FORMATION AND DECOMPOSITION OF ICY CLATHRATES

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Submitted on 1 Jan 1987

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FORMATION AND DECOMPOSITION OF ICY CLATHRATES

R. Smoluchowski

To cite this version:

R. Smoluchowski. FORMATION AND DECOMPOSITION OF ICY CLATHRATES. Journal de

Physique Colloques, 1987, 48 (C1), pp.C1-687-C1-688. �10.1051/jphyscol:19871109�. �jpa-00226250�

JOURNAL DE PHYSIQUE

C o l l o q u e

s u p p l e m e n t au n o 3, Tome 48, m a r s 1987

FORMATION

DECOMPOSITION O F I C Y CLATHRATES

R. SMOLUCHOWSKI

University of Texas, Austin, TX 78712, U.S.A.

A b s t r a c t

R e c e n t e x p e r i m e n t a l s t u d i e s (1) o f t h e f o r m a t i o n and decomposition of CD2 - c l a t h r a t e s , from and i n t o H e i c e and m,

^{to indicate}

an extremely low apparent a c t i v a t i o n energy (2) f o r t h e s e processes. A rough estimate of t h e probable energy associated with volume diffusion of (332 molecule through hexagonal openings i n the c l a t h r a t e cage has been made. It suggests t h a t these reactions occur primarily on surfaces of ice grains. The observed suppression of decomposition of c l a t h r a t e s by pressure (3) is i n q u a l i t a t i v e agreement with these conclusions. These and e a r l i e r r e s u l t s (4) indicate t h a t porosity of cometary n u c l e i should play an important r o l e i n a f f e c t i n g t h e r a t e of escape of t h e entrapped m l e c u l e s i n t o t h e cometary

References

(1) Davidson D.W. e t al;, J. Incl. Phen., 2, (1984), 231.

(2) Smluchowski R.,

Nov. 12 (1985) .

(3) Davidson D.W. e t dl., t o appear i n Geochem. C o m h e m . A c t a .

(4) Klinger J. e t al., t o appear i n Astroroids, Comets and Meteors, Uppsala (1985).

COMMENTS

Name of Speaker :E. WHALLEY for SMOLUCHOWSKI

Relative to Prof. Glen's request for comment on formation of the empty clathrate hydrate, I believe that non polar guest molecules in thin deposite offer the hope of seeing this in the

120 K range. We have observed that warming of a 6 micron thick H2S hydrate into the 140 K range causes the HOD 0-D stretch band to move to a position between the

2925 c m ' l hydrate value and the 2410 cm-I ice value (10K values). However, the sharpness of the band resembled ice Ic and leads us to believe it was not the empty clathrate structure.

Name of Speaker :E. WHALLEY for SMOLUCHOWSKI

Is it known what happens to a clathrate hydrate when it is cooled to a low temperature and then placed in a low-pressure environnement

Do the guest molecules leave

If so what happens to t'ne cage structure

Does it survive in the same way as the high presure phases of ice at low temperature

If not does it become amorphous

Does this not have important implications for the behaviour of clathrates in astrophysical situations such as those discussed by Dr. Smoluchowski

If the cage does survive it might be most interesting to measure its thermal

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19871109

C1-688

JOURNAL DE PHYSIQUE

properties in the light of Dr. Whites'explanation of the thermal properties of clathrates.

Answer by M.A.MHITE

It has been found that pumping on clathrate hydrates at too high a temperature ( T ) N 150 or 200 K) causes the guest molecules to pump away, leaving ice Ih.

(However it is possible to maintain clathrate hydrates under vacuum at low temperatures for a long time without loss of guest molecules. For example, in air thermal conductivity experiments we maintained an sample at T

K for some months without measureable loss of guest molecules). It does not appear possible to stabilize the empty host lattice by this method, nor by any other method yet known.

However if one was able to find a way to produce the empty lattice this would be a very important contribution to this field, as it would allow further experiments w3ich would answer many outstanding questions about the unusual properties of clathrate hydrates.

Name of Speaker :E. WHALLEY for SMOLUCHOWSKI Comment of K. ROESSLER

There of so many gas phase reaction which build-up organic and inorganic molecules,

that a conclusion from the composition of the coma for the presence of the molecules

in form of clathrates in the nucleus is not possible.