Soft Confinement Effects on Dynamics of Hydrated ... - ACS Publications

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Soft Confinement Effects on Dynamics of Hydrated Gelatin P. M. Geethu,† Indresh Yadav,‡ S. K. Deshpande,§ V. K. Aswal,‡ and D. K. Satapathy*,† †

Soft Materials Laboratory, Department of Physics, IIT Madras, Chennai 600 036, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India § UGC-DAE Consortium for Scientific Research, R-5 Shed, BARC, Trombay, Mumbai 400085, India ‡

S Supporting Information *

ABSTRACT: Soft materials under geometrical confinement are ubiquitous and known to exhibit fascinating properties, sometimes even antagonistic to their bulk counterpart. Here, we explore the structural relaxation dynamics (α-relaxation) of the biopolymer gelatin dissolved in water (bulk form) and under soft spatial confinement using dielectric relaxation spectroscopy over a wide frequency range starting from 1 Hz up to 2 GHz. The gelatin−water mixture (hydrated gelatin) is geometrically restricted by the soft fluctuating surfactant monolayer of water−AOT (sodium bis(2-ethylhexyl)sulfosuccinate)-n-decane reverse microemulsions (MEs), where the core size of microemulsion droplets varies from 3.7 to 5.0 nm. The stability of the droplet phase of microemulsion after the incorporation of gelatin is confirmed by the small-angle neutron scattering (SANS) experiment. Notably, the hydrated gelatin in soft confinement exhibits faster relaxation dynamics in comparison to its bulk counterpart, and it further gets accelerated with reduction in the confining volume. Our combined results imply that the properties of confining boundary strongly influence the dynamics of the enclosed material.

1. INTRODUCTION The effect of spatial confinement on the dynamics of soft materials has gained considerable momentum over the past decade owing to their manifold applications in science and technology.1,2 Spatial confinement induced dynamics of soft matter is also very useful from the basic science point of view in understanding the intriguing phenomena, for instance, confinement induced crystallization, self-assembly, glass transition, and cooperative dynamics.1,3 A variety of confined systems such as organic and inorganic liquids, water, polymers, and biological systems have been investigated in conjunction with an equally impressive array of confining geometries like, porous silica, clays, reverse microemulsions, and thin films.4 In most of these cases, dramatic changes in dynamic properties of materials have been observed in comparison to their bulk counterpart.5 In addition to the dimensionality and finite size effects, the interfaces, surfaces, and their interaction with the confined system are also found to play a decisive role in controlling the dynamics of soft materials in confinement.6,7 The quest for understanding the nature of confining surfaces on the dynamics of materials resulted in extensive investigations on molecular liquids and polymers enclosed in static-hard to fluctuating-soft boundaries. Hard confinement has been realized for polymers in the form of thin films deposited on solid substrates and for molecular liquids in narrow pores and spherical cavities.8−11 Slowdown of structural relaxations for fluids confined between two hard walls in comparison to its bulk has been recently reported.12 On the other hand, prominent examples of soft confinement are polymers in the form of free-standing films © 2017 American Chemical Society

and molecular liquids located in the core of the reverse microemulsion droplets.7,13−15 Reverse microemulsions stabilized by surfactant molecules present an efficient system to explore the consequences of soft confinement, where the polar nanoscopic core is surrounded by the rapidly fluctuating surfactant monolayer.7,14−16 The effects of soft confinement on structural relaxation dynamics of molecular liquids like propylene glycol,7 glycerol,15 etc., near their glass transition temperature have been investigated using different techniques such as quasielastic neutron scattering (QENS),14,15 small-angle neutron scattering (SANS),15 triplet state solvation dynamics,7,16 and dielectric relaxation spectroscopy (DRS).9,17 It has been reported that the supercooled nanodroplets of propylene glycol suspended in a more fluid environment (in soft confinement) display an accelerated relaxation dynamics, associated with a reduction in its glass transition temperature Tg (ΔTg = −7 K) in comparison to its bulk counterpart. Interestingly, the α-relaxation time of propylene glycol in soft confinement is observed to be 1.5 decades faster in comparison to its bulk state.7 Further, the dynamics of supercooled glycerol in soft confinement within reverse microemulsion droplets has been investigated. Indeed, the glycerol in soft confinement displays faster relaxation dynamics in comparison to the bulk, and this acceleration in dynamics becomes more pronounced on increasing the degree Received: July 18, 2017 Revised: August 16, 2017 Published: August 30, 2017 6518

DOI: 10.1021/acs.macromol.7b01521 Macromolecules 2017, 50, 6518−6528

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Macromolecules of confinement.15 Moreover, a crossover from Vogel−Fulcher to an Arrhenuis-like temperature dependence of the relaxation times has been reported for glycerol when it changes from bulk phase to the soft-confined state.15 Although the relaxation dynamics of molecular liquids in soft confinement has been studied widely, a systematic investigation on the effect of soft confinement on structural relaxations of polymer molecules, especially when the polymer is confined in the core of the reverse microemulsion droplet surrounded by a flaccid surfactant layer, is still lacking. The aim of this work is to explore the consequences of soft confinement on relaxation dynamics of polymer molecules. We investigate the soft confinement effect on a biopolymer gelatin for the following reasons: (i) hydrophilic nature of gelatin ensures its water solubility, (ii) gelatin exhibits structural relaxation (αrelaxation) in the measured frequency range, and (iii) gelatin polymer can be confined inside the nanodroplets of microemulsions since its Rg is comparable to the ME droplet size. In this paper, we present the effect of soft confinement on αrelaxation of hydrated gelatin polymer, where the soft confinement is achieved by the fluctuating surfactant shell of reverse microemulsions. Strikingly, hydrated gelatin in soft confinement is found to display faster relaxation dynamics. Moreover, with increase in degree of confinementachieved by tuning the core radius of water dropletsthe relaxation dynamics associated with the cooperative movement of gelatin and water is further accelerated, which is in line with the previous reports for molecular liquids in soft confinement.7,15 Besides, the concomitant effects of gelatin polymer chains on polarization mechanisms and dielectric relaxation processes of microemulsions have also been investigated and are discussed in detail. Furthermore, we explore the structural properties of pure and gelatin loaded reverse microemulsions using SANS. The analysis of neutron scattering profiles suggests that the droplet structure remains intact after the incorporation of gelatin polymer. The paper is organized as follows: First we discuss about the structural relaxations in the gelatin−water mixture investigated using DRS. The distinct polarization mechanisms and the associated dielectric relaxation processescore and shell relaxationsin water−AOT (sodium bis(2-ethylhexyl)sulfosuccinate)-n-decane reverse microemulsions are also presented in this section. Next, we turn to discuss the influence of gelatin polymer on structure and dielectric relaxation dynamics of reverse microemulsions probed using SANS and DRS. Finally, we discuss the effect of soft fluctuating surfactant shell boundary and the degree of confinement on the relaxation dynamics of gelatin polymer chains in water (hydrated gelatin).

concentrations. From geometrical considerations, the radius of the water core of AOT stabilized microemulsions is estimated as18,19

For all the samples in the present study, the volume fraction of microemulsion droplets was kept constant at ϕ = 0.1. Droplets having different sizes were prepared by varying W from 25 to 35. In SANS experiments a better contrast is obtained by replacing H2O with D2O (Sigma-Aldrich, 99.9%). Gelatin-loaded microemulsions were prepared by mixing the stock solution of AOT in decane and a solution of gelatin in water at T = 60 °C and then cooled down to room temperature. Gelatin polymer purchased from Alfa Aesar, having a molecular weight of 40 000 g/mol and bloom strength of 150, was used without further purification. The radius of gyration of a gelatin molecule Rg obtained from SANS measurement is about Rg = 27.3 ± 3.1 Å, which is smaller than the water core radius of reverse microemulsions (measured SANS profile together with fit for gelatin in D2O is shown in Figure S1 of the Supporting Information). The concentration of gelatin polymer in reverse microemulsion is quantified as the average number of polymer chains per droplet given by Z = Npolymer/Ndroplet. For all gelatin-loaded microemulsion samples, the average number of polymer chains per droplet was kept constant at Z ≃ 1. 2.2. Dielectric Relaxation Spectroscopy. Dielectric relaxation spectroscopy (DRS) is an efficient technique for investigating the properties of the individual constituents of a complex system by monitoring the cooperative processes at the molecular level. DRS is sensitive to the dynamics of dipolar species as well as the localized charges. In DRS, the complex dielectric permittivity ε*(ω) is derived by measuring the complex impedance Z*(ω) of the sample, which are related by

ε*(ω) = ε′(ω) − iε″(ω) =

σ *(ω) = iωε0ε*(ω)

Vtotal

=

Vwater + VAOT , Vwater + VAOT + Vdecane

(2)

(3)

where ε0 is the dielectric permittivity of a vacuum. The dielectric measurements were performed in a frequency range starting from 1 Hz up to 2 GHz at different temperatures, combining two different equipment. In the frequency range from 1 Hz to 1 MHz, the measurements were conducted using a high-resolution Alpha dielectric analyzer (Novocontrol, Germany) with active sample cell ZGS as the test interface. A liquid sample cell supplied by Novocontrol (BDS 1308) with a diameter of 20 mm and thickness of 100 μm (obtained using silica spacers) was used for low-frequency measurements. For high frequencies starting from 1 MHz up to 2 GHz, the measurements were carried out by means of a coaxial reflectometer (Novocontrol, Germany) based on the Agilent E4991 analyzer. The microemulsion samples were mounted between two gold-plated electrodes having a diameter of 3 mm and a spacing of 100 μm (obtained using silica spacers). The Novocontrol Quatro Cryosystem with a temperature stability of 0.1 K is employed for temperature control, where nitrogen gas is used as the heating agent. Rigorous calibration procedures were followed in order to obtain correct values for Z*(ω). 2.3. Small-Angle Neutron Scattering (SANS). SANS has been widely used for characterizing self-assembled systems such as micelles and microemulsions.20,21 In the SANS experiment, the coherent differential scattering cross section per unit volume (dΣ/dΩ) is measured as a function of scattering vector Q. For a collection of monodisperse, interacting particles (water droplets of microemulsion) dΣ/dΩ is given as22

2.1. Sample Preparation. Water-in-oil reverse microemulsions were prepared by mixing suitable amounts of AOT surfactant (purity 98%, purchased from Sigma-Aldrich) and n-decane (purity 99%, purchased from Alfa Aesar), into which deionized Millipore water (σ = 5.5 × 10−8 S/cm) was added and shaken well for several minutes until single-phase and optically clear samples were obtained. The composition of water−AOT−decane microemulsions is characterized by the parameters W and ϕ, where W denotes the molar ratio of water to AOT surfactant given as W = n[H2O]/n[AOT] and ϕ represents the volume fraction of droplets, defined as Vdroplets

−i ωZ*(ω)C0

where ω is the angular frequency, C0 is the empty cell capacitance, and ε′ and ε″ are the real and imaginary parts of complex dielectric permittivity which are related by the Kramers−Kronig relation. Fourier correlation analysis is employed to obtain Z*(ω). The complex conductivity σ*(ω) of the sample is obtained from ε*(ω) using the relation

2. MATERIALS AND METHODS

ϕ=

(1)

R ≈ 1.4W

where V is the volume of corre-

sponding component. The size of the water droplet core depends linearly on W and is independent of ϕ for moderate droplet 6519


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Figure 1. (a) Real (ε′) and imaginary (ε″) part of complex dielectric permittivity (ε*) as a function of applied frequency for 18 wt % gelatin in water at 20 °C. The solid lines represent the calculated values for ε′ and ε″ according to eq 7. (b) Characteristic relaxation time τ fitted using eq 8 as a function of temperature for hydrated gelatin (18 wt%). Symbols represent the structural α-relaxation time of hydrated gelatin, and the line represent the VF fit. dΣ (Q ) = ϕV (ρP − ρS )2 P(Q )S(Q ) + B dΩ

droplets. Data were corrected for background and empty cell contributions and normalized to absolute cross-sectional units using the standard procedure. SASfit software was used for fitting the measured scattering profiles.25

(4)

where V is volume of individual particle (water droplet) and ϕ is their volume fraction. ρP and ρS are scattering length densities of particles and solvent, respectively. P(Q) is the intraparticle structure factor representing the interference of neutrons scattered from different parts of the same object, and it gives the information about shape and size of the particles. S(Q) is the interparticle structure factor coming from the interference of neutrons scattered from different particles, and it describes the interactions between particles and their spatial arrangements. The constant term B represent incoherent scattering. Here, the nonlinear least-squares method has been used to fit the experimental scattering data with different theoretical models. The intraparticle structure factor P(Q) is related to the single particle form factor F(Q) as P(Q ) = ⟨|F(Q )|2 ⟩

3. RESULTS AND DISCUSSION 3.1. Dielectric Relaxation Investigations on Hydrated Gelatin and Water−AOT−n-Decane Reverse Microemulsions. The primary goal of this work is to investigate the effect of soft confinement on dielectric relaxation dynamics of gelatin−water mixture, where soft confinement is achieved via the fluctuating surfactant boundary provided by the microemulsions. Prior to the discussion on relaxation dynamics of gelatin-loaded reverse microemulsions (GLMEs), we address the dielectric behavior of gelatin dissolved in water, i.e., gelatin−water mixture in bulk state. Additionally, the dielectric relaxation mechanisms in pure/polymer-free reverse microemulsions are also presented in this section. The real (ε′) and imaginary (ε″) parts of complex dielectric permittivity (ε*) as a function of frequency for 18 wt% gelatin−water mixture at 20 °C are shown in Figure 1a. A broad dielectric relaxation process is observed centered in the hundreds of kilohertz range and is characterized by fitting with Havriliak−Negami relaxation function26,27 given as

(5)

When the scatterers are spherical particles of radius R, F(Q) is expressed as F(Q ) =

3{sin(QR ) − QR cos(QR )} (QR )3

(6)

The interparticle structure factor S(Q) describes the correlation between the particles and for noninteracting dilute systems S(Q) = 1. SANS profiles for pure and gelatin loaded microemulsions are fitted by assuming S(Q) ∼ 1 due to the following reasons. (i) In the measured SANS profiles for pure and gelatin-loaded microemulsions, there is no clear signature of S(Q). (ii) The scattering profile for ϕ = 0.1 is found to be scaling with the measured SANS profile for a lower concentration ϕ = 0.05 (Figure S2). Consequently, SANS profiles have been fitted only by taking the contribution of P(Q). Polydispersity of droplets is taken into account while modeling the SANS data by assuming a log-normal distribution of droplet sizes. The contribution toward scattering from hydrated gelatin polymer is negligible compared to water core, and hence the scattering from polymer is neglected while modeling the SANS profiles.23 SANS experiments were carried out using SANS-I instrument at the Dhruva Reactor, Bhabha Atomic Research Centre (BARC), Mumbai, India.24 The mean wavelength (λ) of the incident neutron beam was 5.2 Å with a resolution Δλ/λ of 15%. The scattering data were collected in the wave vector transfer, Q range of 0.017−0.35 Å−1 (Q = 4π sin(θ/2)/λ, where θ is the scattering angle). Samples were held in HELMMA quartz cells having thicknesses of 2 mm, and a onedimensional (1-D) position sensitive detector was used to record the scattering data. The temperature of the samples was kept constant at 30 °C for all the measurements. Water−AOT−n-decane microemulsions were prepared by replacing H2O with D2O. Thus, the scattering is nearly exclusively due to D2O core of microemulsion

ε*(ω) = ε∞ +

σ u Δε + DC + EPn α β i ωε ω (1 + (iωτ ) ) 0

(7)

where ω is the angular frequency, ε∞ is the limiting value of high-frequency dielectric permittivity, Δε is the dielectric strength, τ is the relaxation time, α is the asymmetric broadening parameter (0 < α ≤ 1), β is the symmetric broadening parameter (0 < αβ ≤ 1), σDC is the dc conductivity, and uEP and n are the parameters describing the electrode polarization. Further, the influence of temperature on the observed dielectric relaxation process for gelatin−water mixture is investigated, and the characteristic relaxation time is found to be decreasing with the increase in temperature. Figure 1b shows the dielectric relaxation time for gelatin−water mixture at different temperatures ranging from T = 276 K (∼3 °C) to T = 329 K (∼56 °C) and is found to be following the phenomenological Vogel−Fulcher (VF) behavior given by27,28

⎛ A ⎞ τ = τ0 exp⎜ ⎟ ⎝ T − T0 ⎠ 6520

(8) DOI: 10.1021/acs.macromol.7b01521 Macromolecules 2017, 50, 6518−6528

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Figure 2. Real (ε′) and imaginary (ε″) parts of complex dielectric permittivity as a function of applied frequency for microemulsion with W = 25 and ϕ = 0.1 at two different temperatures below percolation temperature (TP ∼ 39 °C). (a) At T = 20 °C and (b) at T = 35 °C. The arrows indicate core and shell/cluster relaxations, and the solid lines represent the calculated values for ε′ and ε″ according to eq 9.

where T is the temperature and τ0, A, and T0 are the empirical VF parameters. The VF model is the most widely accepted empirical temperature law which is capable of reproducing the molecular dynamics of liquids as a function of temperature (τ(T) behavior) in the vicinity of glass transition. From the literature, it can be inferred that the glass transition temperature of hydrated gelatin is Tg ∼ −73 °C.27,29,30 Thus, in the measured temperature range the hydrated gelatin polymer chains are well-above their glassy state, and therefore a hierarchy of molecular relaxations from fast (secondary βrelaxation) to slow (main chain α-relaxation) could possibly occur. However, it has been reported previously that the dielectric relaxation observed for the gelatin−water mixture (Figure 1a,b) originates from the cooperative motion of gelatin and water and is designated as the α-relaxation in hydrated gelatin.27 To paraphrase, the observed α-relaxation can be interpreted as the micro-Brownian motions of main-chain segments of hydrated gelatin. In the remainder of this section, the results related to the different polarization mechanisms and associated dielectric relaxation dynamics in water−AOT−n-decane reverse microemulsions are described. The typical dielectric spectra of reverse microemulsion with W = 25 and ϕ = 0.1, at two different temperatures (below the percolation threshold temperature, TP) given as T = 20 °C and T = 35 °C are shown in Figures 2a and 2b, respectively. Two distinguishable dielectric relaxations are observed in the measured frequency range as indicated by the arrows in Figure 2a,b and are known in the literature as slow core and fast shell/cluster relaxations.18,31−34 The core relaxation having a rather low relaxation strength is visible only in the real part of dielectric permittivity since the imaginary part of dielectric function is dominated by DC conductivity contribution. The shell relaxation having a strong dielectric strength takes place around 0.2 GHz and can be seen in both real and imaginary part of dielectric permittivity. The Cole−Cole equation is used to model the core and shell relaxations31,32,35 and is given as ε*(ω) = ε∞ + +

where Δεcore, τcore, αcore and Δεshell, τshell, αshell represent the dielectric strength, relaxation time, and asymmetric broadening parameter (0 < α ≤ 1) for the dielectric relaxation of the core and the shell, respectively. The core and shell relaxations are associated with distinct polarization processes that occur in microemulsions and are discussed in the literature.18,31−34 In reverse microemulsions, the ionic head of AOT surfactant having a sodium sulfonate (Na+SO3−) group will be oriented toward water core where the headgroup dissociates as Na+SO3− ⇌ Na+ + SO3−. Subsequently, Na+ ions will diffuse into water, and SO3− ions will remain as attached to the surfactant head. In the presence of an external electric field, an apparent dipole moment will be created due to the asymmetrical distribution of Na+ counterions at surfactant−water interface, leading to a Maxwell−Wagner type relaxation which is referred to as core relaxation in microemulsions.32,36 According to Maxwell−Wagner theory, the relaxation time of the interfacial polarization is given as37,38 ⎛ ε + ε2 ⎞ τMW = ε0⎜ 1 ⎟ ⎝ σ1 + σ2 ⎠

where εi and σi represents the dielectric permittivity and DC conductivity of the respective medium. Considering a conductivity value of σcore ∼ 10−5 S/cm for the water core and a negligible conductivity for the oil continuous medium, Maxwell−Wagner relaxation time is calculated using eq 11 as τMW ≃ 10−4−10−5 s, which corresponds to a frequency of f MW ≃ 103−104 Hz. This further justifies the occurrence of core relaxation in connection with the interfacial polarization occurring in microemulsions. The high-frequency dielectric dispersion associated with the shell relaxation is thought to arise from the rotational diffusion of AOT ion pairs at the surfactant−water interface. Here, the correlated movement of the anionic head groups with respect to the apolar part of the surfactant molecules at the water droplet interface imparts a fluctuating electric dipole moment to the microemulsion droplets.31,36,39 Further discussions regarding the microscopic origin of shell relaxation suggest that the dynamics of the surfactant monolayer is highly correlated to the structure, phase behavior, and interparticle interactions in reverse microemulsions.33,36,40 Thus, in the present study, where the interfacial self-assembled surfactant monolayer is been perturbed by the introduction of gelatin polymer, a comprehensive analysis on shell relaxation dynamics

Δεcore Δεshell αcore + 1 + (iωτcore) 1 + (iωτshell)αshell

σDC u + EPn iωε0 ω

(10)

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Figure 3. (a) Dielectric strength (Δεshell) and (b) characteristic relaxation time (τshell) of shell relaxation as a function of reduced temperature (|TP − T|/TP) below TP for microemulsion with W = 25 and ϕ = 0.1.

water core of microemulsions. It is evident that AOT surfactant shell has very small contrast (ρAOT − ρn‑decane)2 compared to D2O droplets (Table S1), and thus the scattering from AOT surfactant shell has a negligible contribution. The model functions described in section 2.3 are used for fitting the scattering profiles for pure and GLMEs. The analysis of SANS profiles reveal that the spherical droplet structure remains intact after the incorporation of gelatin polymer chains. The mean radius (R) of the water core and the polydispersity of droplets evaluated from the best fit of measured SANS profiles are given in Table 1. The width of the form factor for spherical droplets is inversely proportional to the size of the droplet. A noticeable shift for the width of form factor toward lower scattering vector Q, on introducing gelatin polymer into the microemulsion droplet is observed, which signifies the increase in droplet radius with the incorporation of gelatin polymer. Moreover, a slight enhancement in scattering intensity in the low-Q region is observed for GLMEs and is attributed to the increase in droplet volume due to the presence of gelatin. In order to highlight the small changes in the low-Q region due to the slight increase in droplet volume, the scattering profiles are shown in semilogarithmic scale in respective insets of Figure 4. Further, the polydispersity of droplets is found to increase on introducing gelatin polymer into the droplet core. It is worth noting that the observed increase in polydispersity is higher for smaller droplets and is in line with the previous reports for PEG-loaded reverse microemulsions.23,43 The increase in droplet radius and polydispersity on introducing gelatin strongly suggest that the polymer chains are located inside the microemulsion droplets. Specifically, it can be stated that the gelatin polymer chains are confined by the fluctuating surfactant monolayer (soft confinement). Next, the influence of the biopolymer gelatin on dielectric properties of reverse microemulsions is investigated. Initially, the conventional direct current (DC) electrical conductivity (σ′) spectra as a function of temperature in pure and GLMEs have been studied. It has been fairly established in the literature that with the increase in temperature microemulsions undergo percolation transition, where the water droplets come in close contact and form transient droplet clusters.44−47 During this process, hopping of charges across the surfactant monolayer occurs, resulting in a monumental increase in the electrical conductivity value. The variation of conductivity σ′ measured at 104 Hz as a function of temperature for a pure and GLMEs with ϕ = 0.1 and W = 30 is shown in Figure 5. The percolation

in GLMEs is desirable and is presented in the upcoming section (section 3.2). Further, the effects of temperature on core and shell relaxations are investigated below TP. The core relaxation shows only a weak temperature dependence, whereas the shell relaxation exhibits a significant variation with the evolution of temperature. The dielectric strength and relaxation time for shell relaxation are plotted as a function of reduced temperature |TP − T|/TP below TP for microemulsion with W = 25 and ϕ = 0.1 and are shown in Figures 3a and 3b, respectively. A substantial increase in dielectric strength is observed with the increase in temperature and can be attributed to the net increase in dipole moment associated with the water droplets due to the formation of transient clusters. In addition, a considerable increase in relaxation time with the increase in temperature is noted which indicate the slower movement of AOT head groups owing to the increased intermolecular correlations induced by the temperature-dependent clustering of droplets.41,42 Having been investigated the distinct polarization mechanisms and associated dielectric relaxation processes in hydrated gelatin and reverse microemulsions exclusively, next we proceed to explore the influence of soft-fluctuating surfactant shell boundary on α-relaxation dynamics of hydrated gelatin and the concomitant effects of gelatin polymer on polarization mechanisms in microemulsions itself. Accordingly, gelatin polymer is introduced into the microemulsion samples with different droplet sizes (droplet radius > Rg of gelatin), and the polymer molecules are found to be completely dissolved in the water core of the reverse microemulsions. Neither the precipitation of the added gelatin nor the turbidity of microemulsions is noticed upon addition of gelatin polymer (photographs showing the optically clear pure and GLMEs are given in Figure S3). 3.2. Structural and Dielectric Relaxation Investigations on Gelatin-Loaded Reverse Microemulsions. Before exploring the dielectric behavior of gelatin-loaded microemulsions, we attempt to reveal the structural properties of microemulsions in the presence of gelatin polymer which has been probed using SANS. The measured SANS profiles together with the fitted model curves for pure and GLMEs with droplet volume fraction ϕ = 0.1 and water-to-surfactant molar ratio W = 25, 30, and 35 are shown in Figures 4a, 4b, and 4c, respectively. In all these experiments D2O is used instead of H2O to obtain the contrast (ρD2O − ρn‑decane)2, mostly from the 6522


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Table 1. Droplet Radius (R) and Polydispersity of Pure (Z = 0) and Gelatin-Loaded (Z = 1) Reverse Microemulsions, with Water-to-Surfactant Molar Ratio W = 25, W = 30, and W = 35 As Estimated from SANS molar ratio of waterto-surfactant (W) 25 30 35

no. of polymer chains per droplet (Z) 0 1 0 1 0 1

mean droplet radius R (Å) 37.5 42.6 43.7 47.2 47.6 49.5

± ± ± ± ± ±

0.5 0.6 0.5 0.7 0.7 0.8

polydispersity index 0.24 0.33 0.24 0.32 0.25 0.31

± ± ± ± ± ±

0.04 0.05 0.03 0.05 0.04 0.04

Figure 5. Electrical conductivity σ′ as a function of temperature measured at 104 Hz for pure and gelatin loaded reverse microemulsions with water-to-surfactant molar ratio W = 30 and volume fraction of droplets ϕ = 0.1. The inflection point marks the percolation threshold TP.

frequency dependences of real (ε′) and imaginary (ε″) parts of complex dielectric permittivity ε* for pure and GLMEs are analyzed in detail. The dielectric relaxation of microemulsions occurring at higher frequency starting from 1 MHz up to 2 GHz is found to be significantly affected by the addition of gelatin polymer. The high-frequency dielectric loss (ε″) peak for pure and gelatin-loaded microemulsions with ϕ = 0.1 and W = 25, 30, and 35 are shown in Figures 6a, 6b, and 6c, respectively. Interestingly, a shift of the dielectric relaxation peak toward lower frequencies, increase in the intensity of dielectric peak and a dramatic broadening in the frequency domain upon incorporating the gelatin polymer are noted. These observations provide the main impulse for the subsequent investigations on dielectric mechanisms occurring in GLMEs in a comprehensive manner. The experimental dielectric spectra of GLMEs are fitted to eq 9 by using nonlinear least-squares minimization procedure, and the fitting parametersΔεshell, τshell, and αshellcharacterizing the relaxation process in the gigahertz frequency domain are obtained. In particular, the broadening parameter for shell relaxation αshell is found to be within the range 0.6−0.7, which suggests the possibility that the spectra could result from the partial overlapping of more than one dielectric relaxation processes.36,48 Consequently, the dielectric spectra for GLMEs are fitted by assuming the contribution from two relaxation processes as shown exclusively in eq 11. Here, it is worth noting that the broadening and shifting of dielectric relaxation spectra, similar to the ones observed here, are attributed to the concomitant effect of more than one relaxation processes.36,48

Figure 4. A log−log plot showing measured SANS data together with fits (solid lines) for (a) pure (Z = 0) and gelatin-loaded reverse microemulsions (Z = 1), with water-to-surfactant molar ratio W = 25; (b) pure (Z = 0) and gelatin-loaded reverse microemulsions (Z = 1) with W = 30; (c) pure (Z = 0) and gelatin-loaded reverse microemulsions (Z = 1) with W = 35. Volume fraction of droplets is kept fixed as ϕ = 0.1 for all the three cases. Inset shows the respective semilogarithmic plots.

threshold temperature, evaluated as the inflection point in the temperature-dependent conductivity curve, is found to be decreasing when gelatin polymer is added (ΔTP = −9.9 °C). This observation is expected by considering the significant increase in droplet size on the incorporation of gelatin polymer as obtained from SANS analysis. Indeed, the percolation behavior on GLMEs further support that the gelatin polymer is modifying the dynamics of water droplets. After establishing the percolation dynamics in the microemulsion system loaded with gelatin polymer, next we discuss the dipolar mechanisms and dielectric relaxation processes occurring in pure and GLMEs. To be more precise, the 6523


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Figure 6. Imaginary part of complex dielectric permittivity (ε″) as a function of applied frequency for (a) pure (Z = 0) and gelatin-loaded reverse microemulsions (Z = 1), with water-to-surfactant molar ratio W = 25; (b) pure (Z = 0) and gelatin-loaded reverse microemulsions (Z = 1) with W = 30; (c) pure (Z = 0) and gelatin-loaded reverse microemulsions (Z = 1) with W = 35. Volume fraction of droplets is kept fixed as ϕ = 0.1 for all the three cases.

Figure 7. Typical dielectric spectra for gelatin containing microemulsions with ϕ = 0.1 and (a) water to surfactant ratio W = 25, (b) W = 30, and (c) W = 35. Concentration of gelatin polymer in microemulsion is kept fixed as one polymer chain per droplet (Z = 1) for all the cases.

However, the αshell for pure reverse microemulsions without added gelatin is found to be in the range 0.9−0.98, which excludes the possibility for contribution from more than one relaxation toward the dielectric process. The typical dielectric spectra together with fits for pure and gelatin-loaded microemulsions with ϕ = 0.1 and W = 25, 30, and 35 are shown in Figures 7a, 7b, and 7c, respectively. The corresponding relaxation function is given as Δεcore Δε1 αcore + 1 + (iωτcore) (1 + (iωτ1)α1 )β1 σDC u Δε2 + + EPn α2 + ω 1 + (iωτ2) iωε0

associated with peak:2 are found to be decreasing with increase in W as shown in Figure 9, which is a characteristic behavior of shell relaxation in microemulsions.31,32 This further confirms that peak:2 represents the correlated movement of anionic head groups in the AOT surfactant shell. Next, we address the origin of the dielectric dispersion at lower frequency (peak:1). Here, it is plausible to assume that the peak:1 in the deconvoluted dielectric spectra (Figure 8) arises from the relaxation process occurring due to the dipolar fluctuations in hydrated gelatin polymer present in the intramicellar domain. Further investigations in this direction (regarding peak:1) will allow to perceive the dynamics of gelatin−water mixture in soft confinement. Previous reports on reverse microemulsions with glass-forming liquids as the polar component suggest that the intramicellar liquids exhibit an accelerated dynamics in soft confinement induced by the fluctuating surfactant shell of microemulsions.4,7 Therefore, the dynamics of gelatin in soft confinement will indeed be interesting and is discussed in the upcoming section (section 3.3). Further, the effect of gelatin polymer concentration (Z) on dielectric relaxations of microemulsions is examined. The dielectric strength (Δεcore) and average relaxation time (τcore) of core relaxation are found to be decreasing with increasing gelatin polymer content, and a similar effect is been previously reported for PEG-loaded reverse microemulsions.18 It can be interpreted as with the increase in concentration of gelatin polymer chains, the number of interfaces in the droplet core

ε*(ω) = ε∞ +

(11)

where a further relaxation term described by the Havriliak− Negami (H−N) function is added. The parameters Δε1, τ1, α1, and β1 represent the dielectric strength, relaxation time, and asymmetric and symmetric broadening parameters corresponding to the H−N function. The deconvoluted dielectric spectra illustrating the presence of two distinct relaxations for GLMEs with ϕ = 0.1 and W = 25, 30, and 35 are shown in Figure 8. Here, the first peak at lower frequency (marked as peak:1) represents the H−N function, and peak:2 occurring at higher frequency portrays the Cole−Cole function. It is evident from Figure 8 that the major contribution toward the high-frequency dielectric dispersion is from peak:2 representing the Cole−Cole function and is attributed to shell relaxation in gelatin-loaded microemulsions.18 The dielectric strength and relaxation time 6524


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Figure 9. Dielectric strength Δεshell and characteristic relaxation time τshell corresponding to the shell relaxation in gelatin-loaded microemulsions (represented by peak:2 in Figure 8) as a function of waterto-surfactant molar ratio W. Errors in Δεshell and τshell are smaller than the symbol size and are not shown. The lines are guide for the eyes.

relaxation time associated with core relaxation will decrease as shown in Figure 10a. The concentration of gelatin polymer on surfactant shell relaxation has a converse effect, i.e., the dielectric strength (Δεshell) and relaxation time (τshell) are increasing with amount of gelatin polymer added into the microemulsion. From scattering experiments, it has been reported earlier that the adsorption or anchoring of gelatin onto the AOT monolayers is negligible.49 However, in the presence of gelatin molecules the extent of hydration of polar head groups of AOT surfactant will be considerably affected, owing to the fact that there will be competing interactions for water molecules between the polar cites of gelatin polymer chains and AOT surfactant molecules. This will indeed hinder the free rotational diffusion of anionic groups of AOT surfactant. Accordingly, the relaxation time associated with the shell relaxation will increase. Moreover, in the presence of gelatin polymer the movement of AOT ion pairs is expected to become highly correlated since the water molecules will be almost equally distributed among all the polar sites of AOT surfactant. Thus, the dielectric strength of shell relaxation will increase with increasing gelatin polymer content as shown in Figure 10b. The behavior of shell relaxation with the evolution of temperature from 20 to 35 °C for gelatin-loaded reverse microemulsions is given in Figure S4. The dielectric strength and relaxation time are increasing with the increase in temperature as anticipated for the shell relaxation in microemulsions18 and are shown in Figure S5. Next, we discuss the influence of the fluctuating soft boundary provided by the surfactant monolayer on the relaxation dynamics of hydrated gelatin polymer chains. 3.3. Soft Confinement Effects on Relaxation Dynamics of Hydrated Gelatin Polymer. The dielectric relaxation dynamics of hydrated gelatin in bulk state is previously discussed in section 3.1. A substantially broader relaxation is observed in the frequency range starting from 104 Hz up to 106 Hz and is presented in relation with the α-relaxation of hydrated gelatin. Now, we turn to discuss the effect of soft confinement induced by the fluctuating surfactant shell on the structural relaxation dynamics of hydrated gelatin, where gelatin polymer chains are confined inside ME droplets having core radius ranging from 3.7 to 5.0 nm. Interestingly, the dielectric dispersion corresponding to the α-relaxation in hydrated gelatin in its bulk phase, which appeared in the frequency range 104−

Figure 8. Imaginary part of dielectric permittivity ε″ as a function of frequency for gelatin containing microemulsions with ϕ = 0.1 and (a) water to surfactant ratio W = 25, (b) W = 30, and (c) W = 35. Concentration of gelatin polymer in microemulsion is kept fixed as one polymer chain per droplet (Z = 1) for all the cases. Deconvolution of dielectric peak as described in the text is shown, where peak:1 and peak:2 represent the dielectric relaxation from soft confined gelatin and surfactant shell, respectively.

will increase, leading to the uncorrelated movement of ions at the interfaces. Consequently, the dielectric strength and 6525


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gelatin, whereas the high-frequency contribution is assigned to the polarization of anionic head groups in AOT surfactant shell. Having identified the α-relaxation process in soft-confined hydrated gelatin, next the influence of gelatin concentration Z on relaxation dynamics is investigated. The dielectric strength (ΔεP) and characteristic relaxation time (τP) of α-relaxation in soft-confined hydrated gelatin as a function of Z are shown in Figure 10c. A noticeable increase in dielectric strength and relaxation time is observed with the increase of gelatin polymer concentration. Herein, the increase in dielectric strength of gelatin polymer chains can be described by considering the enhancement in net polarity associated with the gelatin polymer chains with the increase in their concentration. Further, the increase in relaxation time of confined hydrated gelatin can be corroborated with the reduced mobility of polymer chains with the increase of gelatin polymer concentration inside the ME droplets. As mentioned earlier, the α-relaxation in hydrated gelatin shifts to higher frequencies when the polymer is confined in the nanoscopic water domains of reverse microemulsions, in comparison to its bulk state, i.e., a huge shift in frequency window for dielectric relaxation from 104−106 Hz for bulk gelatin to 107−109 Hz for soft-confined gelatin can be noted. We conjecture that the accelerated dynamics observed for confined hydrated gelatin chains is unusual and may be originating form the strong influence of the surrounding surfactant shell. Next, we explored the effect of degree of confinement on the relaxation dynamics of hydrated gelatin. The degree of confinement is expressed by a parameter RF/2R defined as the ratio of the end-to-end distance of gelatin chain in the bulk state to the diameter of the spherical water core of the microemulsion droplet49 and is altered by choosing different W for the microemulsions. The larger the value of RF/2R is, the stronger is the spatial confinement. Here, we assume RF = 6 R g = 6.6 nm for simplicity. The characteristic relaxation time of α-relaxation of hydrated gelatin in soft confinement as a function of degree of confinement RF/2R is shown in Figure 11. Notably, a further acceleration in the dynamics (which corresponds to a decrease in the α-relaxation time) of the soft-confined gelatin polymer chains is observed with increase in degree of confinement. The accelerated dynamics of hydrated gelatin under strong confinement could be due to its increased proximity to the fluctuating surfactant shell. Next, the perturbation of the water core of the droplets by fluctuating dipoles located at the surfactant shell is enhanced upon increase in confinement. Further, the relaxation time of soft confined gelatin obtained by following the aforementioned procedure is plotted as a function of temperature and is shown in Figure S6. Neither Arrhenius nor VF dependence of polymer relaxation on temperature is observed, and this is also in line with the observations reported for molecular liquids under soft confinement. Finally, our findings are discussed in relation with the previous reports discussing the soft confinement effects dynamics of molecular liquids.4,7,15 Further, the effect of hard and soft confinement on relaxation dynamics of glass-forming molecular liquids have previously been investigated widely.4,6,7,16 It has been reported that the molecular liquids in hard confinement exhibits frustrated dynamics, where the interfacial frustration is found to be dominating the confinement effects.10,11 In contrast, an acceleration in relaxation dynamics for molecular liquids is observed in soft confinement

Figure 10. (a) Dielectric strength Δεcore and characteristic relaxation time τcore of core relaxation as a function of gelatin polymer concentration Z. (b) Dielectric strength Δεshell and characteristic relaxation time τshell of shell relaxation as a function of Z. (c) Dielectric strength ΔεP and relaxation time τP of structural relaxation of gelatin polymer in soft confinement as a function of Z. For all the cases W = 30 and ϕ = 0.1 for gelatin-loaded reverse microemulsions. Errors in Δε and τ are smaller than the symbol size and are not shown. The lines are guide for the eyes.

106 Hz, is not detected in the confined state. Moreover, the dielectric spectra for GLMEs reveled substantial modifications to the loss peak representing the shell relaxation in the presence of gelatin polymer. Consequently, the high-frequency dielectric spectra of GLMEs are deconvoluted into two distinct relaxation regions. As we have already noted, the low-frequency contribution is assigned to the structural relaxation of hydrated 6526


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enclosing surfactant boundary. Furthermore, SANS investigations on GLMEs imply that the droplet structure remains intact after the incorporation of gelatin polymer into the microemulsion.

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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.macromol.7b01521. SANS profiles for gelatin polymer dissolved in water; SANS data for microemulsions having volume fraction ϕ = 0.05 and ϕ = 0.1; Optical photographs of pure and gelatin-loaded reverse microemulsion samples; Table showing the neutron scattering length density for different components used in the present study; Dielectric spectra for gelatin-loaded reverse microemulsions with ϕ = 0.1 and W = 25 at different temperatures from T = 20 °C to T = 35 °C; Dielectric strength and characteristic relaxation time of shell relaxation as a function of temperature for gelatin-loaded reverse microemulsions with ϕ = 0.1 and W = 25; Characteristic relaxation time for soft confined gelatin τP as a function of temperature (PDF)

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Figure 11. Characteristic relaxation time (τP) for α-relaxation of hydrated gelatin in soft confinement as a function of degree of confinement RF/2R. Errors in τP are smaller than the symbol size and are not shown.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +91 (0) 44 2257 4899 (D.K.S.). ORCID

V. K. Aswal: 0000-0002-2020-9026 D. K. Satapathy: 0000-0002-3083-655X

attained by means of reverse microemulsions.7,16 Therefore, the observed faster structural relaxation dynamics of hydrated gelatin surrounded by a fluctuating soft boundary is broadly in line with the studies on molecular liquids. Thus, accelerated dynamics of hydrated gelatin when it is surrounded by AOT surfactant shell is not unexpected. It is worth noting that the gelatin polymer molecules are neither in immediate contact nor interacting directly with the AOT surfactant monolayer.49 Therefore, it can be commented that the fluctuating dynamics of surfactant monolayer strongly influences the relaxation dynamics of hydrated gelatin, and the length scale of cooperativity will decide how far the effect will penetrate into the water core and polymer chains.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank Basavraj M. Gurappa, Department of Chemical Engineering, IIT Madras, for helpful discussions. This work is supported by funding from UGC-DAE Consortium For Scientific Research , and NFSC scheme, IIT Madras.

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4. CONCLUSIONS In conclusion, we have investigated the dielectric relaxation of gelatin polymer chains under soft spatial confinement achieved by means of reverse microemulsions. Here, the gelatin polymer chains are enclosed inside the droplet core of water−AOT− decane reverse microemulsions. Notably, the α-relaxation of hydrated gelatin is shifted to higher frequencies when the polymer is confined by the fluctuating surfactant shell. This indicates an accelerated dynamics for hydrated gelatin chains in soft confinement and is broadly similar to the results reported for soft confined molecular liquids.7,15 Moreover, a further enhancement in the dynamics of soft confined hydrated gelatin chains are observed with increase in the degree of confinement obtained by tuning the droplet size of reverse microemulsions. We conjecture that the enhancement in the dynamics of the gelatin polymer chain subjected to increased spatial confinement may arise from its proximity induced cooperativity with the 6527


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