Self diffusion of water

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jean

[SOLVED] self diffusion of water

Hi, I was wondering if anyone could give me a hand- or point in the right direction about a question I have concerning the self diffusion coefficients of HDO vs H2O18. To calculate the effective diffusion coefficients of isotopes of water in groundwater most people use the self-diffusion coefficients in Wang, Robinson and Edelman 1952 in which H2O18>HDO and HTO. I've found more recent references but they compare the diffusion coefficients of D2O and H2O18- I can't seem to find any mention of HDO. So, I guess my questions is; would you expect H2O18 to diffuse faster than HDO?
Thanks!
 
Hmm, that;s a good question. H2O18 is heavier than HDO, so in that aspect I would expect to to diffuse more slowly. Furthermore, you can get deuterium exchange, which is quite fast, and one could think it would move faster than the oxygen in the original HDO. I really don't know.
 

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Intuitively, one bets the month's rent & grocery money on HDO or D2O --- and winds up living under a bridge hungry. What's going on? Autoionization allows O and H/D/T species to diffuse almost independently of each other in water --- "but, that means the lighter species are moving more slowly?" Nope --- cationic species in aqueous solution are very heavily solvated --- if one wishes to apply the various models to calculate/predict diffusion coefficients for H+ (or D, or T) it is necessary to include a solvation mass, which is going to be somewhere between four and six waters, if memory serves --- much heavier than the wandering O18H-, which is essentially unsolvated (charge delocalization leaves no real attractive center for solvation).

Had you simply asked which is faster without explaining what perplexed you, I woulda said HDO --- having to think about what lets O18 move faster, I can explain it --- perhaps not correctly, but, methinks this is what you're seeing --- probably more a matter of retardation of H, D, T for whatever fraction of time they spend in the ionized state than anything else.
 
Re: [SOLVED] self diffusion of water

The self-diffusion coefficient of water has a large drop as the concentration of ethanol increases from 0 to 0.3 and then it nearly keeps constant,while that of ethanol has a minimum around ethanol mole fraction of 0.5.The mutual diffusion coefficient could be divided into two parts,the kinematic factor and the thermodynamic factor.Both the kinematic and thermodynamic factors for ethanol-water mixtures were calculated.It was found that both the numbers of H bonds per water and per ethanol decrease as the mole fraction of ethanol increases. The composition dependences and the relationships between the self-and the mutual diffusion coefficients were further discussed.
 

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