Structure-Dependent Dissolution and Restructuring of Calcite Surfaces by Organophosphonates

Article abstract of DOI:10.1021/acs.cgd.7b00959

Organophosphonates are well-known to strongly interact with the surfaces of various minerals, such as brucite, gypsum, and barite. In this work, we study the influence of six systematically varied organophosphonate molecules (tetraphosphonates and diphosphonates) on the dissolution process of the (10.4) surface of calcite. In order to pursue a systematic study, we have selected organophosphonates that exhibit similar structural features, but also systematic architectural differences. The effect of this class of additives on the dissolution process of the calcite (10.4) surface is evaluated using in situ dynamic atomic force microscopy. For all of the six organophosphonate derivatives, we observe a pronounced restructuring of the (10.4) cleavage plane of calcite, demonstrated by the formation of characteristically shaped etch pits. To elucidate their specific influence on the dissolution process of calcite (10.4), we vary systematically the number of functional end groups (two for the tetraphosphonates and one for the diphosphonates), the spacing between the functional ends through separating methylene groups (2, 6, and 12), as well as the pH of the solution (ranging from 2.6 up to 11.7). For each of the two groups of the organophosphonate derivatives, we observe the very same formation of etch pits (olive-shaped for the tetraphosphonate and triangular-shaped for the diphosphonate molecules), respectively. This finding indicates that the number of functional ends decisively determines the resulting calcite (10.4) surface morphology, whereas the size of the organophosphonate molecule within one group seems not to play any important role. For all of the molecules, the restructuring process of calcite (10.4) is qualitatively independent of the pH of the solution and, therefore, independent of the protonation/deprotonation states of the molecules. Our results reveal a general property of organophosphonate derivatives to induce surface restructuring of the calcite (10.4), which seems to be very robust against variations in both, different molecular structures and different protonation/deprotonation states.

Full text of DOI:10.1021/acs.cgd.7b00959

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Article
Structure-Dependent Dissolution and Restructuring
of Calcite Surfaces by Organophosphonates
Martin Nalbach, Argyri Moschona, Konstantinos D Demadis,
Stefanie Klassen, Ralf Bechstein, and Angelika Kuhnle
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00959 • Publication Date (Web): 02 Oct 2017
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Structure-Dependent Dissolution and Restructuring
of Calcite Surfaces by Organophosphonates
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Martin Nalbach,†,* Argyri Moschona,‡ Konstantinos D. Demadis,‡ Stefanie Klassen,† Ralf
Bechstein,† Angelika Kühnle†
†Institute of Physical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14,
55099 Mainz, Germany
‡Crystal Engineering, Growth and Design Laboratory, Department of Chemistry, University of
Crete, Voutes Campus, Heraklion, Crete, GR-71003, Greece
Keywords: mineral-water-interface, calcite (10.4), surface restructuring, phosphonate,
dissolution, in-situ atomic force microscopy
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ABSTRACT
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Organophosphonates are well known to strongly interact with the surfaces of various minerals,
such as brucite, gypsum and barite. In this work, we study the influence of six systematically varied
organophosphonate molecules (tetraphosphonates and diphosphonates) on the dissolution process
of the (10.4) surface of calcite. In order to pursue a systematic study, we have selected
organophosphonates that exhibit similar structural features, but also systematic architectural
differences. The effect of this class of additives on the dissolution process of the calcite (10.4)
surface is evaluated using in-situ dynamic atomic force microscopy. For all of the six
organophosphonate derivatives, we observe a pronounced restructuring of the (10.4) cleavage
plane of calcite, indicated through the formation of characteristically shaped etch pits. To elucidate
their specific influence on the dissolution process of calcite (10.4), we vary systematically the
number of functional end groups (two for the tetraphosphonates and one for the diphosphonates),
the spacing between the functional ends through separating methylene groups (two, six and
twelve), as well as the pH of the solution (ranging from 2.6 up to 11.7). For each of the two groups
of the organophosphonate derivatives, we observe the very same formation of etch pits (olive-
shaped for the tetraphosphonate and triangular-shaped for the diphosphonate molecules),
respectively. This finding indicates that the number of functional ends decisively determines the
resulting calcite (10.4) surface morphology whereas the size of the organophosphonate molecule
within one group seems not play any important role. For all of the molecules, the restructuring
process of calcite (10.4) is qualitatively independent of the pH of the solution and, therefore,
independent of the protonation / deprotonation states of the molecules. Our results reveal a general
property of organophosphonate derivatives to induce surface restructuring of the calcite (10.4),
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which seems to be very robust against variations in both, different molecular structures and
different protonation / deprotonation states.
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INTRODUCTION
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Organophosphonic acids are molecules that possess a direct phosphorus-carbon (P-C) bond
between the phosphonate tetrahedral moiety and the organic part, in contrast to organophosphates,
which possess a phosphorus-oxygen-carbon (P-O-C) linkage.1 The presence of the robust P-C
bond (dissociation energy of 513 kJ/mol2) renders these molecules resistant to hydrolysis and
thermal decomposition.3 More specifically, all organophosphonates studied here belong to the
aminomethylene-phosphonate family, bearing the amino-methylenephosphonate (–N+(H)-CH2-
PO3H-) zwitterionic moiety. Due to the high affinity of the phosphonate group towards metal ions
and mineral surfaces, organophosphonates have been used extensively in the construction of metal-
organic frameworks,4 as scale growth inhibitors,5 and they have been studied intensively on
various mineral surfaces like calcite,6–8 gypsum,9 barium sulfate,10 and brucite.11
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Figure 1. (a) Model of the calcite (10.4) cleavage plane. (b) Schematic structure of the
tetraphosphonates. We study molecules with a number of methylene spacing groups of n = 1, 3, 6.
(c) Schematic structure of the diphosphonates. Here, the number of methylene spacing groups
equals m = 1, 5, 11. The scale bar in (b) applies to all subfigures.
Organic additives are known to alter the surface morphology of minerals in various ways, e.g.
affecting dissolution and growth, as well as inducing a restructuring of the surface.12–18 For
example, ancient polysaccharide16 adsorbs with a high affinity to calcite step edges, resulting in a
modified and characteristically restructured surface morphology during both, calcite dissolution
and calcite growth.
Here, we study the influence of three tetraphosphonate and three diphosphonate molecules on the
dissolution process of the calcite (10.4) surface, the thermodynamically most stable cleavage plane
of calcite (Figure 1a). The schematic structures for each family of organophosphonates are shown
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in Figure 1b and 1c, the detailed molecule names, abbreviations and schematic structures are given
in Table 1.
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Table 1. Names, abbreviations and schematic structures for the organophosphonate molecules. The
abbreviation includes the acronym for the name, the number of methylene spacing groups (Cx), as
well as a labeling for the number of phosphonic acid end groups. T denotes tetra for the four
phosphonic acid groups in the tetraphosphonates and D denotes di for the two phosphonic acid
groups in the diphosphonates.
Additive
Abbreviation
Additive Name
Additive Structure
O
-O
P
OH
O-
NH+
P
O
OH
Ethylenediamine-tetrakis(methylenephosphonic acid)
EDTMP-C2-T
O
P
NH+
P
OH
O-
HO
O-
O
O-
P
O
-O
P
O
OH
NH+
OH
Hexamethylenediamine-tetrakis(methylenephosphonic acid)
Dodecamethylenediamine-tetrakis(methylenephosphonic acid)
HDTMP-C6-T
DDTMP-C12-T
NH+
OH
HO
O
P
P
O-
O-
O
O
-O
O-
P
P
OH
OH
P
NH+
NH+
OH
O
HO
P
O-
O-
O
Ethylamine-bis(methylenephosphonic acid)
EABMP-C2-D
HABMP-C6-D
n-Hexamethylamine-bis(methylenephosphonic acid)
n-Dodecamethylamine-bis(methylenephosphonic acid)
DABMP-C12-D
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For our studies, we focus on two principal structural differences of the additive molecules. On the
one hand, within each group of organophosphonate molecules, we study the influence of the
spacing between the functional ends through a variation of the number of methylene (-CH2-)
groups. We compare molecules with only two separating methylene groups (EDTMP-C2-T and
EABMP-C2-D), six (HDTMP-C6-T and HABMP-C6-D) and twelve (DDTMP-C12-T and
DABMP-C12-D). On the other hand, we compare for each of the three pairs of organophosphonate
molecules the difference in the number of functional end groups, which consists of phosphonic
acid (-PO3H2) groups connected via a tertiary amine (R-N(CH2)2) group. The tetraphosphonate
molecules possess two amino-bis(methylenephosphonate) functional ends (Figure 1b), the
diphosphonate molecules only one identical amino-bis(methylenephosphonate) functional end
(Figure 1c), respectively. Finally, we systematically vary for all molecules the solution pH to
elucidate the pH dependence of the surface restructuring process of calcite (10.4) in the presence
of organophosphonate molecules.
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We find that independent of the solution pH, all six organophosphonate molecules significantly
restructure the (10.4) surface of calcite by the formation of characteristically shaped etch pits. This
observation indicates a general property of molecules with functional ends consisting of
phosphonate groups connected via a tertiary amine group to enable surface restructuring of the
calcite (10.4) cleavage plane. The number of phosphonate end groups alone seems to exert no
influence on the ability of the molecules to restructure the calcite (10.4) surface, however, it seems
to determine decisively the resulting surface morphology. For the three tetraphosphonate
molecules we observe characteristic olive-shaped etch pits, while for the diphosphonate molecules
a triangular etch pit geometry is predominant on the calcite (10.4) cleavage plane. The results of
the surface restructuring within each group of organophosphonate molecules are independent of
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the spacing between the functional ends, revealing that the size of an organophosphonate molecule
does not have an influence on the dissolution process of calcite (10.4) and only the presence of at
least one functional end plays a decisive role.
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MATERIALS AND METHODS
Organophosphonate synthesis. Ethylenediamine (99%), 1,6-diaminohexane (> 98%), 1,12-
diaminododecane (> 98%), ethylamine (70%), n-hexylamine (99%) and n-dodecylamine (97%)
are purchased from Alfa Aesar, formaldehyde (36.5% aqueous solution) is purchased from Riedel-
de Haen and phosphorus (99%) as well as hydrochlorid acid (37 % aqueous solution) are purchased
from Sigma-Aldrich. The reactants are all used without further purification. All organophosphonic
acid molecules are synthesized via the Mannich-type (Irani-Moedritzer) reaction. In principle, this
reaction allows the clean transformation of a primary amine to an amino-bis(methylenephosphonic
acid) moiety. Literature procedures are followed.19–21 All organophosphonic acids are isolated as
high-purity solids in > 50 % yields, except EDTMP-C2-T which is kept dissolved in water and
used as a 25.4 % w/w stock solution.
Solution preparation. Pure water (18.2 MΩ∙cm) is produced from a Merck Millipore purification
setup. Hydrochloric acid (HCl, 0.1 M and 1 M) and sodium hydroxide (NaOH, 0.1 M and 1 M)
standard solutions are purchased from Carl Roth GmbH & Co. KG and used to adjust the pH of
the solutions.
Instrumentation. All pH measurements are conducted using a Schott laboratory pH meter (CG
842) equipped with a BlueLine pH electrode (Schott Instruments, 18 pH). The pH electrode is
calibrated weekly utilizing buffer standard solutions with a pH value of 4 and 7 (HANNA
instruments, type Hi6004 and Hi6007).
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In-situ dynamic atomic force microscopy. For all measurements, we use calcite crystals with a
sample size of 4 x 4 mm² purchased from Korth Kristalle GmbH. Prior to each experiment, a calcite
crystal is freshly cleaved and cleaned under a nitrogen flow. The measurements are conducted at
a constant temperature of 28 °C with a commercial atomic force microscope (AFM) from Bruker
Corporation (MultiMode V with Nanoscope V controller) that has been modified for high-
resolution imaging in liquid environment.22,23 Unless otherwise stated, all AFM images shown
here are taken using the frequency modulation (FM) mode in a liquid cell from Bruker Nano
Surfaces Division, with gold-coated and p-doped silicon cantilevers (PPP-NCHAuD, Nanosensors
and Tap300GD-G, BudgetSensors). The cantilevers possess a typical eigenfrequency of 100-150
kHz in liquids and a spring constant of ~40 N/m. Initially performed high-resolution measurements
enabled atomic resolution of the calcite (10.4) substrate surface, but at the same time, no adsorbed
molecular structures on calcite (10.4) step edges or terraces could be observed. As we focus in this
study on the dissolution process of calcite (10.4) on a large-scale, especially looking at etch pit
sizes and shapes, we concentrate here on AFM images with µm size. For all in-situ AFM
experiments, the oscillation amplitude of the cantilever is kept constant at 1 nm. In all AFM images
displayed here, the slow and fast scan direction, as well as the measured channel (zp is the relative
piezo displacement) are shown in the schematics in the upper right corner.
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Measurement conditions. We conduct all in-situ AFM measurements in an, with respect to the
solubility of calcite, undersaturated solution. The solution only contains pure water, the solute
organophosphonate molecule and, depending on the starting conditions, hydrochloric acid or
sodium hydroxide, respectively. An immersed calcite crystal in this undersaturated solution
dissolves at all times, independent of the pH and until reaching the dynamic carbonate equilibrium.
2-
During calcite dissolution calcium (Ca2+) and carbonate (CO3 ) ions are released from the surface
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continuously, the latter causing the pH of the surrounding solution to converge to a value of around
8.3,24 corresponding to the carbonate equilibrium. As we observe a continuously dissolving calcite
(10.4) surface within the measuring time of the experiments, we can conclude that the surrounding
solution is still undersaturated and that all images are taken at non-equilibrium conditions. The
small injection volume (approximately 0.3 mL) of the liquid cell disables pH measurements during
and after the in-situ AFM experiments. Therefore, all pH values stated in this study correspond to
the initial pH of the solution, which is adjusted and measured prior to the injection into the liquid
cell. Furthermore, as the used liquid cell is open to air, the solution evaporates undoubtedly at all
times during the in-situ AFM experiment. Hence, this evaporation increases the concentration of
all components in the solution. We note that the indicated organophosphonate concentrations also
refer to the initial concentrations as prepared.
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RESULTS AND DISCUSSION
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We study aqueous organophosphonate solutions with additive concentration ranging from 0.19
µM up to 309 µM. For all additives, we find that a dissolving calcite (10.4) surface restructures
under shrinkage of calcite terraces and the formation of characteristically shaped etch pits (Figure
2). Compared to a calcite (10.4) surface dissolving in pure water (in the absence of additives), the
calcite (10.4) surface appears to be much more rough and uneven in the presence of the three
tetraphosphonate molecules (Figure 2a-c). We observe irregular shaped calcite (10.4) terraces and
several, but small etch pits. During the experiment, these etch pits grow mainly in size while
exposed calcite terraces vanish almost completely. For all three tetraphosphonate molecules
(EDTMP-C2-T, HDTMP-C6-T and DDTMP-C12-T), the very same restructured surface
morphology of calcite (10.4) is revealed. This indicates a general property for the group of
tetraphosphonate molecules to be able to restructure calcite (10.4) during dissolution. Therefrom,
the spacing between the functional ends seems to have no influence on the surface restructuring
process as no differences between two, six and twelve separating methylene groups are observed.
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Figure 2. Representative AFM image series of a calcite (10.4) surface in the presence of the
organophosphonate molecules demonstrating calcite dissolution and etch pit formation. Images (a-
c) are taken in the presence of 0.37 µM EDTMP-C2-T at a pH of 3.1. A rough and uneven calcite
(10.4) surface with predominant etch pits and less distinct terraces is found. Perimeters of a
shrinking calcite terrace and a growing etch pit are exemplarily marked by white lines. The scale
bar in image (a) applies to all images in the series (a-c). Images (d-f) are taken in the presence of
258 µM EABMP-C2-D at a pH of 6.7. Here, the area of distinct calcite terraces is larger compared
to image series (a-c). The dimension of one dominant growing etch pit is marked by white arrows.
The scale bar in image (d) applies to all images in the series (d-f). The images color scale represents
a height difference of 3.3 nm (a, e), 4.3 nm (b), 5.0 nm (c), 3.5 nm (d) and 2.8 nm (f).
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For the three diphosphonate molecules (EABMP-C2-D, HABMP-C6-D and DABMP-C12-D), a
very similar situation can be found during calcite dissolution. We also observe a restructured
calcite (10.4) surface, however, compared to the tetraphosphonate molecules, now with a larger
area of distinct terraces and triangular-shaped etch pits (Figure 2d-f). Here, during the course of
the experiment the etch pits also grow in size and depth as the confining step edges retreat.
However, the calcite (10.4) substrate appears to be less rough and uneven as compared to when
the diphosphonate molecules are present. Variation of the spacing between the functional ends
within the group of diphosphonate molecules (i.e. the length of the alkyl side-chain) again shows
no difference in the surface restructuring of a dissolving calcite (10.4) surface. This finding also
points towards a general property for diphosphonate molecules to restructure calcite (10.4) during
dissolution.
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As all six organophosphonate molecules studied here show this very same influence on calcite
(10.4) dissolution, we can conclude that the existence of at least one functional end (two
methylenephosphonate groups connected via a tertiary amine group) enables calcite (10.4) surface
restructuring. Furthermore, as we can see qualitative differences in the surface morphology
induced by tetra- and diphosphonate molecules, but no differences within each group, we can
deduce that the number of functional end-groups has a significant influence whereas the spacing
between the functional ends seems not to play an important role.
Etch pit geometries on calcite (10.4). The dissolution process of calcite (10.4) in pure water is
characterized by the formation of rhombohedral etch pits (Figure 3a). These etch pits are
̅̅
̅
]
[
]
[
terminated by the thermodynamically most stable step edges running along the 441 and 481
directions (Figure 3b). All four step edges of an etch pit are neutral because of an alternating
arrangement of calcium ions and carbonate groups. Tilting of the carbonate groups in the calcite
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(10.4) surface structure results in two different edges pits sides, namely acute and obtuse. The two
obtuse step edge sides are known to exhibit a more rounded shape, resulting in the asymmetric
etch pit appearance in Figure 3a.25–27
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Figure 3. Etch pit geometries observed on the calcite (10.4) surface in the absence and presence
of the organophosphonate molecules. (a) Amplitude modulation atomic force microscopy (AM-
AFM) image of the calcite (10.4) surface in pure water showing characteristic rhombohedral etch
pits. (b) Model of the calcite (10.4) surface illustrating the etch pit termination by the
̅̅
̅
]
[
]
[
thermodynamically most stable step edges running along the 441 and 481 directions. (c)
Characteristic olive-shaped etch pit in the presence of the three tetraphosphonate molecules
EDTMP-C2-T, HDTMP-C6-T and DDTMP-C12-T. The long diagonal of the etch pit is running
̅
[
]
along the [010] direction, the short diagonal along the 421 direction. The obtuse step edge sides
are rounded similarly to the one in etch pit in (a). (d) A characteristic triangular etch pit geometry
is found when the diphosphonate molecules EABMP-C2-D, HABMP-C6-D and DABMP-C12-D
̅
[
]
are present. The etch pits observed here only show the 481 directions as one of the usually seen
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acute step edges directions. The angle α to the other acute edge is measured to be 75°. The obtuse
step edge side is slightly rounded too. The images color scale represents a height difference of 3.8
nm (a), 3.5 nm (c) and 3.0 nm (d).
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In the presence of the three tetraphosphonate molecules EDTMP-C2-T, HDTMP-C6-T and
DDTMP-C12-T etch pits with an olive shape are observed all over the calcite (10.4) surface (Figure
3c). The geometry of the olive-shaped etch pits is similar to etch pits on the calcite (10.4) substrate
in pure water. The long diagonal is also running along the [010] direction, likewise the short
̅
[
]
diagonal is running along the 421 direction. The two obtuse step edge sides are rounded in a
similar way as it is observed in the asymmetric etch pit appearance in pure water. In contrast to
the etch pit shape in pure water, the two acute edges of the olive-shaped etch pit are modified,
resulting in a very symmetric etch pit geometry.
For the three diphosphonate molecules EABMP-C2-D, HABMP-C6-D and DABMP-C12-D, we
find etch pits with a predominant triangular etch pit geometry (Figure 3d). These etch pits only
show one of the commonly seen thermodynamically most stable acute step edges directions, the
̅
[
]
481 direction. The obtuse step edge side is slightly rounded too and, therefore, comparable to
the etch pit curvature for pure water and for the presence of the tetraphosphonates. The pronounced
angle in the acute step edge corner is measured to be 75°.
pH-independent dissolution process. Organophosphonic acids produce a plethora of chemical
species in aqueous solutions, depending on the pH.28 Popov et al.29 have summarized a large
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portion of the available literature on critical stability constants for a number of phosphonic acids.
Based on the reported data, we draw Scheme 1, illustrating the protonation / deprotonation of
EDTMP-C2-T, exemplarily for the tree tetraphosphonates, as well as Scheme 2 showing the
protonation / deprotonation of EABMP-C2-D, exemplarily for the tree diphosphonates.
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Scheme 1. Protonation / deprotonation sequence for EDTMP-C2-T, showing eight experimentally
detected species. The stepwise deprotonation behavior, starting from the three-fold deprotonated
(H7EDTMP-C2-T-) species to the fully deprotonated (EDTMP-C2-T8-) species, is displayed from
left to right and top to bottom. Values for the logarithmic acid dissociation constant (log Ka) for
each protonation / deprotonation step are provided.29 Relevant protons for each protonation /
deprotonation step are highlighted in red.
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Such data and related speciation diagrams30 can be very useful in interpreting experimental results,
particularly for mineral scale inhibition31 and decalcification of biominerals32,33 but should be used
with caution due to the different experimental conditions employed in measuring the pKa’s of the
phosphonic acids.
Scheme 2. Protonation / deprotonation for EABMP-C2-D, showing five experimentally detected
species. The stepwise deprotonation behavior, starting from the neutral (H4EABMP-C2-D) species
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to the fully deprotonated (EABMP-C2-D4-) species. Relevant protons for each protonation /
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deprotonation step are highlighted in red.
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To study the pH-dependence of calcite (10.4) surface restructuring in the presence of the
organophosphonate molecules, we perform in-situ AFM experiments with solution pH values
ranging from 2.6 up to 11.7. The results for the measurements with varying solution pH are
illustrated in Figure 4. Here, we show representative results for the tetraphosphonate EDTMP-C2-
T at solution pH values of 4.1, 8.2 and 11.7 (Figure 4a-c), and for the diphosphonate EABMP-C2-
D at solution pH values of 3.5, 5.7 and 9.5 in Figure 4d-f. We find no significant differences in the
surface restructuring, depending on the pH value and regardless of the specific molecule studied.
We observe for both families of organophosphonate molecules the very same etch pit shape (olive
for the tetraphosphonate and triangular for the diphosphonate molecules) as the result of the
dissolution process, independent of solution pH. This observation indicates that the surface
restructuring process seems to be insensitive against variations in both factors, i.e. structural
differences due to the alkyl spacing between the functional ends and different protonation /
deprotonation states caused by the solution pH. The pH independence of calcite (10.4) surface
restructuring has been recently also observed in in-situ AFM studies in the presence of organic dye
molecules34 and, hence, might indicate a more general surface restructuring mechanism in the
presence of chemical additives.
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Figure 4. Representative AFM images demonstrating the pH independence of the surface
restructuring of calcite (10.4) in the presence of the organophosphonate molecules. The images
(a), (b) and (c) are taken in the presence of 0.19 µM EDTMP-C2-T at solution pH values of 4.1,
8.2 and 11.7, respectively. The images (d) and (f) are taken in the presence of 309 µM EABMP-
C2-D, image (e) in the presence of 258 µM EABMP-C2-D at solution pH values of 3.5, 5.7 and
9.5, respectively. The scale bar in (a) applies to all images. The images color scale represents a
height difference of 3.0 nm (a), 2.5 nm (b, d), 5.0 nm (c), 3.3 nm (e) and 2.1 nm (f).
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SUMMARY AND CONCLUSION
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Herein, we systematically study the impact of six different organophosphonate molecules, three
tetraphosphonates (EDTMP-C2-T, HDTMP-C6-T and DDTMP-C12-T) and three
diphosphonates (EABMP-C2-D, HABMP-C6-D and DABMP-C12-D) on the dissolution process
of calcite (10.4) using in-situ dynamic AFM. For each molecule, a pronounced surface
restructuring is observed, resulting in the formation of characteristically shaped etch pits for each
group of organophosphonate molecules. From that, we can conclude that the existence of at least
one functional end (two phosphonate groups connected via methylene linkers to a tertiary amine
group) is sufficient to enable calcite (10.4) surface restructuring. This observation points towards
a general property of organophosphonate molecules to alter and restructure the (10.4) surface of
calcite during dissolution. Furthermore, the difference in the number of functional phosphonate
end groups (four for the tetraphosphonates and two for the diphosphonates) decisively determines
the resulting surface morphology. We observe olive-shaped etch pits for the tetraphosphonate and
triangular-shaped ones for the diphosphonate molecules. In contrast to the number of functional
end groups, changing the spacing between the functional ends through a variation of the number
of methylene groups (two, six and twelve) has no influence on the dissolution process of calcite
(10.4). For all three tetraphosphonate as well as for the three diphosphonate derivatives, we
observe the very same etch pit geometry, respectively. Finally, the dissolution process of calcite
(10.4) in the presence of each of all six molecules is independent of solution pH and, therefore,
not affected by the protonation / deprotonation state of the molecules. The surface restructuring
process of calcite (10.4) during dissolution in the presence of the organophosphonate molecules
studied here appears to be very robust against variations in both, additive molecular structure and
solution pH. Although precise information on the interaction of the phosphonate group with the
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calcite surface at the molecular level cannot be drawn from AFM studies, it is well established that
the doubly-deprotonated R-PO32- group interacts much more strongly with the calcite surface than
the singly-deprotonated R-PO3H- group.7 This is consistent with the pH-independent restructuring
process observed for all organophosphonate additives. Furthermore, the second additive side
moiety (a second amino-bis(methylenephosphonate) group for the tetraphosphonates and an alkyl
group for the diphosphonates) does not seem to directly interact with the surface of calcite (10.4).
However, it may indirectly influence the interaction of the entire molecule by creating a
hydrophilic (for tetraphosphonates) or a hydrophobic (for diphosphonates) microenvironment on
the calcite surface. The structural differences in the non-interacting side-groups between
tetraphosphonates and diphosphonates are consistent with the morphology of the etch pits
observed. The important observation that within the same family the side chain-length does not
seem to influence the restructuring process confirms that one bis(methylenephosphonate) moiety
alone is necessary for the process to occur.
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AUTHOR INFORMATION
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Corresponding Author
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*nalbach@uni-mainz.de
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Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
ACKNOWLEDGMENT
AK acknowledges financial support by the German Research Foundation through project
KU1980/7-1 and KDD thanks the University of Crete for financial support through Research Grant
KA 3517.
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ABBREVIATIONS
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EDTMP-C2-T, ethylenediamine-tetrakis(methylenephosphonic acid);
HDTMP-C6-T, hexamethylenediamine-tetrakis(methylenephosphonic acid);
DDTMP-C12-T, Dodecamethylenediamine-tetrakis(methylenephosphonic acid);
EABMP-C2-D, ethylamine-bis(methylenephosphonic acid);
HABMP-C6-D, n- hexamethylamine-bis(methylenephosphonic acid);
DABMP-C12-D, n- dodecamethylamine-bis(methylenephosphonic acid);
AFM, atomic force microscopy;
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(33) Ehrlich, H.; Koutsoukos, P. G.; Demadis, K. D.; Pokrovsky, O. S. Principles of
demineralization: Modern strategies for the isolation of organic frameworks. Part II.
Decalcification. Micron 2009, 40, 169–193.
(34) Nalbach, M.; Klassen, S.; Bechstein, R.; Kühnle, A. Molecular Self-Assembly Versus
Surface Restructuring During Calcite Dissolution. Langmuir 2016, 32, 9975–9981.
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Structure-Dependent Dissolution and
Restructuring of Calcite Surfaces by
Organophosphonates
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Martin Nalbach,†,* Argyri Moschona,‡ Konstantinos D. Demadis,‡ Stefanie Klassen,† Ralf
Bechstein,† Angelika Kühnle†
In-situ atomic force microscopy demonstrates the decisive impact of six different
organophosphonates on the dissolution process of calcite (10.4). Independent of the solution pH,
a pronounced surface restructuring is observed for each molecule, resulting in the formation of
characteristically shaped etch pits. Our observation points towards a general property of
organophosphonates to alter the (10.4) surface of calcite during dissolution.
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Additive
Abbreviation
Additive Name
Additive Structure
O
-O
P
OH
O-
NH+
P
O
OH
Ethylenediamine-tetrakis(methylenephosphonic acid)
EDTMP-C2-T
O
P
NH+
P
8
9
OH
O-
HO
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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27
28
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33
34
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O-
O
O-
P
O
-O
P
O
OH
NH+
OH
Hexamethylenediamine-tetrakis(methylenephosphonic acid)
Dodecamethylenediamine-tetrakis(methylenephosphonic acid)
HDTMP-C6-T
DDTMP-C12-T
NH+
OH
HO
O
P
P
O-
O-
O
O
-O
O-
P
P
OH
O
OH
P
NH+
NH+
OH
O
HO
P
O-
O-
O
Ethylamine-bis(methylenephosphonic acid)
EABMP-C2-D
HABMP-C6-D
n-Hexamethylamine-bis(methylenephosphonic acid)
n-Dodecamethylamine-bis(methylenephosphonic acid)
DABMP-C12-D
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