Small-Angle Scattering from Confined and Interfacial Fluids

The phase behavior of bulk fluids is now well understood and their properties can be predicted accurately using equations of state over a wide range of pressures and temperatures. The behavior of bulk fluids changes dramatically when they are injected into small pores, due to increasing importance of the boundary conditions and molecule-surface interactions. Thus, confinement leads to the emergence of a new set of variables that impact the phase behavior in tight pores but may be neglected in the thermodynamic limit. Examples of such variables are the pore size, shape, and interconnectivity as well as the chemical composition of the pore walls and fluid-surface interaction potential. Due to involvement of numerous systemspecific parameters, a comprehensive understanding of the influence of confinement on the fluid behavior is only beginning to emerge. In addition to their fundamental interest, the ability to understand and predict the phase behavior and dynamics of fluids in natural and engineered porous solids is crucial for a variety of the environment- and energy storage-related technologies. These include the capture and sequestration of anthropogenic greenhouse gases, hydrogen storage, membrane separation of gases, environmental remediation, and catalysis. Until recently, the adsorption of fluids and structure of pores in various porous materials have been routinely explored using volumetric and gravimetric methods, mercury porosimetry, and sorption isotherms. These traditional tools, however, have their limitations. First, they provide data averaged over the entire sample volume and thus fail to elucidate how pores of different sizes contribute to the integral parameters as a function of pressure and temperature. Second, they are invasive, which eliminates contribution from the closed-off regions of the pore space and may affect the integrity of the solid matrix. In contrast, noninvasive small-angle scattering (SAS) techniques offer the unique opportunity to “look but not touch” inside pores and monitor changes in the adsorption behavior of fluid molecules confined in pores of different sizes and topology, as well as to detect the pores inaccessible to the invading fluid. For this reason, a few years ago researchers began to develop and refine scattering techniques and their interpretations as a reliable tool for probing properties of confined and interfacial fluids in natural and engineered porous materials with different structural properties. These efforts resulted in evolving new methods of the SAS data analysis and interpretation as well as developing the new generation of the state-of-the-art high pressure cells that are being used by researchers interested in studying structural and adsorption properties of confined supercritical fluids and gases under pressure. This book examines the macro-, meso- and microscopic aspects of the fluid behavior in porous solids using noninvasive methods of small-angle neutron and x-ray scattering (SANS and SAXS) as well as ultra small-angle neutron and x-ray scattering (USANS and USAXS). There is a great deal of similarity between the x-ray and neutron scattering and therefore both methods are presented and discussed together so that readers may become familiar with both and appreciate the advantages and disadvantages of each type of radiation for the specific system or type of experiment. Both neutrons and x-rays penetrate porous solids and are scattered on the solid/void interface. At ambient conditions, the resulting scattering patterns are governed by the geometry and topology of the pore space on the scale from about 1 nm to about 10 μm and provide quantitative data about the total porosity, pore size distribution, and the specific area of the scattering interface. SAS experiments performed on fluid saturated samples maintained in cells with controlled pressure and temperature conditions, combined with isotopic substitution of invading fluids, facilitate contrast variation experiments. The simplest application— direct contrast matching between the solid matrix and the invading fluid— discriminates between the open (accessible) and closed (inaccessible) porosity. Both SANS/USANS and SAXS/USAXS enable the observation of pore-size-specific invasion of the pore space by fluids and help to access important information on the volume fraction of the adsorbed phase and its average physical density. For geological samples, this can be done in situ at a subsurface-like temperature and pressure conditions. The book is meant as a reference for active researchers in the field, but also may serve as a comprehensive guide for university faculty members and students, who may be insufficiently aware of the range of opportunities provided by the smallangle scattering techniques. The book commences with introductory chapters, which describe major principles of SAS techniques and are sufficiently comprehensive to be useful to researchers interested in structural characterization of various types of materials in different fields of science. Chapters 1–4 introduce the basic properties of neutrons and x-rays, provide brief description of the available neutron and x-ray sources, and give illustrative examples of SAS instrumentation and sample environment. This is followed by discussion of the practical aspects of SAS experiments, sample preparation methods, optimal instrument configurations, and basic principles of the data reduction and analysis presented in Chaps. 5 and 6. Chapter 7 deals with the SAS structural characterization of various porous solids, and Chap. 8 describes studies of confined vapors below saturating pressure with the emphasis on vapor adsorption and capillary condensation. Chapter 9 is concerned with studies of confined and interfacial liquids. Experimental SAS investigations of high-pressure adsorption of supercritical fluids and gases in various engineered and natural porous materials are discussed in Chap. 10. The author has enjoyed and benefitted from longstanding collaboration with T.P. Blach, N.C. Gallego, C.I. Contescu, M. Mastalerz, J.R. Morris, A.P. Radlinski, J.A. Rupp, L.F. Ruppert, R. Sakurovs, and G.D. Wignall. Special thanks are due to my younger colleagues J. Bahadur, S.M. Chathoth, G. Cheng, and L. He who contributed their enthusiasm and talent in many studies of confined fluids described in this book. It is a great pleasure to acknowledge M.M. Agamalian, J.M. Carpenter, A.P. Radlinski, and G.D. Wignall for reading select chapters of the manuscript and offering valuable comments. The assistance of Renee´ Manning and Genevieve Martin in preparing high quality artwork is greatly appreciated.