Summary

By focusing trapping laser to the air/solution interface, photon pressure and heat

generation are induced. Heating caused decrease of surface tension made solution surface

deformed. Slower surface deformation than that of glycine/D O solution is ascribed to  

Fig. 3.9 (a) Initial crystal shape. (b) Laser on, induced crystal growth mainly along the original direction. (c) and (d) show further crystal growth and its local dissolution near laser spot.

higher surface tension of solution surface due to walled container shape and resistance of

high viscosity Further irradiation after solution height became quite near to the bottom glass

substrate gave two results; one is flat polycrystal formation and another is local dry spot

formation.

Crystallization always took place when solution deformed to very thin (<5 m). We

consider major driving forces for flat polycrystal formation are thermal capillary force and

drastic decreasing of molecular moving space, although quite complex reasons for the

crystallization is considered. Probability of crystallization is proportional to laser power but

poorly correlated with solution concentration.

Low collection efficiency of molecules by photon pressure, which reflected in poor

crystal formation probability, can be attributed to strong interaction of solute and solvent

molecules which makes difficult to trap solute molecule. Local dry spot formation is

interpreted as a result of coupled movement of solvent and solute molecules from the laser

spot.

3.7 References

1. T. Sugiyama, T. Adachi, and H. Masuhara, Crystallization of Glycine by Photon Pressure of a Focused CW Laser Beam. Chemistry Letters, 2007. 36: p. 1480-1481.

2. E.J.G. Peterman, F. Gittes, and C.F. Schmidt, Laser-Induced Heating in Optical Traps. Biophysical Journal, 2003. 84: p. 1308-1316.

3. G.D. Costa and J. Calatroni, Transient deformation of liquid surfaces by laser-induced thermocapillarity. Applied Optics, 1979. 18: p. 233-235.

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. 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.

6. M. Jelifiska-Kazimierczuk and J. Szydlowski, Isotope Effect on the Solubility of Amino Acids in Water. Journal of Solution Chemistry,, 1996. 25(12): p. 1175-1184.

7. K. Kar and N. Kishore, Enhancement of Thermal Stability and Inhibition of Protein Aggregation by Osmolytic Effect of Hydroxyproline. Biopolymers, 2007. 87: p.

339-351.

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. Zhang, S. Han, Y. Zhang, R.C. Ford, and J. Li, Neutron spectroscopic and Raman studies of interaction between water and proline. Chemical Physics, 2008. 345: p.

196-199.

10. B. Schobert and H. Tschesche, Unusual solution properties of proline and its interaction with proteins. Biochimica et Biophysica Acta, 1978. 541: p. 270-277.

4. Laser Trapping Crystallization of L-proline in Mixed Solvent (D

O & EtOD)

According to the last chapter, crystal formation in D2O by laser trapping is quite difficult

for L-proline. As we suggested that strong interactions between solute and solvent

molecules may prevent efficient crystallization. We can expect better crystallization

probability by changing the solvent, therefore, we examined different solvent for proline

crystallization by laser trapping.

Decrease in a solubility of proline by changing or adding other solvent may make higher

probability of cluster formation due to increasing in interactions of solute/solute molecules,

leading to the higher probability of laser trapping crystallization. Here we chose deuterated

ethanol, EtOD, as an additional solvent. We used not ethanol but deuterated ethanol in order

to suppress temperature elevation due to the absorption of trapping laser light. The mixed

solvent of D2O and EtOD (0.4 g proline, 200 μL D2O + 400 μL EtOD, supersaturated value

for this mixed solvent is unknown) is employed for trapping experiments. The solution was

dropped to a cover glass and places a cover to prevent evaporation. Crystallization was

examined by focusing trapping laser to the air/solution interface.

4. 1 Solution surface height change and crystallization

By giving irradiation, solution surface height was continuously elevated despite a local

heating took place as shown in Fig. 4.1. Solution height was simply elevated to twice of

initial height. It should be noted that lateral size of solution shrank during irradiation.

Solution covered whole substrate surface before irradiation as Fig. 4.2a but its size became

smaller after starting irradiation as seen in Fig. 4.2b. It is considered due to fast evaporation

of EtOD. Proline/EtOD has lower viscosity and surface tension than proline/D2O [1]. It can

be easily confirmed by dropping both solutions on the glass substrate. EtOD solution of

proline forms widely spread thin solution layer, but D2O solution forms deformed

hemisphere on the substrate. After the evaporation of EtOD, viscosity of solution became

higher since the solution consists less EtOD. Thus elevation of solution height is explained

due to the surface energy and viscosity change induced droplet shape change.

Fig. 4.1 Solution height change recorded during irradiation of 1 W trapping laser.

Subsequently we observed crystallization as seen in Fig. 4.3a to 4.3d. Crystal grew longer

than 100 m within 10 seconds. Its crystal growth rate is between that of polycrystal in D2O

and that of single crystal in EtOD (see Chapter 5). Polycrystals were usually formed and the

rate of crystal growth was relatively fast. We frequently observed unstably trapped small

particles around the laser spot before crystallization. It is probably an aggregate or cluster of

proline molecules that trigger the crystallization.

Fig. 4.3 Pictures of crystal formation and growth. (a) Before crystallization, (b) 2 s, (c) 6 s, and (d) 12 s after starting crystallization. Quite rapid crystal growth was observed.

Fig. 4.2 Mixed solution (a) before and (b) after trapping laser irradiation. Solution size was shrunk in (b).

4. 2 Complicated crystallization behavior

From the Fig. 4.2a we can find the solution surface is quite rough. It implies high

inhomogeneity of distribution of molecules in solution. In addition, we can guess that

solution property such as viscosity, ratio of D2O to EtOD and surface tension were changing

quite a lot during the irradiation. So we regard this crystallization as very complicated

behavior and it is difficult to investigate. For example, the ratio of D2O to EtOD is probably

not constant anywhere and it determines the supersaturated value and crystallization can

occur or not. Laser irradiation not only induces the photon pressure but also heating.

Heating probably induced huge solvent ratio change and inhomogeneous mixing of both

solvent molecules even right after preparation of the sample can be suggested. Solubility of

proline in D2O is 100 times higher than that of EtOD (EtOH) [2, 3]. If there is

inhomogeneous segregation of D2O and EtOD during irradiation, huge difference of

solubility can cause drastic saturation elevation and result in crystallization.

Even contributions of thermal capillary or volume compaction, which are important for

the crystallization in D2O, are considered less effective for crystallization in mixed solution

because no thin layer was formed in mixed solution. Additionally photon pressure probably

works more efficiently since interaction of solute and solvent is weak. However, it is still

regarded as complex contribution of crystallization as we mentioned.

Although organized understanding of crystallization dynamics and mechanism is difficult,

crystallization in mixed solvent always occurred at the focal spot and crystals were trapped

and growing. It is quite alike a typical behavior of laser trapping crystallization. This

strongly indicates that decreasing a solubility of proline will give higher probability of laser

trapping crystallization. Based on this assumption, we intend to shift the experiment to pure

EtOD solvent.

4. 3 Summary

In contrast to D2O solution case, surface height was continuously elevated and

subsequently crystal was formed by focusing trapping laser to the air/solution interface of

mixed solvent. Crystallization at higher solution height suggests that different mechanism of

crystallization unlike to proline in D2O.

Even though observed crystallization is similar to that observed in laser trapping

crystallization of glycine in a sense of non-drying crystallization, crystal formation in mixed

solvent seems more complicated and difficult to elucidate mechanism. Although

understanding of crystallization in mixed solution is in progress and we need more

experiments, mixed solution results gave very important suggestion: We may improve the

probability of laser trapping crystallization by decreasing the solubility with varying

solvent.

4. 4 References

1. G. Vhquez, E. Alvarez, and J.M. Navaza, Surface Tension of Alcohol + Water from 20 to 50 "C. J. Chem. Eng. Data, 1995. 40: p. 611-614.

2. M. Jelifiska-Kazimierczuk and J. Szydlowski, Isotope Effect on the Solubility of Amino Acids in Water. Journal of Solution Chemistry, 1996. 25(12): p. 1175-1184.

3. H.D. Belitz, W. Grosch, and P. Schieberle, Food Chemistry 2009: Springer.