Stimulus‐Responsive Gas Marbles as an Amphibious Carrier for Gaseous Materials

Abstract Gas marbles are a new family of particle‐stabilized soft dispersed system with a soap bubble‐like air‐in‐water‐in‐air structure. Herein, stimulus‐responsive character is successfully introduced to a gas marble system for the first time using polymer particles carrying a poly(tertiary amine methacrylate) (pK a ≈7) steric stabilizer on their surfaces as a particulate stabilizer. The gas marbles exhibited long‐term stability when transferred onto the planar surface of liquid water, provided that the solution pH of the subphase is basic and neutral. In contrast, the use of acidic solutions led to immediate disintegration of the gas marbles, resulting in release of the inner gas. The critical minimum solution pH required for long‐term gas marble stability correlates closely with the known pK a value for the poly(tertiary amine methacrylate) stabilizer. It also demonstrates amphibious motions of the gas marbles.


Experimental Section
Characterization of poly [2-(diethylamino)ethyl methacrylate] (PDEA) homopolymer Gel-permeation chromatography (GPC) measurement for PDEA was performed using a Jasco RI-2031 refractive index detector (Jasco, Tokyo, Japan) equipped with a CTO-10AS VP Shimadzu column working at 40 °C under a flow rate of 0.60 mL/min.A 0.3 M Na2SO4 aqueous solution containing a 0.5 M acetic acid was used as eluent and delivered to column by a Jasco PU-2080 Plus pump (Tokyo, Japan).Sample solution for the GPC measurement was filtered using 0.2 µm pore size membrane filter.The molecular weight of the polymer was calibrated with poly(2-vinylpyridine) standard samples.The Mn and Mw/Mn value for the polymers were estimated as 9,600 g/mol and 1.26, respectively.

Particle size analysis
The size of the particles was determined using a laser diffraction particle size analyzer (Malvern Mastersizer 2000) equipped with a small volume sample dispersion unit (Hydro 2000SM; ca.150 mL including flow cell and tubing), a HeNe laser (633 nm) and a solid-state blue laser (466 nm).The stirring rate was adjusted to 2000 rpm.The raw data were analyzed using a Malvern software.The mean particle diameter was taken to be the volume equivalent sphere mean diameter (Dv), which is mathematically expressed as Dv = Di 4 Ni /  Di 3 Ni, where Di is the diameter of individual particles and Ni is the number of particles corresponding to the specific diameter.The resulting data are presented as mean diameter ± standard deviation.The laser diffraction method determined particle size and its distribution by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample.Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large angles.The angular scattering intensity data is then analyzed to calculate the size of the particles responsible for creating the scattering pattern based on the Mie theory.

Chemical composition
CHN elemental microanalyses were conducted using an Element analyzer 2400II (Perkin Elmer, Yokohama, Japan) at Nagasaki University (Office for Research Initiatives and Development, Nagasaki University, Nagasaki, Japan).
The PDEA loading percentage was determined by comparing the nitrogen content of the particles with that of the PDEA homopolymer (N = 0.1 wt% for the PDEA-PS particles and 7.6 wt% for the PDEA homopolymer).

H NMR spectroscopy
1 H NMR spectra were obtained using a JEOL JNM-ECZ 400 MHz NMR. 1 H NMR samples solutions were prepared in CDCl3.

Zeta potential
Zeta potentials were calculated from the electrophoretic mobility, measured using a Malvern Zetasizer Nano ZS with an MPT-2 Multi-Purpose Titrator.Measurements were conducted as a function of pH with diluted dispersions (approximately 0.02 w/v%) by gradually adding an aqueous solution of NaOH, starting from an initial pH of approximately 3. Zeta potentials were averaged over 3 runs at each pH.
The synthetic protocol was as follows.PNVP (nominal molecular weight = 360,000; 2.5 g; 10 wt% based on styrene) was added to isopropanol (250 mL) in a roundbottomed 500 mL flask with a magnetic stirrer bar and stirred vigorously at 70 °C until the PNVP had dissolved completely, followed by degassing with a nitrogen purge.Polymerization commenced after the addition of 0.25 g AIBN dissolved in 25.0 g styrene.The reaction was allowed to proceed for 24 h with continuous stirring at 250 rpm under a nitrogen atmosphere.The PS particles were then purified by repeated centrifugation-redispersion cycles, replacing successive supernatants with deionized water, followed by freeze drying.

Preparation of gas marbles
Air bubbles with controlled volumes were injected below the planar air-water interface covered by PDEA-PS particle raft using the syringe, resulting in the formation of bubbles whose upper surfaces are covered by the particle raft.The bubble was then pushed using a dispensing spoon toward the surrounding particle raft and was rolled over it to cover its whole surface.The PDEA-PS particles autonomously coat the aqueous bubble and render it non-wetting.(See Figure S5 and Movie S1)

Stability of gas marbles at various relative humidities
The gas marbles (bubble volume, 20 L) were placed in the enclosed dessicator inside of the relative humidity was controlled by saturated aqueous solution of salts [17b] .The relative humidities were tuned to be 40, 50, 62, 74, 80, and 91% using saturated aqueous solutions of magnesium chloride, potassium carbonate, sodium bromide, sodium chloride, potassium chloride, and potassium sulfate, respectively.

Determination of the mean particle mass per one gas marble
After complete evaporation of the water by drying in air at room temperature for 24 h, the residual mass of PDEA-PS particles was determined gravimetrically using a balance (AB135S Analytical Balance, Mettler Toledo).The accuracy of weight is in the order of 0.01 mg.

Configurations of PDEA on the particle surface
The area occupied by a PDEA chain at the surface of the PS particle was calculated to be 4.37 nm 2 based on the elemental microanalysis results.The square root of the occupied molecular area (2.09 nm) exceeds the diameter of gyration (1.27 nm) of a PDEA chain (degree of polymerization = 51), indicating the random-walk configurations of PDEA on the particle surface.Thickness of PDEA-PS particle layer on the gas marble From the weight of PDEA-PS particles and the diameter of gas marbles, we estimated that, on average, the gas marble coating comprises 21-39 particles, which corresponds to thicknesses of 34-62 µm.

Motion transfer between gas-solid and gas-liquid interfaces
When gas marbles are placed on a water film prepared on a solid substrate (poly(methyl methacrylate)) followed by the induction of sliding motion by air blow, they can move from the water surface to the solid substrate surface (Movie S9).This indicates that the location of gas marble motion can be transferred from the gas-liquid surface (fluid field) to the gas-solid surface (elastic field) by the kinetic energy generated by the air blow and the potential energy due to the water film thickness.This motion transfer can be reversed: gas marbles can move from a solid substrate to a water surface (running up the meniscus of the water film and moving to the horizontal water surface) if the gas marbles on the air-solid surface are given more kinetic energy than the potential energy given by the height of the water film by air blowing (Movie S9).The volume-average diameter (Dv) in IPA was 1.58±0.35m, measured by laser diffraction particle size analysis (Figure S1a).The particle size was confirmed by SEM experiments (Figure S1b).Aqueous electrophoresis studies indicated the existing of PDEA on the surface of PS particles (Figure S1c).The iso-electric point of the PDEA-PS particles was approximately pH 9.5.Given that the XPS sampling depth is typically only 2-5 nm, these observations provide good evidence that the PDEA stabilizer is present at the surface of the PDEA-PS particles.Moreover, the intensity of the N1s signal obtained for the PDEA-PS particles can be compared to that of the PDEA homopolymer in order to estimate a surface coverage of 18.7 % for the PDEA stabilizer chains on the particle surface.Notably, at pH 10, substantial flocculation was observed.This phenomenon occurs because the PDEA stabilizer chains become non-protonated and neutral, rendering them hydrophobic under basic conditions.Conversely, protonation of the PDEA chains at pH 3 results in highly cationic particles with significant electrosteric repulsion, leading to a high degree of dispersion.Consequently, the PDEA-PS particles can exist in two distinct states in aqueous media: (i) as colloidally stable particles with protonated, highly cationic PDEA chains in acidic solutions, and (ii) as flocculated particles with neutral PDEA chains in either neutral or alkaline solutions.The cross-sectional SEM image allows determination of the contact angles (θ) of PSEA-PS particle in the array at air-water interface [17c] .Here, θ is the contact angle measured through the water phase.The contact angles of the particles at the air-water interface was arithmetically determined to be 74 ± 3º.

Figure S9
. SEM images of planar air-water interface covered by PDEA-PS particle powder after ethyl cyanoacrylate vapor treatment, followed by the evaporation of water.The ethyl cyanoacrylate vapor treatment was conducted (a) before and (b) after application of physical force (knocking the particle powder from the air phase using a plastic spatula).The images were observed from air-phase faced side.
Before the application of physical force, the particle aggregates were mainly found in the PDEA-PS particle raft on planar air-water interface, because particle-particle interaction dominates over gravity for a few micrometer-sized polymer particles.These aggregates should cover the gas marbles.The particle array monolayer was formed after the application of physical force.These results could suggest that the particle array monolayer could be formed during the preparation of gas marbles by rolling, which could break the aggregates partially to the independent PDEA-PS particles.

Figure S1 .
Figure S1.(a) Laser diffraction particle size distribution curve obtained for PDEA-PS particles in isopropanol (IPA), (b) SEM image of the PDEA-PS particles and (c) zeta potentials of the PDEA-PS particles depending on pH for aqueous dispersions.

Figure
Figure S2. 1 H NMR spectra for (a) PDEA and (b) PDEA-PS particles in CDCl3 at room temperature.The resonance band observed at 1.64-2.05ppm (peak a) and 0.87 ppm (peak b)were attributed to main chain methylene protons and α-methylene protons, respectively.The resonance band at 1.04 ppm (peak f) was associated with the pendant methylene protons from the side chain of the polymer. 1 H NMR spectrum for PDEA-PS particles is indicated in FigureS2b.The main chain methylene protons were contributed to the signal bands at 1.23-2.22ppm (peak g, h), and the phenyl protons were associated with the resonance bands at 6.40-7.10ppm (peak k, i).

Figure S4 .
Figure S4.Optical microscopy images of aqueous dispersions of PDEA-PS particles at pH 3 and 10.

Figure S10 .
Figure S10.(a) Disruption of gas marble (bubble volume, 20 L) by mechanical compression.(b) Inner wall of gas marble observed after disruption.Water was dyed using rhodamine B.

Table S1 .
Quantitative surface composition of PS homopolymer, PDEA homopolymer and PDEA-PS particles determined by XPS