250 nm glycine-rich nanodroplets are formed on dissolution of glycine crystals but are too small to provide productive nucleation sites

Anna Jawor-Baczynska, Jan Sefcik, Barry D. Moore

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36 Citations (Scopus)

Abstract

Recent theoretical and experimental studies have proposed a two-step mechanism for crystal formation in which crystal nucleation is preceded by formation of disordered molecular assemblies. Here, we investigated whether similar intermediates might also form as crystals dissolve, effectively the reverse process. A model system of glycine in water was studied, and the resultant solutions were characterized using small-angle X-ray scattering, dynamic light scattering, and nanoparticle tracking analysis. Invariably, dissolution of glycine crystals into water was observed to produce scattering nanospecies with liquid-like properties and a mean diameter of about 250 nm, at near saturation concentration. The nanospecies persisted indefinitely in solution in the presence of excess glycine crystals and were identified as glycine-rich nanodroplets with an equilibrium population of about 109 per mL. The time to appearance of glycine crystals from quiescent supersaturated solution (S = 1.1) containing either a low population of nanodroplets (nanofiltered) or a high population of nanodroplets (unfiltered) was indistinguishable with typically only a single crystal forming after about 30 h. However, a very significant acceleration of crystal formation was observed whenever a gently tumbling stirrer-bar was introduced into the vial; thousands of microcrystals appeared after an incubation period of only 3-5 h. The possibility of this being caused by factors such as secondary nucleation, bubbles, or glass splinters or scratches was eliminated via control experiments. Further investigation of the glycine solution, just prior to appearance of microcrystals, revealed an additional subpopulation of extremely large glycine-rich nanodroplets (diameter >750 nm), not observed in quiescent solutions. It is proposed that productive nucleation of glycine crystals occurs exclusively within these larger glycine-rich nanodroplets because a critical mass of glycine is required to form nascent crystals large enough to survive exposure to bulk more dilute solution. We hypothesize that nucleation occurs frequently but nonproductively within subcritical mass nanodroplets and infrequently but productively within very rare critical mass solute-rich nanodroplets. Such a model provides a new compelling way of bridging classical mechanisms of crystal nucleation with the more recently proposed two-step processes.

LanguageEnglish
Pages470-478
Number of pages9
JournalCrystal Growth and Design
Volume13
Issue number2
Early online date6 Dec 2012
DOIs
Publication statusPublished - Feb 2013

Fingerprint

glycine
Glycine
Amino acids
dissolving
Dissolution
Nucleation
nucleation
Crystals
crystals
critical mass
Microcrystals
microcrystals
subcritical mass
Barreling
Saturation (materials composition)
Water
Dynamic light scattering
X ray scattering
scattering
assemblies

Keywords

  • nanodroplets
  • glycine
  • crystal nucleation

Cite this

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title = "250 nm glycine-rich nanodroplets are formed on dissolution of glycine crystals but are too small to provide productive nucleation sites",
abstract = "Recent theoretical and experimental studies have proposed a two-step mechanism for crystal formation in which crystal nucleation is preceded by formation of disordered molecular assemblies. Here, we investigated whether similar intermediates might also form as crystals dissolve, effectively the reverse process. A model system of glycine in water was studied, and the resultant solutions were characterized using small-angle X-ray scattering, dynamic light scattering, and nanoparticle tracking analysis. Invariably, dissolution of glycine crystals into water was observed to produce scattering nanospecies with liquid-like properties and a mean diameter of about 250 nm, at near saturation concentration. The nanospecies persisted indefinitely in solution in the presence of excess glycine crystals and were identified as glycine-rich nanodroplets with an equilibrium population of about 109 per mL. The time to appearance of glycine crystals from quiescent supersaturated solution (S = 1.1) containing either a low population of nanodroplets (nanofiltered) or a high population of nanodroplets (unfiltered) was indistinguishable with typically only a single crystal forming after about 30 h. However, a very significant acceleration of crystal formation was observed whenever a gently tumbling stirrer-bar was introduced into the vial; thousands of microcrystals appeared after an incubation period of only 3-5 h. The possibility of this being caused by factors such as secondary nucleation, bubbles, or glass splinters or scratches was eliminated via control experiments. Further investigation of the glycine solution, just prior to appearance of microcrystals, revealed an additional subpopulation of extremely large glycine-rich nanodroplets (diameter >750 nm), not observed in quiescent solutions. It is proposed that productive nucleation of glycine crystals occurs exclusively within these larger glycine-rich nanodroplets because a critical mass of glycine is required to form nascent crystals large enough to survive exposure to bulk more dilute solution. We hypothesize that nucleation occurs frequently but nonproductively within subcritical mass nanodroplets and infrequently but productively within very rare critical mass solute-rich nanodroplets. Such a model provides a new compelling way of bridging classical mechanisms of crystal nucleation with the more recently proposed two-step processes.",
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TY - JOUR

T1 - 250 nm glycine-rich nanodroplets are formed on dissolution of glycine crystals but are too small to provide productive nucleation sites

AU - Jawor-Baczynska, Anna

AU - Sefcik, Jan

AU - Moore, Barry D.

PY - 2013/2

Y1 - 2013/2

N2 - Recent theoretical and experimental studies have proposed a two-step mechanism for crystal formation in which crystal nucleation is preceded by formation of disordered molecular assemblies. Here, we investigated whether similar intermediates might also form as crystals dissolve, effectively the reverse process. A model system of glycine in water was studied, and the resultant solutions were characterized using small-angle X-ray scattering, dynamic light scattering, and nanoparticle tracking analysis. Invariably, dissolution of glycine crystals into water was observed to produce scattering nanospecies with liquid-like properties and a mean diameter of about 250 nm, at near saturation concentration. The nanospecies persisted indefinitely in solution in the presence of excess glycine crystals and were identified as glycine-rich nanodroplets with an equilibrium population of about 109 per mL. The time to appearance of glycine crystals from quiescent supersaturated solution (S = 1.1) containing either a low population of nanodroplets (nanofiltered) or a high population of nanodroplets (unfiltered) was indistinguishable with typically only a single crystal forming after about 30 h. However, a very significant acceleration of crystal formation was observed whenever a gently tumbling stirrer-bar was introduced into the vial; thousands of microcrystals appeared after an incubation period of only 3-5 h. The possibility of this being caused by factors such as secondary nucleation, bubbles, or glass splinters or scratches was eliminated via control experiments. Further investigation of the glycine solution, just prior to appearance of microcrystals, revealed an additional subpopulation of extremely large glycine-rich nanodroplets (diameter >750 nm), not observed in quiescent solutions. It is proposed that productive nucleation of glycine crystals occurs exclusively within these larger glycine-rich nanodroplets because a critical mass of glycine is required to form nascent crystals large enough to survive exposure to bulk more dilute solution. We hypothesize that nucleation occurs frequently but nonproductively within subcritical mass nanodroplets and infrequently but productively within very rare critical mass solute-rich nanodroplets. Such a model provides a new compelling way of bridging classical mechanisms of crystal nucleation with the more recently proposed two-step processes.

AB - Recent theoretical and experimental studies have proposed a two-step mechanism for crystal formation in which crystal nucleation is preceded by formation of disordered molecular assemblies. Here, we investigated whether similar intermediates might also form as crystals dissolve, effectively the reverse process. A model system of glycine in water was studied, and the resultant solutions were characterized using small-angle X-ray scattering, dynamic light scattering, and nanoparticle tracking analysis. Invariably, dissolution of glycine crystals into water was observed to produce scattering nanospecies with liquid-like properties and a mean diameter of about 250 nm, at near saturation concentration. The nanospecies persisted indefinitely in solution in the presence of excess glycine crystals and were identified as glycine-rich nanodroplets with an equilibrium population of about 109 per mL. The time to appearance of glycine crystals from quiescent supersaturated solution (S = 1.1) containing either a low population of nanodroplets (nanofiltered) or a high population of nanodroplets (unfiltered) was indistinguishable with typically only a single crystal forming after about 30 h. However, a very significant acceleration of crystal formation was observed whenever a gently tumbling stirrer-bar was introduced into the vial; thousands of microcrystals appeared after an incubation period of only 3-5 h. The possibility of this being caused by factors such as secondary nucleation, bubbles, or glass splinters or scratches was eliminated via control experiments. Further investigation of the glycine solution, just prior to appearance of microcrystals, revealed an additional subpopulation of extremely large glycine-rich nanodroplets (diameter >750 nm), not observed in quiescent solutions. It is proposed that productive nucleation of glycine crystals occurs exclusively within these larger glycine-rich nanodroplets because a critical mass of glycine is required to form nascent crystals large enough to survive exposure to bulk more dilute solution. We hypothesize that nucleation occurs frequently but nonproductively within subcritical mass nanodroplets and infrequently but productively within very rare critical mass solute-rich nanodroplets. Such a model provides a new compelling way of bridging classical mechanisms of crystal nucleation with the more recently proposed two-step processes.

KW - nanodroplets

KW - glycine

KW - crystal nucleation

UR - http://pubs.acs.org/journal/cgdefu

U2 - 10.1021/cg300150u

DO - 10.1021/cg300150u

M3 - Article

VL - 13

SP - 470

EP - 478

JO - Crystal Growth and Design

T2 - Crystal Growth and Design

JF - Crystal Growth and Design

SN - 1528-7483

IS - 2

ER -