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| 2. |
- Gracin, Sandra, et al.
(author)
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4-aminophenylacetic acid
- 2005
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In: Acta Crystallographica Section E. - : International Union of Crystallography (IUCr). - 1600-5368. ; 61:6, s. O1536-O1537
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Journal article (peer-reviewed)abstract
- Crystals of the title compound, C8H9NO2, were obtained from ethyl acetate. The structure consists of the acid in its zwitterionic form. In the crystal structure, each molecule interacts through strong N-H center dot center dot center dot O hydrogen bonds with six adjacent molecules, yielding a three-dimensional network.
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| 3. |
- Gracin, Sandra, et al.
(author)
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4-Hydroxyphenylacetic acid
- 2005
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In: Acta Crystallographica Section E. - : International Union of Crystallography (IUCr). - 1600-5368. ; 61:6, s. O1919-O1920
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Journal article (peer-reviewed)abstract
- The crystal structure of commercially available 4-hydroxyphenylacetic acid, C8H8O3, is non-centrosymmetric, with four hydrogen bonds between each molecule and adjacent molecules. The hydrogen bonds link the molecules in the crystal structure into an infinite three-dimensional framework.
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| 4. |
- Gracin, Sandra, et al.
(author)
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Controlling polymorphism of p-aminobenzoic acid by sonication
- 2005
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In: the 16th International Symposium on Industrial Crystallization. - : VDI verlag Dusseldorf. ; , s. 677-682
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Conference paper (peer-reviewed)abstract
- The influence of ultrasound on the nucleation of p-aminobenzoic acid in supersaturated aqueous solutions has been investigated. The induction time and the solid phase structure has been determined in experiments with and without ultrasound. Different sonication schemes and intensities have been evaluated. It is found that sonication leads to a much shorter and reproducible induction time. In addition, it has been found that sonication preferentially favor the formation of one of the polymorphs.
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| 5. |
- Gracin, Sandra, et al.
(author)
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Influence of ultrasound on the nucleation of polymorphs of p-aminobenzoic acid
- 2005
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In: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 5:5, s. 1787-1794
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Journal article (peer-reviewed)abstract
- p-Aminobenzoic acid crystallizes in two different polymorphic forms: the alpha-form and the beta-form. The alpha-form crystals are needle-shaped, while the beta-form crystals have a more favorable prismatic shape. The system is enantiotropic with the transition temperature at approximately 25 degrees C. Below the transition temperature, the beta-form is the thermodynamically stable polymorph but can only be produced at very slow supersaturation generation either in water or in ethyl acetate. In the present work, the influence of ultrasound on the nucleation of p-aminobenzoic acid polymorphs has been investigated by use of several different sonication intensities and schemes. It is shown that sonication significantly reduces the induction time for nucleation. By using controlled sonication, we were able to more reproducibly crystallize the beta-form at more reasonable cooling rates. In addition, sonication is found to quite selectively favor the appearance of the beta-polymorph. It is even possible to produce the pure beta-form above the transition temperature where it is the metastable form and impossible to produce without sonication. The alpha-form structure is based on centro symmetric dimers formed by the association of carboxylic acid groups, while the beta-form contains four-membered hydrogen-bonded rings of alternating amino and carboxylic acid groups. It is suggested that ultrasound disturbs the building up of the dimers in the solution and thus favors the crystallization of the beta-polymorph.
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| 6. |
- Gracin, Sandra
(author)
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Polymorphism and Crystallization of p-Aminobenzoic Acid
- 2004
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Licentiate thesis (other academic/artistic)abstract
- Polymorphs are solid phases where the chemical composition isequal but the crystal structure differs. Many organic compoundsmay appear in more than one crystalline structure. The differentpolymorphs of a givensubstance may have significantly differentphysical properties (packing, thermodynamic, spectroscopic,kinetic, surface and mechanical properties). Sometimes the moststable polymorph is difficult to produce or a metastable form hasfavourable properties. This thesis focuses on the crystallization of p-aminobenzoicacid. This model compound crystallizes in two differentpolymorphic forms: 1) the α-polymorph, which isthe commercially available form and appears as long, fibrousneedles 2) theβ-polymorph, that appears in the form ofprisms. The thermodynamic stability and crystallization from differentsolvents have been studied experimentally. The system is found tobe enantiotropic with a transition temperature of 25 °C,below which theβ-form is the stable polymorph. The compoundhas been crystallized from thirteen different solvents, either byslow cooling after which the product is allowed to mature insuspension, or by rapid cooling followed by immediate isolation.Needles were obtained from all solvents by both methods. In waterand ethyl acetate below 20°C the prismaticβ-form isobtained however, often together with the needles. Theβ-formcrystals usually needed hours or days to grow at the very slowcooling used, while needles usually appeared in seconds. Bycareful control of supersaturation and temperature coolingcrystallization can be performed to produce the pure β-formin water and in ethyl acetate. The influence of the solvent isexplained by analysis of the crystal structures versus thepossible interaction of the solvent molecules with the solute insolution. The α-form structure is governed by dimers and iskinetically favoured because the dimers easily form in thesolution especially in less polar solvents. The crystal structureof the β-modification is not based on dimers but on fourmembered rings with alternating amino and carboxyl groups. Keywords:controlled crystallization, polymorphism,p-aminobenzoic acid
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| 7. |
- Gracin, Sandra, et al.
(author)
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Polymorphism and Crystallization of p-Aminobenzoic Acid
- 2004
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In: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 4:5, s. 1013-1023
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Journal article (peer-reviewed)abstract
- p-Aminobenzoic acid (PABA) crystallizes in two different polymorphic forms: the alpha-polymorph, which is the commercially available form and appears as long, fibrous needles, and the beta-polymorph, which appears in the form of prisms. The thermodynamic stability and crystallization from different solvents have been studied experimentally. The system is found to be enantiotropic with a transition temperature of 25degreesC, below which the beta-form is the stable polymorph. The compound has been crystallized from 13 different solvents, either by slow cooling after which the product is allowed to mature in suspension, or by rapid cooling followed by immediate isolation. Needles were obtained from all solvents by both methods. In water and in ethyl acetate, at slow cooling below 20degreesC, the prismatic beta-form is obtained, however, often together with the needles. The beta-form crystals usually needed hours or days to grow at the very slow cooling used, while needles usually appeared in seconds. By careful control of supersaturation and temperature, cooling crystallization can be performed to produce the pure beta-form in water and in ethyl acetate. The influence of the solvent is explained by analysis of the crystal structures versus the possible interaction of the solvent molecules with the solute in solution. The alpha-form structure is governed by carboxylic acid dimers and is kinetically favored since it is believed that the corresponding dimers easily form in the solution, especially in less polar solvents.
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| 8. |
- Gracin, Sandra, et al.
(author)
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Prediction of Solubility of Solid Organic Compounds in Solvents by UNIFAC
- 2002
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In: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 41:20, s. 5114-5124
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Journal article (peer-reviewed)abstract
- Predictions of solubility of nine different solid organic fine chemical compounds in water and organic solvents of relevance to industrial processing are examined. UNIFAC interaction parameters are taken from standard reference literature, extracted from liquid-vapor equilibria. For most systems, predicted solubilities deviate more than 15% from experimental values. Deviations are due to uncertainties in the estimation of the activity of the pure solid as well as to deficiencies in the estimation of activity coefficients in the solution. By comparison with results from ab initio quantum chemical calculations of the elecrostatic potential on the molecular surface of the solutes, it can be shown that a key assumption of the UNIFAC approach is not necessarily fulfilled. The properties of a functional group may depend significantly on the properties of the rest of the molecule.
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| 9. |
- Gracin, Sandra, et al.
(author)
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Redetermination of the beta-polymorph of p-aminobenzoic acid
- 2005
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In: Acta Crystallographica Section E. - : International Union of Crystallography (IUCr). - 1600-5368. ; 61:5, s. O1242-O1244
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Journal article (peer-reviewed)abstract
- Single crystals of p-aminobenzoic acid, C7H7NO2, were grown from water. In the structure, there is one molecule of the acid present in the asymmetric unit. Hydrogen bonds between adjacent molecules lead to the formation of a three-dimensional network.
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| 10. |
- Gracin, Sandra, 1968-
(author)
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Solubility and polymorphism of molecular compounds
- 2005
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Doctoral thesis (other academic/artistic)abstract
- This thesis deals with the controlled crystallization of small organic molecules and is focused on solubility and polymorphism. The solubility was determined for phenylacetic acid, p-hydroxyphenylacetic acid, p-aminophenylacetic acid, p-hydroxybenzoic acid and ibuprofen in both water and in a range of organic solvents. Data is discussed from the standpoint of molecular aspects of solute – solvent interactions and by estimated solid phase activity. It was shown that better understanding could be acquired by making a qualitative analysis of the molecular interactions in the solution and the crystal structure of the compounds in question. Solubility predictions that are carried out by the UNIFAC method are not sufficiently accurate to serve as a basis for a reliable design of a crystallization process or selection of a suitable solvent since they deviate more than 15% from experimental values. The reason for the discrepancies are related to uncertainties in the prediction of activity coefficients by UNIFAC, as well as, difficulties in the estimation of the activity of the solid state. p-Aminobenzoic acid (PABA) has been crystallized from thirteen different solvents either by slow cooling, after which the product is allowed to mature in suspension, or by rapid cooling followed by immediate isolation. Two different polymorphs have been crystallized. The system is found to be enantiotropic with the transition temperature of 25 °C, below which the β-form is the stable polymorph. The α-form was obtained from all solvents by both methods. The β-form is obtained only in carefully controlled conditions from water and ethyl acetate, well below the transition temperature. Often the α-form appears concomitantly. It is shown in this work that sonication significantly reduces the induction time for nucleation. The β-form crystallizes more reproducibly and at higher cooling rates when controlled sonication is used. In addition sonication is found to selectively favor the appearance of one of the polymorphs. Producing the pure β-form was possible even above the transition temperature where other crystallization techniques were only capable of producing the stable α-form. The α-form structure is based on centro symmetric dimers formed by association of carboxylic acid groups. It is suggested that the preference for nucleation of the α-polymorph is related to the formation of dimers in the supersaturated solution. Only at the condition where the formation of dimers is reduced sufficiently, (i.e. in the polar solvents or when sonication is applied) the nucleation of the β-form is favored.
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