A brief survey is given of the structural characteristics of water at low temperatures in the normal and super-cooled states using neutron diffraction. Similar studies axe presented for D2O water in mesoporous sol-gel silicas showing the depression of the nucleation temperature, the effects of fractional filling and the formation of cubic ice I-c in the pores. Preliminary data is also presented for both neutron and x-ray diffraction studies of water in the ordered MCM41 silica, showing the deep super-cooling and the reversible phase transformation between water and cubic ice. The importance of hydrogen-bonded networks and the development of long-range correlations axe used to provide an explanation of the observations. Further work is in progress

Behrens, Peter

Dore, John C.

Haggenmuller, C

Montague, C.

Webber, J. Beau W.

English

Kent Academic Repository

PHASE RELATIONS FOR WATER AND ICE IN CONFINEDGEOMETRYJOHN DORE, BEAU WEBBER, PETER BEHRENSy, CHRISTIAN HAGGENMULLERyANDDAN MONTAGUEzShool of Physial Sienes, University of Kent, CanterburyCT2 7NR, UK.yAnorganishe Chemie, Universitaet Hannover, D-30167, Germany.zWillamette University, Oregon, USA.Abstrat.A brief survey is given of the strutural harateristis of water at lowtemperatures in the normal and super-ooled states using neutron dira-tion. Similar studies are presented for D2O water in mesoporous sol-gelsilias showing the depression of the nuleation temperature, the eets offrational lling and the formation of ubi ie Iin the pores. Prelimi-nary data is also presented for both neutron and x-ray diration studiesof water in the ordered MCM41 silia, showing the deep super-ooling andthe reversible phase transformation between water and ubi ie. The im-portane of hydrogen-bonded networks and the development of long-rangeorrelations are used to provide an explanation of the observations. Furtherwork is in progress.1. IntrodutionThe phase diagram for ie onsists of a large number of rystalline phasesdependent on various geometries of hydrogen-bonded networks. The im-portane of the hydrogen-bond remains in the disordered phases of liquidwater and amorphous ies. The eets are learly displayed in the super-ooled regime where there are signiant strutural hanges as a funtionof temperature. The liquid density is redued towards that of ie I andthere is a steady evolution of the hydrogen-bonded onnetivity towardsthe ontinuous random network of tetrahedral bonds that haraterises thestruture of low-density amorphous ie.2If the water is onned within the geometry of a mesoporous solid (usu-ally silia), the nuleation temperature is depressed by an amount that isinversely proportional to the pore size. Furthermore, the ie formed underthese onditions is not normally of the form ie Ihprodued from the bulkphase. The phase relationships for ie in onned geometry are thereforeof interest and have been investigated by neutron and x-ray dirationstudies. The following setions desribe some of the reent results.The phase diagram for ie onsists of a large number of rystalline phasesdependent on various geometries of hydrogen-bonded networks. The impor-tane of the hydrogen-bond remains in the disordered phases of liquid waterand amorphous ies. The eets are learly displayed in the super-ooledregime where there are signiant strutural hanges as a funtion of tem-perature. The liquid density is redued towards that of ie I and there is asteady evolution of the hydrogen-bonded onnetivity towards the ontin-uous random network of tetrahedral bonds that haraterises the strutureof low-density amorphous ie.If the water is onned within the geometry of a mesoporous solid (usu-ally silia), the nuleation temperature is depressed by an amount that isinversely proportional to the pore size. Furthermore, the ie formed underthese onditions is not normally of the form ie Ihprodued from the bulkphase. The phase relationships for ie in onned geometry are thereforeof interest and have been investigated by neutron and x-ray dirationstudies. The following setions desribe some of the reent results.2. Diration formalismFor neutron diration by heavy water, D2O, the measured moleular stru-ture fator SM(Q) may be written asSM(Q) = f1(Q) + DM(Q)where f1(Q) is the form-fator andDM(Q) represents inter-moleular termsthat dene the liquid struture. The omposite pair orrelation funtion,g(r), may be evaluated from the Fourier-Bessel transformd(r) = 4 r M[g(r)  1℄ =2Z10Q DM(Q) sinQr dQwhere Mis the moleular density. For D2O, the partial funtions are weightedaording tog(r) = 0:092 gOO(r) + 0:422 gOD(r) + 0:486 gDD(r):For temperature variation studies it is onvenient to use a dierenefuntion analysis proedure where3SM(Q;T; T0) = SM(Q;T )   SM(Q;T0) = DM(Q;T; T0)for a temperature T relative to a referene temperature T0. The orrespond-ing spatial funtion, dL(r;T; T0), may be evaluated from the transformof DM(Q;T; T0). Equivalent expressions may be written for the x-rayase where the sattering is predominantly from the oxygen atoms and theb-values are replaed by the appropriate atomi form-fators, whih areQ-dependent.3. Strutural Charaterisation of Low-temperature Water andAmorphous IeNeutron diration studies have been made of `bulk' super-ooled waterdown to temperatures 35obelow the normal nuleation temperature. Thedensity dereases by 8% and the struture hanges signiantly with tem-perature due to the development of long-range orrelations [1℄ as shownin Fig 1. Low-density amorphous ie is made by vapour-deposition ontoa old substrate plate. The neutron diration pattern has a pronounedpeak at 1.7A 1and the d(r) funtion has well-developed features extendingto 20A and beyond. The struture orresponds to that of a fourfold o-ordinated random network based on approximately linear hydrogen-bonds.All of the spei features in the d(r) urve (Fig.1) an be explained byorrelations within the network. In this ontext it an be seen that thestrutural hanges in super-ooled water an be viewed as an inreasingbuild-up of hydrogen-bond onnetivity as the temperature is redued. Ineet the struture of the deeply-superooled liquid is evolving towards thatof amorphous ie but this metastable state is interrupted by the proess ofnuleation and rystallite growth. It is also lear that the amorphous iephase undergoes a phase transition to ubi ie at 140K so although thestrutural harateristis seem losely related there is a temperature regimewhere neither phase an exist on an experimental timesale.4. Experimental results for water/ie in mesoporous silias4.1. SOL-GEL SILICASNeutron diration results for D2O in sol-gel silias have been presentedpreviously [2℄ and also overed in a reent review [3℄. The main eet isa shift in the position of the main diration peak Q0(T )as a funtionof temperature. This feature indiates an enhaned hydrogen-bonding forthe onned water ompared with that of the bulk liquid and orresponds4Figure 1. Spatial orrelations in deeply-superooled water and low-density amorphousie from neutron diration measurements[1℄.to a shift of approximately 15oC for 90A pores, as shown in Fig 2. Thenuleation temperature is also depressed and depends on the pore size.4.2. PARTIALLY-FILLED PORESIt has been suggested that water in the interfaial region lose to the hy-drophili interfae has dierent properties due to the bonding to surfaesiloxyl groups. The eetive range of this inuene has remained a on-troversial topi for many years but the urrent onsensus is that it is re-strited to a few moleular layers. Comparative studies [3, 4℄ using the tem-perature dierene approah for frational llings of 0.4, 0.6 and 1.0 wereunable to detet any signiant variations and showed that the separateDM(Q;T; T0) and dL(r;T; T0) urves saled almost exatly aordingto the lling fator. This surprising result suggests that there is minimalperturbation to the water struture in the interfae region. The signianeof this observation will be disussed in Se 5.5Figure 2. Variation in the position of the main diration peak Q0(T) as a funtion oftemperature for bulk and onned water.4.3. ICE NUCLEATION IN SOL-GEL SILICASNeutron diration results [4, 5℄ have shown that the super-ooled waternuleates with the formation of ubi ie Irather than hexagonal ie Ihformed from the bulk state. This unexpeted behaviour appears to be re-lated to the nuleation and growth mehanism in the pores as ie Ihisformed in large pores. The eet an be most onveniently demonstrated forthe partial lling of dierently-sized pores and the use of a high-resolutiondiratometer. Fig.3 shows the diration pattern in the region of 1.7A 1from several samples where the eetive thikness of the water layer is ap-proximately 20A. The harateristi signal for ie Ihonsists of a tripletwhereas that of ie Iomprises a single entral peak. The results showthat a thin water layer in a large pore nuleates to form pure hexagonal iebut in a more restrited volume there is an asymmetri peak prole sug-gesting a defetive form of ubi ie. There is a possibility that this prolean be explained on the basis of staking faults in the `abab' layers but6the situation is ompliated and there is no onvining explanation of theobserved results. Further disussion is deferred to Se 5.Figure 3. The main diration peak after nuleation of ie in dierent pore-size silias.4.4. WATER/ICE IN MESOPOROUS SILICAS; NEUTRON DIFFRACTIONThe MCM-type silias [6℄ provide an important lass of ordered mesoporoussolids where the geometry is well dened. They are produed via the sol-gelproess using a liquid rystal template and are alined to give an orderedarray of hannels with diameters in the 25-35A range. There are two formsshown in Fig.4 based on the hexagonal array of parallel pores (MCM41) oran inter-twined array of branhed pores based on the ubi phase (MCM48).Neutron diration data [3, 6℄ have been taken for both systems and re-sults are shown in Fig.5a for D2O in MCM41at several temperatures aftersubtration of the substrate sattering ontribution. The liquid undergoesdeep superooling and nuleates to ubi ie at approximately 45oC belowthe normal freezing point of the bulk sample. Furthermore, the transforma-tion between the liquid and rystal phases is ompletely reversible withoutany hysteresis. Consequently, there appears to be a rst-order phase tran-sition between two states that would both be in a metastable ondition forthe bulk state at this temperature. The use of a dierene funtion proe-dure gives the dL(r;T; T0) funtion shown in Fig.5b, where the build-upof long-range spatial orrelations an be seen to be similar to the features7of Fig 2. It is lear that there is a ontinual enhanement of the hydrogen-bonding network as the temperature is redued.Figure 4. The mesoporous struture of MCM41 and MCM48 silias.4.5. WATER/ICE IN MESOPOROUS SILICAS; X-RAY DIFFRACTIONIn order to study other aspets of the deeply-superooled state and nu-leation harateristis, an x-ray diration study [7℄ using synhrotronradiation was undertaken .A sample of the preliminary results is given inFig.6 based on a dierene funtion analysis. It was found that ie nu-leation ourred at a higher temperature than expeted but it is unlearwhether this ourrene is due to the perturbation of the sample by thex-ray beam or due to exess water on the outside of the pores. The gureshows that hexagonal ie is initially formed but also that the entral peak8a) diration results.b) the hange in the spatial funtion dL(r;T; T0).Figure 5. Neutron diration data for superooled water in MCM41 silia.9of the triplet inreases in intensity at the lower temperatures orrespondingto the formation of ubi ie. A puzzling feature of the measurements wasan apparent hange in the relative intensities of the triplet peak even whenthe temperature was stable. This behaviour suggests that there are dynamihanges in the ie struture on a timesale of minutes. The beam-size was2 x 3 mm so many rystallites ontribute to the diration pattern andit seems unlikely that this observation an be the result of any preferredorientation in the sample. The natural onlusion is that the ie strutureis itself hanging and that the average over the utuating Bragg intensityis being observed in the datasets. Clearly, a more detailed investigation willbe needed to isolate the separate time and temperature dependene but theobvious onlusion is that the nuleating ie is not in a stable equilibriumstate.MCM41–C16 0.9{ml/g} H2O  Vs Run 14@286.2K            100             50          50000          40000          30000          20000          10000      1 : 264.7K      2 : 240.1K      3 : 215.2K      4 : 190.4K      8 : 182.3K      9 : 198.6K     10 : 215.1K     11 : 231.6K     12 : 248.1K     13 : 264.6KFigure 6. Temperature dierene funtions for x-ray sattering by water in MCM41silia.The hanges with temperature an also be seen in the small-angle sat-tering region. At low Q-values, the SAXS intensity prole results from theordered array of the ylindrial pores and has a similar shape to that forthe liquid rystal used as the template. The intensity for this region showsa systemati variation with temperature and is highest below 200K, thelowest temperatures of measurement. Furthermore the intensity falls as thesample is warmed to its original temperature.The origin of the SAXS signal is from the ontrast between the -valueof silia and ie; typial relative values are 9.3 for ie Ihand approximately24 for the silia substrate. The large hange in intensity is therefore diÆult10to understand as the -values are not expeted to hange muh with tem-perature. The only other possibility is that there are inhomogeneities withinthe ie itself leading to a variation in density aross the pore volume that isthen onvoluted with the liquid rystal form-fator. If so, this phenomenonwould imply a partial fragmentation of the ie at low temperatures and ahealing or annealing eet on warming. It is, of ourse, possible that the ieis under pressure from the hanges in density within the restrited volumebut it is still diÆult to provide a self-onsistent piture that fully explainsthe observations. Further measurements under arefully ontrolled ondi-tions and heks for reproduibility will be required to understand theseomplex new results. It is hoped to make these measurements in the nearfuture [8℄Conlusions and Future WorkThe piture that emerges from these investigations is that the deeply-superooled liquid state depends on the systemati evolution of the hydrogen-bonded network with reduing temperature. The properties vary in a pre-ditable manner despite the density maximum and the unusual behaviourof the super-ooled state. In a similar manner, the properties of low-densityamorphous ie an be understood in terms of a ontinuos random networkof hydrogen-bonds. Although quantitative agreement with the experimen-tal results has not been obtained from simulation studies, it is well reog-nised that the spae-lling harateristis of the tetrahedral bonding playa entral role in the behaviour. What remains as a hallenge to presentunderstanding entres on the phase relations between the ordered and dis-ordered states, whether in the rystalline solid, amorphous solid or liquid.The present work addresses the relationship between water and amorphousie with rystalline ie I, using onned geometry to provide deep super-ooling of the liquid.Under bulk onditions, hexagonal ie Ihforms naturally as water isooled. However, it appears that the initial phases of nuleation and growthinvolve the formation of a defetive seed rystallite of ie I. The reasonsfor this behaviour are unlear and are probably linked to the stability ofsmall H-bonded lusters in the liquid. As the temperature is further re-dued, the network onnetivity is inreased and the longer-range orrela-tions beome more important. The eets of the onned geometry give aneetive temperature shift so that the network beomes more developed,moleular motion is more restrited and the nuleation proess is inhibited.The eet of pressure within the apillaries annot be ignored but it annotbe simply evaluated. The most important parameter is the density of theonned water in the pores and its variation with temperature but experi-mental methods to determine this value are problemati; one possibility isurrently under investigation [9℄.11The eet of the pore size (Fig 3) suggests that ie Ihis the preferredrystalline phase for large extensions of onneted water, whether as a vol-ume (3D) or a thin layer (2D). It seems likely that this priniple will re-main for hydrophobi as well as hydrophili interfaes but there is, as yet,no experimental veriation sine most of the studies are naturally basedon the wide range of silias that are available. Some additional studies onpartially-hydrophobi samples are needed but are diÆult and have notbeen attempted in any systemati manner. The obvious extension to sys-tems of biologial signiane is a natural development and also the asso-iated prevention of rystallization in ryo-protetion proesses.Another puzzling aspet of the urrent studies is the apparent hangesthat our in the diration [and SAS℄ pattern below the onset of ie nu-leation. This phenomenon suggests that there is a muh more omplexbehaviour of the onned ie than might have been expeted. The fat thatthe water/ie system ontinues to give unexpeted results is, itself, possiblyno surprise { it seems that the more we learn about this fasinating system,the less we understand about the nature of olletive hydrogen-bonded sys-tems. The next deade should see further advanes in the linking together ofexperimental, simulation and theoretial developments to provide a learerpiture but it seems inevitable that water and ie will ontinue to baeus for a little while longer. The mysteries of water will remain as a majorsienti hallenge for the foreseeable future.5. AknowledgementsThe work presented here has developed over an extended period of timewith the help of several EPSRC researh grants, studentships and aessto international failities. More reently, it has been supported by an In-terreg grant for the study of `metastability in moleular systems' with theuniversity of Lille and a joint Royal Soiety/DAAD travel grant for ollab-orative work with Professor Peter Behrens group in Munih and Hannover.One of us (JBWW) wishes to aknowledge support from the Interreg pro-gramme [projet KN/C8/01 272-31503℄. We also wish to thank Henry Fis-her, Thomas Hansen, Angel Mazuelas and Peter Boeseke for assistaneduring various experimental measurements at the ILL and ESRF.Referenes1. M-C. Bellissent-Funel et al, Europhys. Lett. 2, 241 (1984)2. D.C.Steytler, J.C.Dore and C.J.Wright, Mol. Phys. 56,1001 (1985) andpapers ited therein3. Strutural studies of water in onned geometry, J.Dore, Chem.Phys.258, 327 (2000).4. J.M.Baker, PhD thesis, University of Kent, (1996)125. J.M.Baker, J.C.Dore and P.Behrens, J. Phys. Chem. B1001, 6226 (1997)6. P.Behrens and G.Stuky Angwe. Chem. Int Ed Engl. 32, 696 (1993)7. J.C.Dore, J.B.W.Webber, M.Hartl and A. Mazuelas, unpublished data;report in ESRF Annual Review 20018. Neutron and x-ray experiments for water in MCM silias are alreadyplanned.9. The use of variable isotopi substitution for H2O/D2O mixtures an beused in SANS studies to give the temperature dependene of the densitybut the determination of absolute values poses signiant problems.

Phase relations for water and ice in confined geometry

http://kar.kent.ac.uk/13455/1/ARWwater.pdf

Phase relations for water and ice in confined geometry

Abstract

Similar works

Full text

Available Versions

Kent Academic Repository