WITH TWO PLATES SummaryScanning electron micrographs show that the reaction products of solution phosphate and calcite are hemispherical, coral-like growths on calcite surfaces. Electron probe micro-analysis indicates that these are a calcium phosphate, and solution data plotted against solubility isotherms suggest that dicalcium phosphate (DCP) is formed rapidly and slowly changes to octa-calcium phosphate (OCP). X-ray diffraction shows that DCP and OCP are present with DCP predominating. The ratio of Ca:P in the treated calcite, after allowing for the CaC03 present, is that in DCP. and a surface coating of OCP on DCP is likely. The exchangeability of the reacted P falls from 100 per cent for small amounts (0-10 pg P per g calcite) to a constant 30 per cent when larger amounts are present (200-1000 p d g ) resulting from the porous structure of the coral-like growths. These cover only a small fraction of the calcite surface even when large amounts of P are present so allowing calcite to control solution pH. IntroductionTHE reaction of phosphate with calcite surfaces involves adsorption of small amounts of phosphate followed by precipitation of calcium phosphate at higher levels (Cole et al., 1953). The adsorption has been described by the Langmuir isotherm (Kuo and Lotse, 1972), with better fit when the isotherm is divided into two linear sections (Griffin and Jurinak, 1973) and has been described as a heterogeneous nucleation process leading to about 5 per cent of the calcite surface being covered with phosphate.The initial adsorption is thought to occur at certain sites where lateral interaction between phosphate ions produces surface clusters which then act as nuclei for subsequent crystal growth. These nuclei are presumably an amorphous calcium phosphate (Stumm and Leckie, 1970) which is slowly transformed into crystalline calcium phosphate, the form of which seems to depend on the adsorbing surface and on the solution conditions. For example Cole et al. (1 953) found evidence that dicalcium phosphate was formed, Clark and Peech (1955) concluded that octacalcium phosphate or dicalcium phosphate were formed depending on the conditions, Stumm and Leckie (1 970) showed that apatite formed in calcareous sediments, and Holford and Mattingly (1975) found octacalcium phosphate in soils on Oolitic Limestone. Time of reaction is also likely to be important, for Arvieu and Bouvier (1974) state that dicalcium phosphate is the first reaction product which by hydrolysis gives octacalcium phosphate, and

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Interest in deep eutectic solvents (DESs), particularly for electrochemical applications, has boomed in the past decade because they are more versatile than conventional electrolyte solutions and are low cost, renewable, and non‐toxic. The molecular scale lateral nanostructures as a function of potential at the solid–liquid interface—critical design parameters for the use of DESs as electrochemical solvents—are yet to be revealed. In this work, in situ amplitude modulated atomic force microscopy complemented by molecular dynamics simulations is used to probe the Stern and near‐surface layers of the archetypal and by far most studied DES, 1:2 choline chloride:urea (reline), at the highly orientated pyrolytic graphite surface as a function of potential, to reveal highly ordered lateral nanostructures with unprecedented molecular resolution. This detail allows identification of choline, chloride, and urea in the Stern layer on graphite, and in some cases their orientations. Images obtained after the potential is switched from negative to positive show the dynamics of the Stern layer response, revealing that several minutes are required to reach equilibrium. These results provide valuable insight into the nanostructure and dynamics of DESs at the solid–liquid interface, with implications for the rational design of DESs for interfacial applications.