Adsorption of poly(ethylene oxide)-containing amphiphilic polymers on solid-liquid interfaces: Fundamentals and applications
Graphical abstract
Introduction
Academic and industrial research has long been engaged in the study of new materials to address societal, economic, and environmental needs. However, emerging technologies frequently involve unique and challenging conditions which traditional engineered materials can no longer meet. These materials were typically processed at the micrometer length scale and higher. Today the capability exists for precise manipulation of matter at the nanometer scale, meaning that materials can be built from the ground up. These changes in methodology and mindset allow for tailoring of materials to better suit their end use. These formulated materials derive often unique (and therefore valuable) properties from their internal organization at the sub-micron length scale [1], [2], [3]. Due to effects such as quantum confinement, for example, previously unrealized optical and electronic properties can be achieved [4], [5]. Nanoparticle–polymer composites, a broad and ubiquitous class of materials, represent a significant thrust in composite materials research with a rich diversity of applications [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. They rely on the uniform dispersion of sub-micron particles into a continuous phase to elicit enhanced and novel properties. These composites also break the common formulation rule that when one property is improved, a related property suffers (e.g., increasing tensile strength also increases brittleness). This means that property-tradeoffs can be minimized or fully avoided, allowing greater flexibility in choice of raw materials [19]. These engineered colloidal systems contribute towards a large fraction of goods which we interact with on a daily basis (e.g., coatings, detergents, biomedical devices, electronics, and foodstuffs) and appear in many different forms (e.g., structured liquids, foams, semi-solids). They are also driving innovation in emerging fields such as nanoparticle synthesis [20], [21] and targeted drug delivery [22], [23], [24], [25]. The promise of nanoparticle-polymer composite materials motivates the study of their underlying fundamentals.
The hallmark of colloidal systems is their interfaces, which are typically high in energy and abundant in surface area. These interfaces enable interaction among many different types of dispersed colloids and solvents [26], [27], [28], [29]. The interactions are dictated in turn by a variety of forces, including van der Waals, hydrogen bonding, hydrophobic, depletion, and electrostatic [26]. The multitude of forces at play renders the design of nanostructured materials complex and their behavior in various environments difficult to predict. Even small changes to the environment can cause aggregation of the dispersed phases, typically to the detriment of performance. However, this sensitivity to environmental variables also creates the opportunity to build systems that are stimuli-responsive (e.g., to change in pH or temperature).
The aggregation of dispersed liquids or particles occurs as the system free energy is minimized. Some examples of destabilization mechanisms include: Ostwald ripening (inclusion of smaller particles into larger particles), coalescence (combination of dispersed liquid droplets), and flocculation (combination of dispersed solid particles). Aggregation can be mitigated by the introduction of amphiphilic macromolecules which spontaneously adsorb on the interfaces between the continuous and dispersed phases. The adsorbing molecules provide steric or electrostatic repulsion between the dispersed particles or liquids, thereby kinetically stabilizing the system [30], [31], [32], [33], [34]. In many cases these amphiphiles are synthetic, however there are natural amphiphiles which are now being investigated as sustainable alternatives [35], [36], [37]. Colloidal particles with adsorbed polymers can be used to prepare stable liquid-liquid emulsions [38], [39], [40]. Liquid-liquid microemulsions are characterized by droplets which are typically a few nanometers in diameter (smaller than a typical emulsion) and are thermodynamically stabilized (in contrast to common emulsions) by adsorbed amphiphiles. The amphiphilic polymers we describe in this review are either homopolymers or block copolymers. We focus mainly on block copolymers, which comprise two or more chemically different monomers in a single chain. This characteristic allows block copolymers to be designed with wide ranging hydrophobic-lipophilic balance (HLB) and other special functionalization. Block copolymers can be synthesized in different architectures, including diblock, triblock, graft, and star [41], [42]. Their amphiphilic nature allows them to self-assemble into well-defined nanostructures in different solvents and solvent mixtures [43]. They can also self-assemble on solid-liquid and liquid-liquid interfaces. The solvent and/or surface type dictates the properties of these supramolecular structures. Surfactants, typically having a lower molecular weight and comprising a hydrophilic head group and a hydrophobic alkyl chain, can also spontaneously self-assemble into nanostructures in solvents and on interfaces [44], [45].
The manner in which amphiphiles adsorb on an interface dictates how well the system is dispersed and the subsequent system properties. The interaction between particle and amphiphile is affected by solvent quality (pH, ionic strength, and temperature) [46], [47], [48], [49], molecular characteristics (architecture, moiety affinities) [50], [51], [52], and surface properties (geometry, chemistry) [53], [54], [55]. One can imagine that, based on all of the available combinations of the aforementioned attributes, an enormous number of tunable and responsive systems exists. Their design will require improved understanding of fundamental mechanisms of interaction between amphiphiles, solvents, and particles, such that predictive models and guidelines may be constructed which are applicable to broad classes of materials.
Reviews have been published in the previous three decades which cover theoretical treatments and experimental results for nonionic and ionic amphiphile adsorption [56], [57], [58], [59], [60], [61], [62], [63]. In particular the roles of surface type and molecular architecture have been investigated. This has led to improved understanding of the various physical and chemical mechanisms which govern adsorption kinetics and adsorbed layer structures. Recent reviews have covered complex fluids such as surfactant-polymer mixtures [31], [64]. The adsorption of surface active block copolymers on biological interfaces (e.g., proteins) is another rapidly growing field, which is related to drug delivery [65], [66], [67]. However, the need exists to present recent findings in block copolymer adsorption behavior with a focus on how system components (e.g., solvents and particle type) can be selected to purposefully modulate the nanoscale adsorbed structure. Interpretation of recent data with a focus on the underpinning thermodynamics allows for common trends to be extracted, which can be applied across diverse and nascent fields. This review focuses on the adsorption properties of PEO-based amphiphilic polymers, which have become popular because of their sensitivity to environmental changes and rich phase behavior. Several reviews exist which describe the adsorption of low molecular weight PEO-containing surfactants such as alkyl ethoxylates [56], [60], [61], [62], [63], [68]. On the other hand, a similar review on block copolymer adsorption on solid-liquid and liquid-liquid interfaces has not been undertaken recently [69], [70], [71], [72].
Poly(ethylene oxide) (PEO) is a nonionic polymer that is compatible with a variety of solvents, including water, chloroform, and dimethyl formamide [73]. These properties have made PEO suitable for a wide range of industrial applications. PEO has also received attention in the bioengineering community because of its high interfacial activity, benign interaction with proteins and cells, and potential use as a drug release agent [74], [75]. PEO homopolymers are also known to associate with ionic surfactant micelles, such as those formed by anionic sodium dodecyl sulfate (SDS); such association can be reversed by the introduction of simple alcohols, indicating the potential use of PEO as a tunable additive [76]. Structurally, PEO homopolymer is represented by H-(O-CH2-CH2)n-OH, and has a mildly amphiphilic character. PEO is available in a broad range of molecular weights (100 s to many million Da) spanning physical states from a viscous liquid (low molecular weight) to a durable thermoplastic (high molecular weight) [73]. PEO-containing nonionic amphiphilic polymers, especially symmetric A-B-A triblock copolymers of PEO and poly(propylene oxide) (PPO), have been well-studied [77], [78], [79]. Due to their low toxicity, wide range of PEO and PPO composition, and varying molecular weights, they are suited for many applications which require fine control and tunability [80]. PEO-PPO-PEO block copolymers will spontaneously self-assemble into a wide array of microphase-separated structures in mixed solvent systems and non-aqueous solvents [81], [82], [83], [84].
This review is structured in the following manner. First, we introduce general characteristics of homopolymer and block copolymer adsorption on solid-liquid interfaces. Second, we highlight key features of PEO homopolymer adsorption. PEO homopolymer serves as a foundation for discussion of the adsorption properties of more complex, larger PEO-containing block copolymers. Next, we focus on the topic of amphiphile adsorption, and in particular, that of PEO-PPO-PEO block copolymers. Here we cover factors influencing PEO-PPO-PEO adsorption, including particle size and solution conditions. We also discuss recent results from our research group and others on various methods for elucidating polymer-particle-solvent interactions towards better control of the adsorbed amount, ad-layer thickness, and macromolecular organization on the solid-liquid interface. Finally, we draw connections between fundamental adsorption properties discussed throughout the text and practical applications. We conclude with a summary and outlook on the state of the art.
Section snippets
Fundamentals of polymer adsorption
The variety of forces and interactions at play, electrostatic, hydrogen bonding, hydrophobic, and van der Waals, among others, can be a complicating factor in the study of colloidal systems. The summation of these forces dictates whether the net interaction between two components of a mixture will be repulsive or attractive [85]. Determining which behavior will be displayed is not straight forward, but the DLVO theory (not described here) provides general guidelines for aqueous dispersions by
Solid-liquid interfaces
The adsorption of PEO homopolymers on the solid-liquid interface is straight forward compared to that of PEO-containing block copolymers. However, a brief discussion of PEO homopolymer interfacial behavior is instructive, as PEO is a common constituent in many nonionic surfactants and polymers. The hydrophilic PEO blocks in amphiphiles play an integral role in their microphase separation properties in the bulk. PEO also contributes significantly to the thickness and structure of adsorbed layers
Adsorption of PEO-containing amphiphiles
Nonionic surfactants and block copolymers commonly incorporate PEO groups as a hydrophilic component. Examples include linear CiEOj and CiPOyEOj surfactants, PEO-PPO di- and triblock copolymers, and siloxane graft-PEO amphiphiles. The most significant difference between PEO homopolymers and nonionic, PEO-containing amphiphiles, is the latter's ability to self-assemble into various three dimensional structures in solvents and mixtures of solvents. In the case of PEO-PPO-PEO block copolymers,
Select applications for PEO – containing amphiphiles
Polymer adsorption on solid-liquid and soft interfaces can alter system properties to the benefit of an array of emerging applications [293]. The subsequent sections on coatings, emulsification, and drug delivery, serve to illustrate how amphiphilic polymer adsorption can improve material performance and open up new avenues in already well-established areas of application.
Conclusions
In this review we have discussed several key facets of PEO-containing amphiphile adsorption on the solid-liquid interface. As a baseline, fundamentals of polymer adsorption from theory and modeling were discussed. The adsorption of PEO homopolymers on various interfaces was then reviewed to serve as a background for the discussion of PEO-containing amphiphiles, which comprise diverse chemical groups and thus present more complex adsorption properties. PEO is a ubiquitous polymer which, when
Acknowledgements
We thank Professor Marina Tsianou for helpful discussions. Partial support of this work from the Gulf of Mexico Research Initiative (C-MEDS: Consortium for the Molecular Engineering of Dispersant Systems) and the European Union 7th Framework Programme (GA No. 312139) (Kill●Spill: Integrated Biotechnological Solutions for Combating Marine Oil Spills) is gratefully acknowledged.
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