3. Results and discussion
3.2 Pseudopolymorphism: Concentration dependence
In our experiment, it is too difficult to trap the monomer, so liquid-like cluster is the
trapped target which is spontaneous aggregation of L-Phe molecule and D2O solvent due to
concentration fluctuation. Size and amount of liquid-like cluster strongly depends on solution
concentration, and it becomes more with concentration increase. We confirmed that the
spontaneous crystallization at room temperature always provides the monohydrate crystal, 2.2
and 2.7 × 10-2 wt% are unsaturated and supersaturated solution, and the probabilities of
anhydrous and monohydrate crystal formation at room temperature in different concentrations.
After laser irradiation, either supersaturation or saturation even in unsaturation were observed
monohydrate and anhydrous crystal formation in all sample within 30 minutes depending on
the solution concentration, respectively. I summarized all the experiment results in Fig. 3.7. The
Fig. 3.7 Probability of plate-like crystal formation (blue line) and spontaneous crystallization (red line) depending on concentration.
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probabilities of anhydrous and monohydrate crystal formation are calculated by the number of
the crystal form prepared firstly after starting laser irradiation, and the irradiation was often
turned off immediately after the crystal generation. Compared with spontaneous crystallization,
our crystallization time is much shorter. It confirmed that all the crystals were induced by laser,
and they were not spontaneous one even in supersaturation. The probability of anhydrous
crystal formation became clearly high at the low concentration, it indicates anhydrous crystal
formation is dominant in the solution. At high concentration, laser induced spontaneous
crystallization occurred easily in the solution, but anhydrous crystal was still observed at 2.7 ×
10-2 wt%. These results are much different from results in H2O, which indicates the
pseudopolymorphism ware induced in different process again.
Under supersaturation, it shows laser enhanced spontaneous crystallization of
monohydrate crystal always takes place in the solution, but one anhydrous crystal was still
formed in 2.7 ×10-2 wt%. It means some competition may occur between monohydrate and
anhydrous crystal formation, and also illustrates the ability of polymorph controlling and
dehydration induced by laser as described later. As shown in Fig 3.8, initial solution of 2.9 ×
10-2 wt% is full of liquid-like clusters and these liquid-like clusters are efficiently trapped by
laser leads to the aggregate of liquid-like cluster in the focal spot like stage (i). Through the
liquid nucleation, it goes to the stage (ii) where aggregate of liquid-like clusters is transformed
to assembly of liquid-like clusters. In the stage (ii), assembly of liquid-like clusters starts to
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become well-ordered, higher concentration and less solvent molecules. Assembly of liquid-like
clusters which is trapped at focal spot enlarge the trapping potential and enhance trapping force,
and then make high concentration propagating to the outside of focal spot and entering stage (iii). Here, we propose “laser pinning effect.” The alignment of molecules in the focal spot is
reflecting to the polarization of laser, and the effect decrease with the distance from the focal
spot. The alignment pinned by laser in the focal spot is not suitable for monohydrate crystal
formation. As a result, monohydrate crystal always form not at the focal spot, but monohydrate
crystal formation is achieved when molecular alignment become disordered far away from focal
spot. On the other hand, molecules pinned by laser in the focal volume perform the large-linked
molecules or clusters which is the precursor of anhydrous crystal. As local concentration is
Fig. 3.8 Crystallization process of two crystal pseudopolymorphism under different solution concentration.
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increased by laser trapping, the distance between L-Phe molecules becomes closer and the
intermolecular interactions between L-Phe molecules becomes prominent. As a result, L-Phe
molecules themselves directly linked with each other through relatively strong hydrogen bonds,
and it causes local dehydration where strong interaction excluded the solvent, leading to
anhydrous crystal formation. As described in section 2.4, dehydration in H2O is induced by
laser heating which is enough to exceed the transition point 37°C. It is quite different
mechanism between these two, which one is due to temperature elevation and another is
because of intermolecular interaction. In the case of PNIPAM, phase transition is induced when
critical temperature 34°C is achieved. Tsuboi et al. reported laser-induced reversible volume
phase transition of PNIPAM in D2O.59 PNIPAM gel is in a swollen hydrate phase below a
critical temperature, and in a collapsed dehydrate phase above the temperature. Photon pressure
accelerates the dynamic of phase transition, and shift the transition point to lower temperature.
At the focal point, where the photon pressure is exerted, dehydration is promoted, based on
which they suggest that laser-induced shrinkage. This is quite similar with L-Phe, where phase
transition can be regarded as crystal pseudopolymorphism transformation, so we suggest that
laser can exclude the heavy water away from the focal spot, leading to anhydrous crystal
formation. From another viewpoint, this may be another reason that monohydrate crystals were
always generated not at the focal spot.
Under unsaturation (1.7 × 10-2 wt%), there are no L-Phe liquid-like clusters in the initial
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solution, but sometimes L-Phe liquid-like clusters are trapped at the focal spot due to
concentration fluctuation and laser reveals liquid-like clusters where monomer is changed down
the motion at the focal spot and formed local high concentration area (Fig. 3.8, stage (i)).
Through the liquid-nucleation like of supersaturation, assembly of L-Phe liquid-like cluster is
induced by laser and grew up. It is worth to mention that the size and formation rate of assembly
is much smaller and slower than that in supersaturation. It is not large enough to generate
monohydrate crystal at 1.7 × 10-2 wt%, so there is no monohydrate crystal formation as shown
in Fig. 3.8. On the other hand, anhydrous crystal formation still can be triggered by laser
irradiation, where the assembly of large-linked molecules are excluded the heavy water at the
focal spot. We conclude that “laser trapping-induced dehydration”
At the middle concentration, liquid-like clusters are less than supersaturation but higher
than unsaturation in the initial solution, and the expending rate of assembly of liquid-like
clusters are also between 1.7 × 10-2 wt% and 2.9 × 10-2 wt%. Through the same process as
shown in Fig. 3.8, monohydrate and anhydrous crystal are generated in the solution,
respectively. There is sharp decreasing of probability of anhydrous crystal formation in this
concentration range. As we could predict, strong competition between these two crystal
formations should happen in this sharp decreasing area. In the case of polarization dependence,
there is also the biggest difference between three kinds of polarization and the sharp decreasing at the middle concentration. The detail dynamic and mechanism is discussed as follow.
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