Anti-Ice Nucleating Activity of Surfactants Against Silver Iodide in

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Anti-Ice Nucleating Activity of Surfactants Against Silver Iodide in Water-in-Oil Emulsions Takaaki Inada, Toshie Koyama, Hiroyuki Tomita, Takuya Fuse, Chikako Kuwabara, Keita Arakawa, and Seizo Fujikawa J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b02644 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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The Journal of Physical Chemistry

Anti-Ice Nucleating Activity of Surfactants against Silver Iodide in Water-in-Oil Emulsions Takaaki Inada,*,† Toshie Koyama,† Hiroyuki Tomita,† Takuya Fuse,‡ Chikako Kuwabara,§ Keita Arakawa,§ and Seizo Fujikawa§

†

National Institute of Advanced Industrial Science and Technology (AIST), Namiki 1-2-1, Tsukuba, Ibaraki 305-8564, Japan

‡

Research Laboratories, DENSO CORPORTION, Minamiyama 500-1, Komenoki, Nisshin, Aichi 470-0111, Japan

§

Research Faculty and Graduate School of Agriculture, Hokkaido University, Kita-9, Nishi-9, Kita-ku, Sapporo 060-8589, Japan

*

Phone: +81-29-861-7272. E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: Various water-soluble substances are known as anti-ice nucleating agents (antiINAs), which inhibit heterogeneous ice nucleation initiated by ice nucleating agents (INAs). Among them, several surfactants are reportedly effective as anti-INAs especially against silver iodide (AgI), which is a typical inorganic INA that induces heterogeneous ice nucleation at relatively high temperatures. In this study, the anti-ice nucleating activities of seven surfactants were examined in emulsified surfactant solutions containing AgI particles. Among previously reported anti-INAs (e.g., antifreeze proteins (AFPs), polyphenol compounds and synthetic polymers), a cationic surfactant used in this study, hexadecyltrimethylammonium bromide (C16TAB), showed the highest anti-ice nucleating activity against AgI. Based on the unique concentration-dependent dispersibility of AgI particles in C16TAB solution, the anti-ice nucleating activity of C16TAB must be caused by the adsorption of C16TAB molecules on AgI surfaces either as a monolayer or a bilayer depending on the C16TAB concentration.


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INTRODUCTION A water droplet of several micrometers in diameter can be supercooled down to around −38 °C by eliminating insoluble solid particles.1-4 In general, however, it is difficult to eliminate solid particles completely from a relatively large water droplet and to eliminate solid walls that retain water unless the water droplet is somehow levitated. Liquid water that has interfaces with some solid particles or walls is susceptible to heterogeneous ice nucleation at temperatures higher than −38 °C.3,4 In this study, such solids that induce heterogeneous ice nucleation are referred to as ice nucleating agents (INAs). Various water-soluble substances that inhibit heterogeneous ice nucleation by inactivating INAs have been reported, including some that are biological in origin,5-21 and others that are synthetic polymers.11,14,22-26 These substances are hereafter referred to in this study as anti-ice nucleating agents (anti-INAs). Anti-INA molecules inactivate INAs by masking ice nucleating sites distributed on the INA surfaces.14 Recently, numerous surfactants also have been reported as anti-INAs.27-29 A droplet freezing assay, in which ice nucleation is examined by using droplets with diameters on the order of millimeters, showed that those surfactants are effective as antiINAs, especially against silver iodide (AgI),29 which is a typical inorganic INA that induces ice nucleation at relatively high temperatures.30 The droplet freezing assay is a convenient technique to examine anti-INAs.5,11,17-21,25,26,29 However, this assay is not always suitable for investigating the effect of an anti-INA on a single specific INA, because contamination of water with unidentified INAs is unavoidable due to the large volume of a droplet. On the contrary, an emulsion freezing assay is relatively unaffected by unidentified INAs,2,14,15,31 because this assay uses emulsified aqueous droplets with diameters on the order of at most a few tens of micrometers; thus the volume of a droplet used in the emulsion


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freezing assay is at least four orders of magnitude smaller than that used in the droplet freezing assay.15 Furthermore, the emulsion freezing assay enables control of the number or total surface area of INA particles in a droplet, and thus the effect of an anti-INA on a specific INA can be quantitatively analyzed one by one based on the same criteria.14,15 In this study, by using the emulsion freezing assay, we examined the anti-ice nucleating activities of seven surfactants (Table 1), all of which were reported to have relatively strong antiice nucleating activities against AgI in a previous study using the droplet freezing assay.29 We also discuss in detail the anti-ice nucleating activity of hexadecyltrimethylammonium bromide (C16TAB), which showed the highest anti-ice nucleating activity against AgI among these surfactants in this study.

EXPERIMENTAL SECTION Materials. The seven surfactants used as anti-INAs are summarized in Table 1. Among these surfactants, Triton X-100 (TX-100), Emulgen 2025G (E2025G), Tween 80 (TW80), and polyethylene glycol monostearate (n ≈ 40) (PEGMS40) are nonionic, C16TAB is cationic, sodium cholate (SC) is anionic, and myristyl sulfobetaine (MS) is amphoteric. The structural details of these surfactants were reported elsewhere.29 Aqueous solutions of these surfactants were prepared by using purified water as solvent (Millipore, Elix-UV-3 and Academic-A10). Water-in-oil (W/O) emulsions were prepared using n-heptane (Wako Pure Chemical Industries, spectrochemical analysis grade) as a continuous phase and sorbitan tristearate (Sigma-Aldrich, SPAN 65) as an emulsifier.

Table 1. Surfactants Used as Anti-INAs in This Study.


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Substance

Abbreviation Classification

Molecular weight

Origin

CMC a, mg mL−1 0 °C

25 °C

Triton X-100 (polyoxyethylene octylphenyl ether)

TX-100

Nonionic

647

Nacalai Tesque (Japan)

3.2 × 10−1

2.1 × 10−1

Emulgen 2025G (polyoxyethylene octyldodecyl ether)

E2025G

Nonionic

1398

Kao (Japan)

8.9 × 10−2

1.9 × 10−1

Tween 80 (polyoxyethylene sorbitan monooleate)

TW80

Nonionic

1309

Wako Pure Chemical 1.5 × Industries (Japan) 10−2

1.5 × 10−2

Polyethylene glycol monostearate (n ≈ 40)

PEGMS40

Nonionic

2448-3264 Tokyo Chemical Industry (Japan)

4.4 × 10−1

3.3 × 10−1

Hexadecyltrimethylammonium bromide

C16TAB

Cationic

364

Tokyo Chemical Industry (Japan)

−b

3.9 × 10−1

Sodium cholate

SC

Anionic

431


5.3 × 100

5.1 × 100

Myristyl sulfobetaine

MS

Amphoteric

364


1.7 × 10−1

1.6 × 10−1

a

CMC is the critical micelle concentration. b CMC of C16TAB at 0 °C was not measured due to crystallization.

The INAs used here were AgI particles (Sigma-Aldrich, 99%), whose median diameter was 1.65 µm.15 Because the ice nucleating activity of AgI would change depending on the treatment of AgI particles before and during the experiments, a reagent bottle of AgI was resealed immediately after each usage and then stored under a light-shielded and dry condition. Emulsion Freezing Assay. Details of the methods to prepare W/O emulsions and to measure the ice nucleation temperature of emulsified droplets were reported elsewhere,14,15 and thus only a brief description and modifications are given here. First, 0.2 g of AgI particles in 4 mL of purified water was sonicated for 60 min at room temperature by using an ultrasonic cleaner (Emerson Electric, Bransonic model 1510), and then the suspension was filtrated through filter paper (Whatman, ashless grade 41) to remove AgI particles larger than 3 µm in diameter. Then, 2 mL of this suspension was mixed with 1 mL of


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aqueous surfactant solution prepared at a given concentration. This mixture was used as the dispersed phase of W/O emulsions. For emulsified solutions of all surfactants except C16TAB, the emulsifier SPAN 65 was dissolved at 4 wt% in the continuous phase (n-heptane), based on previous studies.14,15 Only for C16TAB solution, 2 wt% SPAN 65 was dissolved in the continuous phase to ensure a stable emulsion. For all surfactants, the dispersed phase was emulsified in the continuous phase by stirring with a homogenizer. The emulsion sample, which contained numerous aqueous droplets with diameters ranging roughly from 2 to 28 µm, was enclosed between two cover glasses and then cooled at 4 °C min−1 on a temperature-controllable copper stage. Ice nucleation was detected for each aqueous droplet by optical microscopy, and the ice nucleation temperature Tf for each droplet was measured by a thermocouple embedded in the copper stage. Tf was analyzed only for the droplets that satisfied 5 < d < 25 µm and 2 ≤ NAgI ≤ 8, where d is the equivalent diameter of equal volume sphere of the droplets, and NAgI is the number of AgI particles inside a droplet. Solubility of Surfactant. All surfactants except C16TAB were promptly and completely dissolved at 0 °C at a concentration as high as 10 mg mL−1, whereas C16TAB was not dissolved in water under the same conditions (0 °C and 10 mg mL−1). Therefore, the solubility of C16TAB in water was measured by using the same apparatus as that used to measure Tf. A 0.06-µL C16TAB solution at a given concentration was enclosed between two cover glasses, resulting in a thin film about 80 µm thick and 1 mm in diameter, and then cooled on the temperature-controllable stage. As the temperature decreased, C16TAB at relatively high concentrations sometimes crystallized, probably as hydrated crystals,32 before ice formation. However, the precipitated C16TAB crystals were usually too fine (< 1 µm) to be detected by optical microscopy. Therefore, the solution was first cooled at 4 °C min−1 until ice formed


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(usually at between −20 and −30 °C), and then heated to just above 0 °C to completely melt the ice. During ice formation, C16TAB (either dissolved molecules or precipitates) was concentrated in the remaining solution, and then, after the ice melted, relatively large C16TAB crystals (> 10 µm) remained if the dissolution temperature of C16TAB in the solution was higher than 0 °C (Figure S1). Then, the C16TAB crystals were gradually dissolved by increasing the temperature by 1 °C increments, and finally, two temperatures were determined: the highest temperature T1 at which C16TAB crystals existed in the solution, and the lowest temperature T2 at which C16TAB crystals were completely dissolved in the solution. Even if the C16TAB crystals seemingly disappeared, “invisible” crystals smaller than the detection limit of microscopy (about 1 µm) might remain. Therefore, after the disappearance of the crystals in the observed image, the temperature of the C16TAB solution was decreased by 3 °C and maintained for 30 min to allow invisible crystals (if any) to grow, and thus the complete dissolution of C16TAB crystals was confirmed. In this study, the dissolution temperature of C16TAB was defined as the average of T1 and T2. Surface Tension of Surfactant Solution. The surface tension of the seven surfactant solutions was measured at various concentrations by the pendant drop method, using a surface tensiometer (Kyowa Interface Science, DM300, FAMAS software). The surface tensiometer was placed in a constant-temperature room to control the temperature of all the equipment and materials, including droplets of the surfactant solutions. For each surfactant solution, a 2-mm-diameter droplet suspended by a needle was enclosed in a rectangular glass cell (10 × 10 × 45 mm) during the measurement to prevent evaporation. The surface tension was recorded at a given temperature after it was sufficiently stabilized.33 The wait time for stabilization was between 3 and 90 min, depending on the type of surfactant. For C16TAB solution, the surface tension was


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measured at temperatures above the Krafft temperature, which was determined by the solubility data obtained here, whereas for the other solutions, the surface tension was measured at 0 and 25 °C.

RESULTS Figure 1 shows the fraction f of unfrozen droplets for different surfactant solutions containing AgI particles as a function of the sample temperature T, for the surfactant concentration c of 1.0 × 10−1 mg mL−1 (Figure 1a) and 1.0 mg mL−1 (Figure 1b). For comparison, also shown are the f−T curves for purified water droplets containing AgI particles (solid circles), which served as control in this study, and for purified water droplets containing no AgI particles (solid line). For reference, these two f−T curves were compared with previous results obtained under the same conditions by using the same apparatus,2,14 and there was no significant difference between the previous and present results (Figure S2), indicating good reproducibility of the emulsion freezing assay.


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Figure 1. Fraction f of unfrozen droplets of different surfactant solutions at concentration c of (a) 1.0 × 10−1 mg mL−1 and (b) 1.0 mg mL−1, as a function of sample temperature T. Number of AgI particles contained in each droplet was between 2 and 8. Numbers of droplets n analyzed under each experimental condition are summarized in Table S1. Solid circles represent results for purified water containing AgI particles. Line represents results for purified water without AgI particles. For all droplets, diameter d satisfies 5 < d < 25 µm.

The f−T curves for all the surfactant solutions containing AgI particles shifted toward lower temperatures compared to that of the control, indicating that all the surfactants inhibited the ice nucleating activity of AgI, although the effectiveness depends on the type of surfactant. There is a similarity in the order of anti-ice nucleating activity of the surfactants between c = 1.0 × 10−1 mg mL−1 (Figure 1a) and 1.0 mg mL−1 (Figure 1b). Among the seven surfactants, C16TAB showed the highest anti-ice nucleating activity; especially at 1.0 mg mL−1, 95% of droplets were supercooled to the same temperature level as that for purified water containing no AgI (solid line).


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For C16TAB solution, as c was decreased, the f−T curves shifted toward higher temperatures, approaching that of the control (Figure 2). Similar tendencies were observed also for the other surfactant solutions (Figure S3). At 1.0 × 10−4 mg mL−1 (Figure S3), most of the surfactants were no longer effective in inhibiting the ice nucleating activity of AgI. In the C16TAB solution, when c > 1.0 × 10−1 mg mL−1, C16TAB possibly crystallizes below 0 °C (the solubility curve for C16TAB is seen later in Figure 5). However, the emulsion freezing assay using 1.0 mg mL−1 C16TAB solution without AgI particles confirmed that C16TAB crystals had no ice nucleating activity (Figure S4).

Figure 2. Fraction f of unfrozen droplets of C16TAB solutions at different concentrations, as a function of sample temperature T. Number of AgI particles contained in each droplet was between 2 and 8. Numbers of droplets n analyzed under each experimental condition are summarized in Table S1. Solid circles represent results for purified water containing AgI particles. Line represents results for purified water without AgI particles. For all droplets, diameter d satisfies 5 < d < 25 µm.


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Due to the surface charge of the particles, most of the AgI particles were stably dispersed in emulsified droplets of the surfactant solutions, except in the C16TAB solution. Similar stable dispersibility of AgI particles were also observed in emulsified droplets of aqueous solutions of other anti-INAs, such as antifreeze protein (AFP) type I and type III, poly(vinyl alcohol) (PVA), poly(vinylpyrolidone) (PVP), and poly(ethylene glycol) (PEG).14 When using C16TAB solution as emulsified droplets, however, a large majority of AgI particles were sometimes precipitated in the droplets, depending on the concentration of C16TAB. We classified the emulsified droplets of C16TAB solution containing AgI particles into two cases: a non-dispersible case in which all AgI particles in a droplet were precipitated and a dispersible case in which at least one AgI particle in a droplet moved around. The fraction of droplets in the non-dispersible case, nnd/(nnd + nd), at different concentrations of C16TAB is summarized in Table 2, where nnd is the number of droplets in the non-dispersible case, and nd is the number of droplets in the dispersible case. This fraction, nnd/(nnd + nd), is an index of the dispersibility of AgI particles. When c < 1.2 × 10−3 or c > 1.2 × 10−1 mg mL−1, AgI particles were relatively well dispersed in emulsified droplets, and thus nnd/(nnd + nd) was relatively small. On the contrary, when 1.2 × 10−3 < c < 1.2 × 10−1 mg mL−1, the dispersibility of AgI particles was low, and thus nnd/(nnd + nd) was relatively high.

Table 2. Dispersibility of AgI Particles in Emulsified Droplets of C16TAB Solution at Different Concentrations. c a, mg mL−1

nnd b

nd c

nnd/(nnd + nd)

1.0 × 10−4

10

62

0.14

1.0 × 10−3

19

71

0.21


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1.2 × 10−3

63

73

0.46

1.0 × 10−2

79

8

0.90

1.0 × 10−1

93

6

0.94

1.2 × 10−1

120

61

0.66

1.0 × 100

7

80

0.08

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a

c is the concentration of C16TAB solution. b nnd is the number of droplets in the nondispersible case in which all AgI particles in a droplet were precipitated. c nd is the number of droplets in the dispersible case in which at least one AgI particle in a droplet moved around.

To examine the relationship between the dispersibility of AgI particles and the anti-ice nucleating activity of C16TAB, the f−T curves were summarized separately for the dispersible case and non-dispersible case at c = 1.2 × 10−3 and 1.2 × 10−1 mg mL−1 (Figure 3), at which nnd and nd become comparable to each other (Table 2). Also shown in Figure 3 are the f−T curves for the control (solid circles), and for purified water droplets containing no AgI particles (solid line). When c = 1.2 × 10−3 mg mL−1, there are distinct differences in the f−T curves between the nondispersible (open circles in Figure 3a) and dispersible (open squares in Figure 3a) cases. In the dispersible case, the f−T curve approached that of the control, whereas in the non-dispersible case, the f−T curve shifted to lower temperatures compared to that of the control. On the contrary, when c = 1.2 × 10−1 mg mL−1, there is no difference in the f−T curve between the nondispersible (open circles in Figure 3b) and dispersible (open squares in Figure 3b) cases. In both cases, the f−T curve shifted to lower temperatures compared to that of the control.


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Figure 3. Fraction f of unfrozen droplets of C16TAB solutions at concentration c of (a) 1.2 × 10−3 mg mL−1 and (b) 1.2 × 10−1 mg mL−1, as a function of sample temperature T. Number of AgI particles contained in each droplet was between 2 and 8. Data are classified into two groups; non-dispersible case in which all AgI particles in a droplet were precipitated, and dispersible case in which at least one AgI particle moved. n is the number of droplets, nnd is the number of droplets in the non-dispersible case, and nd is the number of droplets in the dispersible case. Solid circles represent results for purified water containing AgI particles. Line represents results for purified water without AgI particles. For all droplets, diameter d satisfies 5 < d < 25 µm.


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Figure 4 shows the surface tension of C16TAB solution for different temperatures as a function of the concentration. The surface tension is almost independent of temperature within the range between 25 and 35 °C. The critical micelle concentration (CMC) was taken as the concentration where the surface tension exhibited pronounced change (shown by the arrow in Figure 4).34

Figure 4. Surface tension of C16TAB solution at different temperatures as a function of concentration. Critical micelle concentration (CMC) is indicated by the arrow.

Figure 5 shows the phase diagram of C16TAB solution, including the CMC curve obtained from Figure 4 and the solubility curve. The Krafft point TP was defined as the intersection of the solubility and CMC curves.34,35 The solubility rapidly increased above TP due to the micelle formation. The data in Figure 5 agree well with existing data for CMC,35-37 and with TP for C16TAB solution.32


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Figure 5. Phase diagram of C16TAB solution: (a) solubility curve and (b) critical micelle concentration (CMC) curve. Krafft point is indicated by the arrow.

The surface tension of the other six surfactant solutions was measured at 0 and 25 °C as a function of the concentration (Figure S5). The orders of magnitude of CMC, determined from Figure S5 and then summarized in Table 1, agree well with those in the literature for solutions of TX-100,35,37,38 TW80,33,35,38 SC,35,39 and MS.40 To our knowledge, CMC of E2025G and PEGMS40 solution has not been published.

DISCUSSION All seven surfactants used in this study inhibited the ice nucleating activity of AgI in the emulsion freezing assay, as well as in the droplet freezing assay.29 These anti-ice nucleating activities are comparable to those of previously reported anti-INAs, such as AFPs (type I and III),14 synthetic polymers (PVA, PVP and PEG),14 and several polyphenol compounds,15 when compared at the same mass concentration. Among all these anti-INAs, C16TAB shows the highest anti-ice nucleating activity, inhibiting the ice nucleating activity of AgI almost completely at 1.0 mg mL−1 (Figure 1b).


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In this section, first the present results obtained by the emulsion freezing assay are compared to those previously obtained by the droplet freezing assay,29 both at c = 1.0 × 10−1 mg mL−1 (Figure 6). Here, Tf50 is defined for each solution as the temperature at which 50% of the droplets were frozen (f = 0.5), and ∆Tf50 is defined as the difference between Tf50 for the control and that for the surfactant solution containing AgI particles. The higher the ∆Tf50, the higher the anti-ice nucleating activity. Figure 6 shows that the order of anti-ice nucleating activity of the surfactants differs somewhat between the droplet and emulsion freezing assays; for example, in the droplet freezing assay, MS > TX-100 > TW80, whereas in the emulsion freezing assay, TX-100 > TW80 > MS. Such a difference in the order of anti-ice nucleating activity between the two assays was also reported in a previous study using polyphenol solutions.15

Figure 6. Comparison of ∆Tf50 for 1.0 × 10−1 mg mL−1 surfactant solutions obtained in this study by emulsion freezing assay and those obtained by droplet freezing assay.29 ∆Tf50 is the difference between Tf50 for the control and for the surfactant solutions containing AgI particles, where Tf50 is the temperature at which 50% of the droplets were frozen.


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Here, we discuss why the order of anti-ice nucleating activity differs between the droplet and emulsion freezing assays. When surfactant solutions are used, the difference might be caused by the formation of surfactant micelles (except for C16TAB due to its high TP of 21 °C), because the temperature level of ice nucleation is different between the two assays; in the emulsion freezing assay, Tf50 for the control was −16.1 °C (Figures 1-3), whereas in the droplet freezing assay, Tf50 for the control was −3.2 °C.29 However, the temperature dependence of the CMC of the surfactants used here was so small (Table 1 and Figure S5) that the difference in the order of anti-ice nucleating activity between the two assays was not likely caused by the formation of surfactant micelles, although it was difficult to measure the CMC below 0 °C in this study. It should be noted that the effect of the formation of surfactant micelles on the anti-ice nucleating activity is still a subject of future studies, because it was reported that in solutions of some surfactants at concentrations higher than CMC, ice nucleates at higher temperatures than in purified water, due to the ice nucleating activity of micelles themselves.28 The difference in the order of anti-ice nucleating activity between the droplet and emulsion freezing assays is therefore attributed to the difference in the amount, or the total surface area, of AgI particles in a droplet, as discussed in a previous study.15 The amount of AgI particles in a droplet in the droplet freezing assay was four orders of magnitude higher than that in the emulsion freezing assay. Generally, the ice nucleating activity at the surface of a solid particle is not uniform, because ice nucleating sites with different ice nucleating activities are distributed inhomogeneously even on the surface of a particle.41-45 Therefore, the properties of ice nucleating sites, which are the target of inactivation, in the droplet and emulsion freezing assays are different, because the total surface area of AgI particles in a droplet differs significantly


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between the two assays. This explains the difference in the results between the droplet and emulsion freezing assays. Next, we discuss the mechanism of anti-ice nucleating activity of C16TAB, which showed the highest anti-ice nucleating activity against AgI. As background for the discussion, first the unique dispersibility of AgI particles in C16TAB solution should be explained. Among the antiINAs against AgI so far reported,14,15 only C16TAB affects the dispersibility of AgI particles in emulsified aqueous droplets (Table 2). When c < 1.2 × 10−3 mg mL−1, as in purified water, the dispersibility of AgI particles remained high in the emulsified droplets. However, the dispersibility drastically decreased when 1.2 × 10−3 < c < 1.2 × 10−1 mg mL−1. When the concentration increased further (c > 1.2 × 10−1 mg mL−1), the dispersibility of AgI particles returned to its high level. This concentration dependence of the dispersibility can be explained by the adsorption characteristics of C16TAB on solid surfaces in water. For example, on smooth mica surfaces,46 little adsorption of C16TAB molecules occurs at concentrations lower than 7 × 10−3 mg mL−1. When the concentration increases to 2 × 10−2 mg mL−1, a close-packed monolayer is completely adsorbed, thus exposing hydrophobic chains to water. Then, just below the CMC (4 × 10−1 mg mL−1), an additional second layer starts to form on the monolayer. Finally, bilayer adsorption is complete at the CMC, thus exposing polar-head groups of the second layer to water. Similar concentration dependences of the adsorption of C16TAB molecules has been observed on other surfaces, such as silica,47 and silicon surfaces.48 These previous studies on C16TAB adsorption suggest that the adsorption of C16TAB molecules on AgI surfaces would also have a similar concentration dependence; the adsorption layer changes from a hydrophobic monolayer to a hydrophilic bilayer as the concentration


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increases. The relationship between the adsorption characteristics of C16TAB and the value of nnd/(nnd + nd), which is an index of the dispersibility of AgI particles, is illustrated in Figure 7. Because AgI surfaces are negatively charged in general,30,49 AgI particles were stably dispersed in C16TAB solutions in the absence of adsorbed C16TAB molecules at low concentrations (c < 1.2 × 10−3 mg mL−1) (Figure 7a). As the C16TAB concentration increased (1.2 × 10−3 < c < 1.2 × 10−1 mg mL−1), the monolayer adsorption rendered AgI surfaces hydrophobic, thus decreasing the dispersibility of AgI particles (Figure 7b). As the C16TAB concentration increased further (c > 1.2 × 10−1 mg mL−1), conversely, the bilayer adsorption rendered AgI surfaces hydrophilic, thus recovering the dispersibility of AgI particles (Figure 7c). The concentration where the monolayer changes to the bilayer was 1.2 × 10−1 mg mL−1, which is close to the CMC measured in the present study (3.9 × 10−1 mg mL−1).

Figure 7. Concentration dependence of Tf50 and nnd/(nnd + nd), where Tf50 is the temperature at which 50% of the droplets were frozen, nnd is the number of droplets in the non-dispersible case in which all AgI particles in a droplet were precipitated, and nd is the number of droplets in the dispersible case in which at least one AgI particle in a droplet moved around. Lines are visual guides only. Inset illustrations show concentration dependence of adsorption characteristics of C16TAB.


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The anti-ice nucleating activity of C16TAB can be clearly explained based on the abovementioned adsorption characteristics of C16TAB molecules on AgI surfaces. Primarily, heterogeneous ice nucleation on solid surfaces must be attributed to the ordered ice-like structure of water molecules near the solid surfaces.50,51 On AgI surfaces, the structure of ice-like water molecules would be formed not only by matching the lattice constant of AgI crystals with that of ice Ih or Ic,30,52-55 but also by a proper distribution of electric potential near the AgI surfaces.30,49,56-60 Therefore, the most efficient way of inhibiting ice nucleating activity of AgI is masking the ice nucleating sites distributed on AgI surfaces by the adsorption of other molecules (here, C16TAB molecules). Also illustrated in Figure 7 is the relationship between the adsorption characteristics of C16TAB and the value of Tf50, which is an index of the anti-ice nucleating activity of C16TAB. When c < 1.2 × 10−3 mg mL−1, because there is little or no adsorption of C16TAB molecules on AgI surfaces, Tf50 was as high as −14 °C, indicating the low anti-ice nucleating activity (Figure 7a). When c was increased to 1.2 × 10−3 mg mL−1, at which the monolayer is formed on AgI surfaces, Tf50 started to decrease, indicating that the anti-ice nucleating activity of C16TAB increased. Figure 3a shows that when c = 1.2 × 10−3 mg mL−1, the anti-ice nucleating activity of C16TAB strongly depended on the dispersibility of AgI particles; in the non-dispersible case, the ice nucleating activity of AgI was well inhibited by C16TAB (open circles in Figure 3a), whereas in the dispersible case, the ice nucleating activity of AgI was as high as that of the control (open squares in Figure 3a). These results show that when the hydrophobic monolayer was completely formed on AgI surfaces resulting in the low dispersibility of AgI particles, the


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ice nucleating activity of AgI was inhibited because the C16TAB monolayer masked the ice nucleating sites on the AgI surfaces. When 1.2 × 10−3 < c < 1.2 × 10−1 mg mL−1, due to the complete adsorption of C16TAB monolayers on AgI surfaces, Tf50 decreased to as low as −37 °C, indicating the high anti-ice nucleating activity of C16TAB (Figure 7b). When c was further increased higher than 1.2 × 10−1 mg mL−1, at which the C16TAB monolayer changed to the bilayer structure on AgI surfaces, Tf50 remained at about −37 °C, indicating that the anti-ice nucleating activity of C16TAB remained high. Figure 3b shows that when c = 1.2 × 10−1 mg mL−1, the ice nucleating activity of AgI was effectively inhibited, regardless of the dispersibility of the AgI particles (open circles and squares in Figure 3b). These results show the ice nucleating activity of AgI was efficiently inhibited either by the monolayer or bilayer adsorption. Finally, C16TAB showed the extremely high anti-ice nucleating activity at relatively high concentrations above the CMC; about 95% of droplets at c = 1.0 mg mL−1 were supercooled below −36 °C (Figures 1b and 2), which is the same temperature level as that for purified water containing no AgI. This high anti-ice nucleating activity might be attributed to the formation of the bilayer structure of C16TAB on AgI surfaces, although in the bilayer structure, the adsorption of the second layer is generally not as stable as that of the first layer.46

CONCLUSIONS Anti-ice nucleating activities of seven surfactants (Table 1) known as anti-INAs that are especially effective against AgI, were examined in emulsified aqueous droplets containing AgI particles. Results showed that C16TAB has the highest anti-ice nucleating activity against AgI among the seven surfactants and also among previously reported anti-INAs, including AFPs,


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polyphenol compounds and synthetic polymers. Especially at 1.0 mg mL−1, C16TAB showed extremely high anti-ice nucleating activity. To understand the anti-ice nucleating activity of C16TAB, the dispersibility of AgI particles in C16TAB solution was observed and the solubility and CMC of C16TAB were measured. The results suggest that the anti-ice nucleating activity is caused by the adsorption of C16TAB on AgI surfaces. Dispersion characteristics of AgI particles indicate that at a concentration of 1.0 mg mL−1, which is higher than the CMC, C16TAB molecules were adsorbed as the bilayer structure. Therefore, the extremely high anti-ice nucleating activity of C16TAB might be attributed to the bilayer structure of C16TAB adsorbed on AgI surfaces. The results obtained in this study clearly showed that C16TAB is extremely effective in inhibiting the ice nucleating activity of AgI. However, the anti-ice nucleating activity of C16TAB depends on the target INA; for example, C16TAB is reportedly not so effective against an ice nucleating bacterium, Erwinia ananas.29 Such dependence is probably due to the different interactions of C16TAB molecules with various solid surfaces. Finally, we should note that C16TAB is also known to modify the morphology of various inorganic crystals.61-65 Such effects of C16TAB as a crystal modifier might be related to the antiice nucleating activity studied here.

ASSOCIATED CONTENT Supporting Information Photograph of C16TAB crystals, results of fraction of unfrozen droplets for various surfactant solutions and purified water, results of surface tension for various surfactant solutions, numbers of droplets analyzed (PDF)


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AUTHOR INFORMATION Corresponding Author *

Tel: +81-29-861-7272. Fax: +81-29-851-7523. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was supported by JSPS KAKENHI (Grant Number 26289050) from the Japan Society for the Promotion of Science (JSPS).

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An Investigation into the Inorganic-Organic Interfacial Interactions. Cryst. Growth Des. 2016, 16, 1463-1471.


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Table of Contents


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Anti-Ice Nucleating Activity of Surfactants Against Silver Iodide in

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