Hydrophobic Effect of Alkyl Groups Stabilizing Self-Assembled

Article pubs.acs.org/JPCB

Hydrophobic Effect of Alkyl Groups Stabilizing Self-Assembled Colloids in Water Nimer Murshid and Xiaosong Wang* Department of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada S Supporting Information *

ABSTRACT: Self-assembling behavior of hydrophobic molecules in water confirms that the hydrophobic effect (HE) of alkyl groups stabilizes the aqueous colloids. Fe(CO){CO(CH2)n−1CH3}(Cp)(PPh3) molecules (FpCn) with a Fp head and alkyl tail are water-insoluble. FpC1 aggregates in water precipitated in hours, whereas the hydrophobic interaction (HI) of long alkyl groups drove FpCn (n = 6−18) molecules assemble into colloids in water. The alkyl tails interacted and stretched as indicated by IR analysis, while the separation of Fp head groups at the surface of colloids was indicated by the redox behavior of Fe elements. The molecules therefore arranged into a proposed “Y” shape with hydration cavities. The IR and cyclic voltammetry (CV) analyses indicated that the longer the alkyl tail, the larger the hydration cavity and the stronger the HI. Both HI and hydrophobic hydration (HH) were identified as the HE stabilizing the colloids in water. This knowledge is applicable for a broad range of disciplines, including biology, chemistry, and material science.

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INTRODUCTION The hydrophobic effect (HE) is a ubiquitous phenomenon in biological systems,1−3 supramolecular chemistry,4 and material science5 but is not fully understood.6−9 Although the hydrophobic interaction (HI) of apolar hydrocarbon groups has been accepted as a driving force for the micellization of surfactants and other aqueous self-assembling behavior,7,10 it remains elusive whether and how this effect contributes to the stability of colloids in water. The micellar cores assembled from alkyl groups have been investigated to probe their HE.11,12 It is generally accepted that a substantial amount of hydrophobic nonpolar groups in the core, especially those near the head groups, are exposed to water.11−15 Particularly, the Menger model describes that the micellar core has a quite small hydrophobic domain, with water deeply penetrating into the structure.16,17 The hydrophobic core is, therefore, considerably hydrated. This model supports the experimental result of aqueous solution of sodium octanoate surfactants measured by small-angle X-ray scattering techniques.18 Ben-Amotz et al. have recently investigated the hydrophobic hydration (HH) of surfactants with various aliphatic lengths using Raman spectroscopy with multivariate curve resolution (Raman-MRC).19 They discovered a significant penetration of water into the micellar core assembled by HI of the aliphatic groups. The longer aliphatic tail created a larger size of the hydration cavity.19 This HE resulting from the aggregation of alkyl groups may be a factor stabilizing aqueous colloids. It, however, cannot be affirmed and investigated © 2017 American Chemical Society

because micelles are a dynamic equilibrium system mainly due to a strong interaction between the headgroup and water. An assembling system with weak interaction of head groups with water will render the HE of the alkyl group prominent, which can serve as a model system facilitating the study. The Fe(CO){CO(CH2)5CH3}(Cp)(PPh3) molecule (FpC6) and its analogues, including small and (macro-) molecules, are hydrophobic but able to assemble in water into vesicles.20−23 Water−carbonyl interaction (WCI), a relatively weak interaction widely existing in biological systems and synthetic molecules,24,25 has been identified as one factor stabilizing the colloids.20−23 However, FpC6 vesicles enlarged over time,21,23 and completely precipitated after a few months. On the basis of this phenomenon, we speculate that the balance between the HE of the hexyl group and WCI of the Fp head groups is crucial for the stability of FpC6 colloid. To adjust this balance, FpCn (n = 1−18) molecules with the same Fp headgroup and various lengths of the alkyl groups were synthesized and their aqueous behavior was subsequently investigated to understand the HE of alkyl groups. Herein, we elucidate the HE of alkyl tails on the colloidal stability in water using hydrophobic FpCn as a model system. We first experimentally confirmed that the longer hydrophobic alkyl tails drove the assembling into more stable colloids. Received: May 8, 2017 Revised: May 31, 2017 Published: June 1, 2017 6280

DOI: 10.1021/acs.jpcb.7b04353 J. Phys. Chem. B 2017, 121, 6280−6285

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The Journal of Physical Chemistry B

dissolved in the DMF solution, and the water solution was subsequently added to prepare the mixed solutions of FpCn in water/DMF. Transmission electron microscopic (TEM) characterization was performed using a Philips CM10 transmission electron microscope (TEM), TEM samples were prepared by dropping the solution onto a carbon-coated copper grid (Cu-300CN, Pacific rid Tech), and the grid was then left to dry at ambient temperature. Cryo-TEM images were obtained using a high voltage (200 kV) field emission FEI Tecnai G2 F20 Cryo-TEM microscope. The cryo-TEM sample was prepared by applying 5.0 μL of an aqueous solution of FpCn (0.1 mg/mL) onto a glow discharged copper grid with holey carbon film (Quantifoil Multi A) and thinned by blotting with a filter paper. The grid was then quickly plunged into a liquid ethane bath and transferred under liquid nitrogen to a Gatan 914 cryo-holder and viewed at −179 °C. Preparation of the Aqueous Colloids of FpCn. The aqueous colloids of FpCn (0.1 mg/mL) were prepared by fast addition of 10.0 mL of distilled deionized water to 1.0 mL of THF solution of FpCn (1.0 mg/mL). THF was then removed through nitrogen bubbling for 90 min. Under the same conditions, the same experiments were performed in D2O and examined using 1H NMR, which suggested that THF is completely removed.

Subsequently, cyclic voltammetry (CV) and IR techniques were used to characterize how the molecules were packed within the colloids. Finally, the influence of the HE on the stability is discussed in the context of the nanostructure of FpCn colloids.

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EXPERIMENTAL SECTION Synthesis of FpCn Molecules. Syntheses and characterizations were performed similar to that reported for FpC6.21 FpCn molecules (n: number of carbons in the alkyl chain) were synthesized via the migration insertion reaction (MIR) of Fp alkyl derivatives in the presence of triphenyl phosphine (PPh3) (Figures 1 and S1).43 The molecules have been well characterized using 31P NMR, 1H NMR, and FT-IR spectroscopies (Figures S2−S5 in the Supporting Information).

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RESULTS AND DISCUSSION FpCn (n = 1−18) molecules (Figure 1a) were synthesized via a migration insertion reaction (MIR) of Fp alkyl derivatives in the presence of triphenyl phosphine (PPh3). Synthesis and characterization are discussed in the Supporting Information (Figure S1−S5 in the Supporting Information). The resultant molecules contain the same headgroup (Fe(CO)(CO−)(Cp)(PPh3)) and alkyl tails with various lengths (Cn−1CH3) (Figure 1a). The head−tail structure of FpCn is reminiscent of conventional surfactants, such as sodium dodecyl sulfate (SDS, CH3(CH2)11SO4Na), but the Fp headgroup only weakly interacts with water via WCI and is water-insoluble.21,22 Consequently, FpCn molecules do not behave like amphiphiles.21,22 Surface tension and DLS measurements indicated that the molecules are non-surface-active and aggregate in water without a detectable critical aggregation concentration (CAC) (Figures S6 and S7).21 The assemblies in water are highly integrated,21 and the 1H NMR spectra of the molecules in D2O only show one signal due to the residual H2O (Figure S8). The hydrophobic nature of the molecules accounts for this solution behavior.20−23 As shown in Figure 1b, FpCn (n = 1 and 18) molecules are THF soluble but water insoluble. By adding water to a THF solution of FpCn, the molecules aggregated at a critical water content (CWC). The CWCs are similar for FpCn (n = 1−18) with various lengths of alkyl groups (ca. 60 ± 5 vol %) (Figure S9).21 This similarity is attributed to the WCI of the acyl CO group that accounts for the solubility of the molecules in THF/water mixed solvents.21−23 As shown in Figure 1b, after removal of THF, FpC1 aggregates precipitate in a few hours, whereas FpC18 molecules form relatively stable colloids. The long tail (C18), which is supposed to consume more unfavorable hydration energy,26,27 apparently enhances the stability of the assemblies. This seemingly unreasonable result illustrates that the HE is a conditional effect in self-assembling depending on assembled structures.6,8,26 This conditional effect can therefore be explored using hydrophobic FpCn molecules with various lengths of alkyl tails as a model system.

Figure 1. (a) Synthesis and chemical structure for FpCn. (b) Photographs illustrating the aqueous behavior of FpC1 and FpC18.

Characterization. 1H and 31P NMR (CDCl3) spectra were recorded on a Bruker-300 (300 MHz) spectrometer at 25 °C. Attenuated total reflection-Fourier transform infrared (ATRFTIR) spectra of MpCn in THF and THF/water solutions were recorded on a Bruker Tensor 27 spectrophotometer with a germanium crystal Pike MIRacleTM ATR Attachment using Pike Technologies. DLS results were obtained using a Zetasizer Nano Series (Nano-S90, Malvern Instruments) with a laser wavelength of 633 nm at a fixed angle of 90°. SLS measurements were performed using a Brookhaven Laser Light Scattering System with a BI-200 SM goniometer. A vertically polarized helium−neon diode laser with a wavelength of 636 nm was used as the light source. Measurements were taken at scattering angles (θ) between 50 and 130° with 10° intervals. Surface tensions of pure water and MpCn aqueous solutions with different concentrations were measured at 24.0 °C using a Data Physics DCAT 21 tensiometer system. Cyclic voltammetry (CV) experiments of FpCn solutions (0.1 mg/ mL) in DMF and water/DMF solutions were performed at room temperature using a DY2000 Multi-Channel Potentiostat (Digi-Ivy Inc.) workstation with a scan rate of 50 mV/s and silver wire as a reference electrode. A water solution of KCl (2 mg/mL) and a DMF solution of tetrabutylammonium perchlorate (TBAP) (2 mg/mL) were prepared. FpCn was 6281

DOI: 10.1021/acs.jpcb.7b04353 J. Phys. Chem. B 2017, 121, 6280−6285

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The Journal of Physical Chemistry B

Figure 2. Time-dependent Dh of FpCn (n = 6−18) colloids in water (0.1 M). The arrows in part a indicate the appearance of precipitates.

Figure 3. Cartoon illustration and CV curves for FpC6 molecules in (a) DMF solution and (b) colloids in DMF/water solutions. (c) Redox coupling (ΔE1/2) of FpCn (n = 6−18) colloids in DMF/water. Error bars represent the standard deviations calculated from five repeated measurements. The absence of reduction peaks in part b is due to the absorption of the colloids on the electrode.

The relative stability of the colloids assembled from FpCn (n = 6−18) molecules was compared. Figure 2 shows the hydrodynamic diameter (Dh) of the colloids as a function of aging time. As shown in Figure 2a, the aqueous colloids of FpC6, FpC8, and FpC10 enlarge over time and start to precipitate at the 7th, 15th, and 30th days, respectively, after the preparation, so the aggregates with longer alkyl tails are more stable. This claim is further supported by the solution behavior of FpC14 and FpC18. As shown in Figure 2b, these two colloids in water are extremely stable. No enlargement in Dh

and no precipitation are observed even after the solutions are aged for 3 months (Figure 2b). The microstructure of hydrophobic alkyl domains in micelles is caused by the HE and has been extensively studied using advanced techniques and computer simulation.11−19 The presence of a transition metal element in the Fp headgroup offers a facile approach to probe this structure using electrochemistry. The CV experiment of FpCn molecules in DMF shows a reversible redox cycle due to the iron element (Figure 3a). Upon the aggregation, the close proximity of the 6282

DOI: 10.1021/acs.jpcb.7b04353 J. Phys. Chem. B 2017, 121, 6280−6285

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Figure 4. ATR-FTIR spectra (C−H stretching region) for FpCn. (a) FpC18 in THF and THF/D2O with 60 vol % D2O. (b) The shift of the absorption (2867 cm−1) due to symmetric stretching of CH2. (c) The relative intensity for the absorption due to the asymmetric stretching of CH2 (2923 cm−1) upon the aggregation of FpCn (n = 6−18) in THF/D2O (60 vol % D2O).

because the alkyl chain in good solvents has a lower trans/ gauche bond ratio and is conformationally disordered.34 Upon the assembling in water, the intensity of this absorption is enhanced significantly (Figure 4a). This enhancement is due to the increase in the trans/gauche bond ratio for the alkyl chain.34 In other words, the alkyl chains are stretched and less fluctuated. This effect of water on alkyl chains has been frequently reported before,35,36 and can be attributed to the HH. The hydration water has a relatively slower relaxation, which restricts the motion of the alkyl chain.37 This hypothesis has been experimentally proved as the dynamic of interfacial hydration water is coupled with the fluctuation of the hydrophobic chains with the same energy barriers.38 The relative intensity of the IR absorption peak (2923 cm−1) for FpCn (n = 6−18) resulting from the HH is compared in Figure 4c. As shown in Figure 4c, the intensity increases as the length of alkyl tails increases, supporting that the degree of HH is larger for the FpCn colloids with longer alkyl tails. The nanostructure-dependent HE of alkyl groups on the colloidal stability is discussed. As shown in Table 1, the Dh for FpCn (n = 1−18) ranges from 93 to 270 nm, which is too large to maintain the hydrogen bonding network of water molecules

adjacent Fp head groups on the surface of colloids results in two oxidation potentials separated by a redox coupling (ΔE1/2) (Figure 3b),21,22 because the oxidation of the iron element next to the oxidized one requires a higher potential.28,29 As shown in Figure 3c, the ΔE1/2 for FpCn colloids decreases as the length of alkyl tails increases. The ΔE1/2 value is inversely proportional to the distance between the adjacent Fp heads (dFp−Fp),28,29 so the dFp−Fp for the assemblies increases as the length of the alkyl tail increases. On the other hand, the alkyl tails hydrophobically interact, which is verified by FTIR analysis. Figure 4 illustrates the IR spectra of FpCn (n = 6−18) molecules in THF and corresponding aqueous aggregates. As shown in Figure 4a, the absorption peak at 2867 cm−1, due to the symmetric stretching of CH2,30−32 shifts to 2860 cm−1, when FpC18 molecules associate into aggregates. This shift is caused by the HI of alkyl chains.30,32 As shown in Figures 4b and S10, the shift resulting from the HI of alkyl tails is also 7 cm−1 for FpC14 but relatively smaller for FpC10 (3 cm−1), FpC8 (2 cm−1), and FpC6 (2 cm−1). This result suggests that the shorter tails weaken the HI. On the basis of the CV and IR analyses, FpCn (n = 6−18) molecules, upon aggregation, arrange into a “Y” shape with the Fp head groups separated and the tails interacted at the other end (Figure 3b). The “Y” shape arrangement creates a water-filled cavity, which hydrates the alkyl tails.19 The cavity formed from longer alkyl tails is larger, which allows more methylene groups to be hydrated. This result is in line with the structure of micellar cores assembled from either carboxylates or trimethylammonium surfactants with various lengths of alkyl chains as reported by Ben-Amotz et al.19 The HH of the tail is also supported by the FTIR experiments. As shown in Figure 4a, the asymmetric stretching for CH2 (2923 cm−1)32,33 is barely observed for FpC18 in THF,

Table 1. DLS/SLS Results for FpCn (n = 1−18) in Water

a

6283

FpCn

Dha (nm)

PDI

Rg/Rh

morphology

FpC1 FpC6 FpC8 FpC10 FpC14 FpC18

270 144 123 93 109 142

± ± ± ± ± ±

0.75 0.03 0.06 0.05 0.05 0.04

1.06 1.09 1.04 1.54 1.46

vesicles vesicles vesicles Gaussian chain Gaussian chain

15 5 5 6 3 3

The concentration is 0.1 M. DOI: 10.1021/acs.jpcb.7b04353 J. Phys. Chem. B 2017, 121, 6280−6285

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The Journal of Physical Chemistry B

Figure 5. (a) Cryo-TEM and (b) conventional TEM images for FpC18 colloids.

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around the interface.26,39,40 This hydrophobic force apparently is larger than the WCI of CO groups, which explains the precipitation of FpC1 in water shortly after the aggregation. However, the HE of alkyl chains in FpCn (n = 6−18) is able to compensate the breakage of the water hydrogen bonding network, resulting in assembled colloids in water. The HI of alkyl chains in FpCn (n = 6−18) drives the molecules to assemble into the colloids with “Y” shaped hydration cavities, resulting in HH of alkyl chains. The longer the alkyl tails, the stronger the HI and HH, so the stability of FpCn (n = 6−18) colloids is enhanced with an increase in the length of alkyl tails (Figure 2). As shown in Figure 2 and Table 1, the Dh for the freshly prepared aqueous solution of FpCn (n = 6−10) decreases (from 144 to 99 nm) with increasing length of alkyl chain. This phenomenon can be rationalized by the faster aggregation rate for the molecule with a longer alkyl group due to stronger HI. However, the Dh for FpCn (n = 14 and 18) colloids is relatively larger than that for FpC10 (Table 1). This incongruous behavior in Dh is attributed to the difference in their morphologies as discussed below. The self-assembled morphologies were analyzed using SLS/DLS and TEM. The Rg/Rh for the freshly prepared FpCn (n = 6, 8, and 10) colloids is ca. 1.0 (Table 1), suggesting that vesicles are formed. The vesicular morphology for FpC6 colloids has been reported before.21 The cryo-TEM images for FpC8 and FpC10 colloids also reveal the similar vesicular morphology (Figure S11). On the other hand, the Rg/ Rh for both FpC14 and FpC18 colloids is ca. 1.50 (Table 1), indicating that Gaussian chains are formed (Table 1). Although it is challenging to image the detailed structure of Gaussian chains, we have performed TEM to reveal the overall shape of the assemblies. As shown in the cryo-TEM image (Figure 5a), the FpC18 colloids appear to be particles with deformed spherical shapes, reminiscent of the images for globular proteins.41,42 The average size of the particles is ca. 135 nm, which is similar to the Dh of the colloids (Table 1). The conventional TEM image of the colloids dried on a copper grid shows no individual nanoparticles. The flexible Gaussian chains tend to fuse into films with connected networks (Figure 5b). The globular Gaussian chains, like proteins, may involve bridging water buried within the random coils2 to suspend the colloids in water. Thus, the difference in morphologies explains that the FpCn (n = 14, 18) colloids (Figure 2b) are much more stable than those assembled from FpCn (n = 6, 8, 10) (Figure 2a).

CONCLUSIONS In summary, the solution behavior of the hydrophobic FpCn (n = 1−18) molecules confirms that the HE of alkyl chains is nanostructure-dependent and contributes to the stability of aqueous colloids. This contribution results from the “Y” shape arrangement of the molecules with long alkyl tails. The HI of the alkyl tails integrates the molecules, while the separation of Fp head groups, at the surface of colloids, allows water to penetrate into the structure. The longer alkyl tail generates stronger HI and a larger hydration cavity, resulting in more stable colloids. The methyl groups in FpC1 molecules are too short to interact with each other, so no hydration cavity is created and the assemblies are not stable in water. The colloids assembled from hydrophobic molecules are therefore an ideal model system, complementary to micellar structures, for the elucidation of HE that is a crucial effect in biological systems, supramolecular chemistry, and material science.1−9

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b04353. Synthesis scheme, supporting NMR and IR spectra, surface tension measurements, and DLS results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nimer Murshid: 0000-0002-8329-239X Xiaosong Wang: 0000-0002-6415-4768 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Nature Science and Engineering Research Council of Canada (RGPIN-2016-04497) and the University of Waterloo are acknowledged for financial support.

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