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Morphology and growth of methyl stearate as a function of crystallisation environment Diana M. Camacho, Kevin J. Roberts, Frans Muller, Danielle Thomas, Iain More, and Ken Lewtas Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01436 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016
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Crystal Growth & Design
MORPHOLOGY AND GROWTH OF METHYL STEARATE AS A FUNCTION OF CRYSTALLISATION ENVIRONMENT
Diana M. Camachoa, Kevin J. Robertsa*, Frans Mullera, Danielle Thomasa, Iain Moreb Ken Lewtasb,c [a] Institute of Particle Science and Engineering, School of Chemical and Process Engineering, University of Leeds, Leeds, LS2 9JT, UK
[b] Infineum UK Ltd, Milton Hill Business and Technology Centre, Abingdom, OX13 6BB, UK
[c] Current address: Lewtas Science & Technologies Ltd., Oxford, OX2 , UK
Keywords: Biodiesel cold-flow behaviour, crystal growth kinetics and mechanism, morphological indexing, methyl esters, solvent effect, phase contrast, in-situ microscopy
*Corresponding author
To be submitted to Crystal Growth & Design
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ABSTRACT
In-situ studies of methyl stearate growing from supersaturated n-dodecane, kerosene and toluene solutions reveal strong evidence that solvent choice influences the crystal morphology and crystal growth kinetics.
Crystals with similar habit are observed in all solvents, with the exception of lower supersaturations in kerosene, where a less symmetric morphology was observed. BFDH analysis based on the monoclinic 2 crystal structure of methyl stearate, yielded the morphological indexation to be (110), (1-10), (-110) and (-1-10) for the dominant observed habit and (110) (1-10) (-1-10) (-240) (-3-10) for the less symmetric habit observed in kerosene solvent. Measurements of the growth rate for the (110) and (1-10) faces are similar for all solutions ranging from 0.02 to 1.13 /,
for significantly lower values of
supersaturation in the case of toluene. The tendency of the growth rates’ dependence on was consistent with the Burton-Cabrera-Frank (BCF) growth mechanism in n-dodecane, the Birth and Spread (B&S) mechanism in kerosene and diffusion controlled in toluene solvent.
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1. INTRODUCTION Wax formation in diesel and biodiesel fuels at low temperatures is one of the major problems faced by the fuels industry as crystallisation of the saturated compounds present in these solutions can plug up filters and obstruct pipelines. Preventing wax formation in these multicomponent mixtures requires a good understanding of the crystallisation behaviour of saturated compounds such as Fatty Acid Methyl Esters (FAMEs) present in biodiesel fuel. Notably, growth kinetics and morphological studies can provide the basis to develop adequate technology, such as additives, that can modify the crystals´ habit to allow for the fuel’s adequate flow and settlement properties.
Relevant progress has been made in this area
[1]
including: the use of molecular dynamics
(MD) to study crystal nucleation and growth [2, 3] and the prediction of crystal’s growth from the assessment of the solid/solution interface molecular structure
[4, 5]
. The prediction of the
crystals´ habit, using the Extended Interface Structural Analysis (EISA) shows that interfacial molecular events can be efficiently incorporated to assess crystal growth and morphology when co-solvents, impurities and additives are used
[6]
.
Similarly, molecular based
simulations have been used to control and predict the crystal shapes of organic compounds based on energetic and crystal structure features e.g. the control of AmB crystals’ shape using tailor-made additives
[7]
; the effect of hydroxyl-propyl methylcellulose (HPMC)
concentration and solvent on the crystal habit of Nifedipine (Nif) shape of Para-mino benzoic acid (PABA) crystals
[8]
; the prediction of the
[9]
, 6-methyl-2-thiouracil
[10]
.
Methodologies for the study of face specific growth kinetics include novel techniques such as the use of a growth cell coupled with phase contrast microscopy imaging to analyse the effect of solution flow on crystal growth
[11, 12]
and stereovision
[13]
. The publication by Van Page 3 of 42
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Driessche A.E.S. et.al. (2008)
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[14]
, provides a thorough review of measuring techniques for
the study of crystal growth kinetics.
Despite the developments in this area, thus far there have been no relevant fundamental studies for crystallisation of saturated methyl esters, such as methyl palmitate and stearate, due to the complex nature of crystallisation of these materials. Morphological and growth kinetic studies in saturated methyl esters are scarce, not only due to the difficulty of obtaining reliable crystallographic information for these compounds, but also due to the difficulty of growing observable faceted crystals.
Limited structural information, obtained through powder X-ray diffraction on methyl stearate, suggests this compound crystallises in two polymorphic forms: monoclinic and orthorhombic. The unit cell parameters of the stable monoclinic
[15]
form are thought to be
a=5.61 Å, b=7.33 Å, C=106.6 Å, β=116.47º with crystal space group 2/ . Similarly the unit cell parameters of the orthorhombic
[16]
form are a=5.61 Å, b=7.35 Å, C=95.15 Å with
crystal space group . Observations of methyl stearate crystals have shown this compound crystallises in a plate-like morphology in which the expected dominant (001) face is believed to grow via a screw dislocation mechanism because of spiral growth on this face [17]
.
Given the lack of studies on the crystallisation of saturated methyl esters, it is the aim of this study to deliver fundamental information on the morphology and crystal growth kinetics of methyl stearate as a function of solution environment. These findings are complemented and cross-correlated in section (3.3) with results presented in Camacho D. et al, “Solubility and Page 4 of 42
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nucleation of methyl stearate as a function of crystallisation environment”, manuscript submitted for publication to Cryst. Growth Des ID cg-2016-014359, where a fundamental analysis of methyl stearate nucleation was carried out.
2. MATERIALS AND METHODS 2.1. Materials The solute methyl stearate and solvents n-dodecane and toluene were purchased from SigmaAldrich. The purity of the methyl stearate used was 96% and that of the two solvents was higher than 99%. No further purification was carried out. Kerosene was supplied by Infineum Ltd. Its hydrocarbon composition and n-alkanes chain length distribution is summarised in Table 1. and Fig. 1 respectively. Table 1. Composition of kerosene from 2D Gas Chromatography analysis performed by Infineum UK
Paraffins Cycloalkanes
Aromatics
Hydrocarbon n-alkanes Iso-paraffins Naphthenes Alkyl Benzenes Benzocycloparaffins Naphthalenes Biphenyls/acenaphthenes Fluorenes
Mass % 16.29 23.04 42.40 7.60 6.80 3.43 0.30 0.15
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4
3
mass %
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2
1
0
C6
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
n-alkane Fig. 1 Kerosene n-alkane mass fraction distribution as obtained by 2D Gas Chromatography analysis performed by Infineum UK
2.2. Equipment and experimental procedure In-situ crystal growth studies were carried out using an experimental set-up comprising an optical microscope (Olympus BX51), operated in Differential Interference Contrast (DIC) mode, which was integrated with a QImaging/QICAM camera which captured crystal images as a function of time. The images were then analysed using the QCapture Pro software. The associated growth cell comprised a simple temperature-controlled rectangular tank (10 X 12 cm, depth 1.5 cm) sealed with two removable rectangular glass plates. The solution was secured within a 0.5 ml sealed UV glass cuvette with a path length of 1 mm which was placed within the cell as close to the objective lens of the microscope as feasible. The temperature within the cell is controlled using a Huber Ministat 125 circulating water bath that circulates water through the growth cell. The overall system is shown in Fig. 2.
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Fig. 2 Experimental set up for crystal growth rates measurements, after [11]. (a) Olympus BX51 optical DIC microscope integrated with QImaging/QICAM camera. (b) Enlarged picture of the crystal growth cell
Due the operational working temperature range of the growth cell used, different concentrated solutions were chosen for the analysis. In the case of n-dodecane and kerosene systems, solutions with concentrations of 350 g/L of solvent were chosen. Likewise a solution with concentration of 538 g/L in the case of toluene was selected. This allowed for an operation temperature range above 10 °C to avoid any condensation on the walls of the growth cell.
The supersaturation required for crystallisation was created by decreasing the solution´s temperature from the equilibrium temperature to different chosen temperatures within the metastable zone. Although the supersaturation is set by decreasing the solution’s temperature, circulating water through the cell, the growth of the crystals is only measured once the targeted temperature has been established.
The supersaturation level at each
temperature is calculated using expression (1)
1
(1)
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Where is the solution concentration and is the molar fraction of the solute in the solution at equilibrium, obtained from the van’t Hoff equation at the temperature of measurement. The crystal morphology and subsequent growth of the observed crystals was followed by recording images at equal time-intervals, every 5-20 seconds depending of the speed of the crystal growth in each system. The growth rates of the individual faces were obtained by following the increase with time of the normal distance from the centre of the projected two dimensional 2 crystal to the faces as shown in Fig. 3. The crystals´ centres were defined by drawing lines that connected the crystals´ corners defined by the two most important observed faces. Ten to thirteen measurements of the normal distance increase were recorded.
Fig. 3 Example of measurement of normal distances from the centre of the crystal to the faces. The distances were obtained using QCapture Pro software by drawing a perpendicular line to each face from the centre of the crystal
Due to the experimental challenges involved, notably due to these crystallisation systems having a very small metastable zone width (MSZW), it was not feasible at this stage of experimental methodology and data acquisition, to make a statistically significant number of measurements for these systems (only two growth measurements were possible at each of the selected supersaturations). However, for a less challenging system such as Ibuprofen [11], this has been assessed previously where the standard deviation between measurements was found to be quite low. Similarly, the methodology has shown to be reproducible as it has been
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previously applied in the assessment of the growth kinetics of other organic molecules such as n-docosane [12] and alpha-para amino benzoic acid (PABA) [9].
2.3.Data analysis 2.3.1. Morphological predictions The morphological analysis was carried out making use of a methodology presented elsewhere
[12]
. This methodology relies on the iteratively prediction of the Bravais-Friedel-
Donnay-Harker (BFDH) morphology using pair-wise Miller indices and comparing these predictions with the micrographs obtained experimentally. A summary of this methodology is given in Fig.1 in the Supporting Information (SI) to this paper.
2.3.2. Crystal growth kinetics Given the experimental method used to collect crystals´ growth rates, the measured growth rates are not only influenced by the incorporation of growth units into the crystal surface, but also by the diffusion of the growth units within the bulk of the solution. Thus, growth models that combine these two effects acting in series have been derived as part of this work [18]. This uses and analogy to a circuit as shown in Fig. 2 of the SI delivering specific kinetics models for the dependence of growth rate on supersaturation as described by: a power law the Birth & Spread (B&S) and Burton-Cabrera-Frank (BCF) models
[19]
,
[20]
. These models are
given by expressions (2), (3) and (4) respectively. A value of 1 in expression (2) corresponds to the case of Rough Interface Growth (RIG)
[21]
. The complete derivation of
these models is presented in section 3 of the SI.
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1 1 1 + ´ !"#
(2)
1
1 1 + ´ "#/$ exp #
(3)
1
1 1 + ´ ( ℎ *
(4)
Where is the solution´s relative supersaturation, is the growth rate constant, is the
growth exponent in the RIG interface growth kinetic model and # and * are thermodynamic parameters in the B&S and BCF interface growth kinetic models respectively. ′ is related the coefficient of mass transfer within the bulk of the solution, through expression (5) ,
´ -
./0 12 34 54
(5)
In this expression 6- is the solute density, 78- the solute molecular weight and the equilibrium concentration (solubility)
3. RESULTS AND DISCUSSION 3.1. Morphological observations as a function of solution environment and crystal indexing Page 10 of 42
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Using the set up for the measurements of crystal growth rates, images of C18:0 crystals growing from three solvents were obtained. Depending on the initial assessment of the solutions behaviour, different range of supersaturations within the MSZW were chosen for this analysis. In the case of n-dodecane solutions, growth was difficult to observe above 21°C
< 0.3 and would occur too fast to be recorded below 20.2°C > 0.39. Similarly for kerosene solutions growth was difficult to observe above 18.3°C < 0.45 and would
occur too fast below 17.8°C > 0.52. For toluene systems the growth would occur very fast even at temperatures close to the solubility line and therefore the range of assessed supersaturations was limited. A summary of the width of the metastable zone at the corresponding solution concentration, together with the parameters used during the growth measurements for each system is presented in Table 2.
Table 2. Parameters used for the collection of crystals micrographs for methyl stearate crystallising in n-dodecane, kerosene and toluene solutions
n-dodecane Kerosene Toluene
A(ℎBC (A (A D E 350 350 538
MSZW (°C)
Temperature (°C) range
Supersaturation
range
4.3 (19.8 – 24.1) 4.3 (17.7 – 22.0) 4.3 (10.2 – 14.5)
20.2 to 21.0 17.8 to 18.3 13.5 to 13.9
0.30 to 0.39 0.45 to 0.52 0.04 to 0.08
Selected micrographs of the crystals observed in the range of supersaturations studied for the three solvents are shown in Fig. 4.
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a1)
a2)
b1)
b2)
c1)
c2)
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Fig. 4 Optical micrographs of methyl stearate crystals growing from supersaturated n-dodecane, kerosene and toluene solutions for selected solution supersaturations (σ). (a1) n-dodecane 20.5ºC (σ=0.36) (a2) 20.9°C (σ=0.31); (b1) kerosene: 18ºC (σ=0.49) (b2) >18.3°C (σ
