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1.
Sci Rep ; 8(1): 10689, 2018 Jul 11.
Article in English | MEDLINE | ID: mdl-29993006

ABSTRACT

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

2.
Sci Rep ; 7(1): 7592, 2017 08 08.
Article in English | MEDLINE | ID: mdl-28790315

ABSTRACT

The structure and the strength of organic compound adsorption on mineral surfaces are of interest for a number of industrial and environmental applications, oil recovery, CO2 storage and contamination remediation. Biomineralised calcite plays an essential role in the function of many organisms that control crystal growth with organic macromolecules. Carbonate rocks, composed almost exclusively of calcite, host drinking water aquifers and oil reservoirs. In this study, we examined the ordering behaviour of several organic compounds and the thickness of the adsorbed layers formed on calcite {10.4} surfaces. We used X-ray reflectivity (XRR) to study calcite {10.4} surfaces that were prepared in three alcohols: methanol, isopropanol and pentanol and one carboxylic acid: octanoic acid. All molecules adsorbed in self-assembled layers, where thickness depended on the density and the length of the molecule. For methanol and isopropanol, molecular dynamic simulations (MD) provided complementary information, which allowed us to develop a surface model. Branching in isopropanol induced slightly less ordering because of the additional degree of freedom. Pentanol and octanoic acid adsorbed as single monolayers. The results of this work indicate that adhered organic compounds from the surrounding environment can affect the surface behaviour, depending on properties of the organic compound.

3.
Langmuir ; 31(13): 3847-53, 2015 Apr 07.
Article in English | MEDLINE | ID: mdl-25790337

ABSTRACT

Molecular dynamics (MD) simulations were used to explore adsorption on calcite, from a 1:1 mixture of ethanol and water, on planar {10.4} and stepped, i.e. vicinal, surfaces. Varying the surface geometry resulted in different adsorption patterns, which would directly influence the ability of ethanol to control calcite crystal growth, dissolution, and adsorption/desorption of other ions and molecules. Ethanol forms a well-ordered adsorbed layer on planar faces and on larger terraces, such as between steps and defects, providing little chance for water, with its weaker attachment, to displace it. However, on surfaces with steps, adsorption affinity depends on the length of the terraces between the steps. Long terraces allow ethanol to form a well-ordered, hydrophobic layer, but when step density is high, ethanol adsorption is less ordered, allowing water to associate at and near the steps and even displacing pre-existing ethanol. Water adsorbed at steps forms mass transport pathways between the bulk solution and the solid surface. Our simulations confirm the growth inhibiting properties of ethanol, also explaining how certain crystal faces are more stabilized because of their surface geometry. The -O(H) functional group on ethanol forms tight bonds with calcite; the nonpolar, -CH3 ends, which point away from the surface, create a hydrophobic layer that changes surface charge, thus wettability, and partly protects calcite from precipitation and dissolution. These tricks could easily be adopted by biomineralizing organisms, allowing them to turn on and off crystal growth. They undoubtedly also play a role in the wetting properties of mineral surfaces in commercial CaCO3 manufacture, oil production, and contamination remediation.

4.
Langmuir ; 30(22): 6437-45, 2014 Jun 10.
Article in English | MEDLINE | ID: mdl-24823316

ABSTRACT

We have used density functional theory and the implicit solvent model, COSMO-RS, to investigate how the acidity constant, pKa, of organic acids and bases adsorbed at the organic compound-aqueous solution interface changes, compared to its value in the aqueous phase. The pKa determine the surface charge density of the molecules that accumulate at the fluid-fluid interface. We have estimated the pKa by comparing the stability of the protonated and unprotonated forms of a series of molecules in the bulk aqueous solution and at an interface where parts of each molecule reside in the hydrophobic phase and the rest remains in the hydrophilic phase. We found that the pKa for acids is shifted by ∼1 pH unit to higher values compared to the bulk water pKa, whereas they are shifted to lower values by a similar amount for bases. Because this pKa shift is similar in magnitude for each of the molecules studied, we propose that the pKa for molecules at a water-organic compound interface can easily be predicted by adding a small shift to the aqueous pKa. This shift is general and correlates with the functional group. We also found that the relative composition of molecules at the fluid-fluid interface is not the same as in the bulk. For example, species such as carboxylic acids are enriched at the interface, where they can dominate surface properties, even when they are a modest component in the bulk fluid. For high surface concentrations of carboxylic acid groups at an interface, such as a self-assembled monolayer, we have demonstrated that the pKa depends on the degree of deprotonation through direct hydrogen bonding between protonated and deprotonated acidic headgroups.

5.
Langmuir ; 29(35): 11062-73, 2013 Sep 03.
Article in English | MEDLINE | ID: mdl-23919655

ABSTRACT

The adsorption behavior of calcium carbonate is an important factor in many processes in nature, industry, and biological systems. We determined and compared the adsorption energies for a series of small molecules of different sizes and polarities (i.e., water, several alcohols, and acetic acid) on three synthetic CaCO3 polymorphs (calcite, aragonite, and vaterite). We measured isosteric heats of adsorption from vapor adsorption isotherms for 273 < T < 293 K, and we used XRD and SEM to confirm that samples did not change phase during the experiments. Density functional calculations and molecular dynamics simulations complemented the experimental results and aided interpretation. Alcohols with molecular mass greater than that of methanol bind more strongly to the calcium carbonate polymorphs than water and acetic acid. The adsorption energies for the alcohols are typical of chemisorption and indicate alcohol displacement of water from calcium carbonate surfaces. This explains why organisms favor biomolecules that contain alcohol functional groups (-OH) to control which polymorph they use, the crystal face and orientation, and the particle shape and size in biomineralization processes. This new insight is also very useful in understanding organic molecule adsorption mechanisms in soils, sediments, and rocks, which is important for predicting the behavior of mineral-fluid interactions when the challenge is to remediate contaminated groundwater aquifers or to produce oil and gas from reservoirs.

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