Organisers

• Flor Siperstein (University of Manchester)
• Miguel Jorge (Laboratory of Separation and Reaction Engineering at Universidade do Porto)
• Nigel Seaton (Institute for Materials and Processes at the University of Edinburgh)

Supports

 CECAM

 SimBioMa

Description

Surfactants are molecules composed of a hydrophilic (water-loving) part and a hydrophobic (water-hating) part, which makes them align in water/oil interfaces or form a fascinating range of nanostructures in solution, depending on their concentration, the temperature of the system, and the presence of additives in solution. Materials science exploits the use of surfactants to create self-assembled nanostructures, and a good example is the synthesis of templated mesoporous silica based materials, introduced by Kresge et al. [1]. Creating inorganic materials with well controlled pore sizes and pore connectivity can be important in many fields, from catalysis and separations, to sensors, electronic applications, and biomaterials. [2-4]
Understanding the formation of these materials has not been easy, and although significant progress has been made over the last decade, from an experimental [4-8] and modelling [9-14] and theoretical [15] perspectives, many issues are not well understood. One of the major challenges that the formation of surfactant templated materials poses is that it is an inherently multiscale problem.
At the smallest scale we have the condensation reactions of the inorganic precursor, whose mechanism is strongly affected by the pH of the solution. Some understanding of these processes comes from the extensive work that has been done to understand the formation of zeolites [16-18], although understanding how the presence of surfactants affects such processes is a problem that has not been addressed.
The next scale deals with the interactions between small inorganic oligomers and surfactant self assemblies. It is now understood that the presence of silica oligomers is necessary to drive the phase separation that leads to the formation of templated materials [10,19], and simulations also confirm that strong interactions between inorganic and surfactant heads lead to a phase separation where a sufficiently high surfactant concentration can form ordered phases. Even when some approximations can be made for systems at equilibrium, understanding the evolution of these structures is far from understood. No reports have been made for any successful coarse graining approach that will successfully describe the structural evolution observed in these materials, as state of the art molecular dynamics simulations can not reach sufficiently long times [20].
At the largest scale we have the effects of hydrodynamics (stirring as reported in experiments), which is known to have some effect that can favour or not the ordering of nanostructures, but our understanding of such phenomena is not sufficient to model or predict it.
A parallel problem to understanding the synthesis of surfactant templated materials is to describe the structures formed with sufficient accuracy to understand their applications and limitations. Extensive work has been done in trying to determine pore size distributions of materials containing cylindrical pores, using theory and simulations [21-24], but more limited work has addressed the effect of surface roughness and variations of pore size in the axial direction of a given cylinder that leads to constrictions [25-27].
We consider that the importance and potential impact that surfactant templated materials can have in science and technology demand a better understanding of their synthesis and characterisation, which requires the development of appropriate models and simulation approaches that can deal with the different scales involved.

Scientific Objectives

The purpose of this workshop is to bring together scientist interested in the formation of surfactant self-assembled materials, their characterisation and applications, to discuss the outstanding issues that need to be addressed in order to gain a better understanding on how the materials are formed, how can be controlled and their structure and properties for specific applications. Discussion on different topics will attempt to answer the following questions:

1. Ab initio simulations: what can we learn from them with respect to the reaction mechanisms? Which are the stable species in solution? How large are the energy barriers? What is the role of the surfactant? What are the current computational limitations? What is the information that can be transferred to a “many-scale” simulation approach and what is the need for a true “multi-scale” approach?

2. Force fields: which one should we use and how are they parameterised? How transferable are the potentials for different species? How sensitive are the physical properties that determine organic-inorganic self-assembly to variations in the force fields? What information is needed from first principle calculations or experimental data to generate more accurate force fields? What is the trade-off this community is willing to pay between accuracy and transferability?

3. Atomistic simulations: what are the time and length scale limitations? How to deal with reactive systems? Do we know how different are the characteristic time scales for surfactant rearrangements, or exchanges with the bulk and the condensation reaction rates? Which components or species are essential and which can be ignored? How does the formation of alcohols during the silica condensation affect the self-assembled structures? What can we learn from molecular dynamics, Monte Carlo or kinetic Monte Carlo simulations? What is the information that can be transferred to a “many-scale” simulation approach and what is the need for a true “multi-scale” approach?

4. Mesoscopic simulations: Can we really do multiscale simulations for these systems? What is the information that can be transferred to a “many-scale” simulation approach and what is the need for a true “multi-scale” approach? How can we do effective coarsegraining? What are the necessary features that need to be present in a mesoscopic simulation to model the structure and evolution of surfactant aggregates in the presence of condensing silica species? Are hydrodynamic effects important?

5. Characterisation: what features are necessary to understand the properties of the materials? How important is the pore geometry and surface chemistry? What information is obtained from experiments that is useful for modell methods are available for the material characterisation
6. Experimental evidence: what is the simulation community ignoring that experimentalists consider essential? Is it possible to measure properties that would validate simulations at different scales? How can we improve the exchange of information and knowledge?

Format
The dialogue between scientists working on different aspects of surfactant templated materials is often hindered by the lack of a common language due to the variety of computational strategies needed and an insufficient understanding of experimental evidence by the modelling and simulation community. Therefore we have selected a format that will have 30 min oral presentations followed by 15 min discussion, to facilitate the communication between scientists in different communities.

References

[1] Kresge CT, Leonowicz M, Roth WJ, Vartuli JC, Beck JS Nature 359, 710-712 (1992)
[2] Sanchez C, Julian B, Belleville P, Popall Journal of Materials Chemistry 15, 3559-3592 (2005)
[3] Sanchez C, Arribart H, Guille MMG Nature Materials 4, 277-288 (2005)
[4] Soler-illia GJD, Sanchez C, Lebeau B, Patarin J Chemical Reviews 102, 4093-4138 (2002)
[5] Hoffmann F, Cornelius M, Morell J, Froba M, Angew Chem Int Ed Engl, 45, 3216-51 (2006)
[6] Imperor-Clerc M, Davidson P, Davidson A Journal of the American Chemical Society, 122, 11925-11933 (2000)
[7] Liang, Y., M. Hanzlik, and R. Anwander, Periodic mesoporous organosilicas: mesophases control via binary surfactant mixtures, Journal of Materials Chemistry, 16, 1238-1253, (2005)
[8] Epping JD, Chmelka BF, Current Opinion in Colloid and Interface Science 11, 81-117 (2006)
[9] Auerbach, SM; Ford, MH; Monson, PA Current Opinion in Colloid and Interface Science 10, 220-225 (2005)
[10] Patti, A; Mackie, AD; Siperstein, FR Langmuir 23, 6771-6780 (2007)
[11] Siperstein, FR; Gubbins, KE Langmuir 9, 2049-2057 (2003)
[12] Jorge, M; Auerbach, SM; Monson, PA Journal of the American Chemical Society 127, 14388-14400 (2005)
[13] Schumacher C, Gonzalez J, Wright PA, Seaton NA Journal of Physical Chemistry B 110, 319-333 (2006)
[14] Schumacher C, Gonzalez J, Perez-Mendoza M, Wright PA, Seaton NA Industrial and Engineering Chemical Research 45, 5586-5597 (2006)
[15] Gov, N. Itamar, B, Goldfarb, D. Langmuir 22, 605-614 (2006)
[16] Rao, NZ; Gelb, LD Journal of Physical Chemistry B 108, 12418-12428 (2004)
[17] Van Erp TS, Caremans TP, Kirschhock CEA, Martens JA, PCCP 9, 1044-1051 (2007)
[18] Trinh TT, Jansen APJ, van Santen RA Journal of Physical Chemistry B 110, 23099-23106 (2006)
[19] Firouzi A, Atef F, Oertli AG, Stucky GD, Chmelka BF, Journal of the American Chemical Society 119, 3596-3610 (1997)
[20] Jorge M, Seaton NA, Fundamentals of Adsorption 2007.
[21] Kruk M, Jaroniec M, Sayari A, Chemistry of Materials 11, 492-500 (1999)
[22] Maddox MW, Olivier JP, Gubbins KE, Langmuir 13, 1737-1745 (1997)
[23] Ravikovitch PI, Neimark AV Langmuir 22, 11171-11179 (2006)
[24] Thommes M, Smarsly B, Groenewolt M, Ravikovitch PI, Neimark AV, Langmuir 22, 756-764 (2006)
[25] Coasne, B; Pellenq, RJM, Journal of Chemical Physics 120, 2913-2922 (2004)
[26] Salazar, R; Gelb, LD Langmuir 23, 530-541 (2007)
[27] Coasne B, Hung FR, Pellenq RJM, Siperstein FR, Gubbins KE, Langmuir 22, 194-202 (2006)