Colloids and Surfaces, 54 (1991) 313-319
Elsevier Science Publishers B.V., Amsterdam
!•
313
A study of the relationship between the chernical
structure of sorne carboxylic acids and their capacity to
inhibit the crystal growth of calciurn fluoride
F. Grases, A. Garcia-Raso,
Department
of Chemistry,
J. Palou, A. Costa-Bauza
University
and J.G. March
of Balearic Islands, 07071-Palma de Mallorca (Spain)
(Received 8 May 1990; accepted 20 July 1990)
Abstract
The influence of oxalic acid, malonic acid, malic acid, tartaric acid, succinic acid and glycolic
acid on the growth of calcium fluoride has been studied and interpreted in terms of Langmuirtype adsorption isotherms. Oxalic acid was the most active inhibitor of calcium fluoride growth,
followed by tartaric, malic and malonic acids. Succinic acid caused a very slight effect and monocarboxylic acids caused no inhibitory effects under the conditions studied. An attempt to relate
the chemical structure of the studied carboxylic acids with their inhibitory capacity using molecular models and MNDO calculation has been described. The presence of a sequence constituted
by a carboxylic group and an hydroxy group in the a position, in addition to another carboxylic
group demonstrated a special efficacy to inhibit crystal growth of calcium fluoride crystals due to
structural features. Interactions between inhibitors and the crystal surface were found to be mainly
electrostatic in nature.
INTRODUCTION
Inhibition of crystal growth is an interesting physico-chemical phenomenon
with very important applications in several fields such as medicine (therapy
ofurolithiasis)
[1,2] and industry [3,4]. Nevertheless, the mechanism of crystal growth inhibition is not understood in most cases. Thus, inhibitors are
usually tested individually on the basis of their application, which in this age
of advanced technology seems quite primitive.
Diverse bibliographic information shows that some structural correlations
between inhibitors and the crystals they inhibit or affect must exist [5], thus
even the effectiveness of a given inhibitor can vary when different hydration
forms of the same substance are tested [6]. On the other hand, several interesting studies of inhibitor implications in stereochemistry and materials science can be found in the literature [7-11]. It is evident that a better way to
identify new inhibitors consists of establishing the mechanism of crystal growth
inhibition. In this paper we attempt to relate the molecular structure of several
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© 1991-
Elsevier Science Publishers B.V.
314
carboxylic acids with their inhibitory capacity on calcium fluoride crystal
growth, using molecular models and MNDO calculations.
EXPERIMENTAL
Calcium chloride and potassium fluoride, both purchased from Panreac, were
used to prepare super-saturated solutions. Ammonium chloride (Panreac) was
used to adjust ionic strength. Solutions containing fluoride were prepared and
stored in polypropylene vessels in order to prevent fluoride attack on the glass
surfaces. Seed crystal suspensions were prepared from commercial precipitated calcium fluoride, analytical reagent grade, purchased from Probus. The
reactions were initiated by adding the solution of fluoride to the reaction cell
which already contains Ca (II) ions, calcium fluoride seed crystals and the carboxylic acid under study. The carboxylic acids studied (glycolic acid, oxalic
acid, malonic acid, succinic acid, malic acid and tartaric acid) were purchased
from Alfa Chemical Co. Crystal growth was followed using a potentiometric
technique by means of a calcium specific electrode (Ingold) coupled with a
silver/silver chloride electrode. The other experimental details were described
in a previous paper [12]. Crystallographic data for constructing unit cells of
calcium fluoride [13] and molecular models (Cochranes of Oxford molecular
models, No. 8-61) were used for structural studies. MNDO calculations were
made with a VAX-ll/750 computer.
RESULTS AND DISCUSSION
The crystal growth of calcium fluoride was studied in the presence of several
carboxylic acids (glycolic acid, oxalic acid, malonic acid, succinic acid, malic
acid and tartaric acid) using calcium fluoride seed crystals. Complexation
equilibria between carboxylic acids and calcium were neglected due to the low
carboxylic acid/calcium(II)
molar ratios used. The values of the thermodynamic solubility products (Ps) were obtained from Ref. [14]. Rates of crystallization were calculated from the experimental curves (potential versus time)
by use of the following equation:
R=dTea/dt-
exp [(m V -b)/a]"
a
where a and b are constant parameters
mV =a"ln Tea +b
(1)
d(mV)
dt
deduced from the calibration
graph:
(2)
Equation (1) is deduced from Eqn (2) by derivation.
Selected logarithmic plots corresponding to the kinetics of calcium fluoride
seed crystals growth, in the presence and absence of carboxylic acids are presented in Fig. 1. The rates of crystallization, both in the presence and absence
315
-10
Ln R
¡¡¡
•
a
•
-11
Control
Oxalie aeid'
Malie aeid
Tartarie aeid
-12
-13
-14
-15
-1,0
0,0
1,0
Ln (o)
Fig. 1. Apparent order for crystal growth kinetics in the absence and in the presence ofvarious of
the assayed carboxylic acids. The kinetic data were adjusted to the rate equation: R= -dTca/
dt=k· an where a= [(aca 'a~ )1/3 _P~/3] / P~/3.
,1111
malie
malonie
sueeinie
oxalie
3,5 1 e
la
tartari
•
3,0 -1I 2,5 o
RolR
2,0
1,5
5
1,0
O
5
10
15
20
25
[Inh]
Mol x ¡-1 x 106
Fig. 2. Langmuir adsorption kinetics model applied to calcium fluoride growth in the presence of:
(1) oxalic acid; (2) tartaric acid; (3) malic acid; (4) malonic acid; and (5) succinic acid. Ionic
strength=O.l M (NH4CI), a=2.00, and t=25°C.
316
~~~
O
' o
2.70 A
'
ea,--3.80 A'--r---~-/ea
~1;/
,
A
iJ A. (
3,80 A
O
O
R
"
O
O~O
O
=0
OXALlC ACIO
R = CH2-COo-
MALlC ACIO
R = CHOH-COO-
TARTARIC ACIO
R=
O
MALONIC ACIO
ea
r\ XF,~
F/~t
ea.
~F
ea
~:
ea
\/·)< ';/1\
F~,//
.•
O
§O
,F/F
ea
"C
","
O
"
, '~ ""C'"
O
Fig. 3. Structural
relations between the dicarboxylic acids and a unit cell of CaF 2'
of additives, expressed in terms of the relative supersaturation, show an effective order, n, equal to 2 in all cases and, therefore, crystallization models seem
to indicate spiral growth.
The application of a simple kinetic Langmuir model yielded the straight
lines shown in Fig. 2. Plots like these are use fuI for comparative purposes, since
the slope of such plots is a measure of the affinity of a certain inhibitor for the
particular solid substrate. As can be appreciated, several of the dicarboxylic
317
9
b)
E
<1
>6
4
j
2a)
8
7
>
E
<1
5
0,1
0,2
pH
Fig. 4. Millivolts difference after 5 min fluoride addition between a sample containing oxalic acid
(1.1'10-5 M) and their respective blanks: (a) pH values obtained by adding drops of diluted
ammonia or hydrochloric acid to the super-saturated solution which already contains ammonium
chloride (0.1 M); and (b) ionic strength (ammonium chloride concentration). The other initial
conditions for both (a) and (b) were [F-] =2.8'10-3 M, [Ca2+] = 1.0'10-3 M, added seed=3
mg, final volume=200 mI, and t=25°C.
acids assayed manifested important inhibitory effects; the oxalic acid exhibited the highest effect and succinic acid the lowest effect, whereas glycolic acid
showed no effect under the conditions studied. The distance between the carboxylic oxygen and an a-oxygen is, respectively, 2.86 ± 0.02 and 3.60 ± 0.20 Á
which closely corresponds to the F-F (2.70 Á) and Ca-Ca (3.80 Á) distances
in a unit cell, as can be seen in Fig. 3. Carboxylic acids present very different
energy values and orbital coefficients ofHOMO and LUMO molecular orbitals
and thus it is impossible to propose a general orbital interaction between these
acids and the calcium fluoride crystal. For this reason we can assume that the
main interaction is probably electrostatic. It is noteworthy that when the pH
decreases or the strength increases, the oxalate inhibitory capacity decreases
in both cases, as shown in Fig. 4. These facts are in agreement with the interactions between the carboxylic acids and the calcium fluoride crystals being
fundamentally electrostatic in nature. As is shown in Fig. 5, the presence of
two adjacent active groups permits a very effective adaptation ofthe substance
on the crystal surface as a consequence of two different types of interactions.
The main interaction is by change of two fluoride atoms for the two oxygen
atoms of the acid (oxalic, malic and tartaric present this interaction) and a
secondary electrostatic interaction between the oxygen of the carboxylic acid
and calcium atoms at the surface (Fig. 3).
In conclusion, it is evident that molecular structural models can be used in
explaining the inhibitory behaviour of some substances on the crystal growth
of a particular crystalline structure. Nevertheless, the prediction of the inhibitory behaviour of new non-assayed substances by simple spatial interatomic
318
matching can be difficult or even erroneous in some cases. However, molecular
models can be used for predicting new structures based on the well-known
inhibitory behaviour of a large number of substances.
ACKNOWLEDGEMENT
We acknowledge financial support from the "Dirección General de Investigación Científica y Técnica" (Spain) in grant No 86-0002.
319
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231
"
1451
10
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13
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