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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Externally triggered oscillatory structural forces Sebastian Schön, Marcel Richter, Marcus U. Witt, and Regine von Klitzing Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02284 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018
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Externally triggered oscillatory structural forces Sebastian Sch¨on,† Marcel Richtera,† Marcus Witt,‡ and Regine von Klitzing∗,‡ †Technical University of Berlin, Stranski-Laboratorium, Department of Chemistry, Strasse des 17. Juni 124 D-10623 Berlin, Germany. ‡Technical University of Darmstadt, Soft Matter at Interfaces, Department of Physics, Alarich-Weiss-Strasse 10 D-64287 Darmstadt, Germany. E-mail:
[email protected] Phone: +49 6151 16-25647 a
current address: Hoesch Bausysteme GmbH, Sebenter Weg 41 D-23758 Oldenburg, Germany
Abstract The paper addresses triggering of oscillatory structural forces via temperature variation across an aqueous dispersion of thermoresponsive Poly(N -isopropylacrylamide) (PNIPAM) nanogels confined between silica surfaces. Oscillatory structural forces are a well-known phenomenon in colloidal science, caused by interactions between molecules or colloids. Modulation of these forces usually requires changing the internal parameters of the dispersion, such as ionic strength, particle concentration, and surface charge or changing the properties of the confining walls like surface roughness, potential or elasticity. All these parameters are usually fixed and can only be changed via exchange of the sample or the complete experimental setup. Here, a new approach is presented, combining the characteristics of smart materials with the properties of nanoparticles, using negatively charged PNIPAM nanogels. Aqueous dispersions of these nanogels express no oscillatory structural forces in the initial state (20◦ C), below the volume phase transition temperature (VPTT, 32◦ C). Heating (60◦ C) reduces the nanogel size
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and leads to a more negative zeta potential, which triggers the onset of oscillatory structural forces.
Introduction Controlling the stability of colloidal systems has been a major issue of scientific research due to the ubiquitous nature of colloidal systems in our daily life and in industrial applications. 1 Colloidal systems such as suspensions, emulsions and foams share a common nanostructure. They are built up of solid, liquid or gaseous colloidal particles separated by a thin liquid film. The stability of these thin liquid films on the nanoscale governs the stability of the macroscopic system and is determined by the sum of steric, van der Waals and electrostatic forces. The latter two are summarized by DLVO theory. Adding nanoparticles to the colloidal system can give rise to oscillatory structural forces which dominate the interaction between the confining surfaces, e.g. surfaces of microspheres or planar surfaces. Oscillatory structural forces originate from ordering phenomena of the nanoparticles within the thin liquid film and provide additional force barriers which increases the film stability. 2 For this reason, oscillatory structural forces have been intensely studied in a variety of systems such as colloidal suspensions, 2–9 micellar surfactant solutions, 10–15 polymer suspensions 16–22 and molecular liquids. 23–27 Oscillatory structural forces have also been shown to influence the spreading of nanofluidic films 28–31 which can be utilized in a variety of industrial applications such as enhanced oil recovery, lubrication and soil remediation. One of the latest applications is the use of oscillatory structural forces for separation of a binary polystyrene particles solution. 32 The effect of the oscillatory structural forces on all these applications is strongly related to their magnitude. The magnitude and length scale of the oscillatory structural forces can be influenced by a broad range of parameters such as particle size, ionic strength and surface properties. 33,34 So far, the qualitative impact of the different parameters is quite clear, but the (quantitative) strength of electrostatic repulsion needed for the occur-
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rence of oscillatory forces is quite unclear. Changing the particle charge of Silica particles can be realized by pH variation. This means a simultaneous increase of the ionic strength and a decrease in oscillatory forces. The addition of a small amount of salt like 10−3 mol/l to assure a constant ionic strength would already destroy the force oscillation. 35 Another disadvantage is that usually the sample has to be changed, when a parameter is changed, which changes slightly the conditions of the measurements (e.g. different cantilever). Therefore, the task of the present paper is to change a parameter by an external trigger without affecting the experimental setup. At this point smart materials such as gel particles based on Poly(N -isopropylacrylamide) (PNIPAM) become very interesting, as they can be influenced via a broad range of parameters. 36,37 The most influential and best investigated parameter is the temperature. Increasing the temperature of an aqueous dispersion of PNIPAM gel particles above the volume phase transition temperature (VPTT) of 32◦ C triggers a change in particle volume towards smaller values. 38–41 The change in volume can be explained by the interactions of the PNIPAM units with the water molecules at different temperatures. Below 32◦ C water is a good solvent and the PNIPAM units favor interaction with the water molecules, which results in a swollen state of the particles. Above 32◦ C the gel particles become less hydrophilic and dehydrate. Hydrogen bonds between NIPAM units are favored over NIPAM-H2 O hydrogen bonds. This leads to the removal of water molecules from the particle interior and consequently to a shrinkage. 42 The switching from swollen to shrunken state can also be triggered by changes in ionic strength, 43 solvent quality, 44 surfactants 45 and UV-VIS irradiation. 46 The ability to switch the size of the particles makes them interesting for applications in medicine, for sensors or for contact lenses. 47–49 The most interesting parameter to change the particle size with respect to the oscillatory structural forces experiments is the temperature. Although the temperature does influence the particle interaction via its direct contribution to the Debey screening length and the Hamaker constant, the overall impact is low. The reason for this, is the fact that the relative difference between the low (20◦ C) and high (60◦ C) temperatures used in this work, to trigger the swelling and shrinking
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of particles amounts to a maximum of 13% in absolute temperature (K). On the other hand, shrinking of the particles increases the surface charge density. This change in surface charge density can therefore be used to tune the interaction between the particles via temperature control. Previous attempts to control oscillatory structural forces via temperature used aqueous solutions of responsive block copolymers. 14 At low temperatures the block copolymers are solvated as unimers in the aqueous solution and no oscillatory structural forces can occur. With increasing temperature micelles start to form and with increasing micelles concentration oscillatory structural forces arise. Instead of using temperature to switch the state of a system from containing no colloids to containing colloids, PNIPAM gel particles can be used to change the interaction between the colloids to affect the oscillatory structural forces. To apply PNIPAM gel particles to oscillatory structural force experiments one has to leave the realm of well-studied PNIPAM microgels and synthesize gels of much smaller size. A route to achieve this has been introduced by Arleth et al. 50 who used sodium dodecyl sulfate as emulsifier to obtain PNIPAM gel particles smaller than 100 nm in diameter. In the present paper oscillatory forces across aqueous dispersions of PNIPAM nanogels are triggered by temperature. Starting with the recipe from Arleth et al. including acrylic acid as copolymer to increase the surface potential even smaller PNIPAM nanogels have been synthesized in the present study. Changes with temperature of the PNIPAM nanogel size and surface potential have been measured. Effect of these changes on the interaction between the nanogels and therefore the oscillatory structural forces have been investigated. For this purpose, aqueous dispersions of PNIPAM nanogels have been measured below and above the VPTT via colloidal probe atomic force microscopy, to detect oscillatory structural forces. We report here for the first time of the complete on/off triggering of oscillatory structural forces in a dispersion of PNIPAM nanogels via temperature control. Better understanding of this process would have a major impact on all applications of oscillatory structural forces mentioned above and may result in new applications such as sensors.
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Experimental Materials The monomer N -isopropylacrylamide (NIPAM, 97%), co-monomer acrylic acid (AAc, 99%), the cross-linker N,N ’-methylenebis(acrylamide) (BIS, 99,5%), Sodiumdodecylsulfate (SDS, >99%) and ammonium hydroxide (28-30%) have been obtained from Sigma Aldrich. Potassium persulfate (KPS, 99%) and hydrogen peroxide (30%) have been bought from Fluka and Th. Geyer respectively. All chemicals were used as received. Any water used during synthesis or experimental preparation had Milli-Q grade quality.
Synthesis The synthesis of the PNIPAM nanogels is based on the recipe described by Arleth et al. 50 A first batch (henceforth batch I) has been synthesized using the following recipe: 0.95 g NIPAM (8.4 mmol), 0.20 g SDS (8.3 mol%), 0.26 g BIS (20 mol%) and 0.012 g AAc (2.0 mol%) have been dissolved in 60 mL water. The solution has been heated up to 70◦ C under constant nitrogen stream and equilibrated for 30 min at this temperature. The polymerization was started by injecting 30 mg KPS (1.3 mol%) dissolved in 1 mL water. The colourless solution turned into a milky one within 10 min. After a reaction time of 4 h at 70◦ C, the milky solution was cooled down to room temperature and has been stirred overnight in a nitrogen atmosphere. The obtained nanogels have been cleaned by dialysis (dialysis tube from Visking, cut off at 13500 g/mol) for 10 days with daily water exchange and dried afterwards via freeze drying. The first measurement used to demonstrate a possible effect on the oscillatory structural force consumed the whole batch, therefore a second batch (batch II) has been synthesized. The recipe for batch II used a quadruple amount of NIPAM but otherwise very similar molar ratios of the other ingredients in relation to it. Sole exception is for the AAc content which has been increased for a stronger increase in force oscillation to a molar ratio of 4.3 mol%. 5
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Methods Dynamic light scattering (DLS) Dynamic light scattering has been performed to determine the gel particle size. Two instruments have been used. An ALV goniometer setup, with a Nd:YAG laser operating at a wave length of 532 nm and a Ls Instruments spectrometer with a HeNe laser operating at a wavelength of 632 nm. The measurements have been performed at different scattering angles to determine the diffusion coefficient. Applying this diffusion coefficient to the StokesEinstein equation the hydrodynamic radius of the gel particles at a given temperature was obtained. The data have been analyzed automatically with a custom program (Python, by Anja H¨ohrmann) which uses the Cumulant fit method.
Zeta Potential The zeta potential has been measured in a dilute aqueous dispersion of nanogels (>0.05 wt%) in a temperature range from 20◦ C to 60◦ C with a Zetasizer (Malvern Instruments, Nano ZS ). Zeta potential has been calculated from the electrophoretic mobility using the Smoluchowski approximation. The effect of the increased temperature on the viscosity of water has been taken into consideration.
Atomic force microscopy (AFM) The experiments to measure the oscillatory structural forces have been performed on an Asylum MFP3D atomic force microscope using the colloidal probe AFM technique (CPAFM). 51,52 As cantilevers CSC38/tipless/CrAu (mikromash) have been used. The spring constants were 0.084 N/m and 0.026 N/m, for the first and second batch respectively. A colloidal probe of silicon oxide (bang laboratories) of 6.7 µm diameter has been attached using two-component epoxy glue (UHU endfest). The CP-AFM experiments have been performed against a silicon wafer, which had been chemically cleaned using a mixture 1:1:5
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of ammonium hydroxide, hydrogen peroxide and water at 70◦ C for 20 min. Wafer and cantilever have been plasma cleaned (Diener Electronics, Femto) immediately before the start of the experiment. Temperature control during the experiment was achieved using the Asylum CoolerHeater. Temperature has been measured with an external thermometer (Cole Parmer, Digi-Sense), directly inside the drop (1mL) of the aqueous PNIPAM nanogel dispersion.
Results In order to show the reproducibility of the effect of temperature variation as an external trigger on oscillatory forces the results of both batches are shown although the results obtained with batch II would be sufficient to demonstrate the effect. Fig.1 displays the results of the CP-AFM measurements for an aqueous dispersion of PNIPAM nanogels (batch I) showing the on/off triggering of the oscillatory structural forces. The dispersion has a concentration of 5 wt%. On the left it shows the data at 20◦ C (A, blue) on the right at 60◦ C (B, red). The graph displays the information as force acting on the cantilever over separation between the colloidal probe and the silicon wafer. The blue curve displays the combination of approximately 40 force curves. The black curve shows the binominal smooth of the data. The window of the smooth had a width of less than 0.1% of the data. For more information concerning the treatment of the data see Sch¨on et al. 2018. 53 At 20◦ C the force curve does not show any oscillations. At a range of separation from 1000 down to 50 nm no repulsion is observed. At smaller separations the repulsion starts slowly to increase, with increasing gradient, until the hard contact is reached at zero separation. Fig.1 B displays the results for the same dispersion at 60◦ C. The measurements were performed after the sample equilibrated for 10 min at the new temperature. Again, the red curve shows a combination of approximately 40 force curves, while the black curve shows the binominal smooth (window width
