Troyer et al SolStNMR 28(2005),238

-Topic of the paper: strange liquid states of water in nano-pores

-Tool used: nuclear magnetic resoance (NMR)spin-spin relaxation

- Main message: proton spins make good local probefor determination of amount of waterin surface and in bulk-like state

Message talk: Let’s understand the field, the method,and then let’s evaluate the paper.

Condensed Matter Physics –

Phase TransitionsThe problem of waterThe problem of liquids in confined geometries

Traditionally, three phases of matter: gas, liquid, solid

Every material changes between these phases as functionof: pressure

temperaturevolume

p,V,T dep. are described inthermodynamics

Phase diagram Ice/Water

http://www.lsbu.ac.uk/water/phase.html

Reality isn’t quite as nice: The case of water

closeup

The problem: For water the hydrogen bond is delicate

Liquid water consists of a mixture of short, straight and strong hydrogen bonds and long, weak and bent hydrogen bonds with many intermediate between these extremes.

A whole different animal: Pores, nano-poresand water in (nano)pores

The problem is very relevant for many unsolved issuesIn biophysical fields, technical issues like crude oil harvesting,or academic problems like flow in nano confinements.

Question: What happens to the phase diagram of water in pores? What happens when other phases or solutes are present?

The present paper is cutting-edge in this incomplete story. IE don’t confuseit with textbook knowledge.

And: How does one measure these properties and processes?

a local probe is necessary

NMR is such a probe:

Left to right: 3mm slices, 6mm spaced

Influence T1 2s (above), 0.5s (below)

Influenceof shufflingon water dis.

Spatial Distribution SW

Sample region with SW

smaller than 40 %.

Sample region with SW

bigger than 80 %.

Motion in poresMotion in pores

Diffusion : Random Walks

Influence of local environmenton relaxation.

Influence of changed dynamics.

In nano-pores the ‘moveability’ of a water molecule changes by twoOrders of magnitude between surface and pseudo-bulk water!

MRI:Susceptibility issues

Influence of local structure/geometryon susceptibility, hence lineposition.

Evaluating the paper:

Three principle ways of water filling for nano-pores.(1)All pores contain water but small pores fill faster(2) All pores filled completely(3) Not all pores contain water

Thus the understanding of the ‘virgin’ state of thesample is very important (dry weight, cleaning, checking

for traces of molecular water)

What does the statement mean: ‘At higher hydration levels, the decaycurves exhibited bi-exponential behavior’?

It means there are two phases. But how well can such data beseparated?

Note, these are not raw databut separated curves!

So what would the actual rawdata have looked like?

Puddle model:

Plug model: pores hydrated via filling of surface layer followed bygrowth in thickness ‘the plug’ – this is in agreement with one phase being present

Details not very accessible(published as thesis)

Dimensionless puddle size

Study shows no clear indication which water filling model applies. Same eqn

Troyer et al SolStNMR 28(2005),238