Controlled Nucleation of K3Fe (CN) 6 in Isolated Microdroplets at

May 4, 2009 - Above a threshold octadecylamine (ODA) concentration, Sonset decreased and crystal euhedrality increased, compared to absence of ODA...
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Controlled Nucleation of K3Fe(CN)6 in Isolated Microdroplets at Liquid-Liquid Interface Kristin Allain, Remon Bebawee, and Sunghee Lee* Department of Chemistry, Iona College, New Rochelle, New York 10801

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3183–3190

ReceiVed October 24, 2008; ReVised Manuscript ReceiVed April 14, 2009

ABSTRACT: We have investigated the crystal nucleation behavior of potassium hexacyanoferrate(III) (KFC) in an isolated aqueous microdroplet surrounded by a decanol medium which is isothermally driven to supersaturation by water transport from the droplet, to examine the influence upon KFC nucleation by self-assembled surfactant monolayers at the liquid-liquid interface. The nucleation behavior was studied by measurement of the level of supersaturation required for the onset of crystallization (Sonset) based on microscopic observation. For KFC crystallization in the absence of surfactant, it was found that Sonset was 1.74 ( 0.13. This value was independent of droplet initial size (80-160 µm) and initial KFC concentration (10-40%). In the presence of a minimum threshold concentration of octadecylamine (ODA) in decanol, the Sonset value was diminished significantly (1.41 ( 0.06), indicative of templated nucleation acceleration. Additionally, the crystal habit for KFC grown in the presence of ODA monolayer was markedly more regular than in the absence of monolayer. Neither octadecylammonium chloride (ODA · HCl) nor stearic acid showed any templating effect. Mixed ODA-ODA · HCl monolayers exhibited intermediate effects. The results are explained in terms of ODA monolayer as a template for nucleation due to a combination of both electrostatic interactions and structural matching. Introduction The understanding of, and ability to control, crystal nucleation are vital tasks. Yet, they have not been fully developed, despite their importance to living systems and to pure and applied chemistry. A wide variety of industrial applications, including those in the pharmaceutical, pigment, cosmetic, food, and electronics industries depend upon control of crystal nucleation, growth, size, and form.1-8 One manner in which crystallization has been controlled is through the use of amphiphilic materials having an ability to promote crystal nucleation. The aggregation state and functional groups of such materials are influential in determining the level to which a given amphiphile may promote crystallization. A widely applied mode of presenting amphiphiles to a crystallization system is through use of oriented films;9-26 such modes have been quite effective to control crystallization when used as monolayers at an air-water interface. Less frequently studied are systems in which an amphiphile is presented at a liquid-liquid interface, in order to influence crystallization processes occurring in one of the liquid phases.27-41 One convenient liquid-liquid interfacial system for studying crystal nucleation is the emulsion, both water-in-oil and oil-in-water.27-37,41 Dispersed droplets containing a crystallizable substance are typically observed to nucleate homogeneously, since an assembly of dispersed droplets is expected to avoid sites of unwanted nucleation and thus reduce the probability of a heterogeneous nucleation event in a given droplet.42,43 One of the notable key studies having an active crystallizationinducing amphiphile at a liquid-liquid interface, conducted by Davey,33 involves the crystallization of glycine in confined aqueous droplet geometries ranging from micrometers to nanometer sizes. It was concluded that the polymorphic outcome of the crystallization was likely influenced by the interaction of surfactant-dominated interfacial areas with clusters or nascent nuclei of glycine. In studies of hydrocarbon-in-water emulsion systems, McClements41 found that surfactant type has an effect upon hydrocarbon crystallization temperature, where surfactants * Corresponding author. Mailing address: Department of Chemistry, Iona College, New Rochelle, NY 10801. Tel: 914-633-2638. Fax: 914-633-2240. E-mail: [email protected].

having a structure similar to the crystallizing phase were found to act as nucleation sites for hydrocarbon. Also, Sato34,35 examined the effects of hydrophobic additives upon the crystallization of various oil phases. Under conditions of sufficiently low concentration of additive, Sato concluded that the additive could accelerate nucleation heterogeneously at an oil-water interface of a droplet. Cooper36,37 has systematically studied the effect of both interfacial curvature and the presence of an active interfacial amphiphile (1-heptacosanol) upon nucleation of ice, in both water-in-oil and oil-in-water emulsion systems. In Cooper’s study, ice crystallization was induced by doping nanoemulsions and emulsions with the amphiphile at different concentrations, enabling determination of the effect of amphiphile interfacial concentration upon crystallization temperature. Despite the above-noted work, the mechanisms by which interfacial amphiphiles exert control upon nucleation in systems of confined geometry at a liquid-liquid interface have not been entirely clarified.27,39 Significant numbers of previous studies have dealt with the growth of crystal ensembles, where such systems can become complicated by the exchange of materials within emulsion droplet compartments44 and other neighborhood effects (which catalyze the nucleation of neighboring droplets) inherent to an ensemble of drops in a dynamic state.45 In many prior studies, emulsion droplets had to be stabilized against coalescence by use of a putatively passive surfactant. Distinguishing any effects of a putatively passive surfactant from those of the active amphiphile can be an arduous task, especially when both are simultaneously present in the same interfacial region. These features suggest that the study of crystallization in isolated microdroplets can offer advantages in avoiding dynamic intercompartment exchange and neighborhood effects, and therefore such study may contribute to disentangling the relevant major factors. In our study, we use a crystallization system where isolated aqueous microdroplets are manipulated individually by micropipet under digital video microscopy, in order to monitor the effect of amphiphiles upon nucleation. We have exploited the phenomenon of dewatering of an aqueous microdroplet by a surrounding dehydrating oil phase.45-49 This phenomenon has

10.1021/cg801199e CCC: $40.75  2009 American Chemical Society Published on Web 05/04/2009

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Figure 1. (A) Schematic diagram of experimental setup and (B) chamber structure.

been studied in depth by Duncan et al.,47 and has been used to concentrate solutes by Clinton et al.,50 He et al.48 and Shen et al.49 The slight solubility of water in certain oils is the driving force for the selective extraction of water from the microdroplets. In a series of papers,45,46 Hileman has shown that an ensemble of aqueous microdroplets suspended in silicone oil can be driven to supersaturation and crystallized in an isothermal manner by this phenomenon; however, no amphiphiles were employed in his work. Specifically, we examined the crystallization behavior of potassium hexacyanoferrate(III) (K3Fe(CN)6; KFC).51-53 The importance of KFC is in relation to its close structural analogues, the Prussian Blue compounds, the archetypal example of which is [FeIII]4[FeII(CN)6]3 · nH2O (n ) 14-16). Although the latter compound has long been known as pigment, interest in it has been rekindled due to interesting magnetic and photomagnetic properties exhibited by it and its analogues.26,54,55 Control of the crystallization of simple hexacyanometallate compounds such as KFC will be invaluable to likewise influencing the crystal engineering of other compounds sharing this structural motif. Our focus in this study has been to provide evidence for accelerated nucleation of KFC by an amphiphile arranged as a monolayer at the micrometer sized water-decanol interface. For the amphiphile, we chose octadecylamine (ODA), in view of its extensive prior use as a crystallization template molecule for ionic compounds at an air-water interface.13,15-22 Experimental Section The system for generation and control of a microdroplet was based on a micromanipulation technique which has, as essential components, an inverted microscope combined with micropipet manipulators. The general experimental approach for our micromanipulation technique has been well established and described in detail previously.47,56-59 Briefly, our system, as shown in Figure 1A, was built on an inverted microscope (Olympus IX51) with hydraulic micromanipulators (Narishige) mounted on the opposing side of the microscope stage, supported on a vibration isolated workstation (Newport) to minimize positional

Allain et al. oscillations generated by vibration from external sources. Droplets were viewed by bright field optics with magnification of 40× (in air). The micropipet pressure was controlled by a simple syringe and monitored by an in-line pressure transducer (Validyne). A high resolution digital camera (Olympus DP70) directly attached to the microscope allowed recording of experiments. The recorded videos and images were post analyzed by software to correlate the various parameters such as time, diameter of the droplet, pressure and temperature. To form a chamber, two parallel strips of coverslip glass (Gold seal cover glass no. 1) were supported in such a way as to create a gap of 2 mm between them, as shown in Figure 1B. Within this gap was placed the oil medium reservoir (of volume usually about 200 to 400 µL), held therein by surface tension. The strips of coverslip glass were supported on a base constructed from pieces of a standard glass microscope slide. The completed chamber is placed on the microscope stage. Each experiment employed a fresh reservoir of decanol (or other oil medium, as appropriate), constrained between two strips of freshly cut cover glass. A similarly functional design for this chamber has been described in detail previously.47,58 All chemicals used in this study were purchased from Sigma-Aldrich with the highest purity available and used without further purification. Water used throughout this study was in a neutral pH deionized form (18.2 MΩ · cm). In this paper, where concentrations are reported as “%”, this refers to mass of solute per volume of solution (w/v), unless otherwise specified; for example, “1%” means 1 g of solute per 100 mL of solution. Aqueous solutions of KFC were prepared at different concentrations (10-40%) in deionized water. To produce solutions of 1-octadecylamine (CH3(CH2)17NH2; ODA) in decanol, ODA was initially dissolved in chloroform which was then evaporated, and redissolved in 1-decanol to a concentration of 0.01 to 0.2%, as required. The solution of ODA in decanol was sonicated using a bath sonicator until a clear solution was produced, which required usually 5-10 min of sonication time. All solutions were freshly prepared and filtered through a 0.2 µm Millipore syringe filter before use. Other surfactant solutions were prepared in a similar manner. To prepare decanol solutions of 1-octadecylammonium chloride (CH3(CH2)17NH2 · HCl; ODA · HCl), a mixture of CHCl3:CH3OH (4:1 v/v) was used to initially dissolve the ODA · HCl before it was redried and dissolved in decanol. Stearic acid (CH3(CH2)16COOH; SA) required no predissolution and was dissolved directly into decanol. In view of the reported CO2 sensitivity of ODA and ODA · HCl,60 these solutions were prepared immediately prior to use. For manipulation of an aqueous microdroplet, a micropipet with a desired tip, which had a typical diameter in the range 5 to 30 µm, was achieved by micropipet puller (Narishige PB-7) and subsequently hydrophobized using hexamethyldisilazane ((CH3)3SiNHSi(CH3)3). To achieve a hydrophobic coating of the micropipet, about 2-3 drops of hexamethyldisilazane was added to the center of an enclosed container having freshly pulled micropipets and held for at least 30 min. Video recording of a crystallization event began when the tip of a micropipet containing a solution of interest was under the field of view. Subsequently a microdroplet with a desired initial size was produced by applying slight positive pressure through a syringe connected to the micropipet. The microdroplet was then isolated from the column of solution in the pipet and was held by small suction pressure at the end of the micropipet tip during the entire crystallization experiment. Each crystallization event was invariably completed in under ten minutes for all droplets studied under the conditions employed. All comparative studies of the dynamics of microdroplet dissolution were performed using the same batch of decanol on the same day. Interfacial tension was measured using a tapered micropipet method, in which the equilibrium interfacial tension between the two immiscible phases is determined by measuring the radius of curvature of the interface for a series of pressure changes applied inside a tapered pipet. The details of this method have been previously described.58,59 All experiments were carried out at ambient air temperature of 24 ( 1 °C.

Results and Discussion Typically, we observed that as an aqueous droplet containing a solute of interest became dewatered by surrounding decanol, solute was concentrated to eventually reach an effective supersaturation level for crystallization. Figure 2, center, shows a series of videomicrographs of an aqueous microdroplet contain-

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Figure 3. Effect of initial droplet size on the supersaturation ratio at onset of crystallization (Sonset) for aqueous microdroplets initially containing 10% of KFC, in decanol media at 24 °C.

Figure 2. Microscopic images (I-IV) of a varying stage of shrinkage of an aqueous microdroplet containing 10% initial KFC solution in decanol and its corresponding diameter (b) and concentration (O) as a function of time. Bar represents 100 µm. Conset is defined as the concentration at the onset of crystallization. Representative crystal formed is shown in the inset of Figure 2, bottom.

ing 10% of KFC held by a micropipet within a chamber of decanol at 24 °C. Also shown in Figure 2, top, is a typical plot of the change of droplet diameter as a function of time obtained by measuring the diameter of the shrinking aqueous droplet. An initial diameter of the droplet was measured from the time when the droplet was created and set as time zero. The diameter of a typical droplet decreased rapidly and nearly linearly at the beginning of the experiment (points I-III), but the rate of decrease typically lessened at the end before a crystal appeared in the droplet (point IV). Concurrently, the solute concentration C would increase with time as the droplet shrinkage progressed, according to eq 1,

C ∝

1 3 ) V 4πr3

(1)

where V and r are the volume and the radius of the droplet, respectively, as shown in Figure 2, bottom. The solute concentration at the time of onset of crystallization (Conset) was determined based on the diameter of the droplet measured immediately prior to the point (image IV) where a crystal was first visibly perceptible in a droplet. This determination of solute concentration assumes that there is no significant mass transport of solute to the surrounding immiscible phase.61 Thereafter, growth of this incipiently visible crystal occurred over a time scale of typically 1-10 s, to a point where its longest dimension

equaled the diameter of the remaining solution droplet. In contrast to this rapidly observed growth, typical crystal appearance waiting times within our system were on the order of 300 to 600 s between the initial formation of a droplet and the instant a crystal was visibly perceptible. Thus, some degree of separation was achieved between the crystal appearance waiting time and time needed for full crystal growth. The final solid particle formed in a droplet appeared to consist of irregular aggregates of smaller crystallites, polycrystalline in nature (inset of Figure 2, bottom). Overall, crystals having similar habit were invariably observed for repeated droplet crystallization experiments under the same conditions. We determined the supersaturation ratio (S) at the onset of crystallization (Sonset) for microdroplets initially containing 10% of KFC, in a range of initial droplet diameters from 80 to 160 µm. Supersaturation ratio measured at the onset of crystallization (Sonset) is calculated by

Sonset )

Conset Ceq

(2)

where Ceq is the reported equilibrium solubility of KFC in water, which at 24 °C is 46.0%.62,63 Figure 3 shows the effect of initial droplet size on Sonset. Multiple crystallization events for each initial droplet size converged to a mean value of Sonset of 1.74 in each case, with a standard deviation of (0.13. Similar variations have been reported in cooling crystallization experiments,14,36 in measurements of the amount of undercooling required for onset of crystallization, and interpreted in terms of the stochastic nature64,65 of the nucleation process. In view of this, reported values for Sonset in our paper were averages for at least 50 droplets at identical conditions to obtain sufficient reproducible statistical characteristics of the nucleation process. Although we have shown that the Sonset is independent of initial droplet size, we nevertheless ensured that all subsequent experiments were conducted using droplets having a diameter in the range of 100 ( 10 µm. Figure 4 shows a plot of Sonset vs initial solute concentration in the droplet. The plot shows that the Sonset is essentially independent of initial solute concentration of the droplet. The value of Sonset of 1.74 ( 0.13 reported above for 10% KFC was essentially replicated for droplets having other initial solute concentrations, ranging from 10 to 40%. Figure 5 shows a time sequence of optical videomicrographs of a crystallization event in an aqueous microdroplet with an

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Figure 7. Schematic illustration of surfactant-monolayer-templated nucleation and crystallization. Figure 4. Sonset vs initial solute concentration of KFC (%) (shown below each point) for a microdroplet at 24 °C.

Figure 5. Optical videomicrographs of 10% aqueous KFC droplet held by micropipet in decanol containing 0.1% ODA. Bar represents 50 µm.

Figure 8. Effect of ODA concentration (%) in decanol on the mean values of Sonset for 10% KFC in a microdroplet.

Figure 6. Comparative histograms of the Sonset of KFC at 24 °C with (filled bars) and without (open bars) 0.1% ODA. The presence of ODA appears to facilitate crystallization, resulting in lower Sonset.

initial concentration of 10% of KFC surrounded by decanol containing 0.1% of ODA. It is clearly noticeable that the crystal habit is much more regular than that crystallized in the absence of ODA (see inset Figure 2, bottom). The latter appears pronouncedly polycrystalline, whereas crystals nucleated in the presence of ODA are consistently single crystalline and euhedral (i.e., displaying defined faces) (Figure 5, right). This interesting difference in the habit of the KFC crystal was accompanied by a large difference in Sonset as well. In Figure 6, we present comparative histograms of the relative frequency of Sonset values for a large ensemble of KFC crystallization events, with and without the ODA. The mean Sonset in the presence of 0.1% ODA was found to be 1.41 ( 0.06, as compared to Sonset 1.74 ( 0.13 in the absence of ODA. This frequency distribution plot shows that there is no population overlap between the two systems.

The values of Sonset in the presence of ODA varied up to 0.06 for the population set studied, as compared to 0.13 in the case where ODA was absent. Our findings suggest that KFC nucleation and crystallization in the presence of ODA molecules occurs in a more controlled fashion than in the absence of amphiphilic molecules. We postulate that this may be due to the interaction of hexacyanoferrate(III) anion present in solution and the headgroup of the ODA surfactant in a self-assembled monolayer at the water-decanol interface, as schematically depicted in Figure 7. We investigated the dependence of Sonset on varying concentrations of ODA in the decanol phase to determine if there is a minimum threshold concentration required to exhibit the templating effect. The result is shown in Figure 8. The corresponding Sonset at varying concentrations of ODA, along with crystal habit, are also shown in Table 1. Figure 8 and Table 1 show that the templating effect was seen only in the presence of a threshold minimum concentration of surfactant, i.e., at least about 0.05% of ODA in decanol. This is taken as being indicative of a minimum quantity of surfactant molecules necessary for optimum interaction with the incipient crystal nuclei of KFC to induce the euhedral crystal morphology and lower the Sonset. However, once this minimum quantity of surfactant was supplied, there seemed to be no further diminution of Sonset. It is reasonable to presume that a relatively higher bulk concentration of ODA in the decanol phase should, in general, correspondingly lead to a greater interfacial concentration of ODA due to its amphiphilic character, until a point where sufficient surfactant was available to form a fully covered

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Table 1. Comparison of the Mean Sonset for KFC Nucleation in a Microdroplet as a Function of ODA Concentration in Decanol at 24°Ca

concn of ODA (%)

Sonset

crystal habit

0 0.01 0.02 0.04 0.05 0.06 0.08 0.1 0.2

1.74 ( 0.13 1.70 ( 0.11 1.70 ( 0.11 1.48 ( 0.09 1.46 ( 0.07 1.43 ( 0.07 1.41 ( 0.06 1.41 ( 0.06 1.41 ( 0.06

polycrystalline, habit A polycrystalline, habit A polycrystalline, habit A intermediate, habit B regular, habit C regular, habit C regular, habit C regular, habit C regular, habit C

a Also shown are images of representative crystal habit. Bar represents 100 micrometers.

Figure 9. Aqueous droplet dissolution profile with time at varying concentrations of ODA in decanol phase.

monolayer: degree of amphiphile coverage would not strongly depend on bulk concentration beyond this point. As an independent probe for the presence of ODA at the interface, we studied the dynamics of the dissolution of a pure aqueous droplet in decanol phase in the presence of ODA. The micromanipulation technique has been previously proven to be a convenient method to study the dissolution of a single liquid microdroplet in an effectively infinite medium solution.47 Basically, droplet shrinkage can be conceptualized as a mass transport process of water molecules across an interface into a medium having slight solubility for water; the solubility of water in 1-decanol under ambient conditions is about 3.3 wt %.66 Among a number of factors mediating rate of transport should be presence or absence of molecules which are concentrated at the interface.49 We observed that rate of droplet shrinkage was dependent on concentration of ODA in decanol. Figure 9 shows the droplet dissolution profile for pure aqueous microdroplets surrounded by decanol containing varying concentrations (from 0 to 0.1%) of ODA. All comparative studies of the dynamics of microdroplet dissolution were run using the same batch of decanol on the same day. We observed that, in general, the higher the concentration of ODA in decanol, the more time was required for the dissolution to proceed to completion. For example, a droplet in decanol containing 0.1% ODA shrinks more slowly than a droplet in

decanol containing 0.05% ODA given the same initial droplet size. When the concentration of ODA was lowered to 0.01%, there was no distinguishable difference in dissolution rate versus having no ODA present. Due to the amphiphilic nature of the ODA, it is expected for ODA molecules to be concentrated mostly at the water-decanol interface, and therefore it would seem reasonable that the presence of ODA monolayer would retard the dissolution process of water into the surrounding decanol phase. Similar behavior has been reported in a system wherein an aqueous microdroplet containing a water-soluble surfactant was suspended in silicone oil. In the reported study,49 the dissolution process showed a dependence upon surfactant concentration, where a higher surfactant concentration slowed the dissolution process significantly. This suggests that differences in the rates of droplet shrinkage can be correlated as a convenient indicative measure for coverage of ODA molecules at the interface. Interestingly, we find that minimum ODA concentration needed to observe any nucleation templating effect (Figure 8) is the same value as for concentration exhibiting a significant retardation of the water dissolution process, namely, 0.05% ODA. We have also investigated the interfacial tension at the water-decanol interface. Pure decanol-pure water provided γ ) 8.8 mN/m in our system, which is consistent with the literature value.67 We attempted to measure the interfacial tension for pure water against decanol containing 0.05% ODA, which was the minimum concentration of ODA for which a templating effect was seen. However, a value could not be obtained, due to formation of a flat interface, strong evidence of adsorption of ODA leading to very low interfacial tensions, less than about 1 mN/m. Overall, our observation of nucleation catalysis suggests an interaction between incipient crystal nuclei of KFC and headgroups of ODA molecules present at the interface; while our droplet shrinkage retardation study and very low interfacial tension value tend to provide confirmation for the interfacial presence of ODA. In prior studies at an air-water interface, the mechanism responsible for assisted inorganic crystal nucleation in the presence of a monolayer template has been described as a molecular recognition between the monolayer and the nucleated crystal. Such recognition has been considered to largely depend on the cooperative effects of structural, hydrogen bonding and electrostatic interactions between the monolayer and the nucleated crystal.5,9,11,15 It has also been reported that the structural adaptability of amphiphilic molecules in the monolayer is an additional mechanistic factor,68 as is the mutual influence of the monolayer with inorganic moieties of the adjacent subphase solution.69 In our system, it is plausible to postulate that an electrostatic interaction, between hexacyanoferrate(III) anions in crystal nuclei and cationic headgroup moieties of hydrolyzed ODA molecules, is at least partly responsible for the observed nucleation enhancement. In fact, ample precedent exists in the literature for attraction between monovalent and multivalent anions and ODA monolayers.13,15-22,26,70,71 In particular, a strong interaction between [Fe(CN)6]3- and the ODA molecule has been reported in earlier studies using Langmuir monolayers.21 In the latter system, the surface pressure-area isotherm of an ODA monolayer on pure water was reported as ∼20 Å2/ molecule, while an expanded monolayer of ∼50 Å2/molecule was observed using a KFC solution (0.15 mM) subphase. It is therefore plausible to suggest that a similar interaction is at play in our system. This view is also supported by the fact that use

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Table 2. Summary of Mean Values of Sonset and Crystal Habit of KFC for Selected Amphiphilic Surfactants, Highlighting the Dependence of Sonset of 10% Aqueous KFC Microdroplets upon wt % ODA · HCl in ODA · HCl:ODA (w/w) Mixture in Decanol surfactant

weight ratio (%, w/w)a

Sonset

crystal habit

ODA ODA:ODA · HCl ODA:ODA · HCl ODA:ODA · HCl ODA:ODA · HCl ODA:ODA · HCl ODA:ODA · HCl ODA · HCl SA

100 99.7:0.3 99.4:0.6 99:1 95:5 90:10 80:20 100 100

1.41 ( 0.06 1.43 ( 0.08 1.45 ( 0.08 1.45 ( 0.08 1.50 ( 0.13 1.54 ( 0.13 1.56 ( 0.13 1.76 ( 0.13 1.84 ( 0.12

regular, habit C intermediate, habit B intermediate, habit B intermediate, habit B polycrystalline, habit A polycrystalline, habit A polycrystalline, habit A polycrystalline, habit A polycrystalline, habit A

a

Total surfactant concentration was set to 0.1% in 1-decanol.

of 0.1% stearic acid (SA) as a potential templating molecule, which when hydrolyzed forms a negatively charged monolayer, resulted in no discernible templating effect: neither a lowering in Sonset nor induction of euhedral crystal formation was observed. In fact, it acted more as a nucleation inhibitor since its average Sonset was 1.84 ( 0.12. Based on the foregoing, we sought to determine whether an intensification of positive charge in the monolayer would give rise to an enhancement of the effect, assuming that electrostatic attraction plays a predominant role. Charge enhancement was accomplished by using a mixed monolayer of ODA doped with its conjugate acid, ODA · HCl. We tested a range of concentrations for ODA · HCl doped into ODA monolayer, from 0% (i.e., pure ODA monolayer) to 100% ODA · HCl. The total surfactant concentration was set to 0.1% in decanol. The initial KFC concentration in the microdroplet was 10%. Table 2 shows the mean values of Sonset as a function of varying weight percentage of ODA · HCl, with indication of crystal habit. Our results showed that, with incremental additions of ODA · HCl into ODA monolayer, a significant increase in the Sonset for KFC was observed, which was accompanied by the appearance of polycrystalline KFC in the microdroplet. These results indicate that the addition of ODA · HCl to an ODA monolayer essentially destroys any templating ability of the latter. One possible reason for these results is that a different monolayer packing in ODA · HCl resulted in a structural mismatch with KFC nuclei. Doping ODA · HCl into the ODA monolayer would change monolayer packing characteristics through variation of the average positive charge of the monolayer, which in turn could influence the crystal nucleation process of KFC. The more ODA · HCl in the mixed ODA · HCl: ODA monolayer, the more expanded a monolayer is formed because of Coulombic repulsions between headgroups of like charges. It is expected that amine headgroups of a monolayer of ODA on a neutral subphase, as in our experiments, would be partially ionized,72 since, typically, primary amines have a pKa ∼10. In contrast, a monolayer composed of ODA · HCl is expected to possess a greater apparent positive charge than an ODA monolayer. A monolayer composed of ODA · HCl thus may be expected to exhibit greater headgroup-headgroup repulsion, as evidenced by prior work showing that monolayers of fully ionized amines are more expanded than un-ionized ones due to charge repulsion.72-74 This finding suggests that the driving force for the facilitated nucleation of KFC at the water-decanol interface in the presence of ODA is not solely due to electrostatic interactions, but rather, nucleation is likely fostered due to a combination of both electrostatic interactions and structural matching. Structural matching could be important in the hexacyanoferrate(III) system since its negative charges

Allain et al.

are distributed over many atoms, mainly localizing on the nitrogens in the cyanides. Therefore, in order for a positively charged monolayer to effectively attract hexacyanoferrate, it would require an arrangement of headgroups in a proper position to attract more than one nitrogen of a given hexacyanoferrate trianion. Facilitated nucleation could also conceivably take the form of octadecylammonium ions in a monolayer partially substituting for K+ in a KFC crystal nucleus. In view of the retardation in droplet dissolution engendered by ODA, it is plausible to postulate an alternative rationale in which our observed results may be owed to kinetic effects. That is, a retardation of droplet shrinkage effectively corresponds to a lessening in supersaturation rate (i.e., rate of increase in supersaturation). Indeed, important precedent exists for crystal type control based on differences in supersaturation rate; see, e.g., Kenis et al.,75 which shows the selective growth of γ-glycine polytype through slow, controlled evaporation of water from microdroplets open to air. To establish whether such an effect contributes to the present results, we explored several avenues which would shed light on this. We performed KFC crystallization experiments in microdroplets held in 1-octanol and 1-dodecanol media, as well as in 1-decanol. In each situation, essentially identical crystallization results were obtained, where polycrystalline KFC habit was observed and Sonset was identical within experimental error. However, drastic differences in dissolution rate were manifest, where a KFC-containing microdroplet in 1-octanol would shrink at about a 4-fold faster rate as compared to that of the same size droplet in 1-dodecanol. A rate differential is expected, based on the relative solubility of water in 1-octanol and 1-dodecanol, being about 4.4 wt % and 2.9 wt %, respectively.66,76 We also note that aqueous microdroplets held in 1-decanol shrink at comparable rates regardless of the nature of surfactants we tested (at the same surfactant concentration). Yet, neither SA nor ODA · HCl imparts either euhedral habit or lowering in Sonset. Collectively, these results tend to indicate that the effect of an ODA monolayer is not principally the result of its ability to retard droplet shrinkage; control of habit by supersaturation rate is likely not the mechanism for our results. Overall, this work has clearly demonstrated that the nucleation behavior and final crystal habit may be significantly modified by a surfactant monolayer arranged at the oil-water interface of a microdroplet. Furthermore, the apparent mechanism by which this monolayer exerts its effect has similarities to previous templating systems based on the air-water interface. However, oil-water systems have several dissimilarities compared to air-water systems. Among the expected differences are the existence of a relatively broad interfacial region across the water-organic interface (manifested in our case as a small but still significant intersolubility between water and decanol), and a region of reduced polarity between the polar water phase and bulk alcohol.77 The relative differences in dielectric discontinuity between the organic-water interface and air-water would be expected to lead to important differences in the electrostatics of interaction of hexacyanoferrate(III) ions with interfacial surfactant molecules. Also, the broad interfacial region could influence both the electrostatics and the organization of the surfactant molecules in the interfacial region.78,79 Finally, the penetration of the hydrocarbon tails of interfacial surfactants by oil molecules could be significant,27,39,80 resulting in an expanded monolayer and/or increased disorder of the surfactant molecules with concomitant effects upon structural matching associated with crystal nucleation processes. Indeed, relatively little is conclusively known about the structure of surfactant

Controlled Nucleation of K3Fe(CN)6

assemblies at the liquid-liquid interface compared to that at the liquid-vapor interface. Recent progress in this field, using experimental and theoretical approaches, suggests that the resultant monolayer assembly of amphiphiles at the interface between two immiscible liquids is a result of complex interplay between surfactants (head- and tailgroups) and the organic/oil phase.78,79,81-83 Despite its relative disorder vis-a`-vis the air-water interface, a surfactant monolayer at a liquid-liquid interface may nevertheless be anticipated to have a certain ordered character. Any level of ordering at that interface can have implications for the promotion of nucleation of solutes in one of the phases, just as has been observed for Langmuir monolayer systems.5 An ordered array of surfactant molecules at the water-organic interface should have an ordering influence upon the local environment in the water phase in the vicinity of the interface, leading to changes in the physical character of the water phase which are known to be related to nucleation. Under specified conditions there can exist a complementarity between an ordered surfactant array and an incipient crystal nucleus. It is quite likely that the driving force for the facilitated nucleation which we observe takes advantage of such complementarity as a combination of both electrostatic interactions and structural matching. Although the precise details of the forces responsible for this facilitated nucleation await more in-depth studies, using a wider range of surfactants and conditions, it is clear that our liquid-liquid interfacial system opens exciting new vistas in the engineering of inorganic crystals and other materials. Conclusion We have investigated the nucleation behavior of potassium hexacyanoferrate(III) in an aqueous microdroplet surrounded by a decanol medium which is isothermally driven to supersaturation by water transport from the droplet, both in the absence and in the presence of surfactant monolayers. In this system, Sonset was 1.74 ( 0.13 in the absence of surfactant. This value was independent of droplet initial size (80-160 µm) and initial KFC concentration (10-40%). The presence of ODA molecules as a surfactant monolayer at the liquid-liquid interface lowered the Sonset (1.41 ( 0.06) in the nucleation behavior. By video monitoring, we observed that the habit of KFC crystals formed in the presence of ODA was markedly different from that of KFC crystals in the absence of ODA. Our results demonstrate the role of the ODA monolayer as a template for the formation of regular crystals of KFC at the liquid-liquid interface. Only in the presence of a threshold minimum concentration of surfactant was this effect seen, indicative of the minimum surfactant molecules necessary for an ordered monolayer. Evidence points to an interaction between hexacyanoferrate(III) anion and the cationic headgroups of the surfactant. This study has also demonstrated the convenience of use of an individual, isolated microdroplet system for studying amphiphile templated crystallization at the liquid-liquid interface. Such a system can provide a level of control of important parameters in crystallization, including absence of solid walls, quiescent environment (minimization of mechanical disturbances), and minimization of thermal convection currents. It can also offer an ability to control the size of the droplets and therefore the final crystal size. This study has provided some valuable insight into the nucleation process and in situ formation behavior of crystals at the micrometer sized liquid-liquid interface, which has implications in the design and development of advanced materials for technological use.

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Acknowledgment. The author would like to thank and acknowledge the financial support for this research from the Donors of the American Chemical Society Petroleum Research Fund (Grant PRF#45241-GB9); Camille and Henry Dreyfus Special Grant Program in Chemical Science (SG-07-016); and Iona College.

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