Polymer-mediated phase stability of colloids: Summary of Insights

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“I said to the almond tree, ``Sister, speak to me of God''. And the almond tree blossomed.”
Nikos Kazantzakis
in Report to Greco

One may pretend to understand the complex processes that ultimately give the almond tree flower its functional aesthetic characteristics. Such an act of vanity reflects the opposite of the scientific method. In order to grasp the real world, one must simplify it. An effective approach is to select a small number of relevant parameters, and test how robust the predictions extracted from the experimental, computational or theoretical method followed are compared with real-life observations. Note that even experimental laboratory research provides a simplified picture of real systems.

During my Ph.D. years, we isolated some key parameters governing the (in)stability of colloid-polymer mixtures. The term `colloid’ refers to a state of matter in which a certain amount of material (with one of its dimensions between one nanometre to one micrometre) is dispersed in another medium. Polymers are macromolecules constituted of many repeating units called segments; depending on the number and nature of these segments, polymers may dissolve or phase-separate in solution. Colloid-polymer mixtures are widespread in biological systems (including blood, the cytoplasm of a living cell, and plant sap), as well as in man-made products (such as paints, drinking yogurt, and printing inks). Better control over the stability limits during product development is possible via a fundamental understanding of the effect of some relevant parameters in the system at hand. In the examples given above, multiple colloids, polymers, and other components are often present. Building knowledge on the interactions between components of the same nature, and pairs of different components is a logical starting point. Based on the characteristics of the colloidal particles investigated, we sequester this thesis into three parts. Due to their typical sizes, polymers themselves belong to the colloidal family. Hence, the term colloid-polymer mixture appears convenient, yet a colloid-polymer mixture is a particular mixture of colloids. My thesis is divided in three parts.

In Part I, we took the simplest model system: mixtures of hard  spheres (like billiard balls) with added polymers simplified as ghost-like spheres. We studied how a direct soft interaction beyond the hardcore interaction modulates the phase stability of a model colloid-polymer mixture (Chapter 2). We conclude that soft repulsive interactions widen the stable region and direct soft attractions decrease the stability of a colloid-polymer mixture. We also paid attention to mixtures of such colloidal hard spheres with added tiny polymers (Chapter 3): such a model system may be of relevance for instance in protein crystallization. Upon revisiting a well-established (relatively simple) free volume theory, we improved it for the solid phase state, which brought it closer to more convoluted ones, simulations, and experiments. In Chapters 2 and 3 we (over)simplified the polymers (we took them as a ghost-like sphere), while in Chapter 4 we describe them in more detail. In that Chapter, we extract how the strength of the interaction between the surface of the colloidal particle and the polymer segments affects the phase behavior of a colloid-polymer mixture. We elucidate the possibility of colloid-polymer mixtures which do not phase-separate, even at high polymer concentrations, which is appealing for industrial applications such as paint, coatings, or foodstuff.

Alvaro Figure 2

In Part II we focus on the influence of the shape of the colloidal particle on the phase behaviour of colloid–polymer mixtures. Liquid crystalline phases in nondilute colloidal dispersions may emerge as a consequence of the anisotropic shape of particles. Not surprisingly, the pigment’s shape affects the final properties of paints and coatings. We consider anisotropic hard particles, and study the effects of adding ghost-like spheres to mimic polymer chains. Investigations of cube-like (Chapter 5) and platelet-like (Chapter 6) colloids reveal a rather rich phase behaviour. We highlight the unexpected presence of up to four phases in coexistence in effective two-component systems, reported for the first time in this thesis. Furthermore, we elucidate the relevance of compartmentalisation of tiny compounds in highly concentrated systems. This could be of interest, for instance, in the future development of photonic materials with two different optical paths, and may serve as a model to study crowded living environments.

Alvaro Figure 3

Finally, in Part III, we studied association colloids. We focus on associative colloidal particles formed by diblock copolymers: polymers composed of well-soluble segments and of poorly soluble segments divided into two blocks. In a selective solvent, diblock copolymers can constitute the building blocks of equilibrium structures known as micelles. We focus on self-organised spherical micelles, used in applications ranging from cosmetics to targeted drug delivery to, for instance, tumoral cells. We studied micelle–micelle interactions, and particularly focus on how the building block composition affects colloidal stability (Chapter 7) and how it is affected by the addition of a second (non-blocky) polymer (Chapter 8). The associative and soft nature of association colloids render the problem at hand complex, yet insights could be extracted about the phase stability of micelles. We concluded that spherical micelles resulting from diblocks with a short soluble block are more suitable for applications than those with a large soluble block.

Alvaro Figure 4

By virtue of these simplified models, a collection of predictions governing the (in)stability of colloid–polymer mixtures has been extracted. These may serve for further developments, considering, for instance, not only two but multi-component mixtures. Further tuning of the accuracy of the models could bring them closer to reality. The author hopes that the small pieces that this thesis has added to the puzzle of knowledge may inspire and be of utility to others.

For accessing the complete thesis, click here.

Note: The first and last parts of this Summary are (heavily) influenced by the author’s article in Cultural Resuena, `Nikos Kazantzakis y el espíritu científico’ (only in Spanish):

Nikos Kazantzakis y el espíritu científico


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