Article pubs.acs.org/Langmuir
Colloidal Nanoparticle Interaction Transition during Solvent Evaporation Investigated by in-Situ Small-Angle X‑ray Scattering J. Bahadur,*,†,‡ D. Sen,† S. Mazumder,† G. Santoro,§ S. Yu,§ S. V. Roth,§ and Y. B. Melnichenko‡ †
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Photon Science, Deutsches Elektronen-Synchrotron (DESY), Notkestr. 85 D-22607, Hamburg, Germany ‡
S Supporting Information *
ABSTRACT: In-situ scanning small-angle X-ray scattering (SAXS) experiments have been performed to probe the drying of a single suspended droplet of silica colloids. It has been demonstrated that the formation of a nanoparticle shell during drying can be confirmed just by measuring the temporal evolution of the spatial transmission profile across the drying droplet. The shrinkage of the droplet stops once the shell is formed. The temporal dependence of the shell thickness and droplet radius has been estimated by quantitative analysis of the functionality of the transmission profiles. It is revealed that the position of the correlation peak originating from interactions between silica nanoparticles evolves linearly during the initial stage of drying and exhibits sigmoidal growth behavior in later stages. The interaction between colloidal particles in different drying stages has been investigated. We provide experimental confirmation of the transition from repulsive interaction to a capillary-driven short-range attraction during shell formation. The present work demonstrates that in-situ scanning SAXS on a suspended droplet is an invaluable technique for monitoring the dynamic self-organization of colloids as it probes the drying of complex fluids without the interference of a substrate.
1. INTRODUCTION The drying of colloidal dispersions has been extensively studied not only because of its scientific importance but also for many industrial applications.1 Spray drying,2−4 which is used in the food industry, pharmaceuticals, polymers, and detergents, utilizes the drying of droplets. Recently, evaporation-induced assembly of colloids through droplet drying has also been realized to fabricate nanocomposite grains with tailored morphologies and devices for optical photovoltaic cells.5−10 The understanding of droplet drying is important as far as the optimization of processing parameters is concerned to create better products. The evaporation of colloidal dispersions on a substrate has been extensively reviewed recently.1 Evaporation gives rise to the so-called coffee ring effect11−16 for droplets with pinned contact lines. A ring like stain remains on the substrate once the liquid has evaporated. The mechanism behind this ring-stain formation is explained in the pioneering work of Deegan et al.11 In an evaporating drop with an immobile contact line, capillary flow is generated to replenish the liquid that has evaporated from the edges. This flow drags particles toward the contact line, forming the ring-shaped stain in particle suspensions such as coffee. Although lots of investigations have been devoted to study the morphology of a drying droplet on a substrate, much less effort has been invested to understand the drying mechanism of suspended colloidal droplets. Complex issues, such as the pinning of the contact line and the wetting of a substrate, emerging during the assembly process of the colloids © 2015 American Chemical Society
on a substrate can be avoided in the case of drying a suspended droplet. The drying of a single suspended droplet is an ideal model system for studying the physical processes and interactions among the colloids during evaporation. Extensive efforts17−19 have been applied to study the morphology of drying sessile droplets using optical techniques; however, the self-organization of the colloids and their interactions in the course of drying is not well understood yet. Although optical techniques can probe the morphological changes during drying, they cannot deliver information about the interaction between colloids. In addition, optical techniques are unable to assess the thickness of the shell, which may form during the drying of colloidal droplets. In this sense, in situ Xray scattering experiments20,22,23 on a suspended evaporative droplet, using a microfocused synchrotron beam in scanning mode, is best suited to address the aforementioned issues. X-ray scattering could allow probing of the evolution of droplet shrinkage and the interaction21 between colloids simultaneously. In our recent work,22 we revealed that in situ small-angle X-ray scattering (SAXS) can provide valuable and important information about the isotropic/anisotropic nature of the jamming of colloids in drying droplets. The assembly of the colloids was studied for two different droplets having a distinct concentration of colloid particles. In one case, the size of the Received: December 8, 2014 Revised: January 27, 2015 Published: April 3, 2015 4612
DOI: 10.1021/la504769k Langmuir 2015, 31, 4612−4618
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Langmuir
Figure 1. Schematic of the experimental setup. The droplet is attached to an XYZ translation stage to facilitate the scanning transmission measurements and SAXS across the droplet in both the horizontal and the vertical directions.
Figure 2. Transmission of X-rays across the drying droplet and its evolution with time for up to 16 min of drying (a) and from 18 min (b). Solid lines are fits of the model to the experimental data. Deionized (DI) water is used to dilute the obtained dispersion. The initial volume fraction of nanoparticles in the droplet was kept at 0.05. In situ scanning SAXS experiments on such evaporative droplets were performed at micro and nanofocus X-ray scattering (MiNaXS) beamline P03 at synchrotron source PETRA III, DESY, Hamburg, Germany.24−26 The beam size for SAXS measurements was 32 μm (horizontal) × 23 μm (vertical) with a wavelength of 0.0957 nm. Scattering patterns were recorded using a 2D-pixel detector (Pilatus 300 K, DECTRIS Ltd., Switzerland), and the radial average density I(Q) was calculated for different wave vector transfers Q (Q = ((4π sin(θ))/λ),where 2θ is the scattering angle and λ is wavelength of the X-rays). The transmission of the X-rays across the droplet is monitored simultaneously by a PIN diode during the scanning experiment. The schematic of the experimental setup is shown in Figure 1. More details on the experimental and analysis protocols can be found in the Supporting Information.
colloidal nanoparticles and the colloidal concentration were such that uniform jamming occurred throughout the drying process without shell formation. In the other case, the concentration of the colloids was increased significantly, leading to anisotropic jamming of the colloidal nanoparticles during the early stage of drying. Shell formation during solvent evaporation of a colloidal droplet and its mechanical behavior under capillary pressure can lead to various kinds of grain morphology.6−8 One of the hypotheses about the mechanical characteristic of the formed shell states that the viscoelastic shell transforms into an elastic shell when the capillary attraction between colloids dominates the electrostatic repulsion.17 To date, this is not directly confirmed by experiments. The present work attempts to explore the nature of the interaction between colloids during drying to test this hypothesis. The findings of the present work support this hypothesis. Furthermore, we demonstrate that the formation of a shell can be confirmed just by measuring the temporal evolution of spatial transmission profiles across the drying droplet, whereas the evolution of the intercolloidal interactions should be probed by analyzing SAXS patterns.
3. RESULTS AND DISCUSSIONS 3.1. Analysis of Transmission Profiles. Figure 2a shows the evolution of the spatial transmission profile along the scanning axis of the droplet during the first 16 min of drying. Figure 2b shows the transmission profiles across the droplet beyond 16 min of drying (Supporting Information). The transmission values fluctuate in the regions outside the droplet due to both fluctuations in the beam current and the singlepoint normalization procedure adopted (Supporting Information). It is evident from the figure that the width of the
2. EXPERIMENT METHODOLOGY A colloidal droplet having a 2 μL volume is generated using a silica colloidal dispersion (TM40, LUDOX, average particle radius ∼12 nm, supplied by Sigma-Aldrich, USA) and is suspended on a narrow tip. 4613
DOI: 10.1021/la504769k Langmuir 2015, 31, 4612−4618
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3.2. Interpretation of SAXS Profiles. The scattering profiles acquired in the center of the droplet are shown in Figure 3. Qualitatively, the evolution of the scattering profiles at
transmission profile decreases, maintaining a concave profile for 20 min of drying, indicating an isotropic shrinkage of the droplet. The central part of the transmission profile becomes convex after 20 min. Interestingly, prominent dip points (where concave and convex profiles meet) have been observed in the transmission profiles. The droplet stops shrinking after 20 min of drying. The spatial variation of X-ray transmission can be understood in terms of the variation of path length across the droplet during scanning. The transmission of X-rays in a medium with macroscopic absorption cross-section Σa can be written as T = exp( −Σad)
(1)
where d is the path length of X-rays in the medium. The absorption cross section of the colloidal dispersion depends on the absorption cross section of water and silica colloids. During the drying process, the overall absorption of the system increases as the water evaporates due to an increase in the volume fraction of silica colloids. The path length traversed by X-rays across the spherical droplet having a uniform volume fraction can be written as d = 2 R2 − x 2
Figure 3. Temporal evolution of the scattering profiles measured in the center of the drying droplet.
(2)
the edges of the droplet is similar to that in the center (Supporting Information). However, position-related differences in the effective thickness and volume fraction lead to a significant variation in the scattering intensity. The SAXS profiles in the initial period of drying can be fitted using a charged sphere model.27,28 For example, the SAXS profile at t = 2 min was fitted using Igor Pro-based small-angle neutron scattering (SANS) analysis software29 developed by the National Institutes of Standard and Technology (NIST, USA; Supporting Information). In the intermediate and advanced stages of drying, the SAXS profiles could not be explained by a charged sphere model. A strong correlation peak is observed in the scattering profile due to the spatial correlation of silica colloids. As the droplet dries, the peak position shifts toward higher Q, indicating a closer packing of the silica particles. Besides the shift in the peak position we also observe significant variation in the SAXS profiles at low Q values. The low-Q scattering behavior depends on the type of interaction between the colloids and is addressed in the next section. To extract the peak positions, pseudo-Voigt fitting functions30 have been used. The estimated peak positions both in the center of the droplet and at the droplet edges are presented in Figure 4. The position of the peak increases
where R is the radius of the droplet and x is the distance of the scan position from the center. Equations 1 and 2 have been used to fit the transmission profiles. The fitting of the concaveshaped transmission profiles with the uniform sphere model is quite satisfactory (Figure 2). The fitting parameters are shown in Table S1. As expected, radius R0 decreases and the effective absorption cross section of the system increases with drying. The transmission profiles beyond 20 min of drying cannot be fitted with a uniform sphere model, thus we have employed a core−shell droplet morphological model. The path length of X-rays for a spherical core−shell droplet with outer radius R and inner radius Ri can be written as d = 2 R2 − x 2 , R i < x < R d=
2(R2 − R i 2)
⎡ 2 2 ⎣ (R − x ) +
⎤ (R i 2 − x 2 ) ⎦
, x < Ri (3)
The fitting of the transmssion profiles after 20 min using eqs 1 and 3 is shown in Figure 2b. It is evident that the model fits reasonably well. The fitting parameters are tabulated in Table S1. It is evident that the width of the transmssion profiles during 22 and 38 min remains almost constant. The extent of the concave and convex parts of the transmission profiles depends on the overall radius and shell thickness. The magnitude of the transmission in the concave and convex parts depends on the effective absorption cross sections at the core and shell of the droplet. The prominent dip points in the transmission profiles originate from the well-defined interface between the core and shell in terms of the X-rays absorption coefficient, which is related to the silica volume fraction, ruling out the possibility of a concentration gradient inside the droplet. In the later stages of drying, the water in the core can evaporate only through interstices between the jammed colloids at the shell, ultimately leading to hollow silica grains. In our case, after 38 min of drying time we do not observe any significant modification of the transmission profiles that could point toward the formation of hollow grains.
Figure 4. Evolution of peak position at three selected droplet positions during drying. Solid lines represent the fitting of the experimental data with linear and sigmoidal functions. 4614
DOI: 10.1021/la504769k Langmuir 2015, 31, 4612−4618
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Langmuir Table 1. Fitting Parameters for the Linear and Sigmoidal Peak Position Regime droplet position
x0 (nm−1)
C (min−1)
xi (nm−1)
xf (nm−1)
ti (min)
α (min)
end 1 center end 2
0.12 ± 0.03 0.11 ± 0.03 0.12 ± 0.04
0.004 ± 0.0005 0.004 ± 0.0004 0.004 ± 0.0005
0.15 ± 0.007 0.13 ± 0.02 0.16 ± 0.003
0.30 ± 0.002 0.30 ± 0.003 0.30 ± 0.001
16.4 ± 0.4 16 ± 1 15.4 ± 0.14
2.3 ± 0.3 3.1 ± 0.6 1.5 ± 0.13
where r(t) is the instantaneous radius of a droplet of initial radius r0 and β is the evaporation coefficient which characterizes the evaporation process for a given droplet under given drying conditions. The number of silica particles in the droplet remains constant during the entire drying process, thus
continuously in the early and intermediate stages of drying (
