J N Sherwood

Publications (66)79.78 Total impact

• Article: Hyper-Rayleigh scattering as a means of monitoring crystal nucleation in solution.

  M C D'Arrigo, F R Cruickshank, D Pugh, J N Sherwood, J D Wallis, C Mackenzie, D Hayward
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  ABSTRACT: Hyper-Rayleigh scattering is revealed as a very sensitive monitor of cluster formation in solution, and as a means of studying the mechanism of crystal nucleation in molecular species. Two compounds are selected with particularly high second harmonic generation (SHG) powers in the crystalline state and experimental conditions are defined allowing the measurement of the beta value for one of these as 18+/-1x10(-30) esu. It is found to agree with current theoretical prediction of 20x10(-30) esu. In the more powerful of these, two photon induced fluorescence is found to be partly responsible for the SHG. The solubilities of both compounds in methanol are measured and it is observed that these differ by a factor of ten. When the solution concentration is increased beyond 45% of the saturation value, the quadratic coefficient exhibits non-linear behaviour with respect to concentration. Additionally, the widths of the distributions of the HRS signals increase initially with concentration as expected, but, beyond 45% saturation concentrations, these narrow again. These phenomena are interpreted as indicators of cluster formation in these solutions well below saturation concentrations. A future experimental design is proposed in which the coherent component will yield information on the organisation of the molecules in such clusters.
  Physical Chemistry Chemical Physics 09/2006; 8(32):3761-6. · 3.57 Impact Factor
• Article: Dissolution kinetics of single crystals of alpha-lactose monohydrate.

  S L Raghavan, R I Ristic, D B Sheen, J N Sherwood
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  ABSTRACT: The dissolution kinetics of alpha-lactose monohydrate (alphaLM) single crystals were studied by a flow-cell method at different undersaturations. Linear dissolution profiles were obtained as a function of time for all the faces except the (010) face. The dissolution rates, obtained from these profiles, were anisotropic and varied considerably with undersaturation. At low undersaturations (0-2%), the order of dissolution rate was (110) > (100) > (011) = (110) > (010). This order changed with increasing undersaturation (>5%) to (011) > (100) > (110) > (110) > (010). In alphaLM crystals in which lattice strain was induced by synchrotron X-irradiation, the rates of dissolution of all faces increased with increasing strain. The increase was less significant for the (011) faces than for the remainder. Under this constraint, the (010) face became the fastest dissolving one and the [011]face became the slowest one. The results of all experiments are explained on the basis that although dislocations may act as initiating dissolution centers at very low undersaturations, these sources rapidly give way to two-dimensional nucleation of randomly distributed dissolution sites as the undersaturation is increased. Under these conditions, which better reflect the normal dissolution processes of materials, bulk lattice strain plays the most significant role in defining the dissolution rate. The results show a potential route to the controlled engineering of the dissolution behavior of crystalline materials.
  Journal of Pharmaceutical Sciences 10/2002; 91(10):2166-74. · 3.06 Impact Factor
• Article: Explanation for the Supersaturation-Dependent Morphology of Monoclinic Paracetamol

  S. X. M. Boerrigter, H. M. Cuppen, R. I. Ristic, J. N. Sherwood, P. Bennema, H. Meekes
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  ABSTRACT: The growth morphology of monoclinic paracetamol as a function of the supersaturation is determined using Monte Carlo simulations based on the crystal structure. The results are compared with experimental results reported recently on both the morphology and the relevant growth mechanisms. The change of an elongated to a more bulky habit with increasing supersaturation is reproduced well by the simulations. The method used opens ways to predict the crystal morphology for real crystal structures in dependence of supersaturation once information on the relevant growth mechanism for the various faces is known.
  07/2002;
• Article: Understanding the Solvent-Induced Habit Modification of Benzophenone in Terms of Molecular Recognition at the Crystal/Solution Interface.

  K. J. Roberts, J. N. Sherwood, C. S. Yoon, R. Docherty
  04/2002;
• Article: Morphology and growth kinetics of large sodium chlorate crystals grown in the presence and absence of sodium dithionate impurity

  R. Ristic, J. N. Sherwood, K. Wojciechowski
  04/2002;
• Article: Macro- and Micromorphology of Monoclinic Paracetamol Grown from Pure Aqueous Solution

  R. I. Ristic, S. Finnie, D. B. Sheen, J. N. Sherwood
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  ABSTRACT: The morphology of monoclinic paracetamol crystals has been investigated both theoretically and experimentally. Calculations using the computer program HABIT 95 with both DREIDING II and MOMANY force fields predict prismatic forms in which {100}, {001}, {110}, and {201̄} show approximately equivalent morphological importance. Whereas all of these faces are observed experimentally, the real crystals showed a {110} dominance at low supersaturations which gave way to an increasing {001} dominance as the supersaturation increased. This variation was accompanied by a change from a columnar to a platelike habit. Surface examinations using phase contrast microscopy showed the habit changes to be due principally to changes in the growth mechanism of the {110} faces. A slow growth process involving two-dimensional nucleation at a few growth sources occurred at low supersaturations. This gave way to dislocation growth and finally at high supersaturations, to a fast growing mixed mechanism combining two-dimensional growth from the edges and vertexes of the {110} faces with the operation of dislocation sources at the face center. The increasing dominance of the two-dimensional growth contribution at the highest supersaturations coupled with an increase in macrostep formation resulted in the development of inclusions in the {110} sectors. This phenomenon will result in significant increases in the solvent impurity content of crystals at the high supersaturations normally used in the production of this material. The results of this study show well the dominant part that the growth mechanism can play in the definition of the morphology of crystals and hence the care which must be taken in the interpretation of modeling calculations.
  08/2001;
• Article: The bulk crystallization of alpha-lactose monohydrate from aqueous solution.

  S L Raghavan, R I Ristic, D B Sheen, J N Sherwood
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  ABSTRACT: The bulk crystallization of alpha-lactose monohydrate from aqueous solution by primary nucleation has been studied under controlled conditions of supersaturation, temperature, and pH. The induction times to nucleation were extremely long compared with those generally observed for other materials, even at the high supersaturations used in the experiment. As a result, it was necessary to stir the supersaturated solution vigorously to induce nucleation in a reasonable but still lengthy working time. Even then, nucleation only occurred to a limited extent, following which growth ceased for 8-10 h before resuming. After this period, growth recommenced but again slowed to a low rate after another 8 h. At this stage, the yield of product was low and in most cases the particles had achieved sizes close to the maximum noted. The yields increased with further crystallization time (22-72 h total from the recommencement of growth) to give, under high initial supersaturation conditions, amounts of product close to the theoretical value. For the most part, however, the particle size did not increase with this later increase in yield, showing only significant changes after the extremely long total crystallization times. It is proposed that these extreme properties result from the formation in solution by mutarotation of the anomer alpha-lactose, which inhibits nucleation as well as its previously observed influence on growth.
  Journal of Pharmaceutical Sciences 08/2001; 90(7):823-32. · 3.06 Impact Factor
• Article: Fracture property studies of paracetamol single crystals using microindentation techniques.

  K V Prasad, D B Sheen, J N Sherwood
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  ABSTRACT: To study the fracture behavior of the major habit faces of paracetamol single crystals using microindentation techniques and to correlate this with crystal structure and molecular packing. Vicker's microindentation techniques were used to measure the hardness and crack lengths. The development of all the major radial cracks was analyzed using the Laugier relationship and fracture toughness values evaluated. Paracetamol single crystals showed severe cracking and fracture around all Vicker's indentations with a limited zone of plastic deformation close to the indent. This is consistent with the material being a highly brittle solid that deforms principally by elastic deformation to fracture rather than by plastic flow. Fracture was associated predominantly with the (010) cleavage plane, but was also observed parallel to other lattice planes including (110), (210) and (100). The cleavage plane (010) had the lowest fracture toughness value, Kc = 0.041MPa m1/2, while the greatest value, Kc = 0.105MPa m1/2 was obtained for the (210) plane. Paracetamol crystals showed severe cracking and fracture because of the highly brittle nature of the material. The fracture behavior could be explained on the basis of the molecular packing arrangement and the calculated attachment energies across the fracture planes.
  Pharmaceutical Research 07/2001; 18(6):867-72. · 4.09 Impact Factor
• Article: Microhardness and dislocation identification studies on paracetamol single crystals.

  S Finnie, K V Prasad, D B Sheen, J N Sherwood
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  ABSTRACT: To study the mechanical behaviour of paracetamol single crystals. Microhardness indentation techniques were used to study the hardness anisotropy of paracetamol. Solvent etching technique was used to define the range of plastic deformation and the orientation of the dislocation lines. The orientation dependence of Knoop hardness on the (001), (110) and (201) surfaces was compared with calculated values of the Effective Resolved Shear Stress (ERSS) for plastic deformation by specific dislocation types. The principal habit faces of single crystals using both Vickers and Knoop indenters showed a range of hardness from 235 to 456 MPa depending on the type of indenter used and its orientation on the surface. Solvent etching of the plastically deformed region of the crystal around the Vickers/Knoop indentations confirmed that the slip plane was (010). ERSS analysis suggested that the deformation occurred by the slip of dislocations of the types (010)[001] and (010)[100]. Crystals doped with 0.08-0.8 w/w% p-acetoxyacetanilide showed hardness values similar to the pure material. The low number of distinct dislocation slip systems (two) is characteristic of a brittle material and is consistent with the observation that paracetamol will tolerate only deformations of 1 part in 10(6) before fracture.
  Pharmaceutical Research 06/2001; 18(5):674-81. · 4.09 Impact Factor
• Article: Polymorphism in 2,4,6-trinitrotoluene crystallized from solution.

  R M Vrcelj, H G Gallagher, J N Sherwood
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  ABSTRACT: An examination has been made of the role of solvent type in the definition of the polymorphic nature of 2,4,6-trinitrotoluene precipitated from solution. A combination of calorimetric and structural techniques including in situ crystallization studies using synchrotron radiation has shown that the variations in polymorphic form following precipitation from solution do not arise specifically from any stereospecific guidance that the nature of the solvent might impose on the structural form. Rather the differences are linked to the variations in solubility and hence supersaturation which might build up prior to nucleation and growth and to the isolation of the metastable orthorhombic phase from the solvent. The final conclusion is that the changes fit well with Ostwald's Law of Stages with the orthorhombic form always precipitating initially followed by its conversion to the stable monoclinic form. The previously observed tendency for some solvents to yield one or the other form then becomes attributable to kinetics in solution rather than structural control. It can be associated with the solubility of the material in the solvent used and the role of this factor in a solvent-mediated phase transformation. On this basis rules can be formulated for the isolation of the metastable forms of this and similarly related polymorphic systems.
  Journal of the American Chemical Society 04/2001; 123(10):2291-5. · 9.91 Impact Factor
• Article: Crystallization of paracetamol from solution in the presence and absence of impurity.

  K V Prasad, R I Ristic, D B Sheen, J N Sherwood
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  ABSTRACT: The bulk crystallization of paracetamol has been examined under controlled conditions in the presence and absence of the additive p-acetoxyacetanilide (PAA), as a function of both supersaturation and additive levels. The induction time to nucleation was found to increase with increase in PAA concentration in solution. The product micro-crystals were characterized for shape and strain/defect content using electron and optical microscopy and X-ray Laue diffraction techniques, respectively. A change in crystal habit of the pure crystals from columnar (dominant [110]) to plate-like (dominant [001]) was observed to occur with an increase in supersaturation level, whilst the addition of PAA invariably led to the development of columnar crystals with an aspect ratio that varied with impurity level and supersaturation. HPLC showed the PAA to be incorporated into the crystals with an average segregation coefficient of 14-18% depending on the supersaturation. The ready incorporation of PAA is attributed to the molecular similarity of this molecule to that of the host material. The incorporation is shown to cause a significant increase in the mosaic spread, implying the development of a significant strain/defect content in the crystals. The influence of the impurity on the time to nucleation is probably due to its effect in blocking the development of the critical nucleus. The potential implications of such variations in morphology and strain content in the design of the physical and chemical properties of the resulting particulates are discussed.
  International Journal of Pharmaceutics 04/2001; 215(1-2):29-44. · 3.35 Impact Factor
• Article: Morphology of Crystals of α-Lactose Hydrate Grown from Aqueous Solution

  S. L. Raghavan, R. I. Ristic, D. B. Sheen, J. N. Sherwood
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  ABSTRACT: When dissolved in aqueous solution, α-lactose, whether originally in the anhydrous or the monohydrated form, readily undergoes mutarotation to yield the β isomer. At equilibrium, which is reached in 6.5 h at 292 K and more rapidly with increasing temperatures, the latter is present to the extent of ca. 60% w/w. At saturation, α-lactose hydrate precipitates from this solution being considerably less soluble than the β form. The resulting crystals have a highly asymmetric, tomahawk shaped morphology characterized by large {01̄1} faces. It is confirmed that this shape arises from growth inhibition due to the incorporation of the β isomer in the {01̄1} sectors of the crystals coupled with a characteristic lack of growth in the [01̄0] direction. Growth under conditions that restrict the formation of the β isomer yields more symmetrically shaped, needlelike crystals. These show a gradual transition to the tomahawk shape as the concentration of the β isomer in solution and hence in the crystal is increased. An assessment is made of the degree of incorporation of the β isomer into the crystal, its distribution and its influence on the defect structure of the material.
  11/2000;
• Article: Characterisation of polyphenylene thin films using synchrotron radiation X-ray reflectivity

  K J Roberts, J N Sherwood, T Shripathi, R J Oldman, P A Holmes, A Nevin
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  ABSTRACT: The technique of synchrotron radiation X-ray reflectivity is presented and its application to the non-destructive characterisation of polymer thin films outlined. The technique is applied to an investigation of the structure of polyphenylene thin films prepared on indium/tin oxide coated glass substrates. Samples synthesised from carbonate-based precursors have a higher density than those prepared from acetate-based precursors. Possible molecular origins are discussed.
  Journal of Physics D Applied Physics 11/2000; 23(2):255. · 2.54 Impact Factor
• Article: Organic materials for non-linear optics: inter-relationships between molecular properties, crystal structure and optical properties

  R T Bailey, F R Cruickshank, P Pavlides, D Pugh, J N Sherwood
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  ABSTRACT: An account is given of the origins, in the molecular electronic properties and crystal structure, of the large quadratic optical nonlinearities found in some organic crystals. Particular cases, in which powerful second-harmonic generation or large Pockels effects have been found, chosen to illustrate the influence of different aspects of molecular or crystal structure, are introduced. Three of these, MBANP, NMBA and DAN are described in greater detail in the light of recent experimental and theoretical work.
  Journal of Physics D Applied Physics 07/2000; 24(2):135. · 2.54 Impact Factor
• Article: The Nonlinear Optical Properties of the Crystal (S)-3-Methyl-5-nitro-N-(1-phenylethyl)-2-pyridinamine

  S. Lochran, R. T. Bailey, F. R. Cruickshank, D. Pugh, J. N. Sherwood, G. S. Simpson
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  ABSTRACT: The organic crystal (S)-3-methyl-5-nitro-N-(1-phenylethyl)-2-pyridinamine has been previously identified by the powder technique as a promising nonlinear optical material. Large single crystals (3 × 3 × 2 cm3) of this material have been grown by temperature lowering of a seeded supersaturated solution in acetone. Refined refractive indices were obtained from the predicted angles of incidence of the phase-matched second harmonic signals. Second-order nonlinear optical coefficients, dij, as measured by the Maker fringe technique, are 11, 9, and 7 pm V-1 for d14, d25, and d36, respectively. The results are compared with calculations based on the oriented gas model.
  06/2000;
• Source

  Available from: Susan Elizabeth Girdwood

  Article: Synchrotron Radiation X-ray Multiple Diffraction applied to the Study of Electric Field Induced Strain in MBANP Organic Non-linear Optical Material

  L.H. Avanci, L.P. Cardoso, S.E. Girdwood, D. Pugh, K.J. Roberts, J.M. Sasaki, J.N. Sherwood
  Physical Review B 01/2000; · 3.69 Impact Factor
• Article: Linear and nonlinear optical properties of organic crystals: MBANP and its derivatives

  R T Bailey, F R Cruickshank, S Lochran, D Pugh, J N Sherwood, P J Langley, J D Wallis
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  ABSTRACT: In addition to possessing strong optical nonlinearity, the (−)-2-(α-methylbenzylamino)-5-nitropyridine (MBANP) crystal has been shown to have unusual linear optical properties. Two methyl derivatives of MBANP have now been synthesized and grown as large single crystals; their refractive indices have been measured as a function of frequency and second-harmonic generation, including the determination of the phase matching loci has been investigated. Data is also available on the racemic form of crystalline MBANP. The properties of this family of crystals have been reviewed and an attempt is made to relate the experimental responses of the crystals to their molecular structure and the calculated properties of the molecules.
  Bulletin of Materials Science 04/1999; 22(3):421-430. · 0.88 Impact Factor
• Source

  Available from: Vittorio Degiorgio

  Article: Nonlinear susceptibility and damage thresholds of organic MBA-NP crystals

  S Nitti, H M Tan, G Banfi, V Degiorgio, R T Bailey, F Cruickshank, D Pugh, E A Shepherd, J N Sherwood, G S Simpson
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  ABSTRACT: The nonlinear optical behaviour of the organic crystal MBA-NP has been investigated at 1064 and 532 nm, with both nanosecond and picosecond pulses. The second-order effective susceptibility for second harmonic generation, and the damage thresholds compare favourably with those of most inorganic crystals. Two-photon absorption is rather small, while stimulated Raman scattering, which can be excited only with the 532 nm radiation, is strongly dependent on the orientation of the sample.
  Journal of Physics D Applied Physics 12/1998; 26(8B):B225. · 2.54 Impact Factor
• Article: Assessment of optically nonlinear organic crystals

  R T Bailey, F R Cruickshank, D Pugh, J N Sherwood
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  ABSTRACT: The information required from optical assessment of nonlinear organic materials differs widely depending on the application envisaged for the material in question. Accordingly, a suitable assessment scheme must be devised which will give a reliable indication of the promise of these materials in a wide variety of applications. Such a scheme is described, beginning with the assessment of microcrystalline powders and ending with predictions of wavelength and angle criticality of phase matching in single crystals. Electro-optical assessment at low frequencies is also described. It is shown that the performance of a material cannot be reliably expressed by any single parameter, but that it must be examined afresh for each application considered. Although several examples are discussed, the literature contains very few comprehensive nonlinear optical assessments. Based on the available data, some tentative structure-property relationships are proposed.
  Journal of Physics D Applied Physics 12/1998; 26(8B):B208. · 2.54 Impact Factor
• Article: Refractive Indices of the Optically Nonlinear Organic Crystal (S)-3-Methyl-5-nitro-N-(1-phenylethyl)-2-pyridinamine

  S. Lochran, R. T. Bailey, F. R. Cruickshank, D. Pugh, J. N. Sherwood, G. S. Simpson
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  ABSTRACT: The refractive indices of structurally well-characterized crystals of the new optically nonlinear organic crystal (S)-3-methyl-5-nitro-N-(1-phenylethyl)-2-pyridinamine have been measured from 480 to 660 nm and fitted to four-parameter Sellmeier equations with an accuracy of ±3 × 10-3 to ±5 × 10-3. The value of ny was accurately verified by the measurement of the external angle of incidence required, at the (101) plane, to observe type I phase matching with a fundamental wavelength of 1064 nm. The phase matching locus has been established and is of Hobden class eight. The values of the nx, ny, and nz refractive indices at 532 nm were found from the Sellmeier equations to be 1.690, 1.901, and 1.725, respectively, and their relative magnitudes are correlated with the molecular arrangement in the crystal lattice. This material is a member of a series of pyridinamines that can exhibit the phenomenon of frequency-dependent orientation of their dielectric axes. Since this compound crystallizes in an orthorhombic structure, rotation of the dielectric axes is precluded for symmetry reasons.
  07/1998;