Author
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Laird, David |
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Submitted to: Journal of Applied Clay Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/24/2006 Publication Date: 10/2/2006 Citation: Laird, D.A. 2006. Influence of layer charge on swelling of smectites. Journal of Applied Clay Science. 34:74-87. Interpretive Summary: The ability to swell by adsorbing water is a critical property of soil clays that influences both the ability of soils to retain water and the stability of soil structure, which allows both air and water to move through soils. The swelling properties of clays also determine the suitability of clays for various industrial applications. The six independent processes that control clay swelling are described in this paper along with interactions between these processes. As the surface charge density of a clay increases swelling in general decreases, but not all swelling processes are influenced by surface charge. Several of these processes may occur at the same time, and their combined effects determines the bulk swelling properties of clay in soils and sediments. This paper is of primary interest to scientists who seek to understand the formation and stabilization of soil structure and to engineers who seek to develop new industrial applications of clays. Technical Abstract: Six separate processes control the swelling of smectites saturated with alkali and alkaline earth cations in aqueous systems. Crystalline swelling occurs between smectite layers within quasicrystals and involves the intercalation of zero to four discrete layers of water molecules. A balance between strong electrostatic-attraction and hydration-repulsion forces controls crystalline swelling. The extent of crystalline swelling decreases with increasing layer charge. Double-layer swelling occurs between quasicrystals. An electrostatic repulsion force develops when the positively charged diffuse portions of double layers from two quasicrystals overlap in an aqueous suspension. Layer charge has little or no direct effect on double-layer swelling. The break up and formation of quasicrystals is a dynamic process that controls the average size of quasicrystals in an aqueous smectite suspension. As layer charge increases, quasicrystals tend to become larger and more stable. In smectite suspensions with more than one type of exchangeable cation, the cations can demix due to a complex feedback between cation exchange selectivity and crystalline swelling. Increasing layer charge increases selectivity for alkaline earth cations relative to sodium (Na) or lithium (Li), and hence reduces the breakup of quasicrystals. Co-volume swelling is an entropy driven process caused by restrictions on the rotational freedom of suspended quasicrystals. Brownian swelling is also an entropy driven process resulting from random thermal motion of suspended colloids. There is no reason to believe that layer charge directly influences either co-volume or Brownian swelling. Macroscopic measures of swelling (e.g., change in total volume or water content) necessarily measure the combined effect of all swelling processes occurring within the system |
