Molecular Dynamics Simulations of Template-Assisted Nucleation: Alcohol Monolayers atthe Air -Water Interface and Ice Formation
Yaohua Dai and John Spencer Evans*Laboratory for Chemical Physics, New York UniVersity, 345 E. 24th Street, New York, New York 10010
ReceiVed: May 14, 2001; In Final Form: August 23, 2001
Perhaps the best characterized template nucleation systems are the alcohol monolayer assemblies at the air-water interface. These monolayers, consisting of aliphatic alcohols [CnH2n+1OH, n ) 16-31) have been thesubject of hexagonal ice (Ih) induction experiments over a range of temperatures. To learn more about themolecular basis of alcohol monolayer template-induced ice nucleation phenomena, we performed canonicalmolecular dynamics (MD) simulations (1 ns) of pure C29, C30, and C31 alcohol monolayer-water dropletsystems, using the reversible reference system propagation algorithm (r-RESPA). We find that the MD-determined monolayer physical parameters (at 278.15 K) exhibit reasonable agreement with experimentallydetermined values for crystalline C30 and C31 monolayers at 278.15 K. More importantly, as the simulationtemperatures approach monolayer-specific freezing points, the water layer immediately below the monolayersurfaces adopted “ice-like” lattice parameters and hexagonal or c-centered rectangular geometries that arecharacteristic of the (Ih) {001} plane. The analysis of monolayer-OH headgroup orientations and surfacetopologies reveal that odd-carbon monolayers, and C31 in particular, are more effective nucleation templates,due to the following factors: (1)ab-plane alcohol-OH headgroup geometries which provide a closerab-plane lattice match to the{001} Ih interface and (2) the presence of smoother monolayer headgroup topologies(i.e., azimuthal positioning), which permit a larger percentage of water molecules to form hydrogen bondswith the monolayer alcohol headgroups.
Introduction
The ability of biological macromolecules to induce thenucleation of inorganic solids1 and ice2-4 has formed the basisof synthetic templating techniques.5-10 With regard to nucleationinduction, the role of supamolecular templates, such as com-pressed Langmuir monolayers,11,12 self-assembling mono-layers,13-15 oriented polymer films,16 and surfactant aggre-gates,17,18is to mimic the molecular characteristics of a growingcrystal surface, i.e., provide active sites that stereospecificallyinteract with molecules or atoms in solution.19 Perhaps the bestcharacterized template nucleation systems are the uncompressedaliphatic alcohol [CnH2n+1OH, n ) 16-31] monolayer as-semblies at the air-water interface.5,6,12,20These monolayershave been the subject of ice induction experiments over a rangeof temperatures.5-7,12,20-22
The ice induction or templating capability of alcohol mono-layers has been correlated to the following monolayer proper-ties: (1) geometric matching between the unit cells of themonolayer chains and those of the of hexagonal ice (Ih) {001}plane,21 (2) monolayer alkyl chain length, and (3) monolayerpacking arrangements.5,7,20,21From experimental studies, it isknown that even-carbon alcohol monolayers induce ice nucle-ation at temperatures lower than those required for odd-numbered chains5,6,20,22and that C31 alcohol monolayers induceice nucleation at higher temperatures.5,6,20,22Yet, the packingand intermolecular headgroup-OH distances for C30 and C31alcohol chains are alike.6,20To explain this paradox, it has beenproposed6 that alcohol monolayer-OH orientational differencesin odd- and even-carbon monolayers affects ice nucleation. Thishypothesis is based upon structural studies of alcohol estermonolayers;21 however, it should be realized that free C-O-H
headgroups in a monolayer may not be constrained or orientedto the same extent as the alcohol moieties are in crystallinealcohol esters.21 It has also been hypothesized that “smooth”monolayer surfaces are important for ice nucleation.7 However,to date, alcohol monolayer surface topologies have not beencompletely defined.6,7,20,22
It has been suggested that molecular dynamics (MD) couldbe utilized to resolve the issue regarding the observed temper-ature-related ice nucleation differences in odd- and even-carbonalcohol monolayers.6 Recently, Bell and Rice reported MDsimulations of long chain carboxylic acids and alcohols sup-ported on the{001} basal plane of Ih,23 focusing on the influenceof alcohol headgroup-ice surface interactions on monolayertilt angles. However, the actual ice nucleation process was notinvestigated in these studies. To explore the molecular basis oftemplate-assisted ice nucleation, we preformed canonical NVT(constant particle number, volume, and temperature) MDsimulations (1 ns in duration) of pure C29, C30, and C31 alcoholmonolayer-water droplet systems (Table 1),20 using the revers-ible reference system propagation algorithm (r-RESPA).24-27
These three chain lengths were chosen to explore the relationshipbetween odd/even carbon number and the temperature-depend-ent nucleation of the{001} basal plane of Ih.5,6,20,22The resultsobtained from the r-RESPA MD simulations reveal that thealcohol monolayers achieve chain packing arrangements andlattice parameters that are in reasonable agreement with exper-mental data.6,20 Suprisingly, within 1 ns of simulation time, wewere able to observe the formation of planar water clustersimmediately below the interfaces of the each monolayer.However, not all alcohol monolayers act as an effectivetemplate: the water clusters which are templated by the odd-
carbon C31 and C29 monolayers possess lattice parameterswhich exhibit reasonable agreement with those obtained for Ih
{001} interface,5 whereas the water clusters below the C30monolayer exhibit poor condensation and insufficient latticematching to the Ih {001} plane. We find that the observeddifferences in ice nucleation capabilities are linked not to spatialarrangements of alcohol hydrocarbon chains but to-OHheadgroup arrangement in theab plane and toz axis alcoholheadgroup topology or monolayer “surface roughness”,28 bothof which influence the number of hydrogen bonds that formbetween monolayer alcohol headgroups and the water phase.
Methodology
Simulation Protocol. Each monolayer is modeled as anuncompressed system, i.e., pressure) 1 atm.5,6,20 Because ofinherent problems related to using periodic boundary conditionsfor interfacial systems, we employed three-dimensional bound-ary conditions similar to those utilized in cationic surfactant-hydrophobic suface MD studies.27 Each simulation box (40×25 × 100 Å3, with the largest cell dimension along thez axis)consists of 56 alcohol molecules (all-trans conformation)periodically replicated as parallel chains in thex andy directions(t, δ ) 0°), using published unit cell dimensions.5,6 Eachmonolayer covers a water slab consisting of 2880 molecules.All -OH groups are facing the water interface and aredistributed on thez ) 0 plane. The alcohol monolayer densitieswere 20 Å2/molecule, corresponding to packing densitiesreported in alcohol monolayer ice nucleation experiments.6 Foreach monolayer-water system, the equilibration stage involvedthe following protocol, adapted from r-RESPA studies ofsurfactant-water-hydrophobic surface systems:27 Step 1: Toreduce the computational expense, the initial MD equilibrationis conducted with a half-sized system. This smaller system isheated from 100 to 300 K in increments of 10 K at every 10 psinterval.Step 2: The system is then simulated for 250 ps at theappropriate simulation temperature (300, 278.15, 272.15, or265.65 K). At this point, the system size is doubled.Step 3:After a 10 ps short relaxation period, the MD simulation runsare conducted at constant temperature for a total duration of 1ns. Constant particle, volume, and temperature (NVT) simula-tions are performed using the Nose-Hoover chain thermostatextended system canonical dynamics r-RESPA method24-27
which involved 3-stage force decomposition into intramolecular,short-range, and long-range intermolecular forces. All simula-tions utilized time steps of 6 fs. The minimum image conventionwas used to calculate both the van der Waals interactions andthe real part of the Ewald sum with spherical truncations of 7Å for the short-range and 10 Å for the long-range part of ther-RESPA decomposition.27
Force Field and Atom Parameters. Intermolecular non-bonding potentials involved the pairwise site-site Lennard-Jones12-6 attraction-repulsion term (cross interactions calculatedusing Lorentz-Berthelot combination rules) and Ewald sum-mation27 for electrostatics. For alcohol molecules, methyl andmethylene groups were represented as united atoms (i.e., thesegroups were represented by single interaction sites),-OHheadgroups were represented as explicit atoms; the atomparameter sets, bonding potentials (bond, angle, torsion, andinversion), Lennard-Jones A and B parameters and correspond-ing atomic charges were taken from published liquid alcoholMD simulations.29 TIP3P parameters and charges were utilizedto represent the water solvent.30
Results
Monolayer Physical Parameters: Correlation betweenExperimental and Simulation Data. Our first task is toevaluate the degree to which r-RESPA MD simulations canreplicate experimentally characterized C31 and C30 monolayerassemblies at 278.15 K.6,20 We examine thea andb unit cellparameters, area per molecule [A, ) (a‚b)/2],6,20 and tilt anglest and δ (which describe the angles of tilt of the alkyl chainfrom the normal to the water surface in theb anda directions,respectively),6,20,28for each monolayer system near the conclu-sion of the r-RESPA simulation run (i.e., 950 ps). Thea andbvalues were determined from methyl carbon distances using thefollowing equations:
whered is the distance between two adjacent carbon atoms alongthex or y direction, andNxp, Nyp are the number of atom pairsalong thex direction andy direction, respectively, for all rowsof the monolayer. As shown in Table 1, we find that the MD-determineda, b, A, t andδ parameters are observed to decreaseas the system temperature is lowered. If we compare thea, b,andA parameters obtained for C31, C30, and C29 monolayersat equivalent simulation temperatures, we find close agreementbetween each simulation system (i.e.,(5%). More importantly,our C30 and C31 parameters obtained at 278.15 K exhibitreasonable agreement (10-15%) with grazing incidence X-raydiffraction (GIXD)-determined values for crystalline C31 andC30 monolayers.20 Monolayer chain tilting (Table 1, Figure1)6,20was observed for all monolayer systems; monolayer tiltingwas also reported in previous MD studies of alcohol monolayerson the Ih {001} plane.23 However, compared to GIXD-
a Experimental C31 and C30 values (parentheses) were determined by GIXD at 278 K, in the uncompressed state.6,20 No comparable experimentaldata exists for C29 alcohol monolayers. Boldfaced temperature values indicate the observed freezing temperature in alcohol/water droplet systems.20
All a, b, A, t, andδ values were measured at 950 ps for each simulation run.b For comparison, lattice parameter values for Ih are presented (takenfrom ref 5).
a )
∑N
dy,i
Nxp
; b )
∑N
dx,i
Nyp
(1)
10832 J. Phys. Chem. B, Vol. 105, No. 44, 2001 Dai and Evans
determined tilt angles,6,20 the r-RESPA-determined tilt anglesare observed to be larger (by 30-60%). We attribute these largedeviations to inherent limitations in our simulation protocol (e.g.,united atom description of alkyl chains, three-dimensionalboundary conditions, bonding and nonbonding potentials,etc.)23,27,28and to chain density-related tilting distortions.23
Template-Ice “Matching”, Headgroup Orientation, andMonolayer Surface Topology.If we compare the r-RESPA-determined C31, C30, and C29 monolayer chaina, b, andAparameters at 950 ps with those obtained for the Ih {001} plane,5
we observe reasonable agreement (i.e., within 13-17%) at thelowest simulation temperatures (Table 1). This agreement is alsoconfirmed by visual analyses (Figure 2): we obtain crystallineor periodic chain packing (as viewed along thez axis) for eachmonolayer system that resembles the distorted hexagonal orc-centered rectangular unit cell observed in experimental GIXDstudies of alcohol monolayers6,20,22and in theoretical MD studiesof C22 alcohol monolayers on ice.23 As noted elsewhere, thispacking arrangement exhibits geometric complementarity to theIh {001} plane.5,6,20,22,23
An examination of the-OH headgroupab-plane geometryfor each alcohol monolayer at the observed freezing temperature(Figure 3, at 950 ps) reveals some interesting contrasts tohydrocarbon chain packing geometry. Experimentally, it hasbeen noted that the oxygen atoms of C30 and C31 alcohols inmonolayer assemblies are differentially positioned.6 In ourr-RESPA simulations, a similar result emerges: compared tothe C31 and C29 monolayers, the C30 monolayers possess fourregions per 56 alcohol molecules (7%) where the-OHheadgroup atoms in theab plane exhibit “mismatch” to thehexagonal or c-centered rectangular packing arrangement ob-served for the alkyl chains (indicated by circled areas on thefigures; see Figure 2 for comparison). Two “mismatched”regions were also noted in the C29 monolayer headgroup-OH
atom arrangement (3.6%, Figure 3). By comparison, the C31monolayer headgroup-OH distribution qualitatively appearsto exhibit a closer match to the hexagonl or c-centeredrectangular packing geometry than that observed in either theC30 or C29 monolayers. Hence, qualitative chain-relateddifferences are observed in-OH ab-plane headgroup arrange-ment, with odd-carbon monolayers possessing a higher degreeof -OH headgroup hexagonal ordering and less mismatch,compared to even-carbon monolayer systems.
Next, we focus our attention on monolayer surface topologyor -OH headgroup distribution along thez axis. Typically, thepeak height of the headgroup distribution has been used todescribe the surface roughness in MD simulations of surfactantmonolayers.27 For the purposes of this study, we define thetopology of each monolayer to be dependent upon the positionof the alcohol headgroup oxygen atoms at the monolayersurface. We wish to determine the time-averaged surfaceroughness,R, for each monolayer.R represents the average ofthe absolute value of the distance of each oxygen atom that arefound in a slab of thickness∆z with distanced from thez ) 0plane (i.e., a reference plane):
whered is the z distance of the alcohol oxygen atoms to thereference plane andN is the total number of the alcohol oxygenatoms in the system. For each simulation,R is calculated overthe 700-1000 ps time span (Table 1). Note thatR is temper-ature-dependent; all monolayer surfaces become “smoother” asthe simulation temperature decreases, with the smallest valuesof R observed at the respective freezing points. For each
Figure 1. Representative views of alcohol monolayer chain tilting. CPK representations of C30 and C31 alcohol monolayers (56 molecules/monolayers), as viewed along thea (top) andb (bottom) axes. Snapshots were taken at 950 ps,T ) 300 K, and visualized using MOLMOL. Thewater layers has been deleted for clarity. Color legend: white) hydrogen atoms; red) oxygen atoms; gray) carbon atoms.
R )x∑
N
|d|2
N(2)
MD Simulations of Template-Assisted Nucleation J. Phys. Chem. B, Vol. 105, No. 44, 200110833
monolayer simulation, a significant reduction inRvalues occursfrom 300 to 278.15 K (i.e., by a factor of 3.5-7) However, themost striking observation is the variation ofR as a function ofchain length (Table 1): odd-carbon alcohol monolayers possesslowerRvalues (by a factor of 2 at below-ambient temperatures),with the C31 monolayer possessing the “smoothest” topographi-cal headgroup profile at all simulation temperatures. From this,we conclude that carbon chain length not only influences thetwo-dimensional arrangement of monolayer alcohol groups in
theab plane but also affects the azimuthal positioning of-OHgroups, which, in turn, affects the surface “roughness” ortopology of the monolayer interface.5-7,20
Template-Solvent Ordering and Nonbonding Interac-tions. To complement our analyses of monolayer moleculararrangements, we turn our attention to the water phase and thebehavior of water molecules that are in close proximity to themonolayer surface. In particular, we are interested in the “first”or “contact” water layer that exists immediately below the
Figure 2. Representative snapshots (CPK,z-axis view, 950 ps) of C29, C30, and C31 alcohol monolayer chains (56 molecule clusters, bottom) andcorresponding contact water layer (top) at below ambient temperatures. Simulation temperatures: C29, 272.15 K; C30, 265.65 K; C31, 271.15K.The a- andb-axes orientations are shown. Thez-axis view of the monolayer chains is taken normal to the tilt angle axes. Color legend: white)hydrogen atoms; red) oxygen atoms; gray) carbon atoms. In the case of the contact water layer, for clarity, the hydrogen atoms are deleted andthe oxygen atom van der Waals radii have been scaled by a factor of 2.5. For comparison, a hexagon is positioned over each figure to aid invisualizing hexagonally arranged oxygen atom arrays and monolayer alkyl chain carbon atoms.
Figure 3. Representative snapshots (CPK,z-axis view, 950 ps) of C29, C30, and C31 alcohol monolayer-OH headgroups at below ambienttemperatures. Simulation temperatures: C29, 272.15 K; C30, 265.65 K; C31, 271.15K. Thea- andb-axes orientations are shown. As per Figure2, thez-axis view of the monolayer chains is taken normal to the tilt angle axes. Color legend: black) H atoms; gray) O atoms. Note that, forclarity, both the oxygen and hydrogen atom radii have been scaled equivalently. Circled regions indicate headgroup moieties which exhibit mismatch.
10834 J. Phys. Chem. B, Vol. 105, No. 44, 2001 Dai and Evans
monolayer headgroup surface in each simulation, because,presumably, the ordering of this layer would be influenced bytheab-plane arrangement and surface topology of the monolayer-OH headgroups. The contact or first water layer is defined asthe water molecules immediately below the monolayer surfacethat have at least one hydrogen bond interaction with alcohol-OH headgroups. For the purposes of this investigation, ahydrogen bond exists if the distance between headgroup andwater oxygen atoms is 2-3 Å and the corresponding O-H-Oangle is between 100-180°. Any contact layer water moleculewhich does not meet this criteria is not counted as a monolayerhydrogen-bonded species.
An examination of the contact water layer for each monolayersimulation at 950 ps reveals the presence of water clustering orcondensation below all monolayer surfaces at each freezing point(Figure 2). We did not observe ordered clustering in higher-order water layers below any of the monolayer surfaces (data
not shown). Note that the contact water layer below the C31and C29 monolayers exhibits greater density and the presenceof planar oxygen atom clustering; the cluster geometry coincideswith the hexagonal or c-centered rectangular oxygen atomgeometry found at the Ih {001} interface.6,20,21 Although apercentage of contact layer water molecules below the C31 andC29 monolayers deviate from the perfect{001} plane unit cell(Table 2), it is still evident that the majority of water oxygenatoms are assembling into a structure that resembles the Ih {001}plane. By comparison, the C30 monolayer systems do notpromote water molecule clustering to the same extent (Figure2). These qualitative findings are supported by quantitativedeterminations of the contact water layera, b, andA values forall simulations at all temperatures (Table 2). For all monolayersystems at 300 K, we observe little or no water oxygen atomperiodicity in the contact layer. As the temperature decreases,the contact water layera, b, andA parameters begin to approachthose of{001} Ih (Table 2). Note that, at the observed freezingtemperatures, the contact water layers associated with odd-carbon monolayers exhibit the closest agreement to experimental{001} Ih parameters, with the C31 monolayer-organized contactwater layer possessing lattice parameters which more closelymatch that of the Ih {001} interface, in agreement withexperimental results.5,6,20,22
The above-mentioned numeric and visual findings are sup-ported by analyses of time-dependent hydrogen bond formationbetween the-OH headgroups and the contact water layer(Figure 4). In Figure 4, note the following: (1) as the simulationtemperature decreases, the average number of hydrogen bondsformed between-OH headgroups and the corresponding contactwater layer increases and (2) average hydrogen bond numberis highest for the odd carbon C31 and C29 alcohol monolayers.Hence, the monolayer-OH headgroups do develop nonbondinginteractions with the contact water layer during the r-RESPAsimulation, leading to the ordering of water clusters at themonolayer interface. Interestingly, the odd-carbon monolayers,and C31 in particular, participate in a larger number of solvent
TABLE 2: First Water Layer Parameters Obtained fromCanonical NVT RESPA MD Simulations
a Boldfaced temperature values indicate the observed freezingtemperature in alcohol/water droplet systems.20 All a, b, andA valueswere measured at 950 ps for each simulation run; for the first waterlayer, lattice defects are taken into consideration; that is, all empty latticenodes are filled during parameter calculation.b N/A ) no latticestructure was observed at this temperature for the first water layer (i.e.,no periodicity).c For comparison, lattice parameter values for Ih arepresented (taken from ref 5).
Figure 4. Average number of hydrogen bonds (normalized to a single alcohol headgroup) formed between monolayer and contact water layer asa function of simulation time. A hydrogen bond exists if the distance between headgroup and water oxygen atoms is 2-3 Å and the correspondingO-H-O angle is between 100 and 180°. Any contact layer water molecule which does not meet this criteria is not counted as a monolayerhydrogen-bonded species. (A) C31, 272.15 K; (B) C31, 278.15 K; (C) C31, 300 K; (D) C30, 265.65 K; (E) C30, 278.15 K; (F) C30, 300 K; (G)C29, 271 K (curve denoted in gray); (H) C29, 278.15 K; (I) C29, 300 K.
MD Simulations of Template-Assisted Nucleation J. Phys. Chem. B, Vol. 105, No. 44, 200110835
interactions (Figures 2 and 4) that result in the formation ofplanar “ice-like” water domains (Figure 2 and Table 2). Notethat we also analyzed the second layer water immediately abovethe first or contact water layer, but we did not find evidence ofhydrogen bonding to the monolayer surface or the presence ofhexagonal clustering during the course of the simulations.
Discussion
On the basis of the r-RESPA simulation data, we concludethat there is a strong correlation between carbon length, alcoholmonolayer surface topology, headgroup distribution, and theability of a given monolayer to form hydrogen bonds with thecontact water layer (Figure 4) and to order water clusters intoperiodic arrays similar to Ih {001} interface (Figure 2, Table1), in accord with experimental data.5-7,20,22The key observationin this study is that despite the formation of crystalline alkylchain hexagonal or c-centered rectangular packing arrangementsthat complement the Ih {001} interface (Figure 2 and Tables 1and 2), it is the-OH headgroup distribution and ordering, bothin the ab plane (Figure 3) and along thez axis (Table 1,Rvalues), which apparently influences the ability of each mono-layer to form hydrogen bonds with the solvent layer (Figure4). These simulation results support the hypothesis thatheadgroupab-plane orientation and surface smoothness arekey requirements for nucleation in monolayer templatingsystems.5-7,20,22Moreover, these theoretical studies suggest thatthe ice nucleation process itself is already underway by the endof 1 ns.
Limitations of this Study. Three important caveats regardingthis MD study should be mentioned. First, a word regardingtheoretical simulation data and its agreement with experiment.It is obvious that the monolayer-water data obtained fromr-RESPA simulations using the united atom and TIP3P forcefield parameters does exhibit deviation from experimentalvalues. In particular, the monolayera, b, and A parametersexhibit 10-15% agreement with experimental monolayer valuesand 13-17% agreement with the Ih {001} interface latticeparameters. However, tilt anglest and δ exhibit a 30-60%deviation from experimental values (Table 1).6,20 Given thatthere is no published experimental data forR, the averagenumber of hydrogen bonds per headgroup, or monolayer-OHheadgroup orientations,6,20,22we cannot estimate the degree ofagreement between these particular simulation parameters andthe corresponding experimental parameters. Hence, the readershould expect that simulation-determinedR values will exhibitsome deviation from laboratory-determined values, once theseare obtained via experimentation. Second, our simulation timeframe is limited to 1 ns; obviously, later events, such aspropagation of water cluster formation into higher-order waterlayers, cannot be commented upon in this preliminary study.Thus, our findings are only applicable to the earliest events inthe alcohol monolayer-ice nucleation process. However, evenin this early phase of template-mediated nucleation, it isinteresting to note that MD simulations can provide informationregarding the organization of alcohol monolayers (Table 1 andFigures 1-3)6,20 as well as the formation of hydrogen-bond-stabilized “Ih-like” hexagonally arranged water clusters (Table1 and Figures 2 and 4). These results demonstrate one usefulaspect of MD simulations: the ability to examine templatingprocesses at the atomic level on a time scale not currentlyachievable by experiment. Third, the presence of lattice match-ing between alcohol headgroups and the “templated” watermolecules does not mean a priori that lattice matching is theonly determining factor in induced nucleation. For example,
compressed alcohol monolayers consisting of C18H37OH chainsinduced the nucleation of CaSO4*2H2O from the{010} face;32
however, there is no lattice match between the alcohol mono-layer and the layer structure of CaSO4*2H2O.22,32 It has beenproposed that hydrogen bonding between the alcohol-OHheadgroups and the sulfate groups induced the oriented nucle-ation.22
Factors Involved in Monolayer Topology.On the basis ofthe experimental ice nucleation studies conducted on alcoholmonolayers which contained chain length variations [i.e.,CnH2n+1OH, n ) 16-31], it has been suggested that as the alkylchain length increases the degree of crystalline coherence lengthor lateral order within the monolayer also increases.6,22 Theincreased lateral coherence would be achieved by the increasednumber of weak van der Waals interactions that would takeplace as the number of methylene units increased in adjacentalkyl chains.6,23,31 In part, this rationale would explain theobservation that C31 monolayers induce ice nucleation at highertemperatures compared to C29 monolayers. However, lateralordering within the alkyl chain region cannot fully explain the-OH headgroup mismatch and topological variation that areobserved in the C30 versus the C29 monolayer simulations(Table 1, Figures 3, 4).
This brings us to the concept of headgroup orientation, ormore precisely, the orientation of the C-O-H moiety withrespect to the water surface.6 Given that the azimuthal orientationof the hydrocarbon chains differing in length by one methyleneunit are the same,6,22 it is the difference in orientation of theCH2OH groups that distinguishes even- from odd-carbon alkylchain monolayers.6,22 GIXD measurements and lattice energycalculations of uncompressed monolayers comprised of C19H39-CO2CmH2mOH (m ) 9 and 10) esters indicate that, form ) 9,the C-O-H group is almost perpendicular to the water surface,thereby allowing the-OH group to form three hydrogen bondswith water molecules.6,22Conversely, form ) 10, the C-O-Hbond makes an angle of approximately 20° to the water phase,which limits the-OH group to the formation of two hydrogenbonds with water molecules.6 In fact, this is what we observein our simulations: the odd-carbon C31 and C29 monolayersforms three hydrogen bonds maximally with the solvent,whereas C30 can only form two (Figure 4). Judging from the-OH group arrangements observed in theab plane (Figure 3)and along thez axis (Table 1), our simulations indicate that theeven-carbon alkyl chains C-O-H headgroups become moreheterogeneously oriented over time, compared to their coun-terparts in odd-carbon monolayers. Orientational heterogeneityleads to lattice mismatch between the alcohol headgroups andthe hydrocarbon chains within the monolayer, which, in turn,leads to reduced ice nucleation capabilities. This implies thatthe methylene groups near the monolayer surface (C1, andpossibly C2, C3, and C4) of an even-carbon alcohol moleculemust experience different bonding and/or nonbonding forcesand molecular motions, compared to their counterparts in anodd-carbon alcohol chain. Given that our simulations employedan united atom representation for alkyl chain methylene andmethyl groups, we cannot accurately determine the short andlong-range atom-specific forces involved in headgroup orienta-tion and methylene group motion at or near the monolayersurface at this time. Full atom representation studies will berequired to resolve these important issues.
Beyond Monolayers.Perhaps the most important conceptobtained from this MD study is the effect that reactive chemicalgroup orientations and topological differences can have on thenucleation process. Clearly, manipulation of these factors may
10836 J. Phys. Chem. B, Vol. 105, No. 44, 2001 Dai and Evans
improve the ability of synthetic templating systems to nucleatea wide range of nanoparticles. Moreover, given the molecularparallels between template monolayers and biomineral- and ice-specific proteins, it is likely that proteins which interact withbiological interfaces also exploit topology and reactive donorgroup orientations to achieve stereospecific binding or tonucleate crystalline solids with defined morphologies. As anexample, consider that the ice antifreeze proteins, AFP TypeIII 33 and the spruce budwormâ-helix,34 and the bacterial icenucleation protein fragment35 possess relatively flat ice bindingdomains and side chain arrangements that correspond with Ih
water molecules. Certainly, additional MD studies, coupled toexperimental methods, will facilitate our understanding ofsynthetic and biological templating processes that invokenucleation.
Acknowledgment. We thank Mark Tuckerman (New YorkUniversity) for the use of the r-RESPA code and his helpfuldiscussions. Support for this study has been made possible bygrants from the National Science Foundation (DMR 99-01356;MCB 98-16703) and the Army Research Office (YoungInvestigator Award, DAA19-99-1-0225). This paper representscontribution number 15 from the Laboratory for ChemicalPhysics, New York University.
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