5. Laser Trapping Crystallization of L-proline in Deuterated Ethanol (EtOD)
6.5 Droplet formation: Compared with different solvents
As mentioned above, dense liquid droplet is formed by cooperation of photon pressure
and Marangoni effect. It has been reported that surface deformation is the indispensable
process for droplet formation [2]. We found the solution deformation both in proline/D2O
and proline/EtOD but droplet formation was observed only latter.
To consider different droplet formation tendencies in D2O and EtOD from energetic view
points, a schematic free energy diagram is drawn as depicted in Fig. 6.8. Horizontal and
vertical axes express stage of droplet formation and free energies of the droplet formation in
proline/D2O and proline/EtOD. Droplet formation is influenced on elemental phenomena such
as photon pressure, surface deformation, temperature elevation and molecular interactions.
We assume that the same initial free energy in two solutions.
The droplet formation can be considered as one kind of phase transition from view point of
All liquid
Fig. 6.7 Phase diagram showing temperature and concentration dependent phase transition.
Arrow indicates transition path from liquid to liquid-liquid separation phase.
molecular arrangement conversion. Solute-solute interaction will be strengthened during the
droplet formation because too many molecules are packed in small space of droplet. It has
reported that the effects of hydrogen bonding with a water molecule on the relative stability of
the low energy conformers of proline [11]. On the other hand, the higher energy for droplet
formation(less water) in proline/D2O solution can be suggested. Oppositely, there is no big
influence in EtOD solution on droplet formation so that relatively small energy difference
between initial state and droplet formation is suggested.
Photon pressure which collects molecules at the focal spot is indispensable for droplet
formation and increase energy to overcome energy barrier for droplet formation. Focused
laser induced temperature elevation, and it caused surface deformation with thinning of
solution layer which enhanced mass transfer due to thermal capillary.
On the other hand, temperature increase due to laser irradiation increases thermal
disturbance and suppresses molecular trapping. Thermal interference for droplet formation in
EtOD should be energetically much larger compare with that in D2O. However we did not
observe droplet in D2O but in EtOD. It suggests other negative contribution to prevent droplet
formation and it should be the strong molecular interaction between solute and solvent
molecules. It has reported that strong hydrogen bonding between them so that photon pressure
can not trap proline molecules easily to form droplet [12, 13]. Oppositely, it should be less
influence in EtOD solution on droplet formation.
Combination of above mentioned positive and negative influences determine that it can
overcome the barrier or not to form the dense droplet.
6.6 Summary
We proposed droplet formation dynamics based on experimental observation of its
formation process. Droplet is formed by the cooperation of Managoni effect and photon
pressure. Observed droplet formation of proline in EtOD is similar to that observed in
glycine in D2O. However, temperature elevation in EtOD should be higher than that in D2O
and it prevents effective droplet formation and crystallization in EtOD. For example,
crystallization took place through the liquid droplet reflects high temperature elevation is
Free energy G
Fig. 6.8 Schematic drawing of free energy for droplet formation in different solventnegative effect for crystallization. It increases the molecular motion so that slow droplet
formation and growth. Beside, droplet is difficult to be observed under higher laser power in
proline/EtOD because surface height decreasing is too quick to trap enough molecules for
the droplet formation.
Irradiation induced obvious increase of backward scattering intensity increase reflects
clearly concentration increase due to the laser trapping.
Droplet formation is considered as liquid-liquid separation. It is formed through rapid
concentration increase in short time so that molecules cannot arrange well. A reason of
droplet formation only in EtOD but not in D2O can be explained in view point of high free
energy of droplet formation in D2O; strong interaction between water and proline molecules
need high energy to brake interaction.
6.7 Reference
1. H. Masuhara, T. Sugiyama, T. Rungsimanon, K.-i. Yuyama, A. Miura, and J.-R. Tu, Laser-trapping assembling dynamics of molecules and proteins at surface and interface. Pure Appl. Chem., 2011. 83: p. 869-833.
2. K.-i. Yuyama, K. Ishiguro, T. Rungsimanon, T. Sugiyama, and H. Masuhara, Single droplet formation and crystal growth in urea solution induced by laser trapping.
Proc. of SPIE, 2010. 7762.
3. K.-i. Yuyama, T. Sugiyama, and H. Masuhara, Millimeter-Scale Dense Liquid Droplet Formation and Crystallization in Glycine Solution Induced by Photon Pressure. J. Phys. Chem. Lett., 2010. 38: p. 1321-1325.
4. G.D. Costa, Optical visualization of the velocity distribution in a laser-induced thermocapillary liquid flow. Applied Optics, 1993. 32: p. 2144-2151.
5. G.D. Costa and J. Calatroni, Transient deformation of liquid surfaces by laser-induced thermocapillarity. Applied Optics, 1979. 18: p. 233-235.
6. O.A. Louchev, S. Juodkazis, N. Murazawa, S. Wada, and H. Misawa, Coupled laser molecular trapping,cluster assembly, and deposition fed by laser-induced Marangoni convection. Optics Express, 2008. 16: p. 5673-5680.
7. B.A. WRIGHT and P.A. COLE, Preliminary examination of the crystal structure of l-proline. Aeta Cryst., 1949. 2: p. 129-130.
8. E. Romano , F. Suvire, M.E. Manzur, S. Wesler, R.D. Enriz, and M.A.A. Molina, Dielectric properties of proline: Hydration effect. Journal of Molecular Liquids, 2006. 126: p. 43-47.
9. P.G. Vekilov, Dense Liquid Precursor for the Nucleation of Ordered Solid Phases from Solution. Crystal Growth & Design, 2004. 4.
10. P.E. Bonnett, K.J. Carpenter, S. Dawsona, and R.J. Davey, Solution crystallisation via a submerged liquid–liquid phase boundary:oiling out. Chem. Commun, 2003: p.
689-699.
11. K.-M. Lee, S.-W. Park, I.-S. Jeon, B.-R. Lee, D.-S. Ahn, and S. Lee, Computational Study of Proline-Water cluster. Bull. Korean Chem. Soc., 2005. 26: p. 909-912.
12. M. Civera, M. Sironi, and S.L. Fornili, Unusual properties of aqueous solutions of L-proline:A molecular dynamics study. Chemical Physics Letters, 2005. 415: p.
274-278.
13. B. Schobert and H. Tschesche, Unusual solution properties of proline and its interaction with proteins. Biochimica et Biophysica Acta, 1978. 541: p. 270-277.
7. Summary
In this study we examined laser trapping-induced crystallization of L-proline by focusing
near-infrared laser to the air/solution interface of different solvents such as D2O, EtOD and
its mixture. We carried out microscopic observations of surface deformation, dense liquid
droplet formation and crystallization processes in different solvents and discussed it
dynamics and mechanism of observed trapping laser-induced crystallization and related
phase transition phenomena.
In chapter 3, we examined laser trapping crystallization of proline in D2O. Microscopic
imaging of crystallization process with surface deformation dynamics observation revealed
solution deformation and following crystallization or local dry spot formation. Poor
crystallinity of formed flat polycrystal only within very thin solution layer and frequently
observed local dry spot formation suggests that photon pressure works ineffectively for
forming molecular assembly of proline in D2O due to too strong solute-solvent interaction.
In chapter 4, we examined laser trapping crystallization of proline in mixed solvent of
D2O and EtOD. We succeeded inducing crystallization of proline in mixed solvent. It gives
important suggestion that a photon pressure-induced crystallization possibility can be raised
by adjusting solvent condition such as controlling of solubility and/or of interaction strength
between solute and solvent molecules.
In chapter 5, we examined laser trapping crystallization of proline in EtOD and
successfully demonstrated photon pressure induced crystallization for the first time although
formed crystal disappeared by terminating the trapping laser light. Characterization of
formed crystal done by polarization microscopy, SHG and Raman spectroscopy indicates
formed crystal is proline crystal. We found that crystallization of proline frequently
achieved through dense liquid droplet formation and we succeeded the observation of
primal droplet formation process under microscope. It is also the first report of proline
droplet formation in EtOD. Since previously reported droplet formation of several amino
acids have done with larger scale such as sub-millimeter, microscopic observation of droplet
formation and growth dynamics is one of the significant achievements in this study.
In chapter 6, we have discussed droplet formation dynamics and contribution for
crystallization. Backward scattering spectroscopy at the focal spot during droplet formation
experimentally indicates very local concentration elevation around focal spot of trapping
laser and strongly indicates the formation of dense liquid droplet. Comparisons with the
droplet of glycine in D2O suggest similarity with that observed in proline/EtOD and
suppose similar mechanism of crystallization through liquid-liquid phase separation.
As a result, we successfully demonstrated crystallization of proline by choosing different
solvents. Attempts done in different solvents revealed that solvent dependent obvious
assembling dynamics and crystallization process change. It suggests that crystallization
induced by laser trapping is trigged by local molecular assembling of solute molecules
which exist in quite different environment depending on surrounding solvent molecule and
such molecular environment difference critically affects to crystallization. Results obtained
in this study not only greatly contributed for understanding of laser trapping crystallization
dynamics and mechanism but also clearly demonstrated a novel photoscience of photon
pressure utilized molecular assembly formation.