Electrons in Cement

Science  01 Jul 2011:
Vol. 333, Issue 6038, pp. 49-50
DOI: 10.1126/science.1207837

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A free or ionized electron introduced into a polar molecular solvent such as water or ammonia can be stabilized or trapped by reorientation of the host solvent molecules, leading to the appearance of “solvated” electrons in the liquid (1, 2). For example, in solutions of alkali metals in liquid ammonia, alkali valence electrons are spontaneously ionized by the dissolving metal and released directly into solution. These systems, first described by Sir Humphry Davy in 1808 (3), are brilliant blue, electrolytic conductors when dilute in metal and spectacular golden-bronze and genuinely metallic in concentrated solutions. Kraus, as early as 1914, described the negative carrier of electric current in dilute metal-ammonia solutions as “the negative electron surrounded with an envelope of solvent molecules” (see the figure, panel A) (4). Two years later, Gibson and Argo (5) introduced the term “solvated electrons” to describe this, the simplest anion in solution. On page 71 of this issue, Kim et al. (6) report the synthesis and properties of solvated electrons in an entirely new and highly unusual host solvent—cement. The authors have succeeded in preparing solvated electrons in a high-temperature (1873 K) melt of mayenite, 12CaO·7Al2O3 (abbreviated to C12A7), a component of alumina cement.