| 133 |
|
|
| 134 |
|
Many of the technological applications of the lyotropic mesogens |
| 135 |
|
manipulate the orientation and structuring of the liquid crystal |
| 136 |
< |
through application of local electric fields.\cite{?} |
| 136 |
> |
through application of external electric fields.\cite{?} |
| 137 |
|
Macroscopically, this restructuring is visible in the interactions the |
| 138 |
|
bulk phase has with scattered or transmitted light.\cite{?} |
| 139 |
|
|
| 143 |
|
similar compounds) in 1973 in an effort to find a LC that had a |
| 144 |
|
melting point near room temperature.\cite{Gray:1973ca} It's known to |
| 145 |
|
have a crystalline to nematic phase transition at 18 C and a nematic |
| 146 |
< |
to isotropic transition at 35 C.\cite{Gray:1973ca} |
| 146 |
> |
to isotropic transition at 35 C.\cite{Gray:1973ca} Recently it has |
| 147 |
> |
seen new life with the application of droplets of the molecule in |
| 148 |
> |
water being used to study defect sites and nanoparticle |
| 149 |
> |
strcuturing.\cite{PhysRevLett.111.227801} |
| 150 |
|
|
| 151 |
|
Nitrile groups can serve as very precise electric field reporters via |
| 152 |
|
their distinctive Raman and IR signatures.\cite{Boxer:2009xw} The |
| 186 |
|
bond. This should be readily visible experimentally through Raman or |
| 187 |
|
IR spectroscopy. |
| 188 |
|
|
| 189 |
< |
Herein, we show the computational investigation of these electric field effects through atomistic simulations. These are then coupled with ab intio and classical spectrum calculations to predict changes experiments should be able to replicate. |
| 189 |
> |
Herein, we show the computational investigation of these electric |
| 190 |
> |
field effects through atomistic simulations of 5CB with external |
| 191 |
> |
fields applied. These simulations are then coupled with ab intio and |
| 192 |
> |
classical spectrum calculations to predict changes. These changes are |
| 193 |
> |
easily varifiable with experiments and should be able to replicated |
| 194 |
> |
experimentally. |
| 195 |
|
|
| 196 |
|
\section{Computational Details} |
| 197 |
|
The force field was mainly taken from Guo et al.\cite{Zhang:2011hh} A |
| 299 |
|
after equilibration with zero field applied.} |
| 300 |
|
\label{fig:orderParameter} |
| 301 |
|
\end{figure} |
| 294 |
– |
|
| 295 |
– |
Interestingly, the field that is needed to switch the phase of 5CB |
| 296 |
– |
macroscopically is larger than 1 V. However, in this case, only a |
| 297 |
– |
voltage of 1.2 V was need to induce a phase change. This is impart due |
| 298 |
– |
to the distance the field is being applied across. At such a small |
| 299 |
– |
distance, the field is much larger than the macroscopic and thus |
| 300 |
– |
easily induces a field dependent phase change. |
| 302 |
|
|
| 303 |
|
In the figure below, this phase change is represented nicely as |
| 304 |
|
ellipsoids that are created by the vector formed between the nitrogen |
| 346 |
|
\end{figure} |
| 347 |
|
|
| 348 |
|
Before Gaussian silumations were carried out, it was attempt to apply |
| 349 |
< |
the method developed by Cho et. al. This method involves the fitting |
| 349 |
> |
the method developed by Cho et. al.\cite{Oh:2008fk} This method involves the fitting |
| 350 |
|
of multiple parameters to Gaussian calculated freuencies to find a |
| 351 |
|
correlation between the potential around the bond and the |
| 352 |
|
frequency. This is very similar to work done by Skinner et. al. with |
| 366 |
|
on the $j$th site of the $m$th water molecule and $r_{aj \left(m\right)}$ |
| 367 |
|
is the distance between the site $a$ of the nitrile molecule and the $j$th |
| 368 |
|
site of the $m$th water molecule. However, since these simulations |
| 369 |
< |
are done under the presence of external electric fields and in the |
| 370 |
< |
absence of water the equations must have a correction factor for the |
| 371 |
< |
external field change as well as the use of electric field site data |
| 372 |
< |
instead of charged site points. So by modifing the original |
| 369 |
> |
are done under the presence of external fields and in the |
| 370 |
> |
absence of water, the equations need a correction factor for the shift |
| 371 |
> |
caused by the external field. The equation is also reworked to use |
| 372 |
> |
electric field site data instead of partial charges from surrounding |
| 373 |
> |
atoms. So by modifing the original |
| 374 |
|
$\phi^{water}_{a}$ to $\phi^{5CB}_{a}$ we get, |
| 375 |
|
\begin{equation} |
| 376 |
|
\phi^{5CB}_{a} = \frac{1}{4\pi \epsilon_{0}} \left( \vec{E}\bullet |
| 398 |
|
Since no explicit dependence was observed between the calculated |
| 399 |
|
frequency and the electric field, it was not a viable route for the |
| 400 |
|
calculation of a nitrile spectrum. Instead, the frequencies were taken |
| 401 |
< |
and convolved together. These two spectra are seen below in Figure |
| 401 |
> |
and convolved together with a lorentzian line shape applied around the |
| 402 |
> |
frequency value. These spectra are seen below in Figure |
| 403 |
|
4. While the spectrum without a field is lower in intensity and is |
| 404 |
< |
almost bimodel in distrobuiton, the external field spectrum is much |
| 404 |
> |
almost bimodel in distrobution, the external field spectrum is much |
| 405 |
|
more unimodel. This tighter clustering has the affect of increasing the |
| 406 |
|
intensity around 2226 cm\textsuperscript{-1} where the peak is |
| 407 |
|
centered. The external field also has fewer frequencies of higher |
| 414 |
|
\label{fig:Con} |
| 415 |
|
\end{figure} |
| 416 |
|
\section{Discussion} |
| 417 |
+ |
Interestingly, the field that is needed to switch the phase of 5CB |
| 418 |
+ |
macroscopically is larger than 1 V. However, in this case, only a |
| 419 |
+ |
voltage of 1.2 V was need to induce a phase change. This is impart due |
| 420 |
+ |
to the short distance of 5 nm the field is being applied across. At such a small |
| 421 |
+ |
distance, the field is much larger than the macroscopic and thus |
| 422 |
+ |
easily induces a field dependent phase change. However, this field |
| 423 |
+ |
will not cause a breakdown of the 5CB since electrochemistry studies |
| 424 |
+ |
have shown that it can be used in the presence of fields as high as |
| 425 |
+ |
500 V macroscopically. This large of a field near the surface of the |
| 426 |
+ |
elctrode would cause breakdown of 5CB if it could happen. |
| 427 |
+ |
|
| 428 |
|
The absence of any electric field dependency of the freuquency with |
| 429 |
|
the Gaussian simulations is strange. A large base of research has been |
| 430 |
< |
built upon the known tuning of the nitrile bond as local field |
| 431 |
< |
changes. This differences may be due to the absence of water. Many of |
| 432 |
< |
the nitrile bond fitting maps are done in the presence of liquid |
| 419 |
< |
water. Liquid water is known to have a very high internal field which |
| 430 |
> |
built upon the known tuning of the nitrile bond as the local field |
| 431 |
> |
changes. This difference may be due to the absence of water or a |
| 432 |
> |
molecule that induces a large internal field. Liquid water is known to have a very high internal field which |
| 433 |
|
is much larger than the internal fields of neat 5CB. Even though the |
| 434 |
< |
application of Gaussian simulations followed by mappying to |
| 434 |
> |
application of Gaussian simulations followed by mapping it to |
| 435 |
|
some classical parameter is easy and straight forward, this system |
| 436 |
|
illistrates how that 'go to' method can break down. |
| 437 |
|
|
| 438 |
|
While this makes the application of nitrile Stark effects in |
| 439 |
< |
simulations of water absent simulations harder, these data show |
| 439 |
> |
simulations without water harder, these data show |
| 440 |
|
that it is not a deal breaker. The classically calculated nitrile |
| 441 |
|
spectrum shows changes in the spectra that will be easily seen through |
| 442 |
|
experimental routes. It indicates a shifted peak lower in energy |
| 443 |
< |
should arise. This peak is a few wavenumbers from the larger peak and |
| 444 |
< |
almost 75 wavenumbers from the center. This seperation between the two |
| 445 |
< |
peaks means experimental results will have an easily resolved peak. |
| 443 |
> |
should arise. This peak is a few wavenumbers from the leading edge of |
| 444 |
> |
the larger peak and almost 75 wavenumbers from the center. This |
| 445 |
> |
seperation between the two peaks means experimental results will show |
| 446 |
> |
an easily resolved peak. |
| 447 |
|
|
| 448 |
< |
The Gaussian derived spectra do indicate that with an applied field |
| 448 |
> |
The Gaussian derived spectra do indicate an applied field |
| 449 |
|
and subsiquent phase change does cause a narrowing of freuency |
| 450 |
|
distrobution. |
| 451 |
|
\section{Conclusions} |
| 452 |
< |
Field dependent changes in the phase of a system are |
| 439 |
< |
Jonathan K. Whitmer |
| 440 |
< |
cho stuff |
| 452 |
> |
Field dependent changes |
| 453 |
|
\newpage |
| 454 |
|
|
| 455 |
|
\bibliography{5CB} |
