and molecules are distributed based on their relative solubilities, the system is at dynamic equilibrium and molecular transport rates from oil to interface and interface to water are not rate limiting. For emulsions and microemulsions of the same components, the intermolecular forces between them in all three regions will be the same and if their intermolecular motions are fast in microemulsions, they will be fast in emulsions.
- Reaction Regions. Relative to molecular sizes, the fluid oil, water and interfacial regions are very large and contain many millions of molecules. Consequently, chemical reactants, whose concentrations are orders of magnitude lower, simply dissolve in them and their distributions depend on their relative solubilities in each region (middle row, right). Once reactants (and other components are mixed) reach equilibrium, that is their concentrations are the same in all oil, all interfacial, and all aqueous regions, the entrance and exit rates of reactants between regions is constant and the regions are effectively continuous.
Ergo, light scattering “sees” aggregates of various sizes and shapes and chemical reactants “see” continuous reaction regions of constant reactant concentrations. The answer to the perplexed man’s question is: Dualité Oui!. Microemulsions and Emulsions have dual properties depending on “who is looking,” molecules or light.
The middle row, right shows our arenediazonium ion chemical probe reacting with the antioxidant (AO) in the interfacial region. The chemical equation (bottom left) shows the important steps in the reaction. The measured rate (and rate constant) depend on the concentrations of probe and AO in the interfacial region and the relationship between kobs and the partition constants of the AO between the oil and interface and interface and water (bottom right). A second equation is needed to solve for the partition constants and also the interfacial rate constant and the distribution constants. The results of this work are providing new insight into antioxidant distributions in emulsions and the reasons for their differences in their efficiencies.1
1 To model chemical reactivity in heterogeneous emulsions, think homogeneous microemulsions. Carlos Bravo-Díaz, Laurence S. Romsted, Changyao Liu, Sonia Losada-Barreiro, Maria José Pastoriza-Gallego, Xiang Gao, Qing Gu, Krishnan Gunaseelan, Verónica Sánchez-Paz, Yongliang Zhang and Aijaz Ahmad Dar, Feature Article Langmuir, 2015, Publication Date (Web): March 25, 2015 DOI: 10.1021/acs.langmuir.5b00112