Properties of CO$_2$ clathrate hydrates formed in the presence of MgSO$_4$ solutions with implications for icy moons

0.33 MB·

by  E. Safi

Astronomy&Astrophysicsmanuscriptno.Clathrate˙Paper˙AA (cid:13)c ESO2017
January27,2017
Properties of CO clathrate hydrates formed in the presence of
2
MgSO solutions with implications for icy moons
4
E.Safi1,2⋆,StephenP.Thompson2,AneurinEvans1,SarahJ.Day2,C.A.Murray2,J.E.Parker2,A.R.Baker2,J.M.
Oliveira1,andJ.Th.vanLoon1
1 AstrophysicsGroup,Lennard-JonesLaboratories,KeeleUniversity,Keele,Staffordshire,ST55BG,UK
2 DiamondLightSource,HarwellScienceandInnovationCampus,Didcot,Oxfordshire,OX110DE,UK
7 Version25,January27,2017
1
0 ABSTRACT
2
Context.There is evidence to suggest that clathrate hydrates have a significant effect on the surface geology of icy bodies in the
n
SolarSystem.Howevertheaqueousenvironmentsbelievedtobepresentonthesebodiesarelikelytobesalineratherthanpurewater.
a
Laboratoryworktounderpinthepropertiesofclathratehydratesinsuchenvironmentsisgenerallylacking.
J
Aims.WeaimtofillthisgapbycarryingoutalaboratoryinvestigationofthephysicalpropertiesofCO2clathratehydratesproduced
6 inweakaqueoussolutionsofMgSO .
4
2 Methods.WeuseinsitusynchrotronX-raypowderdiffractiontoinvestigateclathratehydratesformedathighCO pressureinice
2
that has formed from aqueous solutions of MgSO4 with varying concentrations. We measure the thermal expansion, density and
] dissociationpropertiesoftheclathratesundertemperatureconditionssimilartothoseonicySolarSystembodies.
P
Results.Wefindthatthesulphatesolutioninhibitstheformationofclathratesbyloweringtheirdissociationtemperatures.Hysteresis
E
isfound in thethermal expansion coefficients asthe clathrates are cooled and heated; we attributethisto thepresence of the salt
. insolution.WefindthedensityderivedfromX-raypowderdiffractionmeasurementsistemperatureandpressuredependent.When
h
comparingthedensityoftheCO clathratestothatofthesolutioninwhichtheywereformed,weconcludethattheyshouldsinkin
p 2
theoceansinwhichtheyform.WealsofindthatthepolymorphoficepresentatlowtemperaturesisIhratherthantheexpectedIc,
-
o whichwetentativelyattributetothepresenceoftheMgSO .
4
r Conclusions.We(1)concludethatthedensityoftheclathrateshasimplicationsfortheirbehaviourinsatelliteoceansastheirsinking
t
s andfloatingcapabilitiesaretemperatureandpressuredependent,(2)concludethatthepresenceofMgSO4 inhibitstheformationof
a clathratesandinsomecasesmayevenaffecttheirstructureand(3)reportthedominanceofIhthroughouttheexperimentalprocedure
[ despiteIcbeingthestablephaseatlowtemperature.

1 Key words. Methods: laboratory – Molecular data–Planetsand satellites:surfaces – Planetsand satellites:individual: Europa –
v Planetsandsatellites:individual:Enceladus
4
7
6
1. Introduction as CO and CH . sII also form cubic structures and are com-
7 2 4
posed of sixteen small 512 cages and eight large 51264 cages;
0 Clathrate hydrates are formed at high pressures and low
sII clathrates typically host smaller molecules such as O and
1. temperatures and are cage-like structures in which water N .Theleastcommonclathratehydrate,sH,iscomposedo2fone
0 molecules bonded via hydrogen bonds can encase guest lar2ge cage, three smaller cages and two medium 435663 cages.
7 molecules. The conditions on icy Solar System bodies such
sHclathratesformhexagonalstructuresandusuallyrequiretwo
1 as Enceladus, Europa, Mars and comets have long been con-
typesofguestspeciesinordertoremainstable.Recentlyanew
: sidered as potential for clathrate formation (Max&Clifford
v structureofclathratehasbeenproposed(Huangetal.2016);this
2000; Prieto-Ballesterosetal. 2005; Marboeufetal. 2010;
i type of clathrate (“structure III”) is predicted to have a cubic
X Bouquetetal. 2015). Clathrates are leading candidates for
structure and be composed of two large 8668412 and six small
r the storage of gases such as CH4 and CO2 in the Solar 8248cages.
a System (Prieto-Ballesterosetal. 2005; Mousisetal. 2015b;
Bouquetetal. 2015); therefore understanding the kinetics and
thermodynamics of clathrate hydrates under planetary condi- It has been confirmed that water ice and CO2 are present
tionsisimportant. on the surface of Enceladus (Matsonetal. 2013). Among the
The type of guest molecule that can be trapped within a gases such as CH4 present in the plumes emanating from the
clathrate depends on the clathrate structure, of which three satellite’s surface (Waiteetal. 2006), CO2 has poor solubility
are currently known: sI, sII and sH (Sloan&Koh 2007). sI in water. This suggests the trapping of gases in the form of
clathratesformacubicstructurewithspacegroupPm-3n.They clathratehydrates,withsubsequentreleaseduetotheirdissoci-
arecomposedoftwocagetypes;thesmaller512 (12pentagonal ation(Bouquetetal.2015)couldgiverisetotheplumes.Fortes
faces) and the larger51262 (12 pentagonalfacesand 2 hexago- (2007)usedaclathratexenolithmodeltoaccountfortheorigin
nal faces). sI clathrates are constructed of two small cages for of Enceladus’ plumes, suggesting that fluids are able to break
every six larger ones, and host relatively large molecules such through the ice shell, metasomatising the mantle by the em-
placement of clathrates along fractures and grain boundaries.
⋆ email:e.safi@keele.ac.uk The clathrates are trapped in the rising cryomagmas as xeno-
1
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
liths, and are carried upwards where they dissociate, releasing tron diffraction Falentyetal. (2015) studied the dissociation of
theirenclosedgasandformingtheplumes. CO clathratesinpurewatericebetween170–190K,withspe-
2
Thedensityofclathratesis asignificantfactorin determin- cial attention to the polymorphof ice formed.They found that
ingtheirfate.Iftheirdensityishigherthanthatoftheoceansin below 160 K cubic ice (Ic) was the more stable phase, while
which they are formed, they sink to the ocean floor; if lower, between 160–190 K Ic transforms to the more thermodynami-
they rise to the ice/ocean interface. If their destination is the cally stable hexagonalice (Ih). Falentyetal. (2015) concluded
ocean floor then they might be dissociated by heat produced that, due to Ic forming with smaller crystallite sizes compared
from hydrothermal activity. On the other hand if they ascend, toIh,itcouldprovideanadditionalpathwayfortheescapedgas
thenaclathratelayerwouldbepresentattheinterfacebetween molecules originating from clathrate structures, therefore sup-
theiceandoceansurface. portingtheirdissociation.
Bouquetetal.(2015)calculatedthedensityofclathratesas- Animportantfactorwhenconsideringclathratedissociation
suming fully filled cages and a volatile composition based on is ocean salinity. It is well known that saline solutions depress
Enceladus’ plume; they found densities of 1.04 g cm−3 and the freezing point of water (e.g. Duan&Sun 2006), suggest-
0.97 g cm−3 for sI and sII clathrates respectively. When com- ingthatthetemperaturesatwhichclathratesformanddissociate
paringthesetotheircomputedoceandensitiestheydeducedthat within the oceans on planets and satellites will also be lower.
sII clathrates shouldbe buoyantand thereforelikely to ascend. Miller (1961) showed how clathrate dissociation is affected by
Howevertheywereunabletoarriveataconclusionregardingthe temperature and pressure conditions in pure water. However,
sI clathrates, as there was significant uncertainty regarding the when comparing the results obtained by Miller with the more
ocean’ssalinity,andbecausetheclathratedensitywastooclose recent theoretical results obtained by Bouquetetal. (2015) for
to thatof the oceanitself. Clathrate ascensionwould,however, salinesolutions,thereisasignificantdifference,suggestingthat
enableclathratestoplayapartintheformationoftheplumes,as increased salinity may indeed lower the temperature at which
theirdissociationwouldincreasepressureconditionsatthesite clathratesareabletoform.
oftheplume’sorigin(Bouquetetal.2015). In this paper we use synchrotron X-ray powder diffraction
The trapping of gases by clathrates could also have a sig- (SXRPD) to investigate the thermal and physical properties of
nificantimpactonEnceladus’oceancompositionandhencethe CO clathrate hydratesproducedfrom weak aqueous solutions
2
plumesemittedin the southpolarregion(Bouquetetal. 2015). ofMgSO .We replicatepossiblethermalvariationsduetosea-
4
The enclathration of gases would lower the concentration of sonal and tidal changes, ocean depth and salt concentration
volatilesintheoceantobelowthatobservedintheplumes.This and observe the formation and dissociation conditions of CO
2
wouldindicatethatanyclathratesformedwouldneedtodissoci- clathratehydrates.TheSXRPD providesinformationaboutthe
ateinordertoreplenishthevolatileconcentrationoftheplume. temperature-dependence of clathrate densities and hence their
If thisis notthe case thenthe gasconcentrationwouldneed to abilitytoriseorsinkintheoceansinwhichtheyareformed.We
be restoredbyan alternativemechanism,suchashydrothermal alsoinvestigateclathratedissociationkineticsandtheinfluence
activity(Bouquetetal.2015). ofthedifferentpolymorphsofice.
Prieto-Ballesterosetal. (2005) evaluated the stability and
calculatedthedensityofseveraltypesofclathratesthoughttobe
foundinthecrustandoceanofEuropausingthermalmodelsfor 2. Experimentalwork
thecrust.TheyfoundSO ,CH ,H SandCO clathratesshould
2 4 2 2
all be stable in most regions of the crust. They deduced that In this work we use an epsomite (MgSO ·7H O) salt solution
4 2
CH , H S and CO clathratesshould float in an eutectic ocean to form the ice and CO clathrate system. The concentrations,
4 2 2 2
compositionofMgSO -H O,butthatSO clathrateswouldsink. and the temperatureand pressure ranges used, are summarised
4 2 2
Howeverthesinkingandfloatingcapabilitiesofvarioushydrates in Table 1, in which the concentration of MgSO ·7H O has
4 2
willalsolikelydependonthesalinityoftheoceansincethiswill beenconvertedto concentrationofMgSO perkg H O, allow-
4 2
affecttheirbuoyancy. ing for the contribution that the waters of hydration make to
Mousisetal. (2013) investigated clathrates in Lake Vostok the achieved concentrations. In the following we refer to 20g
(Antarctica)usinga statistical thermodynamicmodel.Theyas- MgSO ·7H O/1kg H O as MS10.5, and 5g MgSO ·7H O/1kg
4 2 2 4 2
sumedtemperaturesof276Kandpressuresof35MPaandfound H O as MS3.1. The salt concentrations are similar to those of
2
that Xe, Kr, Ar and CH should be depleted in the lake, while Enceladus (whose salinity is estimated to lie in the range 2–
4
CO shouldbeenrichedcomparedtoitsatmosphericabundance. 10g/kgH O;Zolotov2007)andofEuropa(MgSO concentra-
2 2 4
They also foundthat air clathratesshould float as they are less tion estimated to be between 1–100 g/kg H O; Hand&Chyba
2
dense than liquid water. However, air clathrates have not been 2007).
observedon the surfaceof the ice abovethe lake (Siegertetal. The temperature (T) range we cover is from ∼ 90 K to
2000). To account for this McKayetal. (2013) suggested that ∼ 240 K, and the bulk of our measurements were carried out
largeamountsofCO arealsotrappedwithintheclathrates,in- at pressures (P) of 5, 10 and 20 bar. The range of T is some-
2
creasingtheirrelativedensity. what below that estimated for the sub-surface oceans of (for
Clathrates have been found to form in the Sea of Okhotsk example) Europa and Enceladus (see e.g. Meloshetal. 2004;
(PacificOcean),andTakeyaetal.(2006)haveusedX-Raypow- Bouquetetal. 2015), and is more representativeof these satel-
derdiffractiontostudytheircrystalstructuresandthermalprop- lites’ surfaces.Thepressureswe usedin thisworkwereneces-
erties. They found that four samples from four different loca- sarilyoptimisedtogiveareasonableconversionratetoclathrate
tions each had sI clathrates encaging CH and a small amount with the facility, and within the time, available. In planetary
4
of CO . The small amount of encaged CO is consistent with environments the pressures in sub-surface oceans depend on
2 2
McKayetal.’ssuggestionthatalargeamountoftrappedCO is the depthof the overlyingice sheet, but are typically hundreds
2
necessaryforclathratestosink. of bar (see e.g. Meloshetal. 2004), although the pressures in
It is likely that the type of ice presentduringthe formation Enceladus’ sub-surface ocean may be as low as a few 10s of
ofclathratesalsohasaneffectontheirdissociation.Usingneu- bars(Matsonetal.2012;Bouquetetal.2015).
2
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
Table1.Concentration,temperaturerange,pressure,densityofsaltsolutions.Forcomparison,thewt%salinityofEnceladus’ocean
isestimatedtobeintherange2−10gsaltperkgH O(Zolotov2007),thatofEuropaisestimatedtoliebetween1.1and96.8g
2
salt(MgSO )perkgH O(Hand&Chyba2007).
4 2
Concentration wt% Temperature Pressure Density
gMS7/kgH O MgSO /kgH O range(±5)(K) (bar) (gcm−3)
2 4 2
20 10.5(MS10.5) 90.1–225.02 5±0.01 1.016±0.001
20 10.5(MS10.5) 90.01–240 10±0.01 1.016±0.001
5 3.1(MS3.1) 89.96–245.04 20±0.01 1.003±0.001
SXRPD data were collected using beamline I11 at the 23◦ −24◦, 24.6◦ and 25.2◦ 2θ (Dayetal. 2015). During fitting
DiamondLightSource(Thompsonetal.2009)duringtwelve8- the lattice parametersof Ih ice were initially set to 4.497479Å
hour shifts. The X-ray wavelength was 0.826220Å, calibrated and7.322382Åfortheaandcaxesrespectively(Fortes2007).
againstNISTSRM640cstandardSipowder;thebeamsizeatthe Values for the weighted profile (R ) and background-
wp
samplewas2.5mm(horizontal)×0.8mm(vertical).Thehigh corrected weighted profile (R′ ) fitting agreement parameters
wp
pressure gas cell and the procedureused to form clathrates are between the calculated and experimental diffraction data (see
describedbyDayetal.(2015).A0.8mmdiametersingle-crystal Young1993;McCuskeretal.1999,forfurtherdetails)–exclud-
sapphiretubeisfilledwithsolutionandsealedintothegascell. ingtheicepeaksfromtherefinement–wereR =6.44%and
wp
This is then mounted onto the central circle of the beamline’s 1.21%andR′ =30.71%and27.95%,forthestructuresformed
concentricthreecirclediffractometer,andcooledusingaliquid at90Kand1w8p0Krespectively.Theassociatederrorinthelattice
nitrogen Oxford Cryosystems 700+ cryostream. The latter has
parameterwastypically±0.001Å.
temperaturestability±0.1Kandaramprateof360K/h.
The bulk densities of the MgSO starting solutions were
Once frozen at ∼240 K, CO gas is admitted to the cell 4
2 measured using a 1000µm PhysioCare concept Eppendorf
at the chosen pressure and a fast position sensitive detector
Reference pipette to gather a precise volume of solution and
(Thompsonetal.2011)isusedtocollectinsitupowderdiffrac-
weighed using a Mettler Toledo balance at room temperature.
tion data as the temperature is slowly raised. During this time
ThesolutiondensitiesaregiveninTable1.
iceandclathrateformationissimultaneouslyobserved.Wecon-
tinuetoincreasethetemperatureuntilboththeclathrateandice
arelost,whereuponthetemperaturerampisreversedandthecell 3.1.Inhibitingeffectsonclathrateformation
iscooledoncemore.Dependingonpressureandsolutioncom-
position, either pure-phase clathrates or an ice-clathrate mix is Fig.2comparesthedissociationtemperaturesandpressuresfor
formed.Usingthis“secondcycle”techniqueprovidesincreased the CO2 clathrates formed in the MS10.5 and MS3.1 solutions
clathrate formation(see discussion in Dayetal. 2015). For the tothosereportedbyMiller(1961)forpurewater.Wehavefitted
presentworkwethencycledthetemperaturebetween250Kand thedatafortheMS10.5andMS3.1solutions,forwhichwehave
90 K using the Cryostream to replicate diurnaland tidal varia- dissociationtemperaturesatfourpressures(5,10,15,20bar),to
tionswithappliedCO pressuresbetween5–20bar.Dissociation afunctionoftheform(cf.Miller1961)
2
temperaturesandpressuresweredeterminedbyholdingthesam-
pleatconstantpressureandgraduallyincreasingthetemperature α
log P=− +β , (1)
in5Ktemperaturestepsuntiltherewerenopeaksdiscerniblein 10 T
theX-raydiffractionpattern.
Each SXRPD data-collection cycle, including the time al- where T is in K, P is in bar, and α and β are constants to be
lowedforthe sampleto cometo temperatureequilibrium,took determined.Whilewerecognisethelimitedamountofavailable
approximately20minutes.Oncedatacollectionwascompleted datatodeterminethetwoparametersαandβ,wefindα=1661±
thetemperaturewaschangedtothenewsettinganddatacollec- 292 K and β = 7.74 ± 1.19. These values may be compared
tionrepeated. with those given by Miller (1961) for the dissociation of CO
2
The SXRPD patterns were analysed via Rietveld structure clathratesinpurewater:α=1121.0Kandβ=5.1524;thedata
refinement, using TOPAS refinement software (Coelho 2007) inMiller(1961)arebasedonmeasurementsinthetemperature
and previously published clathrate atom positions and lattice range175–232K.Ourdataconfirmthelikelyinhibitingeffectby
parameters (Udachinetal. 2001) as starting values. Published loweringthetemperatureatwhichCO2clathratesdissociateata
atompositionsandlatticeparameters(Fortes2007)forIhandIc givenpressureoverthetemperaturerange235–260K.
weresimilarlyused.Fromtherefinements,thelatticeparameter,
a,ateachtemperaturestepwasobtainedandhencethethermal
3.2.Thermalexpansion
expansionanddensityofthecubicclathratestructureswerede-
rived.
The thermal expansion of clathrate hydrates is an important
propertythatenablesustounderstandtheirphysicalbehaviour.
Forexample,ithasbeensuggestedthatthe increasein thermal
3. Results
expansion could be due to greater anharmonicity in the crys-
A typical example of a refinement is shown in Fig. 1, which tallattice(Tse1987);thelargerthermalexpansionofclathrates
showsa comparisonoftheSXRPD patternsforIh,IcandCO comparedtohexagonalicecouldbeduetointeractionsbetween
2
clathrates formed in the MS10.5 solution at a CO pressure of theguestmoleculeandhoststructure(Shpakovetal.1998).
2
10bar.Thepresenceoftheclathratesat90Kand180Kisevi- Fig.3showsthedependenceoftheclathratelatticeparame-
dentfromtheformationofmultiplefeaturesat14◦−19◦,21.4◦, terontemperatureatthreepressure-compositioncombinations.
3
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
Table2.CoefficientsofthepolynomialexpressionfordescribinglatticeconstantsofCO clathratehydratesformedintheMS10.5
2
andMS3.1solutions.
Solution Heating Cooling
a (Å) a (10−5ÅK−1) a (10−7ÅK−2) a (Å) a (10−5ÅK−1) a (10−7ÅK−2)
0 1 2 0 1 2
20gMS7/1kgH O(5bar) 11.9187 16.671 — 11.9299 8.01391 —
2
{10.5gMgSO /kgH O} ±0.001418 ±0.8611 — ±0.001149 ±0.8213 —
4 2
20gMS7/1kgH O(10bar) 11.9189 −1.64158 9.91808 11.9343 –19.5623 12.7815
2
{10.5gMgSO /kgH O} ±0.002084 ±2.685 ±0.82 ±0.009007 ±13.77 ±5.077
4 2
5gMS7/1kgH O(20bar) 11.9487 –28.1259 16.2818 11.9294 0.144563 7.38167
2
{3.1gMgSO /kgH O} ±0.002782 ±3.49 ±1.034 ±0.002302 ±3.395 ±1.207
4 2
Wehaveusedapolynomialapproachtodescribethetemperature HereM andM arethemassesoftheCO guestandwa-
CO2 H2O 2
dependencyofthelatticeparameter,usingthefunction ter molecules respectively, and θ and θ are the fractional oc-
1 2
cupanciesofthelargeandsmallcagesrespectively.Ramandata
a=a0+a1T +a2T2 . (2) for CO2 clathrates formed at 20 bar in pure water (Dayetal.
2015)indicatedthatonlythelargeclathratecagesareoccupied
Table2givesthevaluesofthecoefficientsa ,a ,a ,obtainedby
0 1 2 byCO (seealsoRatcliffe&Ripmeester1986).Forthetimebe-
2
fittingEq.(2)tothedata.Althoughthefirstordertermisappar-
ing, we assume in the following that this is also the case for
entlynotsignificantlydifferentfromzerointhebottomtworows
clathrates formedin the presence of MgSO and that therefore
4
oftheTable,itsinclusionwasfoundtosignificantlyimprovethe
θ = 1 and θ = 0. The dependence of the clathrate densities
1 2
fittoourexperimentaldatawheninclusionofthesecondorder
on temperature, calculated using Eq. (3), are shown in Fig. 5.
termisnecessary.
Thededuceddensitiesvarywith compositionand,as wouldbe
TheMS10.5solutionataCO2pressureof10barwascycled expectedfromthe hysteresiseffectin thelatticeparameter(see
onceonly(cf.Section2)andclathratesappearedat185±5Kon
Fig. 3), on whether the clathrate is beingheated or cooled.We
coolingfrom250K.ItseemsevidentfromFig.3thattheexpan-
discuss this further in Section 4.3 below and also consider the
sion of the clathrate on heating does not follow the behaviour effectoffractionaloccupancyofthecages.
on cooling: there is hysteresis in that the cooling and heating
seem notto be reversible.The MS10.5 solutionat a CO pres-
2
sure of 5 bar shows a greater degreeof hysteresiscomparedto 3.4.Weightpercentageofclathrate,Ih,andIcice
the10barsolution.Ittoowasthermallycycledand,oncooling,
Fromtherelativecontributionofeachphasetotheoverallinten-
clathratesappearedat195±5K.Similarly,theMS3.1solution
sity of featuresin the powderdiffractionpattern we can obtain
ataCO pressureof20barwasalsocycledonce,withclathrates
2
the relative fraction by weight of each crystalline component
appearingat247.5±2.5Kwhencooledfrom250K.Thissolu-
present in the sample under study. These are shown in Fig. 6
tion shows a significantlylower degreeof hysteresiscompared
as a function of temperature for the MS10.5 (at 5 and 10 bar
tothesolutionsat5and10barCO pressure.Thedifferencein
2
CO ) and MS3.1 (20 bar CO ). It is immediately evident that
behaviour between heating and cooling may be related to dif- 2 2
the proportionof Ic formedin all three samples is small (typi-
feringlevelsofbondingdisorderwithintheclathratephase(see
cally<5%),evenat90K.
discussioninSection4).
Thecoefficientofthermalexpansionatconstantpressureis Fig. 6 also shows that the relative composition predomi-
nantly depends on pressure and salt concentration. The MS3.1
definedintheusualwayas[(da/dT)/a ] .Inthesimplestcase,
0 P
solutionataCO pressureof20baristhelowestconcentration
the expansion has a linear dependence on temperature and the 2
coefficientofexpansionisa ,whichisindependentoftempera- andhighestpressuresampleandcontainsthehighestproportion
1
ture.Thecoefficientsofthermalexpansionareplottedasafunc- ofclathrates.Theothertwosampleshavethesamesaltconcen-
trationbutareatlowerpressures(5and10bar)andconsequently
tion of temperature in Fig. 4, and exhibit strong pressure de-
showlowerproportionsofclathrate.However,inallthreesam-
pendency. Those CO clathrates formed at the lower pressure
2
ples, the compositionof Ic is similar and always less than 5%;
of 5 bar display a purely linear expansion,while those formed
thisisdiscussedfurtherinSection4.4.
athigherpressureshowmorecomplexbehaviour.Since higher
pressuresresultin highercageoccupancy(Hansenetal. 2016),
ourresultsimplythattheoccupancyofthecagesmayinfluence
thethermalexpansionofclathrates.Thisisdiscussedfurtherin 4. Discussion
Section4.
4.1.InhibitingeffectofMgSO onclathrateformation
4
It is well known that electrolytes have an inhibiting effect on
3.3.Density
theformationofclathratehydrates(Sabil2009).Thisiscaused
The density, ρ, of a clathrate depends on the lattice parame- bytheionsintheelectrolytesolutionloweringthesolubilityof
ter, a, the mass of its water molecules, the mass of the guest the gas, hence lowering the activity of the water, resulting in
molecule and the cage occupancy; it is calculated as follows the clathratehydratesformingatlowertemperaturesrelativeto
(Prieto-Ballesterosetal.2005): their developmentin pure water (Duan&Sun 2006). Also, the
presenceofinhibitorsimpedesthewatermoleculesfromform-
ρ= (cid:16)MCO2 (6θ1+2θ2)+46MH2O(cid:17) . (3) icnlagthhryadterofgoermnabtoionnd.s (Sabil 2009), adding a further obstacle to
a3
4
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
Sodiumchloride(NaCl)andcalciumchloride(CaCl )elec- eutectic freezing out of the pure-phase water ice. This dis-
2
trolytesolutionshavebeenextensivelystudiedasthesearesome placement would cause adjacent water molecules from the
of the major components of terrestrial seawater and rocks and surroundingcages to break hydrogenbonds, hence altering
theirinhibitingeffectiswellknown.Theydecreasethedissoci- neighbouringcages(Shinetal.2012).Suchaprocesswould
ation temperature of clathrates by approximately 5 K in con- beexpectedtoaffecttheelasticpropertiesofthecages,lead-
centrations of 10% by weight of solution and by more than ingtostresshysteresis(Sohetal.2007),andtothehystere-
10Kclosetotheeutecticsolutioncomposition(17%byweight; sis we see in the thermal expansion (see Fig.3). This ef-
Prieto-Ballesterosetal. 2005). In situ studies of clathrate hy- fect should be strongest in those samples with the highest
drates formed in chloride solutions will be reported elsewhere concentrationofepsomite,asis indeedobserved.Thismay
(Safietal.,2017,inpreparation). also be related to our observation(see Section 4.4) that the
Thebestelectrolyteinhibitorswillexhibitmaximumcharge clathrate structure may play a role in stabilising the Ih ice
and minimum radius (Makogon 1981) and, while less is phaseovertheIcphase.
known aboutmagnesiumelectrolyte solutions(e.g. MgCl and
2
MgSO ), they do exhibit an inhibiting effect that is stronger Furthermore, at higher pressures the cage occupancy is
4
than calcium or sodium electrolyte solutions (Sabil2009). The higher and could result in an increase of the lattice parame-
smaller ionic size of magnesium increases the surface charge tersbyseveralthousandthofanÅ(Hansenetal.2016).Indeed,
density, and so attracts more water molecules, thus decreasing Hansenetal. found that CO2 molecules situated in the small
theactivityofwater(Sabil2009). cagesexpandtheclathratelatticeathighertemperatures.Despite
Prieto-Ballesterosetal. (2005) observed a decrease in the thefactthatwehaveassumedθ1 = 1andθ2 = 0,itmaybethat
crystallisation temperature of clathrates due to the presence of theclathratesformedathigherpressureshaveavalueofθ2 >0.
magnesium.However,theinhibitingeffectofthedissolvedmag- Thisiswhatourexperimentaldatasuggestasthethermalcoef-
nesium in their experimentwas small, amountingto about2 K ficientofexpansionforsamplesatthehigherpressureexhibita
at17%MgCl .AlsonotedbyPrieto-Ballesterosetal.(2005)is steepergradient(seeFig4).Howeveritisdifficultwiththedata
2
that the salt depresses the freezing point of water by approx- availabletodrawanyfirmconclusionsabouttheeffectsofpres-
imately 4 K, and so a larger temperature difference between sureandsalinityonthehysteresisinthermalexpansion.Further
icemeltingandclathratedissociationisobservedintheeutectic data,coveringagreaterareaofthepressureandsalinityparam-
salt system. A similar trend was reportedearlier by Kangetal. eterspace,areneededtoaddressthisissue.
(1998) who found that, as they increased the concentration of
MgCl , the amount of hydrate formed at a particular pressure
2 4.3.Clathratedensityandbuoyancy
becomesless.
Fig. 6 shows that there is a larger difference in the wt% of Thevariationofclathratedensity with temperature(see Fig. 5)
clathrates, during both heating and cooling for the samples at has implications for the sinking or rising capabilities of the
a CO pressure of 10 and 20 bar compared to the samples at clathratesinplanetaryoceans.Accordingtoourresultsthegen-
2
5 and 10 bar. This could be due to the fact that the solutions eral implication is that the clathrate density is higher at lower
subjected to 5 and 10 bar CO contain 20 g of epsomite per temperatures and lower at higher temperatures, implying they
2
kg water and the solution subjected to a 20 bar CO pressure havea greaterprobabilityof sinkingatlowertemperaturesand
2
contains5 g of epsomite per kg water, i.e. the epsomite is act- of floating at higher temperatures. However, this will also de-
ing as a clathrate inhibitor. This is further suggested in Fig. 2 pendonthesalinityoftheoceaninquestion.
wherewecompareourclathratedissociationtemperatureswith The MS3.1 and MS10.5 solutions used in this experiment
theCO clathratedissociationcurvegivenbyMiller(1961).As are similar to the salinities of the oceans on Enceladus and
2
discussedinSection3.1,thisshowsthatclathratesformedinthe Europa (see Table 1). If we compare our clathrate densities at
saltsolutiondissociateatlowertemperatures. various temperatures with that of the solutions in which they
were formed, we see that both the measured solution densities
(1.003 g cm−3 and 1.016 g cm−3 for MS3.1 and MS10.5 solu-
4.2.Thermalexpansion
tions,respectively)aremuchlowerthantheCO clathrateden-
2
Asurprisingfeatureofthethermalexpansionbehaviour(Fig.3) sity(Fig.5),irrespectiveofthetemperatureandpressurecondi-
istheapparenthysteresisinthedependenceofaonT,depending tions.Thehigherclathratedensitiessuggesttheclathrateswould
onwhetherthesampleisbeingcooledorheated.Thevariation alwayssink.Indeed,thisisalsotrueifweassumeθ2 isbetween
ofawithT fortheMS3.1solutionataCO2pressureof20baris 0.625–0.688(keepingθ1 =1)whicharethevaluesHansenetal.
almostreversible,withlittledifferencebetweenthecoolingand (2016)obtainedfromtheirexperimentalinvestigation.Therefore
heatingcurves.HoweverfortheMS10.5solutionataCO pres- sinkingofclathratesisthelikelyscenarioforbothEnceladusand
2
sureof10barwebegintoseeadistinctdifferencebetweencool- Europa.
ingandheatingwhilefortheMS10.5solutionataCO2pressure ForCO2 clathratestofloatinanoceanwithsalinitycloseto
of 5 bar the difference is very noticeable. The increase in hys- Enceladus’andEuropa’stheywouldneedlowerguestmolecule
teresiswithdecreasingpressuremaybeduetotwocontributing occupancy.FromEq.(3)theclathratesformedintheMS3.1so-
factors: lutionataCO2pressureof20barwouldrequirethelargercages
to be 73% filled, while the MS10.5 solutionsat CO pressures
2
1. clathratestabilityisgreaterathigherpressures,sothatther- of5and10barwouldneedthelargercagestobe78%and77%
malcyclinghasalessereffect; filled respectively (assuming θ = 0). As mentioned, pressure
2
2. thepossibilitythatduringclathrateformation,theice-phase directly affects the clathrate cage occupancyin that occupancy
water molecules that form the clathrate cages shift in po- (and hence density) is greater at increased pressure. A conse-
sition and form hydrogen bonds with liquid-phase wa- quenceofthisisthatclathratesformeddeeperinanoceanwould
ter molecules. The latter originate from the fluid inclu- have a higher occupancy and would therefore have a greater
sions/channelsrichinMgandSO ionsthatresultfromthe probabilityofsinking.
4
5
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
OurresultsalsosuggestthatifCO clathratesweretoformat 5. Conclusion
2
thebaseofafloatingiceshell(cf.Prieto-Ballesterosetal.2005),
By the use of in situ synchrotron X-ray powder diffraction we
theytoo shouldsink and transportencasedgases to the bottom
havestudiedtheformation,dissociationandthermalexpansion
of the ocean floor. If, on the other hand, the ocean was of eu-
propertiesof CO clathratehydratesformedin MgSO salt so-
tecticcompositionofMgSO (17wt%,assuggestedforEuropa 2 4
4 lutions.Specifically,wehave:
by Prieto-Ballesterosetal. 2005), the ocean density would be
1.19 g cm−3, implying the CO clathrates would in fact float;
2 1. found the dissociation temperatures and pressures of CO
2
thiswouldcausefracturingandgravitationalcollapseoftheter-
clathrate hydrates formed in a salt solution containing ep-
rainduetorapidreleaseofgas(Prieto-Ballesterosetal.2005).
somite (MgSO ·7H O) and that the salt solution inhibits
4 2
clathrateformation.
The effect of pressure on density can be seen in Fig. 5. If
2. computed the density of these clathrates at different tem-
we compare both the MS10.5 solutions at CO pressures of
2 peraturesandpressuresandcomparedthistothe densityof
5 and 10 bar, we see that the sample subjected to 5 bar CO
2 thesolutioninwhichtheywereformed.Whileitwasfound
pressure has a lower density from 95 K to 195 K on heating.
thatthedensityoftheclathratedependsontemperatureand
From 195 K onwards the sample subjected to a CO pressure
2 pressure(andhencelocalfactorssuch asseasonalandtidal
of10barproducesthelowestdensityclathrates.However,con-
changes),whencomparedtothedensityofthesaltsolution
sidering the cooling curves in Fig. 5, the MS10.5 solution at a
they formed in they should in general sink, irrespective of
CO pressure of 10 bar has the highest density throughoutthe
2 thetemperatureandpressure.
entirecoolingprocess.Formostoftheheatingcurvesandallof
3. investigatedthepolymorphsoftheassociatedicephases.We
the cooling curves in Fig. 5 our results are consistent with the
report the dominance of Ih throughout the experiment de-
conclusionthathigherpressureenvironmentsproduceclathrates
spitetheexpectationofIcbeingthethermodynamicallysta-
with higherdensitiesand whichare thereforeless buoyant.We
blepolymorphatlowertemperatures.Thismaybeduetothe
should note that our conclusion regarding buoyancy relates to
saltsolutioninhibitingtheIhtoIctransformation.However
thecaseofCO clathrates.Forthecaseofmultiple-guestoccu-
2 furtherinvestigationintothethermodynamicsandkineticsof
pancy(e.g.CO + CH ) the sinking/floatingcapabilitiesmight
2 4 iceinrelationtoclathratesisneededtoconfirmthis.
wellbedifferent.Howeversuchmultiple-guestclathrateswould
mostlikelybeofthelesscommonsHtype(seeSection1). Theseexperimentalobservationsdemonstratetheimportanceof
understandingtheroleplayedbysalts,clathratesandiceonthe
surfacegeologyandsub-surfaceoceansoficySolarSystembod-
4.4.Thenatureoftheice ies. As a gas transport mechanism the likely sinking of CO2
clathrates formed in saline environmentscould make a signifi-
cantcontributiontooceanfloorgeochemistryonsuchobjects.
Icisthemostcommonpolymorphoficeattemperaturesbelow
160 K. Thereforeinvestigationinto the nature of the ice phase
Acknowledgements. Wethankananonymousrefereefortheircarefulandthor-
thatcoexistswithclathratesisespeciallysignificantinthecon- oughreading ofthepaper, andformakingseveral commentsandsuggestions
textofcoldextra-terrestrialenvironmentsasitwouldimpacton thathavehelpedtoclarifyandimprovethetext.Thisworkwassupportedbythe
theinterpretationofremotelysenseddataandourunderstanding DiamondLightSourcethroughbeamtimeawardsEE-9703andEE-11174.ESis
supportedbyKeeleUniversityandDiamondLightSource.
ofthephysicalprocessingandconditionsintheseenvironments
(Falenty&Kuhs2009).Above240Kwatercrystallisesintothe
thermodynamicallyfavouredIhphase,therateofchangeofIhto References
Icbeinghighestbetween200K–190K,whilebelow∼160KIc
isthestablephase(Falentyetal.2015).Wenotethattheexper- Bouquet,A.,Mousis,O.,Waite,J.H.,Picaud,S.,2015,Geophys.Res.Letts,42,
1334
imentalworkofFalentyetal.wasundertakenat6mbar,typical
Coelho,A.,2007,TopasAcademicVersion4.1,
of Mars’ atmosphere; however the crystallisation temperatures
Day,S.J.,Thompson,S.P.,Evans,A.,Parker,J.E.,2015,A&A,574,A91
ofIhandIcdonotseemtobesensitivetopressuresuptoafew Duan,Z.,Sun,R.,2006,AmericanMineralogist,91,1346
100bar(seee.g.Zhangetal.2015). Falenty,A.,Kuhs,F.,2009,J.Phys.Chem.B.,113,15975
Falenty,A.,Hansen,T.C.,2011,Proceedingsofthe7thInternationalConference
Despitebeingthermodynamicallyfavourableatlowtemper- onGasHydrates
Falenty,A.,Hansen,T.C.,Kuhs,W.F.,2015,arXiv:1510.08004,Proceedingsof
ature,ourdatashowthatthewt%ofIcisnevermorethanabout
the8thInternationalConferenceonGasHydrates
5%, even at 90 K (see Fig. 6) and it may be that the clathrate Fortes,A.D.,Icarus,191,743
structurepreferentiallystabilisestheIhphaseovertheIcphase. Hand,K.P.,Chyba,C.F.,2007,Icarus,189,424
However, it is also possible that, at low temperatures, the rate Hansen,T.C.,Falenty,A.,Kuhs,W.F.,2016,J.Chem.Phys.,144,054301
Huang,Y.,Zhu,C.,Wang,L.,Cao,X.,Su,Y.,Jiang,X.,Meng, S.,Zhao,J.,
oftransformationisslowand,givensufficienttime,alloftheIh
Zeng,X.C.,2016,Sci.Adv.,Vol.2,no.2,e1501010
wouldeventuallytransformto Ic.Furthermore,Ic mightbe the
Kang,S.P.,Chun,M.K.,Lee,H.,1998,FluidPhaseEquilibra,147,229
more favourable phase if the ice were to condense at tempera- Makogon,Y.F.,1981,Hydratesofnaturalgas,PenwellBooks
tureslowerthanthoseusedinthiswork. Marboeuf,U.,etal.,2010,ApJ,708,812
Matson,D.L.,Castillo-Rogez,J.C.,Davies,A.G.,Johnson,T.V.,2012,Icarus,
Also, wenotethatclathratedissociationdoesnotoccurun- 221,53
Matson,D.L.,Davies,A.G.,Johnson,T.V.,Castillo-Rogez,J.C.,Lunine,J.I.,
til the temperaturereachesapproximately200–240K, atwhich
2013,BAAS,416.03
point ice would form as Ih after the clathrates have dissoci- Max,M.D.,Clifford,S.M.,2000,J.Geophys.Res.105,4165
ated(Falenty&Kuhs2009).Therefore,sincetheCO clathrates McCusker,L.B.,VonDreele,R.B.,Cox,D.E.,Loue¨,R.D.,Scardi,P.,1999,J.
2
formed at the the pressures and salinities used in this experi- Appl.Cryst.32,36
McKay, C.P.,Hand,K.P.,Doran, P.T.,Andersen, D.T.,Priscu, J.C.,2013
mentdoesnotseemtodissociateatverylowtemperaturesbelow
Geophys.Res.Letts.,30,1702
240K,wewouldnotexpecttheformationofsignificantquanti-
Melosh,H.J.,Ekholm,A.G.,Showman,A.P.,Lorenz,R.D.,Icarus,168,498
tiesoftheIcphase. Miller,S.L.,1961,PNAS,47,1798
6
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
Mousis, O., Lakhlifi, A., Picaud, S., Pasek, M., Chassefie`re, E., 2013, 35000
Astrobiology,13,380
I
Mousis,O.,Guilbert-Lepoutre,A.,Brugger,B.,Jorda,L.,Kargel,J.S.,Bouquet,
30000
A.,Auger,A.-T.,Lamy,P.,Vernazza,P.,Thomas,N.,Sierks,H.,2015b,ApJ, C
814,L5
25000
Prieto-Ballesteros, O., Kargel, J. S., Ferna´ndez-Sampedro, M., Selsis, F.,
Martınez,E.S.,Hogenboom,D.L.,2005,Icarus,177,491
20000
Ratcliffe,C.I.,Ripmeester,J.A.,1986,J.Chem.Phys.,90,1259
SabNile,tKhe.rMlan.,d2s0.09,Ph.D.dissertation,TechnischeUniversiteitDelft,Delft,The ounts 15000
C
Shin,K.Kumar,R.,Udachin,K.A.,Alavi,S.,Ripmeester,J.A.,2012,PNAS,
10000
109,14785
Shpakov,V.P.,Tulk,C.A.,Kvamme,B.,Belosludov,V.R.,1998,Chem.Phys.
5000
Lett.,1998,282,107
Siegert,M.J.,Kwok,R.,Mayer,C.,Hubbard,B.,2000,Nature,403,643
Sloan,E.D.andKoh,C.A.,2007,Clathratehydratesofnaturalgas,2ndediton, 0
CRCPress,Florida
Soh,T.K.M.,Thurn,J.,Thomas,J.H.,Talghader,J.J.,2007,J.Phys.D:Appl. −5000
10 15 20 25 30
Phys.,40,2176
Takeya,S.,Kida,M.,Minami,H.,Sakagami,H.,Hachikubo,A.,Takahashi,N., 2q degrees
Shoji,H.,Soloviev,V.,Wallmann,K.,Biebow,N.,Obzhirov,A.,Salomatin,
A.,Poort,J.,2006,Chem.Eng.Sci.,61,2670 35000
Thompson,S.P.,Parker,J.E.,Potter,J.,Hill,T.P.Birt,A.,Cobb,T.M.,Yuan, I
F.,Tang,C.C.,2009,Rev.Sci.Instrum.,80,75107 30000
Thompson,S.P.,Parker,J.E.,Potter,J.,Birt,A.,Yuan,F.,Fearn,R.D.,Lennie, C
A.R.,Street,S.R.,Tang,C.C.,2011,J.SynchrotronRadiat.,18,637 25000
Tse,J.S.,1987,JournalofPhysics,48,543
Udachin, K.A.,Ratcliffe, C.I.,Ripmeester, J.A.,2001,J.ofPhys.D:Appl. 20000
Phys.,40,2176 nts
Waite,J.H.,Combi,M.R.,Ip,W.-H.,Cravens,T.E.,McNutt,R.L.,Kasprzak, ou 15000
C
W.,Yelle, R.,Luhmann, J.,Niemann, H.,Gell, D.,Magee, B., Fletcher, G.,
Lunine,J.,Tseng,W.-L.,2006,Science,311,1419 10000
Young,R.A.,1993,inTheRietveldMethod,p.1ed.byR.A.Young,IUCrBook
Series,OxfordUniversityPress 5000
Zhang,X.,etal.,2015,Prog.SolidStateChem.,43,71
Zolotov,M.Y.,2007,Geophys.Res.Letts.,34,L23203 0
−5000
10 15 20 25 30
2q degrees
7000
6000
5000
4000
unts 3000
o
C
2000
1000
0
−1000
10 15 20 25 30
2q degrees
Fig.1.Rietveld refinementsof the MS10.5solutionata 10bar
CO pressureexperimentaldata.Fromtop-bottom:90K,180K,
2
245K.Theexperimentaldataareshowninblack,thecalculated
fit in red, andthe residualsin greybelow.Allprominentpeaks
are labelled where C = clathrate peaks and I = hexagonal ice
peaks.Thelargerresidualsforsomeoftheicepeaksaredueto
poorpowderaveragingduetothewaytheicefreezesinsidethe
cell(preferredorientation)andthe restrictedcellrockingangle
usedtocompensateforthisduringmeasurement.
7
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
3 11.96
heating (2)
11.955
2
11.95
bar) 1 Å) 11.945
g P ( a (
lo 0 11.94
cooling (1)
11.935
-1
11.93
-2 80 100 120 140 160 180 200 220 240
180 200 220 240 260 280 300
T (K)
T (K)
Fig.2. CO clathratedissociation curve(red)forCO clathrate
2 2 11.97
hydrates in MS10.5 (circles) and MS3.1 (squares) solutions,
heating (2)
compared with the CO clathrate dissociation curve for CO
2 2
clathratehyratesinpurewater(Miller1961,bluecurve). 11.96
11.95
)
Å
(
a
11.94
11.93
cooling (1)
11.92
80 100 120 140 160 180 200 220 240
T (K)
11.98
heating (2)
11.97
11.96
)
Å
(
a 11.95
11.94 cooling (1)
11.93
80 100 120 140 160 180 200 220 240
T (K)
Fig.3. The temperature dependence of the lattice parameters
of CO clathrate hydrates formed in solutions of MS10.5 and
2
MS3.1atvariouspressures.Fromtop-bottom:MS10.5at5bar,
MS10.5 at 10 bar and MS3.1 at 20 bar. “(1)” and “(2)” indi-
cate that the cooling was performedfirst, followed by heating.
Foreaseofpresentationbluesymbolsrepresentvaluesobtained
duringcoolingandredvaluesobtainedduringheating.
8
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
6.0E−05 1.075
5.0E−05 2550ggg MM MSSS777//11/1kkkggg HH H22OO2O (( 22(500 bbbaaarrr))) HHC 1.074
20g MS7/1kg H2O (5 bar) C 1.073
−1K) 4.0E−05 2200gg MMSS77//11kkgg HH22OO ((1100 bbaarr)) HC −3m) 1.072 cooling (1)
(0 3.0E−05 (gc 1.071
/a y 1.07
T) 2.0E−05 sit
d n 1.069
da/ 1.0E−05 De 1.068 heating (2)
(
0.0E+00 1.067
1.066
−1.0E−05
80 100 120 140 160 180 200 220 240
80 100 120 140 160 180 200 220 240
T (K)
T (K)
1.076
Fig.4. The thermalexpansioncoefficientfor CO clathrate hy-
2
cooling (1)
dratesformedin solutionsof MS10.5and MS3.1 derivedfrom 1.074
thepolynominalfitstothedatainFig.3.Black:MS3.1/20bar;
green:MS10.5/5bar;orange:MS10.5/10bar.H=Heatingand 3) 1.072
−
C=Cooling. m
c 1.07
g
(
sity 1.068 heating (2)
n
e 1.066
D
1.064
1.062
80 100 120 140 160 180 200 220 240
T (K)
1.074
heating (2)
1.072
3) 1.07
−
m
c 1.068
g
( cooling (1)
sity 1.066
n
e 1.064
D
1.062
1.06
80 100 120 140 160 180 200 220 240
T (K)
Fig.5.Dependenceofdensityontemperature.Fromtop-bottom:
MS10.5 at 5 bar, MS10.5 at 10 bar and MS3.1 at 20 bar.
Symbols/colours,andmeaningof“(1)”and“(2)”,asperFig.3.
9
E.Safietal.:PropertiesofCO clathratehydratesinMgSO solutions
2 4
100
80
Clathrate
60
)
%
(
wt 40
Ih
20
0
Ic
80 100 120 140 160 180 200 220 240
T (K)
100
80 Clathrate
60
)
% Ih
(
wt 40
20
0
Ic
80 100 120 140 160 180 200 220 240
T (K)
100
Clathrate
80
60
)
%
(
wt 40
20
Ih
0
Ic
80 100 120 140 160 180 200 220 240
T (K)
Fig.6. Weighted percentage (wt%) curves for solutions. From
top-bottom: MS10.5 at 5 bar, MS10.5 at 10 bar and MS3.1
at 20 bar. Triangles: clathrates; circles: Ih ice; squares Ic ice.
ColoursareasperFig.3.
10