Ph and warm water

[Tony]

I know there has been independent talk in the pass regarding using warm water and Ph.
I think I understand that using warm water in the rinse stage can increase swelling and aid in producing a more broadband hologram. But the down side is that if left too long, the gelatin can get washed away.

I know even less about the effects of Ph. I am sure it has a lot to do with acid or base levels effecting the gelatin. As well as effecting the "springs" of the gelatin.

Can anyone comment on the idea of changing the Ph such that would allow a warmer water temperature that would allow the gel to swell and not exceed its tensile strength? Can Ph change the tensile strength of the gelatin?

I'm not planning to try it, just wondering. I've read papers that explain that Ph effects the outcome but never read anything on which way Ph effects outcome.

Thanks

[Dinesh]

Well, it's a complex question because the exact mechanisms of dcg hardening is still not completely understood. There are a variety of theories which explain the observed facts.

Before we even expose, the solution is an equilibrium of of dichromate and chromate ions:

2HCrO4(-) -> Cr2O7(--) + H2O
HHCrO2CrO4 -> H + HCrO4(-)
HCrO4(-) -> H + CrO4 (--)

From here it can be seen that an excess of hydrogen (lower pH) speeds up the emulsion. The addition of an alkali results in:
Cr2O7(--) + 2OH(-) -> 2CrO4(--) + H2O
This reduces the sensitivity of the emulsion.

So, the higher the pH, the longer is the exposure time.

During exposure, for the mechanism of hardening, one theory is:
3(NH4)2Cr2O7 + 6H(+) + hf -> (CrO)2CrO4 + 6NH4OH
(CrO)2CrO4 = 2 gelatin = absorption complex

In this scenario, the free hydrogen radicals are being swept away as exposure occurs, which would increase the pH during exposure. This is, in fact, observed.

The pH does increase during exposure.

Another theory is:
H(+) + HCrO4 + hf -> CrO3 + H2O
which also mops up the free hydrogens.

Once cross linked, the bond energy between the ion and the dipole is in excess of 150 kcal/mole, which is more than light can deliver. Hence, once cross-linked, light alone cannot undo the cross linking and we have (optical) stability.

Another effect is that the spectral sensitivity of the emulsion is affected by pH. In the UV - vis range (roughly 300nm - 600nm), the maximum sensitivity is obtained with a pH of 5.6. Below is a curve of spectral sensitvity against pH ( from Wiley Series on Photographic Science and Technology and Graphic Arts)

The pH also has an effect on Dark reaction. The reaction can be expressed:
Cr2O7(--) + 6 H(+) -> Cr(+++) + 3H2)
As the pH is increased, the reaction tilts to the right. Since the hardening is dependent on the Cr2O7(--), the higher the pH, the longer will the emulsion last. Below is a curve of the effective life of an emulsion wrt pH (also from The Wiley Series)

dcgpH1005.jpg (43.75 KiB) Viewed 4669 times

[a_k]

Hello Tony,

The gelatin molecule is a so called zwitterion. Around its isoelectric point (type A: pH 7-9 / type B:4.7-5.4) the net electrical charge is zero and the swellling is minimal.

With increasing deviation from the gelatins isoelectric point the swelling as well as its solubility/washing out are increased.

So, if you want to use high temperatures, adjust the baths pH to be close to the used gelatins isoelectric point.

Ahmet

[Dinesh]

Ahmet
Would the iso-electric point vary once polymerisation has taken place? In fact, once polymerisation has taken place, is it still a zwitterion. My understanding is that because gelatin is a zwitterion, it tends to cross link with the Cr. After that, the charges are neutralised. Some theories of cross linking by gelatin seem to be different from cross linking with PVA, so I was also wondering how similar the cross linking of gelatin and chromium-3 is in the two cases (gelatin and PVA)

the net electrical charge is zero and the swellling is minimal.

Because of the dipole moment - or lack of?

[a_k]

Hello Dinesh,

First of all, i'm not a chemist, just happen to have a strong interest in the subject, so don't take me too seriously

The isoelectric point of gelatin is adjusted during manufacture and basically depends on how the material was extracted (acid for type A-pork, type B-cow).

Gelatin has a very large number of differently charged sites. I think it would require much more drastic measures to change the gelatins isoelectric point to any considerable extent. The cross linking due to Cr3+ ions doesn't greatly change the net charge. It is merely a structural rearrangement of already existing elements and their bonds.

I don't think that gelatin loses its zwitterion nature after Cr cross linkage:

- Cr ions act on the carboxyl groups but there are other groups carrying a charge which are not affected

- Somewhere i read that the amount of Cr3+ needed to account for the observed degree of cross linkage is much higher (a factor around 100 if i recall correctly) than actually is the case. So even the carboxylic groups charge isn't neutralised fully.

- Depending on the pH of the water bath you'll notice large variations of the degree of swelling

With PVA the cross link is between hydroxylic groups, in the case of gelatin it's the carboxylic groups which are involved. Chromium interacts with both groups but maybe differently.

a_k wrote:the net electrical charge is zero and the swellling is minimal.

Because of the dipole moment - or lack of?

It's how the isoelectric point is defined: the pH at which the number of positively charged functional groups is equal to the number of the negatively charged groups, so the net charge is zero.

Ahmet

[a_k]

The paper i mentioned in the previous post is:
Masako Sasaki, Studies on photosensitive dichromated materials, Report of the institute of industrial science - University of Tokyo, Vol 27, No 1, March 1979 http://holoforum.org/data/doc/Sasaki.pdf - Martin, i hope it's ok with you?

On page 35/36 is mentioned, that for the cross linking of 150g gelatin 52g Cr is needed in theory, but the empirical value is found to be 11 times smaller (not 100x).

There is a paper by Lessard about dichromated PVA. According to it, it is Cr(V) which plays an important role in cross linking PVA: http://holoforum.org/data/doc/Lessard-P ... romate.pdf

[Dinesh]

Actually, chemistry is not my strong point either But, I know that gelatin exists in a number of different forms and not all those forms are conducive to cross-linking. So, in thinking about this, it occurred to me that if the swelling at a given pH of water is due to it's net charge being neutralised

With increasing deviation from the gelatins isoelectric point the swelling as well as its solubility/washing out are increased.

So, if you want to use high temperatures, adjust the baths pH to be close to the used gelatins isoelectric point.

then those forms of gelatin that do not partake in cross linking will still have the same charge configuration as before exposure, ie the exposure will not cross link and therefore will not change the charge configuration; the swell rate will not alter from that of unexposed gelatin when in water of a given pH. But those forms that do take part in cross linking will have their charge changed or reduced , even to zero at the iso-electric point, and will behave differently in the same water at a specific pH. However, inside of a dark (crosslinked) fringe, both forms of gelatin would exist. This seems to imply that the bandwidth would vary not only dependent on the density difference (caused by exposure and implicit in the latent image), but also by the ratio of the different allotropes of gelatin in the emulsion and the pH of the water. I wonder if this might be a way of controlling the bandwidth of dcg?

I remember a paper by Kubota in which he mixed several different types of gelatin to control bandwidth (unfortunately I don't remember the reference - Applied Optics, I think). The concept was that the stronger, ie higher bloom strength, gelatin would act as a kind of "scaffolding" to the weaker gelatin. He got the results he was expecting - he did get a lower bandwidth - but, at the molecular level, I didn't that the Van der Waals forces would justify calling the stronger gelatin a "scaffold". However, it did work, so I always guessed that it must be right since it did work. I now wonder whether the mix of gelatin strengths were related more to the allotropes of the gelatin inside the fringe, thus changing the ratio and so the various charge configurations.

[Martin]

The paper i mentioned in the previous post is:
Masako Sasaki, Studies on photosensitive dichromated materials, Report of the institute of industrial science - University of Tokyo, Vol 27, No 1, March 1979 http://holoforum.org/data/doc/Sasaki.pdf - Martin, i hope it's ok with you?

No problem for me - as long as the authors don't object.

As for the swelling/processing baths, I've always been wondering about the loss of material (gelatin that is) during the processing steps. Washing the trays afterwards, I've seen evidence for that occasionally. I don't buy into the assumption of a "polymerization of gelatin" entirely. Of course, there's crosslinking but at the same time there may be loss of material also.

[a_k]

Martin, what do you mean with the 'assumption of a "polymerization of gelatin"', is it the terminology you don't like? Gelatin could even be called a "superpolymer" or "polymer of polymers" since the macromolecules themselves consist of amino acids which are bond together.

A polymer doesn't have to be structured as a chain (molecule connected head to tail) and the subunits may be polymers. So why shouldn't cross linked gelatin be named a polymer?

I think that the loss of material is mainly due to the limited solubility of the gelatin before it's "melting point" is reached. The losses are easy to limit. The sort of gelatin and it's preprocessing (degree of cross linkage) determine how much material is washed out. A low water temperature and a suitable pH would minimise the material loss but also the swelling and the resulting DE.

A thorough soak/prewash of the gelatin before usage with cold, deonised water would remove some of the impurities. A prewash at sligthly elevated temperatures would remove some gelatin along with the impurities but the remaining gelatin would be less prone to get lost in subsequent use.

[Joe Farina]

I remember a paper by Kubota in which he mixed several different types of gelatin to control bandwidth (unfortunately I don't remember the reference - Applied Optics, I think).

Maybe that would be "Control of the reconstruction wavelength of Lippmann holograms recorded in dichromated gelatin," Toshihiro Kubota, Applied Optics, Vol. 28, No. 10 (15 May 1989), pages 1845-9. But it seems to deal more with shifting towards blue than controlling bandwidth.