Ebook: Self-Assembly

In the Spring of 2002, a group of scientists assembled in Massa Marittima, Tuscany, Italy, for a meeting on the topic of ‘Self-Assembly – the Future’. The participants were all experts in the field and came to Massa Marittima from all over the world, and the organisers were Piero Baglioni (University of Florence, Italy), Julian Eastoe (University of Bristol, UK), Alan Hatton (MIT, Boston, USA), Brian Robinson (University of East Anglia, UK) with the help of Stephen Lee and Ray MacKay (US Army).

It was an interdisciplinary meeting that included participation by Physicists, Chemists, Biologists and Chemical Engineers. Those who attended agreed to write up their papers after the meeting in the form of chapters suitable for a book. Such was the enthusiasm about the scope and quality of the programme that we received chapters from almost all the participants and this book is the result. We believe it will be a useful addition to the literature in this important area of research and that it gives readers a helpful account of the state of the field of self-assembly at the present time.

While in Tuscany, we were able to enjoy the hospitality of the region and we are specially grateful for a reception by the Mayor of Massa Marittima.

I would like to particularly thank the authors for their written contributions, Peter Brown for his invaluable advice and help with editing the manuscripts, and Dr Einar Fredriksson, Anne Marie de Rover and the publication team at IOS Press for preparing an excellent book.

Finally, we would very much like to thank the US Army, the Italian Ministry for Research (MUR) through the Italian Center for Colloid Science and Nanotechnology, and the National Science Foundation of the USA for their generous financial support of the meeting.

Brian Robinson, Book Editor, School of Chemistry and Pharmacy, University of East Anglia, Norwich, Norfolk NR9 5NF, UK

A brief overview of three diverse new directions in colloid science is given. Particles adsorbed at liquid surfaces are yielding novel types of emulsion, colloidosome particles and new materials. Phenomena arising from the coupling of DNA hybridisation and liquid crystal phase elastic deformation forces with colloids and surfaces are described. Finally, we compare effects of surfactant aggregates on chemical reactions with the external control of reactions offered by “Lab-on-a-Chip” micro reactors.

Two self-assembled systems have been investigated for potential application to chemical biological (CB) defense; gold nanoparticles and functionalized dendritic polymers (dendrimers). The former system consists of nanometer-sized gold particles encapsulated by monomolecular layers of alkanethiol surfactant deposited as thin films on microelectrodes. These films have been shown to be sensitive as chemical vapor detectors. The latter system consist of antibody-dendrimer conjungates shown to improve the sensitivity of immunoassays for microorganisms as well as dendrimers derivatized with reactive groups which decontaminate 1-chloroethylsulfide. These same materials have been incorporated into a cream to provide topical skin protection against chemical agents.

One of the most dramatic causes of wall painting degradation is the slow transformation of the calcium carbonate binder, CaCO₃ into selenite, CaSO₄.2H₂O, due to the combined action of polluting SO₂ and oxygen. This process is called sulphatisation and the effects on the paintings are loss of colour transparency and loss of cohesion of the painted layers. The present paper reviews the sulphatisation process and the peculiarities of both the Ferroni-Dini method and Ca(OH)₂ nanotechnology for pre-consolidation, cleaning and consolidation of wall paintings affected by sulphatisation. We show that the mechanical stress induced by the volume expansion associated with the transformation calcite ⇒ selenite plays a critical role in wall painting degradation by sulphatisation. The first step of the Ferroni-Dini method, consisting of treatment with ammonium carbonate by means of wood poultice compresses, is shown to be effective in removing selenite and converting it into ammonium sulfate. Furthermore, the consolidation induced by the second step of the Ferroni-Dini method, consisting of barium hydroxide treatment, is shown to be due to two different mechanisms: the ‘voids filling’ by BaSO₄ and BaCO₃ crystal formation and the generation in situ of ‘fresh’ lime responsible for a new setting process. The innovative pre-consolidation treatment by Ca(OH)₂ nanoparticles is shown by examples from real cases in important conservation workshops. Applications of the Ferroni-Dini method and Ca(OH)₂ nanotechnology in other fields of work of art conservation are briefly discussed.

Colloidal systems offer many potential advantages for chemical processing when large interfacial areas, small diffusional resistances and intimate mixing are required, and in microfluidic channels where conventional two phase processes cannot be conducted. The effectiveness of some colloidal systems can be enhanced if they respond to temperature, light, redox potentials or magnetic fields to assist in mediating their interactions with the solutions being treated. A range of such colloidal systems are discussed, and their potential advantages delineated.

Pulsed Gradient Spin Echo, PGSE, NMR self-diffusion studies have revealed a powerful tool to investigate organized systems. The experimental techniques are quite easy and accessible but they need, especially in the case of restricted diffusion, suitable interpretation models of the experimental data. Some applications of our approach are presented in order to elucidate the potentialities and the peculiarities of the method used and are discussed in term of next future perspectives.

This chapter reviews some recent experimental investigations of the orientational behaviors of thermotropic liquid crystals on surfaces that possess well-defined topography and chemical functionality. These orientational behaviors reflect competing long-range (elastic, electrostatic) and short-range, chemically specific (e.g., metal-ligand or hydrogen bond) interactions between the surface and liquid crystal. The defined nature of these surfaces makes possible the rationale manipulation of the orientational behavior of liquid crystals. We illustrate the opportunity in this area by using surfaces that present oriented carboxylic acid groups and metal carboxylates. Potential applications of these systems to gas-phase sensing of organophosphonate compounds are also described.

The present article deals with the synthesis of amphiphilic compounds bearing single hydrophobic parts including hydrocarbon, perfluorocarbon and hydrofluorocarbon chains. From the physicochemical study of their behavior in an aqueous medium it appears that their organization depends mainly on the hydrophobic structure. Dynamically stable aggregates could be obtained with certain hybrid species whereas perfluorinated analogues would lead to micellar systems. Supramolecular organizations such as Newton Black Films (NBF) can be obtained with perfluorocarbon products, on the opposite the presence of an ethyl terminus part would prohibit the formation of films. Hybrid amphiphiles have been found to maintain membrane proteins in aqueous media. This class of compound could be used in the future to form supramolecular assemblies involving biomolecules.

A generic model is presented for the self-assembly of chiral rod-like molecules and its applicability demonstrated by observations of the self-assembly behaviour of β-sheet-forming peptides. It is argued that the model also provides an explanation for the self-assembly behaviour of any chiral molecule that can undergo one-dimensional self-assembly. This generally leads to the formation of “hydrogels” or “organogels” which are believed to be comprised of three-dimensional networks of fibrils interconnected at fibre-like junctions.

Nonionic surfactants containing polyoxyethylene chains can act both as templates and as reducing agents in the synthesis of nanoparticles of noble metals from a solution of the metal salt. In this paper, we show that nanoparticles of platinum can be obtained by mixing one microemulsion containing a water-soluble platinum complex, [PtCl₆]²⁻, with another microemulsion containing a reducing agent, such as sodium borohydride (NaBH₄). The choice of surfactant is decisive in controlling the reaction rate. Whereas an alcohol ethoxylate gives a fast reaction regardless of the hydrophilic-lipophilic balance of the surfactant, reaction in a microemulsion based on the anionic surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT) was relatively sluggish. The difference is attributed to the nonionic surfactant assisting NaBH₄ as reducing agent. We also show that silver nanoparticles can be produced by reduction of a silver nitrate solution with a nonionic surfactant, and a block copolymer of the polyoxyethylene-polyoxypropylene-polyoxyethylene type, without the use of any additional reducing agent. This reaction takes place in the narrow channels of a reverse-hexagonal liquid- crystalline phase and the small silver particles became aligned into millimetre long fibres.

The reversible liquid-to-solid transition of the alkane-continuous phase of a dilute colloidal dispersion (w/o microemulsion or surfactant-stabilised nanoparticles) can be induced by pressure and temperature changes, without destabilising the colloid. The structural changes have been studied by Small-Angle Neutron Scattering (SANS) at high pressure (P = 1–600 bar) and over a range of temperature (T = 3–20°C). In the freezing process two microdomains are formed within the frozen dispersion; one, the pure oil solvent which is selectively solidified (I), the other, a concentrated “liquid” dispersion of particles (II) which separates when the solvent freezes. These two microdomains are intimately mixed and bi-continuous within the frozen colloid and exist in a state of equilibrium at fixed pressure and temperature. The position of equilibrium, represented by the proportion of the solvent which is solidified, and thereby the concentration of particles within the ejected microdomains (II), depends on both temperature and pressure. At constant temperature, increasing pressure results in an increase in the particle concentration such that at high pressure the surfactant layers on adjacent particles become compressed or interdigitated. ombined with SANS measurements, to determine the interparticle separation, an analysis in terms of the osmotic pressure (π) provides a “unification” of the effects of both temperature and pressure on the system. The results provide a measure of the interparticle repulsion forces between the adsorbed monolayers on a 3D configuration of particles as a function of particle separation.

Similar results have now been obtained for certain L₁ self-assembly systems undergoing the water-ice transition. Distinct from this behaviour is that occurring when the aqueous phase is solidified by hydrate formation above the normal freezing point. In this case the slowly propagating clathrate structure “engulfs” the dispersed phase which is not ejected into microdomains i.e. the inter-particle structure is here completely “frozen–in”.

Traditional glycero-phospholipids have been extensively studied with varying head groups and hydrocarbon chains. Glycero-phospholipid backbone modifications represent a smaller family of modified phospholipids, and as discussed in this review, these derivatives were prepared for a variety of different needs and physical studies. The ability to chemically-tailor the structure of these amphiphiles combined with the propensity of these amphiphiles to form supramolecular structures of varying size and complexity provides innumerable opportunities for fundamental studies as well as for applications in medicine and biotechnology.

Recent literature concerning photo-surfactants is reviewed and key publications are highlighted. In addition, the synthesis and properties of a new stilbene-containing gemini surfactant 1 are described. This compound is unusual since in micellar solutions it undergoes dimerization, rather than isomerization, and this has interesting consequences for interfacial and aggregation properties.

An interfacial Maillard reaction between furfural and cysteine in two different food-grade nano-sized self-assembled solutions with two oppositely curved interfaces (W/O and O/W microemulsions) have been studied and compared. These microemulsions are selective microreactors strongly enhancing the generation of sulfur-containing flavors. The Maillard reactions occur at lower temperatures than in water and are much faster. The interfaces of both W/O and O/W microemulsions are capable of enhancing the Maillard reactions in which the selectivity and reactivity are controlled by the composition of the interface and its curvature.

In the W/O microemulsions the Maillard reaction was controlled and enhanced by the interfacial concentration of a co-emulsifier such as butanol and are restricted by the concentration of the core water reservoir.

On the other hand, in the O/W microemulsions, where water is the continuous phase, the reaction rates are enhanced by increase in the water content and the microemulsion curvature. The Maillard product internal composition (regioselectivity and type of products) is dictated by temperature, time, pH and mainly by the nature of the interface, and by the surfactant nature and its interfacial composition.

Cyclodextrins(CDs) form stable supramolecular structures (polypseudorotaxanes) with different polymers, through a threading process, by hosting the hydrophobic chain in their free cavities.

The kinetics of formation of such adducts in different solvents and with different reactants (α-, β-, or γ-CD, and PEG, PPG or pluronics) at different temperatures was studied. The physico-chemical treatment provides the number of CD molecules threaded per single polymer chain, and the corresponding thermodynamic parameters. Addition of salts to the reaction medium reveal a Hofmeister series effect. Induced water structure changes were observed when D₂O, urea and sugars were added to the solvent.

In this chapter is described a novel approach for synthesizing mesoporous silicas with tunable pore diameters, wall thickness and pore spacings that can be used as templates for the assembly of semiconductor nanowire arrays. Silicon and germanium nanowires, with size monodisperse diameters, can readily be formed within the mesoporous silica matrix using a supercritical fluid inclusion technique. These nano-composite materials display unique optical properties such as intense room temperature ultraviolet and visible photoluminescence. The implication of these mesoporous nanowire materials for future electronic and opto-electronic devices is discussed.

In this paper we review our work on the interaction of polymers and surfactants in solution and at interfaces. A common theme is the change in structure found when polymer chains are decorated with surfactants or sugars.