Article pubs.acs.org/Langmuir
Adaptability of Monoglyceride-Induced Crystallization of K2SO4: Effect of Various Anions and Lipid Chain Splay Omoakhe Tisor,† Michelle Muzzio,† David Lopez,‡ and Sunghee Lee*,† †
Department of Chemistry, Iona College, 715 North Avenue, New Rochelle, New York 10801, United States New Rochelle High School, 265 Clove Road, New Rochelle, New York 10801, United States
‡
ABSTRACT: Understanding the interaction of a surfactant assembly with surrounding ionic compositions is critical in controlling the nanostructure formed by amphiphilic systems. In this study, we investigated the influence of specific anions upon the character of the crystallization process of a model inorganic salt (K2SO4) in an aqueous microdroplet (10−100 μm) in the presence of monolayers of monoelaidin (ME), monovaccenin (MV), monoolein (MO), and monolinolein (ML) assembled at a water−decanol interface. In particular, control monolayers of each monoglyceride at the water−oil interface did not accelerate nucleation (i.e., act as a template) of the crystallization of K2SO4. However, in the presence of 1− 10 mM exogenous anions, certain types of monoglycerides did induce nucleation. The influence of the anions was consistent with the Hofmeister series. Monoglyceride monolayers in combination with chaotropic anions, such as SCN−, imparted a euhedral crystal habit with a relatively lower Conset, while kosmotropic anions showed less notable changes in the crystallization characteristics. However, this specific anion effect was seen only for monolayers of ME, MV, and MO but not for ML. This enhancement of nucleation-inducing ability upon addition of chaotropic anions appears to be related to the degree of chain splay: the larger the chain splay, the less anion efficiency in enhancing the templating capability. Our findings indicate that the improved nucleation-enhancing capabilities may originate from mutual interactions between anions and the monolayer, allowing the monoglyceride monolayers to adapt to a modulated self-assembled structure resulting in the greatest degree of control over K2SO4 crystallization.
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INTRODUCTION Many amphiphilic molecules spontaneously self-assemble into ordered nanostructured lyotropic liquid-crystalline phases in an aqueous environment. These lyotropic phases and their unique lipidic structures have been important in various industrial and basic life science fields such as food and cosmetics, drug delivery and medical imaging, and membrane protein crystallization.1−3 Among various amphiphilic molecules, monoglyceride-based polymorphic lyotropic phases in aqueous dispersions have been extensively studied.1,4 In particular, the bicontinuous cubic phases (BCP), composed of a rich variety of bilayer-containing phases, have recently been recognized for their use in the crystallization of proteins. For example, monoolein (MO)-based BCP has been reported to facilitate the crystallization of various integral membrane proteins for X-ray structural studies.3,5−7 Much attention has been given to the structural characteristics and modulation of lyotropic phase of various monoglycerides in order to study nanostructural parameters and control the properties suitable for particular applications. The type of lyotropic nanostructure formed depends on many different contributions, including temperature, pressure, and the addition of chemical components such as solvent or other additives and compositional variations.1,2,8,9 In addition, it has been reported that monoglycerides having various molecular shapes, in terms of number and location of © XXXX American Chemical Society
double bonds in their chain structures, can provide different lipid phase behavior and number density of water channels in BCP.10 The presence of electrolytes is also known to modify the corresponding phase diagrams of monoglycerides significantly. In the case of monoolein (MO)−water mixtures, a swelling or shrinking of the water channel diameters and even phase transitions are observed, depending on the type and amount of added electrolytes. X-ray diffraction studies have shown that the lattice constants of BCP formed by hydrated monoglycerides, from which water channel diameters can be determined, are affected by the presence of ions in water and that the magnitude of the lattice constant follows the specific anion effect (the so-called Hofmeister series). For example, the chaotropic anions expand the lattice constants of the BCP formed by monoglycerides and the kosmotropic anions exhibit an opposite effect, i.e., they reduce the lattice constant.1,2,9,11−13 However, a detailed understanding of how the interactions of ions and self-assembled structures of monoglycerides give rise to specific structural and phase behavior is currently lacking. Our research in recent years has focused on the study of the characteristics of aggregated surfactcants at the liquid−liquid Received: December 19, 2014 Revised: January 31, 2015
A
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which consists of an inverted microscope (Olympus IX 51) combined with a hydraulic micropipet manipulator (Narishige), supported on a vibration-isolated workstation (Newport), with a CCD camera directly attached to the microscope for real time recording of the crystallization events. The recorded videos and images were analyzed afterward to measure the dimensions of droplets and the crystal appearance using custom-built image analysis software. For the formation and manipulation of an aqueous microdroplet, a micropipet with a desired tip, having a typical diameter in the range of 10 to 30 μm, was achieved with a micropipet puller (Narishige PB-7) and subsequently hydrophobized by exposure to vapors of hexamethyldisilazane [(CH3)3SiNHSi(CH3)3); HMDS]. To achieve a hydrophobic coating of the micropipet, about 2 to 3 drops of hexamethyldisilazane was added to the center of an enclosed container having freshly pulled micropipets and held for at least 30 min. This process efficiently inhibits the wetting of the micropipet glass surface by an aqueous solution. The HMDS must be handled using gloves in a fume hood and stored in a refrigerator. It is recommended to read the MSDS information on handling HMDS. Fundamentals of Isothermal Microdroplet Crystallization. We use a crystallization system where an isolated individual aqueous microdroplet, in a dehydrating oil phase, is manipulated by a micropipet under real-time video microscopy. The oil phase contains a lipid or surfactant. The present study employed monoglycerides as an amphiphile. When an aqueous droplet containing a solute of interest becomes dewatered by a surrounding dehydrating oil (in this study, 1-decanol), the droplet undergoes dissolution and shrinks (Figure 2a,b), and the solute is concentrated to eventually reach an effective saturation level for crystallization (Figure 2c). We can modulate the molecular packing of the surrounding monolayer via varying surfactant characteristics. Therefore, a microdroplet composed of ordered surfactant molecules provides a model system for the study of controlled heterogeneous crystal nucleation induced by a surfactant at the liquid−liquid interface, which occurs in the absence of solid walls and with the minimization of thermal convection currents.16 To characterize the relative templating capabilities of different interfacial monolayers, we have used the Conset value, which is the concentration of the solute in the microdroplet at the onset of crystallization. This value is the representation of the supersaturation level at which nucleation is very rapid. The onset of crystallization is typically clearly visible as shown in Figure 3 (an arrow points to the first appearance of a visible crystal). The comparison of the volume of a microdroplet in its initial stage (with its known concentration) to its final stage (at the onset of crystallization) by measuring the radius of the shrinking droplet allows us to determine Conset on the basis of the following simple relation, as shown in eq 1.
interface, with particular attention to the interplay among solutes and surfactant monolayers in affecting crystallization in an isolated aqueous microdroplet. In our earlier investigations of a model inorganic crystallizable solute, K2SO4, we showed that several key crystallization characteristics such as the level of saturation needed for nucleation to occur and the habit and form of the final crystal could be controlled depending on the types of surfactant monolayer and the specific identity of anions present. 14,15 Our findings demonstrate that there are appreciable interactions between certain anions and the surfactant monolayer and that these interactions can lead to different reorganization effects in the monolayer upon binding with specific anions. The consequence of this reorganization of the monolayer eventually influences its templating ability in a characteristic way. Hence, our microdroplet crystallization system has the capability to provide valuable information concerning ion−monolayer interactions at the liquid−liquid interface. In the present study, we seek to increase our understanding of how a monolayer of several monoglycerides, each having various molecular shapes, can be modulated in the presence of specific ionic compositions. Toward this end, we investigated the series of monoglycerides as a surfactant monolayer at the oil−water interface in terms of its role to influence K2SO4 crystallization characteristics and its interaction with salts having different specific identities. It is expected that our findings will provide significant insights regarding the interaction of a surfactant assembly with ionic compositions and provide the possibility of predicting and controlling the final nanostructure formed by amphiphilic systems.
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EXPERIMENTAL SECTION
Experimental System. Our crystallization system has the novel capability of simultaneous manipulation of individual micrometer-sized droplets and in situ observation during the entire crystallization processes. The detailed experimental system has been described elsewhere.16 A brief schematic of the system is shown in Figure 1,
C∝
1 3 = V 4πr 3
(1)
A typical time profile of a droplet radius and corresponding K2SO4 concentration (%) within a droplet is shown in Figure 3 along with videomicrographs. In this paper, Conset concentrations are reported as ‘‘%’’, which refers to solute mass (g) per solution volume (mL), unless specified otherwise. Images were collected with a pixel size of 0.16 μm using the entire field of 1920 × 1080 pixels. Thus, the uncertainty in the diameter measurement is 0.32 μm (2 pixels × 0.16 μm/pixel). Because we generally have maintained droplets in the diameter size
Figure 1. Brief schematics of the experimental setup.
Figure 2. Schematic description of an isothermal microdroplet crystallization process. B
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it crystallizes as anhydrous orthorhombic crystals from aqueous solution under ambient conditions.18 Finally, crystallization systems employing potassium sulfate are seen to show a wide metastable zone width,19 which means that K2SO4 crystallization systems lend themselves to studies of materials which can modify the supersaturation needed for crystallization to occur.
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Figure 3. Typical time profile of a droplet radius and corresponding K2SO4 concentration (%) within a droplet. (Top panel) Videomicrographs of the crystallization process in an aqueous microdroplet containing K2SO4 (in 10 mM KSCN) surrounded by decanol having 12 mM monoolein. In this case, the value of Conset is 18%. A scale bar on videomicrographs represents 100 μm.
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RESULTS Effect of Various Monoglycerides on the Crystallization of K2SO4. Experiments were performed by introducing an aqueous microdroplet containing K2SO4 at an initial concentration of 5% (287 mM) into a 1-decanol medium containing various monoglycerides at ambient temperature. The droplet was allowed to shrink owing to the dehydration capacity of the oil phase. The results are shown in Figure 5, which plots Conset for K2SO4 crystallization as a function of various monoglycerides in decanol (12 mM), along with the typical appearance of the final K2SO4 crystal formed under this condition. As shown in Figure 5, the Conset in the absence of any monoglycerides is 75 ± 10% (control), and the presence of various monoglycerides (ME, MV, MO, and ML) resulted in Conset in the range of 55−70%. Values for the mean Conset were averages for at least 50−100 droplets under identical conditions to obtain sufficiently reproducible statistical characteristics of the nucleation process. On the basis of the high Conset and polycrystalline nature of the crystal formed, it appears that none of the monoglyceride monolayers alone could significantly enhance the nucleation of K2SO4. Effect of Specific Anions on the Templating Ability of Various Monoglyceride Surfactants: Monoelaidin (ME), Monovaccenin (MV), Monoolein (MO), and Monolinolein (ML). We have investigated the influence of specific anions upon the characteristics (Conset and crystal appearance) for the process of crystallization of K2SO4 from an aqueous microdroplet in the presence of monolayers of various monoglycerides. The following anions, X, in a concentration of 10 mM salts, were added to the microdroplet: X = HPO42−, Cl−, Br−, NO3−, I−, and SCN− in the form of their potassium salts. Figure 6 shows the results for the mean Conset value for monoolein (MO) monolayers. The addition of 10 mM potassium salts (KX) to the MO system (blue column) shows that values of Conset are considerably lowered by adding various salts, in a manner that is strongly dependent upon the nature of the anion added. It is seen that in the presence of 10 mM thiocyanate, iodide, or nitrate anions (chaotropic anions), the mean Conset values markedly decreased to a value of nearly 20% or lower, while the values in the absence of an added anion were about 70% (red
range of ∼100 μm, the error associated with the diameter measurement is 0.32%, which propagates to 0.55% relative uncertainty in the volume calculation Through the measurement of Conset, nucleation acceleration or inhibition by various oil-soluble surfactants can be quantified relative to the absence of surfactant, under carefully controlled conditions. For example, crystallization events which are observed at relatively low Conset values are observed only in the presence of effective surfactant monolayers, indicative of templated nucleation. Without an effective surfactant monolayer (or in the absence of any surfactant), only relatively high Conset values were found. This indicates that effective surfactant monolayers do not require as high a level of supersaturation to induce nucleation. System of Study. We have chosen a series of monoglycerides having a C18 carbon atom chain length in the acyl chain and a glycerol headgroup with varying degrees and types of double-bond unsaturation (Table 1 and Figure 4).
Table 1. Monoglycerides Used in This Study lipids 1-(9E-octadecenoyl)-rac-glycerol, monoelaidin 1-(11Z-octadecenoyl)-rac-glycerol, monovacennin 1-(9Z-octadecenoyl)-rac-glycerol, monoolein 1-(9Z,12Z-octadecadienoyl)-rac-glycerol, monolinolein
chemical formula
abbreviations
C21H40O4
ME (C18:1, t9)
C21H40O4
MV (C18:1, c11)
C21H40O4
MO (C18:1, c9)
C21H38O4
ML (C18:2, c9, c12)
MATERIALS AND METHODS
Materials. All series of monoglycerides used in this study were purchased from Nu Chek Prep Inc. and used as received (purity ≥99%). 1-Decanol, potassium salts, and all other chemicals, of the highest purity available, were purchased from Sigma-Aldrich and used without additional purification. Sample Preparation. The monoglyceride-containing organic phase (1-decanol) was prepared by dissolving the monoglyceride directly into 1-decanol, followed by bath sonication for ∼30 min at ambient temperature. For all of the experiments conducted herein, the total monoglyceride concentration in the solution was approximately 10−12 mM. Aqueous solutions using various potassium salts were prepared from purified, deionized water (18.2 MΩ·cm) using a Millipore water-purification system (Direct Q-3). All solutions were prepared immediately prior to use. All experiments were carried out at ambient temperature, ∼23−25 °C.
As shown in Figure 4, a general molecular structure of monoglycerides consists of a glycerol headgroup and an acyl chain, connected at the sn-1 position of glycerol group. Both MO and MV have an acyl chain consisting of 18 carbon atoms; however, MO has one cis CC double bond between the 9th and 10th carbon atoms, and MV has the cis CC double bond located between the 11th and 12th carbon atoms. In the case of ML, it has two cis CC double bonds at C9 and C12, while ME has one trans CC double bond between the 9th and 10th carbon atoms. The K2SO4 crystal system has been extensively utilized for evaluating the crystallization measurement methods and apparatus,17 hence it can be considered to be a suitable model system for crystallization. It is not known to have polymorphic complications, and C
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Figure 4. Molecular structure of monoglycerides used in this study.
significant reduction in the level of supersaturation needed to drive the nucleation. A regular crystal habit for K2SO4 was also observed when thiocyanate, iodide, or nitrate was added, as shown in Figure 6. A regular shape for the final crystal and a low Conset are a distinctly different regime compared to those obtained in the absence of added chaotropic anions. For the monolayer of MO, a definite trend was apparent: certain anions (e.g., chaotropic ones such as SCN−, I−, and NO3−) appear to decrease Conset and promote the formation of crystals with a well-defined habit, while other anions (e.g., very kosmotropic ones such as HPO42−) have lesser effects, as judged by similar high Conset and ill-formed crystals obtained, just as with no salts added to the system. Anions with intermediate ability (e.g., Br− and Cl−) gave intermediate values for Conset (30−40%). The order followed a Hofmeister series (lowest Conset on the right):
Figure 5. Conset for the crystallization of K2SO4 in the presence of various monoglyceride monolayers in decanol. Conset in the absence of any monoglycerides is 75 ± 10% (control). The representative image shows the typical polycrystalline K2SO4 formed. The diameter of the droplet for the image is ∼40 μm.
HPO4 2 − < Cl− < Br − < NO3− ≈ I− < SCN−
It is interesting that the templating ability for a monolayer of MO becomes progressively more enhanced (as judged by the formation of the euhedral crystal and lower Conset) as an anion of higher chaotropic character is introduced into the system, even though the chaotropic anion was present in relatively trace amounts (10 mM anion added vs 287 mM K2SO4). The red square symbols in Figure 6 show the control value in the absence of MO, but with a corresponding 10 mM salt (i.e., no surfactant). In the absence of MO, crystallization occurred at consistently high levels of Conset, and there were no distinguishable differences among various types of added anions upon mean Conset values and the crystal habit. These results strongly suggest that the enhanced templating ability is due to an interaction of the exogenous anions with the MO monolayer. We then repeated the same set of experiments, but with ME and MV monolayers. Overall, a similar tendency is found for the ME and MV systems as shown in Figure 7a. It is evident that certain anions (e.g., chaotropic ones such as SCN−, I−, and NO3−) appreciably improve the ability of some surfactant monolayers (viz., ME, MV, and MO) to enhance the templating capability of potassium sulfate crystallization, while other anions have less of an effect. The addition of potassium salts (10 mM) to the ML system showed no pronounced change in the mean Conset, contrary to the enhancement of crystallization characteristics seen in the ME, MV, and MO systems. The results are shown in Figure 7b, along with the
Figure 6. Effect of anions (10 mM of X) on Conset for K2SO4 crystallization in the presence (blue columns) and absence (red squares) of MO surfactants at 10 mM in decanol. Also shown are the typical crystal images observed for high and low Conset.
squares). Note that the equilibrium solubility for K2SO4 is about 12% at 25 °C.18,20 The lower value of Conset indicates a D
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Figure 7. Effect of anions (10 mM of KX) on Conset for K2SO4 crystallization in the presence of (A) ME, MV, and MO and (B) ML surfactants at 10 mM in decanol.
control (no surfactant). The Conset levels were maintained at about ∼55−70% regardless of the nature of the salt added. In an unsaturated monoglyceride, the cis double bond causes a kink or bend to exist in the acyl chain. Unlike the trans configuraton of the double bond in ME, the curvature imposed by the cis double bond leads to varying degrees of chain splay, hence there is an increased degree of chain splay for MV and MO relative to ME.10 The two cis double bonds of ML provide the largest degree of curvature of the respective tailgroups and hence the most chain splay of the species studied in this work. The “length” of the acyl chain, expressed as a dynamic average, changes in accordance with the position of the double bond. Thus, in comparing MV to MO, for example, MV has been said to be a “slimmer” molecule than MO with a correspondingly smaller shape factor because its cis double bond has been shifted away from the glycerol headgroup.8 Hence, the order of chain splay follows ME < MV < MO < ML. On the basis of Figure 7, it appears that the enhancement of templating capability upon addition of chaotropic anions such as SCN− is related to the chain splay of the monoglyceride monolayer. While ME, MV, and MO have shown anion sensitivities, ML with the most chain splay of the series did not show any changes. To further corroborate whether the anion sensitivity is related to the chain splay of the monolayer, we have used ×10 diluted KSCN concentration (1 mM instead of 10 mM KSCN) and compared the value of Conset, as shown in Figure 8. In the presence of 10 mM KSCN, there is no distinction in Conset values among ME, MV, and MO; all exhibit low Conset
(14−15%). However, the differences in Conset values emerge between various monoglycerides when the concentration of KSCN is lowered to 1 mM. As shown in Figure 8, ME monolayers still exhibit very low Conset (17%), indicative of the enhanced templating capability at this concentration; however, monolayers of MV and MO show higher Conset values (25 and 38%, respectively), indicating less efficiency in the enhancement of the templating capability, compared to the values (∼15%) obtained in 10 mM KSCN. This indicates that the monolayer formed by ME most effectively responds to the addition of trace amount of anions (1 mM), while MV and MO requires a greater concentration of the chaotropic anion in order to see a similar enhancement. Recall that the ML monolayer, having the most chain splay, cannot be induced to exhibit a templating capability, regardless of the anion concentration or type. Hence, taken together, these findings seem to indicate that an enhancement of templating capability upon chaotropic anion addition appears to be related to the degree of chain splay of the monolayer; the larger the chain splay, the less anion efficiency in enhancing the templating capability, judged by the ability to lower the value of Conset.
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DISCUSSION A vast array of examples of ion-specific effects (so-called Hofmeister effects) have been found in colloidal chemistry, interfacial science, and biochemistry.21 For the anions, the general trend is HPO4 2 − < SO4 2 − < F− < Cl− < Br − < NO3− < I− < ClO4− < SCN−
In general, one end of the series usually includes large, polarizable anions such as iodide, thiocyanate, and perchlorate, while the other end of the series generally includes fluoride, sulfate, and hydrogen phosphate. Although the origin of these specific ion effects is still under discussion, it is becoming better understood due to considerable progress made on this topic. An increasing number of experimental and theoretical results have established that Hofmeister effects may best be considered as interfacial phenomena in which specified anions will have their most pronounced effect in the vicinity of interfaces. Numerous studies have appeared relating how the specific forces that drive Hofmeister effects can be used as tools to probe structure and function in various systems including bulk phases, surfactants, polymer systems, interfaces, proteins and enzymes, biology, and medicine (ref 21 and refs therein). By taking the present crystallization studies in conjunction with our prior work,14,15 cumulative evidence now exists to
Figure 8. Comparison of Conset for K2SO4 in the presence of 1 mM vs 10 mM KSCN in ME, MV, and MO monolayers. E
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monoglycerides in aqueous dispersions. Monoolein and many other monoglycerides, when dispersed in pure water, aggregate into lyotropic liquid crystals having well-defined phases amenable to structural analysis by X-ray diffraction. When specific electrolytes are present, however, the shape of the selfassembled structures are markedly affected, having a profound influence on the phase behavior and microstructural parameters. For example, the structural evolution of the monoolein bicontinuous cubic phase (BCP) was reported to be influenced by anions in manner that follows the Hofmeister series. The binding of specific ions to the MO in their aggregate form resulted in an altered lattice parameter for this cubic phase, attributed to a change in curvature of the individual surfactant moiety.13 Similarly, Takahashi et al. reported that the lattice constants of BCP formed by hydrated MO and ME are affected by the presence of ions in water and that the magnitude of the lattice constant change is in line with the Hofmeister series based on X-ray diffraction studies.9,11 The chaotropes expand the lattice constants of the BCP phases formed by MO and ME, and kosmotropes exhibit an opposite effect; i.e., they reduce the lattice constant. It has been interpreted that kosmotropes lead to the dehydration of the MO headgroups at the interface, which in turn reduces its effective occupied area. This results in the shrinking of the water channels, as indicated by a decreasing lattice constant. On the other hand, chaotropic electrolytes lead to an increase in the hydration of the MO headgroups, thus enlarging the interfacial area occupied by an MO molecule. Consequently increasing the lattice parameters indicates a swelling of the water channels of the cubic phase.11 We believe that similar phenomena are at play in our system. The headgroup of our monoglyceride monolayers may bind with the anions in an ion-specific way to lead to a unique change in amphiphile curvature, thereby modulating the aggregate structure of the ordered assembly. For ME, MV, and MO, the binding of chaotropic anions can in turn result in modulated packing parameters for the monolayers, having the capacity to effectively induce crystal nucleation. Furthermore, the extent to which these chaotropic anions affect the templating capability appears to be related to the molecular shape of the monoglyceride. Monoglycerides with low chain splay such as ME showed the most pronounced enhancement in templating capability at lower chaotropic concentrations, followed by MV and MO, in order of increasing chain splay. However, no such enhancement effect at all was seen for ML, having the largest chain splay studied, in the anion concentration range of up to 10 mM. Recall that Kulkarni has reported that different lipid phase behavior and number density of water channels in BCP result from differing tailgroup shape for monoglycerides having various molecular shapes, in terms of the number and location of double bonds in their chain structures.10 Lyotropic liquid crystals of ML in water, in particular, had a significantly lower lattice parameter for their cubic phase than did MO, ME, or MV. Therefore, it would seem that the chain packing for ML in our monolayers may be largely governed by tail−tail interactions and is not susceptible to a change in packing even given the putative enlargement of the interfacial area due to the binding of chaotropic anions at their headgroups. We have observed that nucleation characteristics for K2SO4 are appreciably acccelerated in the presence of a small quantity of added chaotropic anions. It is believed that the improved templating capabilities appear to originate from mutual interactions among the crystallizable inorganic salt present in
show that the effect of specific anions on monolayer structure depends strongly upon the nature of the surfactant as well as the anion. In Figure 9 we present our own data from previous
Figure 9. Effect of anions (10 mM of KX) on Conset for K2SO4 crystallization in the presence of various surfactants in decanol.
work, combined with the results from the present work, in order to illustrate clearly the dramatic and opposing effects a small amount of anions (10 mM) can have, depending on the nature of the monolayer. In the case of cationic monolayers of octadecylamine (ODA)14 and hexadecyltrimethylammonium bromide (CTAB, data not shown),15 more chaotropic anions resulted in a diminution in the efficiency of the templating capability, as evidenced by higher Conset values and the polycrystalline habit; the ability of the anions to depreciate the templating ability varied in a manner consistent with the Hofmeister series. Anions at the chaotropic extreme of the series (e.g., NO3 −, I−, and SCN−) showed the most pronounced effect upon ODA and CTAB. On the contrary, in the case of monoglyceride monolayers of ME, MV, and MO (results for MO are shown in Figure 9), more chaotropic anions resulted in an enhancement in the efficiency of the templating capability, as evidenced by lower Conset values and euhedral crystal formation. The ability of the anions to enhance the templating ability varied in a manner consistent with the Hofmeister series. Electrolytes are known to have a considerable influence on ordered assemblies, charged or uncharged, including monolayers, bilayers, and vesicles. The literature on the effects of ionic compositions on self-assembled monolayers and bilayers shows that they often follow the order in the Hofmeister series, even at low levels of added salts (0.01−10 mM).22−25 In the specific case of monoglyceride monolayers and bilayers, a significant precedent exists for anion binding to monoglyceride headgroups, which could be relevant to our observed results. Garcia-Celma deposited a monoolein lipid membrane upon an alkanethiol-gold solid support and exposed the membrane to rapid exchange with electrolyte solutions.26 An electrophysiological detection technique was used to register charge displacements in the lipid headgroup region of the membrane. It was concluded that chaotropic anions, e.g., SCN−, are pronouncedly more attracted to the lipid interface than are other anions, which was attributed to a binding of these chaotropes to the headgroup.26 Similar binding phenomena between anions and monoglyceride aggregates have been manifested in the phase behavior of self-assembled F
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A cascade of studies of surfactants at the liquid−liquid interface has indicated that the monolayer assembly of amphiphiles at the interface between two immiscible liquids is a result of a complex interplay between surfactants (headgroups and tailgroups) and the organic/oil phase.35 Yet despite recent progress, relatively less is conclusively known about the structure of surfactant assemblies at the liquid−liquid interface compared to that at the liquid−vapor interface. Worthy of mention is the possible effect of the organic phase in our interfacial system. For example, the broad interfacial region between water and decanol could influence the organization of the surfactant molecules in the interfacial region.36 In addition, the penetration of the hydrocarbon tails of interfacial surfactants by oil molecules could be important.37 All of these factors may complicate any straightforward explanation of precisely what changes are imparted to the structure of surfactant aggregates in our system, upon addition of chaotropic anions. Nevertheless, we believe that this work provides further strengthening for the model of monolayertemplated crystal nucleation that requires adaptability in organic soft monolayers as a result of specific coordination with the surrounding ionic compositions.30,33 This cooperative interaction results in the reorganization of the microstructures that eventually influence the templating capability at the liquid−liquid interface in an ion-specific way.
bulk, the added anions, and the monolayer. Such mutual interactions can allow the monolayer to adapt and modulate its aggregate structure in order to result in the greatest degree of control over K2SO4 crystallization. Previous studies on inorganic crystal nucleation processes templated by a monolayer have indicated that the mechanism responsible has its foundation in molecular recognition, as a function of cooperative effects of structural, electrostatic, and hydrogen bonding interactions between the monolayer and the nucleated crystal.27−29 Such structural/geometric effects are generally described in terms of the mutual influence between the structure of an ordered monolayer and that of the crystal nucleus, exerted in a manner effective to attain an energetically and structurally optimum interface. Furthermore, increasing emphasis has been recently placed upon the ability of the monolayer to adapt to its adjacent subphase in order to orient itself to act as a template30 and upon mutual influences of the monolayer with inorganic moieties of the crystallizable substance.31,32 This kind of cooperative reorganization and synergistic activity between the solute and the monolayer has been reported most recently for the nucleation of calcium carbonate.33 In prior work on the nucleation of ice at the oil− water interface by a long-chain (≥C22) alcohol surfactant monolayer, the supercooling necessary to nucleate was found to depend strongly upon the length of the hydrocarbon tail; a varying tail length would naturally imply variations in the monolayer aggregate structure.34 In view of the previous studies on inorganic crystal nucleation processes templated by a monolayer, as described above, the observed nucleation enhancement in our system can be ascribed to the existence of an ordered array of hydrophilic headgroups having geometric complementarity to the crystal nucleus of K2SO4. Electrostatic effects probably do not play a major role in our system because monoglycerides are nonionic lipids, so any observed nucleation enhancement is likely to be a result of a hydrogen bonding interaction.27−29 Yet, in our studies, it is only in the presence of added chaotropic anions that selected monoglycerides would exhibit an enhancement of K2SO4 crystallization. This indicates that a rearrangement of the monoglyceride monolayer (MO, ME, and MV) upon interaction with chaotropic anions may play a role. While the nature of the specific interaction of the anion upon the lipid monolayer is not conclusively known at the liquid−liquid interface, it likely involves an association of these anions with the monolayer that results in a reorganization of assembled molecules. Chaotropic anion binding to the monoglyceride headgroup can change the effective size of the lipid polar moiety relative to that of the long hydrocarbon chain. This in turn would change the headgroup curvature which determines microstructure and global packing constraints, to result in a modulation of the molecular arrangement of the monolayer at the liquid−liquid interface and change the degree of complementarity between the monolayer and the nucleated crystal. For ME, MO, and MV, chaotropic anion binding results in altered monolayer packing characteristics for an enhanced structural match with K2SO4 nuclei. This is not to say that chaotropic anions do not similarly bind to ML. Even so, nucleation enhancement is not observed for ML monolayers in the presence of chaotropic anions. To account for this, we recall that monolayer packing characteristics flow from headgroup geometry coupled with the nature of the hydrocarbon tail. For ML, the disorder in its hydrocarbon tail and large chain splay may frustrate any beneficial effect of the chaotropic anions.
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CONCLUSIONS We have employed various monoglyceride monolayers, at a micrometer-sized water droplet surrounded by oil, as an organized organic assembly for the study of the conditions which induce the nucleation of a model crystal, K2SO4. We have found that the addition of a small quantity of exogenous ions at relatively low concentrations (1−10 mM) could significantly affect important crystallization parameters in a manner consistent with the Hofmeister series. Surprisingly, the chaotropic anions enhance the templating capability of the neutral monoglyceride surfactant, but the extent of this interaction is attenuated in surfactants (ML) having greater chain splay. Because chaotropic anions are known to have a tendency to accumulate at an oil−water interface, they are thus capable of influencing the microstructure of the organized organic assembly of the monolayer. Our work demonstrates that minor amounts of certain anions can exert disproportionate effects on templated nucleation via their influence on the template itself. While a more quantitative description of the detailed underlying mechanism would be welcome, for a more complete understanding of this phenomena our intriguing results demonstrate the flexibility and adaptability of the surfactant monolayer and its exquisite sensitivity to the adjacent medium. Our work can be considered to be an important contribution to the understanding of biomineralization in realistic biomimetic systems because biomineralization phenomena in living systems occur in the presence of multiple dissolved species in addition to the biomineral itself, and multiple kinds of salts can have synergistic effects.
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AUTHOR INFORMATION
Corresponding Author
*Tel: 914-633-2638. Fax: 914-633-2240. E-mail: SLee@iona. edu. Notes
The authors declare no competing financial interest. G
DOI: 10.1021/la5049419 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
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ACKNOWLEDGMENTS We acknowledge financial support from the National Science Foundation (NSF-CHE-1212967). M.M. and O.T. thank the Patrick J. Martin Foundation for scholarships.
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DOI: 10.1021/la5049419 Langmuir XXXX, XXX, XXX−XXX
