Distance Between Ions and Water Molecules in Hydrated Solutions

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Discussion Overview

The discussion revolves around the average distances between ions (specifically sodium and potassium) and water molecules in hydrated solutions. Participants seek to understand these distances under varying conditions of hydration, including scenarios with one to four water molecules.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant requests average distances for sodium and potassium ions when hydrated with different numbers of water molecules.
  • Another participant questions the context of the request, asking if the values are needed for estimation or if specific values are sought.
  • Some participants suggest searching for ionic radii in water, noting that some sources may already account for hydration.
  • A participant shares a detailed reference that includes various ionic radii and distances between water and ions, highlighting the variability in these measurements.
  • Concerns are raised about the reliability of certain papers, with one participant arguing that the binding of ions to water molecules is inconsistent and dependent on factors like temperature.
  • Another participant mentions a specific distance range for K+-H2O interactions, indicating it can vary between 2.5 and 3.5 Å.
  • One participant expresses skepticism about the simplifications in a referenced picture, suggesting that the actual distances may be greater than depicted.
  • A later reply mentions a specific distance of approximately 2.5 to 2.65 Å between oxygen and potassium, while questioning the possibility of finding examples where this distance is significantly shorter, around 1.5 Å.

Areas of Agreement / Disagreement

Participants express differing views on the consistency of ion-water distances, with some arguing that these distances can vary significantly based on conditions. There is no consensus on the exact values or the reliability of the referenced sources.

Contextual Notes

The discussion highlights the variability of ion-water distances and the influence of hydration number and environmental conditions. Some assumptions about the consistency of these distances remain unresolved.

somasimple
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Hi All,

What are the average distance of a ion and a water molecule when it is hydrated.

1/ for sodium?
2/ potassium?

Thanks.
 
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What's the context? Are you asked to estimate it? Or you just need the value?
 
I just need average values for sodium and potassium.
Contexts:
1/ with 1 water molecule until 4 water molecules (see picture for a sodium example).
 

Attachments

  • Na_4water.jpg
    Na_4water.jpg
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Google "ionic radii in water". Just beware, as some of those listed can already take hydration into account.
 
Is this one http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TFN-4KPX2FG-5&_user=2552272&_coverDate=10%2F05%2F2006&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1463541728&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=2552272&md5=83e5857fb22d5ca6a0c410434cee74df&searchtype=a"?
 
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I don't have access, so can't tell you much. But my guess is that it should at least list sources of the radii used.
 
Click on figures and tables. There are many values.
BTW, I may buy it.

Fig. 5. (drawn to scale) The oxygen (O) and two hydrogen (H) atoms of water are shown as circles with covalent radii, d(O) and d(H). The circles on the right represent ions with aqueous ionic radii, d(i), aq = kd(A). The length of the hydration bond, d(–O) is the distance between the center of O(water) and P(i/w) (shown as dotted lines in Figs. I–V): (I) Li+, cdots, three dots, centered, Cs+: k = 0.81 = phi/2, d(–O) = 0.64 (±) 0.03 Å. d(i), aq (in Å): 1.42 (Li+) < 1.65 (Ag+ circle with dashes) < 1.74 (Na+) < 2.16 (K+) < 2.26 (Rb+) < 2.49 (Cs+). (II) F−, cdots, three dots, centered, I−: k = 1.57 = 2/phi1/2, d(–O) = 1.60 (±) 0.04 Å. d(i), aq (in Å): 1.11 (F−) < 1.48 (Cl−) < 1.74 (Br−) < 2.08 (I−). The ‘hydrogen bond’ length d(–H) = d(–O) − d(OH) = 0.63 Å. (III): Ni+2, cdots, three dots, centered, Ba+2: k = 0.80 not, vert, similar phi/2 = 0.81, d(–O) = 1.06 (±) 0.08 Å. d(i), aq (in Å) (only some are shown to avoid overcrowding): 0.98 (Ni+2) < 1.19 (Hg+2) < 1.32 (Mg+2) < 2.06 (Ba+2). (IV): Al+3, cdots, three dots, centered, Eu+3: k = 0.47 = (2/phi3), d(–O) = 1.40 (±) 0.06 Å. d(i), aq (in Å) (only some are shown): 0.55 (Al+3) < 0.63 (Rh+3) < 0.81 (Tl+3) < 0.91 (Yb+3, dashed line) < 1.08 (Eu+3, dash and dot line). (V) Lu+3, cdots, three dots, centered, La+3: k = 0.62 = phi/2, d(–O) = 1.37 (±) 0.05 Å. d(i), aq in increasing order (in Å) (only some are shown): 0.97 (Lu+3) < 1.01 (Y+3, dashed line) < 1.13 (Sm+3) < 1.17 (La+3).
 
I can buy it as well, but I won't buy it just to spend money :-p
 
I think the responses are in my previous message.
 
  • #10
No, that's a silly (to the extent of bordering on crankish) paper.

And your question makes no sense. These ions do not bind consistently to any particular number of water molecules in liquid water, and the coordination structure is dependent on the coordination number. The K+-H2O distance can be anywhere between 2.5 and 3.5 Å, and that's dependent on temperature, etc. See, e.g. http://jcp.aip.org/resource/1/jcpsa6/v132/i12/p124503_s1" . (Which, unlike the other, isn't indulging in numerology.)
 
Last edited by a moderator:
  • #11
and this one
http://www.physics.usyd.edu.au/~serdar/papers/jpc08-pol.pdf
 
  • #12
or this one?
http://pubs.acs.org/doi/abs/10.1021/jp075913v
or these books? ;
http://pubs.acs.org/doi/abs/10.1021/cr00019a014
http://pubs.acs.org/doi/abs/10.1021/cr00090a003
 
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  • #14
What's your point? That you know how to use google scholar/chemfinder/isi? I do too.

So if you've been reading these papers, why are you under the impression that hydration of an ion looks like the picture you posted?
 
  • #15
1/ I was just asking for the relevance/quality of the papers.
2/ The picture is very simplified. It is flat but it is very convenient to draw that way. It seems that the distances from oxygen to a ion is a bit farer than my trial.
 
  • #16
It is said that distance from Oxygen atom and potassium atom is around 2.5, 2.65 Angstroms.
Is it possible to find some example where this distance is far less and around 1.5 Angstrom?
 

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