Amphiphilic Crescent-Moon-Shaped Microparticles Formed by Selective Adsorption of Colloids

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Original Entry by Andrew Capulli, AP225 Fall 2011

Reference

"Amphiphilic Crescent-Moon-Shaped Microparticles Formed by Selective Adsorption of Colloids" S.-H. Kim, A. Abbaspourrad, and David Weitz. Journal of the American Chemical Society 133(14), 5516-5524 (2011).

Key Words

Amphiphilic, Hydrophobic, Hydrophilic, Interfacial Tension, HLB Scale, Emulsion

Introduction: Motivation

There are numerous amphiphilic surfactant molecules available in a wide variety of hydrophilic-hydrophobic balances, shapes, sizes etc as pointed out by the authors. However, as amphiphilic microparticles have become of interest due to their potential use as pigments in display devices and as mentioned by the authors, microparticles can be used as the "building blocks to construct photonic structures through directional interactions." However, surfactant microparticles are of particular interest due to their amphiphilic nature and potential for added increased strength of emulsion. While the authors note that the science behind the creation of such amphiphilic particles is not necessarily novel in this research, the field itself is new and like how their are numerous tunable molecular surfactants (in particular numerous shapes) there is a need in the mircoparticle field for such shape diversity and study into the effects thereof. Dumbbell, mushroom, and spherical shaped amphiphilic microparticles have been studied but there is a need to investigate more and in this methods paper the authors do just that, presenting a new way to form "crescent" (half moon shelled) shaped amphiphilic microparticles.

Summary of Main Experimentation

The paper is essentially a methods paper and the details of the setup are described more thoroughly. Here I summarize them: Weitz et. al. use a microfluidic device (theta injection) to produce their amphiphilic microparticles. Fluorocarbon oil (FC-77) and ethoxylated trimethylolpropane triacrylate (ETPTA) are 'injected' into an aqueous solution via their theta capillary device (see Figure 1 below). Because FC-77 has a higher energy with water than ETPTA, the ETPTA drop will spread around the FC-77 drop. As the authors describe the behavior of the spreading (how much the ETPTA spreads or wets the FC-77 drop) is dependent on the make up of the aqueous solution used as the continuous phase of the set up (blue in Figure 1).

Weitz Fig1.jpg

To give some perspective, full spreading is reportedly observed in 3 wt% PVA solution while only partial spreading (more so drop separation) is observed in 1 wt% Pluronic F-108. Essentially, spreading goes as:Weitz Crescent 1.jpg, where S is the spreading parameter which is dependent on the interfacial tension between phases j and k. The spreading parameter as described by the authors, "reflects the relative values of the surface energies, and the propensity of one fluid to spread on another." If S<0, partial spreading of the ETPTA will occur and if S>0 complete spreading of ETPTA will occur (shelling of the ETPTA over the FC-77). This is shown in figure 2 below.

Weitz Fig2.jpg

For the preparation of amphiphilic crescent shaped particles, partial spreading is desired. The ETPTA solution containes hydrophilic silica particles which, once injected into the aqueous solution, will migrate toward the surface of the ETPTA that is in contact with water and away from the surface of the ETPTA that is in contact with the FC-77. This migration is spontaneous because it lowers the energy between the surface of the ETPTA and the water (aqueous solution). By using photopolymerization (UV), the authors polymerize the ETPTA-silca material in its semi-spread (crescent) shape then disperse the FC-77-ETPTA drops in ethanol which allows for FC-77 removal. This is leaves just the crescent shaped polymer with a convex hydrophilic portion due to the presence of silica particles at this surface and a hydrophobic concave portion where the FC-77 once was. Below I have made a hybrid of Figures 3 and Figure 9; Figure 3a below schematically outlines the process just described:

Weitz Fig3.jpg

In 3a, the blue dots represent the hydrophilic silica particles on the convex portion of the ETPTA microparticle. The hydrophilic tendency of these particle create a gradient within the mircoparticle itself since the particle are only present on the convex surface. The authors prove that the silica particles are only present on the convex surface via SEM imaging. To further improve the amphiphilic gradient, that is to say to increase the 'polarity' of the microparticle, hydrophobic OTMOS-treated silica particles are added to the ETPTA solution before polymerization and similarly gather at the FC-77 interface. Once FC-77 has been removed, the crescent shape has an 'enhance' amphiphilicity where instead of just the convex portion being more hydrophilic, the convex portion is more hydrophobic (thus increasing the gradient and making for a stronger surfactant). This is process is schematically outline above in 9a.

The creation of newly shaped amphiphilic microparticles is interesting on its own but the authors further describe, through a series of small experiments, the advantages of the crescent shaped microparticles. Stabilization of oil in water via these particles is observed by the authors (and documented in the images below: Figure 9). Unlike, say, spherical amphiphilic particles, the crescent shaped microparticles do not go into the oil to stabilize the fluid in the emulsion; these crescent shaped microparticles, because of their shell like shape, "maximizes the fraction of interface that is protected by obviating body hindrance and therefore provides higher stability." Essentially, these crescent amphiphilic microparticles pack tightly on the surface of oil drops in solution which increases the "protection" of the oil drop. The authors even show that their crescent particles "protect" oil droplets from each other (from fusing) because each drop is so well coated with the amphiphilic particles; even when pushed together, sufficiently coated oil droplets do not fuse. There are many figures that show how well they author's microparticles essentially coat and thus stabilize oil in water; Figure 8 below however is the most indicative of the strength of these surfactant microparticles. The authors poke a silicon oil drop coated ('immobilized') by their microparticles and no puncture is observed. The indentation was even present after removal of the glass poker suggesting the strength (and high packing) of the amphiphilic microparticles.

Weitz Fig8.jpg

As discussed, the spreading of the ETPTA over the FC-77 is variable depending on the aqueous solution used during fabrication; thus, varying spreading varies the curvature of the resultant amphiphilic microparticle. The authors discuss how smaller curvature microparticles work well stabilizing larger drops of oil due to their ability to pact tightly at the surface if the drop while the higher curvature microparticles work well stabilizing small oil drops by overlapping.

Connection to Soft Matter

The connection to soft matter is no stretch for this paper, it directly relates to the surfactant physics discussed in class. The amphiphilic crescent microparticles lower the interfacial tension between the water and oil so well by not going into the oil phase itself but rather gathering (densely) on the surface of the oil drop thus creating a barrier between the water and oil, stabilizing the system. Conceptually, these microparticles are like thin sheets that coat the surface of the oil as compared to the more so one dimensional molecular surfactants that stabilize systems via head and tail affinity. Essentially, I mean whereas a typical molecular surfactant may act like a nail holding two repulsive magnets together, these amphiphilic microparticles (because of their size/shape/packing) act as a glue layer holding two repulsive magnets together... the difference being a matter of point force versus a three dimensional distributed force. The implications here are exciting; surfactants are used every day but we're still not sure about all the physics behind them. These microparticle surfactants are exact in their production; knowing the concentration of silica particles (which result in the hydrophilicity and hydrophobicity of the particle) may give more of a quantitative understanding of the surfactant (as compared to the HLB Scale).

More Thoughts

Increasing the effectiveness of surfactants has very big implications. Emulsions are everywhere (medical field, chemical engineering, etc) and controlling the emulsion is crucial to many processes and products. The importance of controlling emulsions for use in medicine/pharmaceuticals goes without saying. Microemulsions used in vaccines need to be highly stabilized and the emulsions are very small (http://en.wikipedia.org/wiki/Emulsion). The amphiphilic microparticles presented in this paper may provide a means of more easily and accurately stabilizing such medicines. Ice cream, which is my favorite emulsion, may be made 'creamier' by the inclusion of better surfactants and emulsifiers such as those in this paper. The authors demonstrated how these microparticles 'protected' oil droplets from fusing with other oil droplets (which is what is desired for a good emulsion!). Maybe its selfish to think of this physics in terms of food but I can't help myself: by making the cream/fat droplets smaller in the ice cream emulsion, a softer, more uniform ice cream can be made. These microparticles may be the the means to a creamier, more stable, ice cream without the use of more fats... just better surfactants.

So... after all, maybe oil and water do mix...