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Nov 2, 2017 - ABSTRACT: Membrane-based liquid air fresheners are attracting significant market interest because of their small size and ability to all...
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Membranes for Continuous Nonenergized Air Freshener Perfume Delivery Gui Min Shi,† Lin Hao,† Kelly Anderson,‡ and Tai Shung Chung*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Singapore Corporate R&D, Singapore Branch, The Procter and Gamble Company, Singapore 138547, Singapore

‡

S Supporting Information *

ABSTRACT: Membrane-based liquid air fresheners are attracting significant market interest because of their small size and ability to allow continuous delivery of perfume compounds without the need of external energy sources. However, the air freshener membranes that enable the nonenergized delivery are rarely reported in the literature except in patents. For the first time, we investigated a commercial air freshener membrane systematically to provide thorough characterizations of the membrane materials, morphology, and pore information. The interaction between a model perfume containing four compounds and the membrane was also studied through liquid wetting and contact angle measurements. Long-term tests to track the air freshener revealed that the perfume release rate was dependent on the perfume reservoir composition in a controlled external environment. Lastly, perfume transport through the membrane was investigated for the correlation of membrane properties and liquid flux through the membrane. This pioneering study may provide essential information for future improvement over the existing air freshener membranes to enable continuous perfume delivery without an energy provision. impermeable film that separated the perfume reservoir and the membrane. The membrane was kept dry until the rupturable film was broken to achieve a more consistent release of volatile materials. The use of membranes for controlled perfume delivery is an emerging field for membrane applications.7−10 An ideal membrane should be able to deliver different perfume compounds at a constant rate throughout the product’s lifetime in changing environments (e.g., air temperature and velocity). However, there are almost no systematic studies on how to design membrane air fresheners except in patents. Therefore, this pioneering study explored the design of a commercial air freshener. Specifically, we aim to systematically investigate the fundamentals of the air freshener membrane. First, the membrane characterizations, including elemental compositions, morphology, pore size distribution, and free volume were examined. Second, the membrane interaction with a model perfume containing four compounds was investigated through membrane wetting and sorption studies. Third, the perfume release rate in a commercial air freshener was studied. Lastly, fundamental sciences will also be conducted to understand the transport mechanisms in commercial membrane air fresheners.

1. INTRODUCTION Air fresheners are air care consumer products that dispense fragrance in homes, restrooms, or cars. The air care products include spray, candle, wick, gel, and membrane air fresheners. The original membrane-based air fresheners were developed in the 1970s to compete with the other air fresheners because they would not spill even if they were dropped.1 The global market value for air fresheners is projected to reach around 10.4 billion USD by the year 2020.2 Among the air freshener products, membrane-based liquid air fresheners have accounted for a significant market share because of their small size and ability to allow continuous delivery of perfume compounds without the need of external energy sources. Currently, there are a few studies on membrane air fresheners in the literature, but almost all of them are in patents. US patent 2,979,268A3 described a combination of package and diffusing device for the diffusion of a deodorant or perfume to the surrounding air, whereas US patent 3,785,5564 disclosed an apparatus and a method for packaging liquids such as a deodorant in an elongated plastic film. US patent 4,145,0015 disclosed a deodorizer sandwiched between laminated layers, in which thermoplastic membranes such as polyethylene and ethylene vinyl acetate copolymer membranes were used for the release of a deodorizer. More recently, US patent 8,931,7116 disclosed an apparatus for delivering volatile materials by means of a microporous membrane. The apparatus included a perfume reservoir, a membrane, and a rupturable © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

October 4, 2017 October 28, 2017 November 2, 2017 November 2, 2017 DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

2. MATERIALS AND METHODS 2.1. Materials. The commercial air freshener membrane was provided by The Procter and Gamble Company (P&G). The perfume compounds ethyl-2-methyl butyrate, benzyl acetate, phenyl ethyl alcohol, and cyclamen aldehyde were purchased from Sigma-Aldrich and Taytonn Pte. Ltd. Their physicochemical properties (obtained from Aspen Properties V9) are shown in Table S1 and Figure 1.

a MEGA-5 MS column (packed with polysiloxanes) and an FID detector. 2.3. Wetting Studies on the Air Freshener Membranes. To evaluate the factors affecting the membrane wetting by various perfume compounds, a simple device was designed and is illustrated in Figure S1. Briefly, two membrane strips with a predetermined length and width were hung in an enclosed container with the bottom of one strip immersed in a liquid while the other strip was not in contact with liquid. The membrane strips were premarked by a pen with 0.5 cm intervals. Then, the membrane strips were put inside the container with 1 cm of the bottom of the strip immersed in the liquid surface while another bottom of the strip was placed above the liquid surface. The time was recorded as the starting point of the wetting process. When wetting happened, the marks on the strips would be partially dissolved by the liquid or vapor and became blurry. The times spent to wet each mark upward were noted and compared later. The wetting of the membrane was measured from the mark change in the membrane strip. 2.4. Static Head Measurements of the Air Freshener Membranes. The static head of the air freshener membranes was defined as the start of leaking out liquid droplets. Figure S2 illustrates the lab-made device placed in a fume hood under an air velocity of ∼0.1 ± 0.02 m/s at about 25 ± 1 °C. A plastic tube was attached to the membrane-enclosed perfume reservoir to allow the liquid perfume to be added into the perfume reservoir at 1 cm liquid head per 10 min. Once the liquid droplets appeared on the air side of the membrane surface, the static head of the air freshener membranes was determined. 2.5. Membrane Characterizations. The membrane morphology was observed by using a JSM-6700F field emission scanning electron microscope (FESEM). The pore size distribution of the membrane was further verified by a capillary flow porometer. Elemental mapping was taken by X-ray energy dispersive spectrometry (EDX) using an Oxford INCA energy dispersion of X-ray system equipped with a super ultrathin window (SUTW) connected to a SEM JEOL JSM-5600LV operating at 15 kV. Wide-angle X-ray diffraction (XRD) measurements of the membranes were carried out by a Bruker X-ray diffractometer (Bruker D8 advanced diffractometer) at ambient temperature using the Cu Kα radiation wavelength (λ = 1.54 Å) at 40 kV and 30 mA. An X-ray photoelectron spectrometer (Kratos XPS System-AXIS His-165 Ultra) was used to measure the surface chemistry of the membranes. The filler loading and thermal stability of the membranes were measured by TGA using a Shimazu Thermal Analyzer (DTG60AH/TA-60WS/FC-60A) with a heating rate of 10 °C min−1

Figure 1. Structure and molecular size of the selected perfume compounds.

2.2. Air Freshener Testing Experiments. A lab-made air freshener module was comprised of a lab-made aluminum device that contained 5 g of model perfume and a piece of the air freshener membrane with an area of 19.63 cm2, as illustrated in Figure 2. The model perfume can be either a mixture of the 4 compounds listed in Section 2.1, each 25 wt %, or a pure perfume compound. All air fresheners were tested under a fume hood with an air velocity at 0.1 ± 0.02 m/s at 25 ± 1 °C. The system was stabilized for 2 h before the air freshener was weighed (W1) by a Mettler Toledo balance. Then, the air freshener was weighed (W2) again to obtain a perfume release rate which was calculated by eq 1. release rate =

W1 − W2 A(t 2 − t1)

(1) 2

where A is the membrane area in cm and t1 and t2 are the times in h when W1 and W2 were measured, respectively. The perfume compositions in the air freshener were analyzed by two duplicate injections by using a Hewlett-Packard GC 7890 with

Figure 2. An air freshener placed in (a) the wet mode with a membrane and (b) the evaporation-only mode without a membrane. B

DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. (A) SEM and (B) FESEM images of the membrane top surface; (C−E) membrane cross-sectional images under FESEM; EDX mapping of (F) Si and (G) Ti.

to 800 °C under air atmosphere. The positron annihilation spectroscopy (PAS) experiments were carried out using a slow positron beam (0−30 keV) in National University of Singapore (NUS) with the detailed description given elsewhere.11 The positron annihilation lifetime spectra (PALS) were recorded with a fast−fast positron annihilation lifetime spectrometer to study the bulk properties of the membrane.12 Contact angles of the membranes were conducted on the membrane surfaces by using a goniometer (VCA Optima, AST Products Inc.) to determine the affinity with various perfume compounds. The membrane porosity, ε, was calculated according to eq 2:13,14 ε=

ρmat − ρmem ρmat

3. RESULTS AND DISCUSSION 3.1. Air Freshener Membrane Characterizations. 3.1.1. Membrane Characterizations. Figures 3A and B display the SEM and FESEM images of the membrane surface, respectively, while Figures 3C−E show the cross-section images. The membrane is generally symmetric, and the particle distribution appears homogeneous. The latter is further proved by energy-dispersive X-ray spectroscopy (EDX) on elements Si and Ti (Figures 3F and G). XRD results are presented in Figure 4 for both top and bottom surfaces. Both surfaces show similar XRD patterns where CaCO3 and TiO2 are detected at both surfaces, while SiO2 exists only in a small amount.15−17 As shown from the TGA data in Figure 5, the membrane starts to decompose at around 260 °C and reaches a weight loss of around 49 wt % at 720 °C. The residual of the membrane, i.e.,

× 100% (2)

where ρmem is the membrane density in g/cm3, which is equal to the membrane weight in g in air divided by its volume in g/ cm3. The density of the membrane material, ρmat, was estimated using a Mettler Toledo analytical balance ML204 and a density kit ML-DNY-43 (Zurich, Switzerland). The density kit applied the Archimedes’ principle based on eq 3 and the measured membrane weights in air and liquid. ρmat =

Mair ρ Mair − Mliq liq

(3)

where ρliq is the density of the applied liquid in g/cm3 (i.e., hexane). Mair and Mliq refer to the membrane weights in g in air and liquid, respectively.

Figure 4. XRD results of the top and bottom membrane surfaces. C

DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research ⎛ 40 ⎞ Z(E+) = ⎜ ⎟E+1.6 ⎝ ρ⎠

(4)

where Z stands for the mean implantation depth in nm, E+ is the incident energy in keV, and ρ refers to the density of the material in g/cm3.22−26 Figure 7 (left) indicates a slight variation between the top and bottom surfaces at the first 0.6 μm depth, and the bottom surface shows more Å-level space (i.e., free volume) compared to the top surface. However, as illustrated in Figure 7 (right), both surfaces show comparable results in terms of R-parameter, which implies that they have similar pore size distributions at the micrometer level. Therefore, the membrane is symmetric at the micrometer level. Figure 8 shows the free volume distribution of the bulk membrane by PALS. Two kinds of free volumes are detected, one with a radius of 2.94 Å and the other with 5.24 Å. The former is possibly from polyethylene chains because it has a dspace value of about 4 Å,27 while the latter may result from the interface between fillers and the polyethylene matrix. Beause the former has a higher intensity of 14.04% than the latter of 1.60%, their corresponding fractional free volumes (FFV) are 2.68 and 1.73%, respectively. Compared to the molecular sizes of perfumes (Figure 1), it is possible that some molecules could be trapped inside the larger free volume (diameter = 10.48 Å). 3.1.3. Membrane Porosity. Porosity can be estimated by the density measurements and the liquid sorption. The density measurement method gives the porosity of 42%, calculated according to eq 2, which is also confirmed by the liquid sorption. As shown in Table S2, the estimated amount of liquid uptake for a porosity of 42% (assuming all pores are filled with perfume compounds) is close to the calculated perfume sorption uptake with a 42% porosity. The small variation could be due to the experimental error or the evaporation of perfume molecules during weight measurements, especially for ethyl-2-methyl butyrate because of its higher vapor pressure compared to the other perfume compounds. 3.2. Wetting Studies. 3.2.1. Membrane Wetting by Pure Perfume Compounds. Table S3 lists the amounts of perfume compounds that one membrane can absorb by the liquid sorption at different temperatures, i.e., 25, 35, and 45 °C. The perfume compounds were specifically chosen to represent the more volatile to the least volatile compounds in a typical

Figure 5. TGA data of the membrane.

SiO2, CaO, and TiO2, is about 51 wt %. The addition of silica particles in the membrane increases its surface area, while the carbonate particles in the membrane reduce its tendency to deform (i.e., higher Young’s modulus)18 and improve its surface gloss.19 The incorporation of TiO2 serves as antimicrobial function20 as the membrane may be exposed to humid environments. In addition, the particles also probably contribute to a narrow pore size distribution.21 3.1.2. Membrane Pore Size and Free Volume Studies. Figure 6 shows the pore size distribution of the membrane. The bubble point pore diameter is 0.0217 μm. It means that the largest pore inside the membrane is around this size, and the smallest pore size is around 0.0187 μm. The average pore size is around 0.0203 μm. These data imply that the membrane has a narrow pore size distribution. To verify the surface structure of the membrane, PAS analyses were conducted. As illustrated in Figure 7, the S and R parameters are sensitive to Å-level space and μm-level pores, respectively. A higher S parameter corresponds to a higher free volume. To estimate the layer thicknesses, the mean implantation depth Z(E+) can be calculated from the equation below:

Figure 6. Membrane pore size distribution determined by the PMI capillary flow porometer. D

DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. Depth profiles of S and R parameters in the top and bottom membrane surfaces measured by PAS.

ethyl-2-methyl butyrate > benzyl acetate > phenyl ethyl alcohol > cyclamen aldehyde. The wetting is contributed by capillary and surface tension effects initially,28 followed by a further ingression of the liquid in the membrane because of its vapor pressure. This is because the higher vapor pressure enables the liquid perfume to enter the membrane pores with a greater distance.29 3.2.2. Membrane Wetting with a Perfume Mixture. When using a model perfume mixture consisting of 25 wt % each perfume compound, the membrane wetting under a saturated vapor condition is affected by the mixed solvent instead of each individual compound. As illustrated in Figure 9d, the rate of membrane wetting falls into a middle range of all four compounds at 25 °C. The predicted wetting rate, which is calculated as the averaged wetting height versus time, is close to the experimental data. Both the predicted and experimental wetting lengths reach ∼4 cm eventually. This implies that the air freshener membrane will remain wet throughout the product’s lifetime even when part of the membrane is not directly in contact with the liquid perfume. The top and bottom portions of the wetted membrane were isolated, immersed into ethanol to extract the absorbed perfume, and then evaluated by GC. Table S4 lists the composition in each portion. The amount of the absorbed perfume is normalized with benzyl acetate. In other words, the benzyl acetate weight is set as 1, and the ratios of other compounds to benzyl acetate are calculated accordingly. A small difference in terms of the perfume composition can be noticed between the top and bottom portions. Both top and bottom portions comprise all four perfume compounds with similar ethyl 2-methyl butyrate:benzyl acetate:phenyl ethyl alcohol ratios of 0.93:1.00:0.94 for the top part and 0.90:1.00:1.03 for the bottom part. Because ethyl 2-methyl butyrate has a higher vapor pressure than others, it may lose more before and during the GC tests. Thus, the actual absorbed amount of ethyl 2-methyl butyrate should be higher inside the mixture. For phenyl ethyl alcohol, it only reaches up to 2.5 cm when conducting the wetting experiment with the pure compound. However, it can be brought up by the other two more volatile chemicals to a height of 4 cm during the wetting experiment in the mixture. Similarly, cyclamen aldehyde, the least volatile compounds of these four, also appears in the mixture with less amounts (i.e., 0.69 and 0.65 for the top and bottom portions, respectively). Clearly, the greater capillary action induced by the more volatile compounds also enhances the wetting length of this less volatile chemical.

Figure 8. Free volume distribution of the membrane obtained from PALS.

perfume.6 For all four individual compounds, when the temperature increases, the total amount of liquid that the membrane can uptake remains constant and follows the trend: benzyl acetate > phenyl ethyl alcohol > cyclamen aldehyde > ethyl-2-methyl butyrate. The weight difference is mainly due to the difference in liquid density. The wetting lengths of the membranes in liquid perfumes under saturated vapor conditions are shown in Figure 9a. Clearly, the wetting length highly depends on the vapor pressure of the liquid: the lower the vapor pressure, the slower the wetting. Taking wetting in liquid ethyl-2-methyl butyrate under the saturated vapor condition as an example, the liquid compound starts to wet the membrane quickly upon contact. Subsequently, the membrane is wetted further due to vapor condensation and reaches a height of 4.5 cm in 2 days. However, the wetting levels for chemicals such as phenyl ethyl alcohol and cyclamen aldehyde after the same duration are much lower and less than 3 cm. The smaller wetting lengths are probably attributed to their low vapor pressures. As the wetting in a liquid under the saturated condition is probably contributed by liquid capillary and vapor condensation, it is therefore important to obtain the membrane wetting due to the vapor condensation (i.e., the membrane is not in contact with the liquid) under the saturated conditions, as shown in Figure 9b. If the wetting length by a vapor is known, the wetting length by liquid capillary can be calculated. As shown in Figure 9c, the wetting lengths by liquid capillary follow the trend: E

DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. Membrane wetting by (a) liquid perfume under saturated pure vapor, (b) pure vapor, and (c) liquid capillary and (d) mixed solvents under the saturated condition at 25 °C.

3.3. Air Freshener Perfume Release As a Function of Time. The perfume release rate of a membrane air freshener is an important parameter to determine if the air freshener can provide the required fragrance in a room. As such, the air freshener with the membrane in contact with the perfume (shown in Figure 2) was investigated and compared with an evaporation-only mode without a membrane. Figure 10 displays

and the evaporation-only mode have similar release rates because the perfume evaporation for a fixed environment primarily depends on its vapor pressure. Although the air freshener membrane with pore sizes of 10−30 nm31 is able to hold the liquid from dripping out from the membrane, it cannot regulate the perfume release rate to a more consistent range. Therefore, the air freshener tests under the wet mode have large inconsistent release rates similar to those of the evaporation-only mode. The detailed variations of the four perfume compositions in the reservoir with time are presented in Figure 11. Under the wet mode, the evaporation rates follow the order: ethyl-2methyl butyrate > benzyl acetate > phenyl ethyl alcohol > cyclamen aldehyde, which corresponds to the descending order of their vapor pressures. The wet mode and the evaporationonly mode have similar composition profiles where the ethyl-2methyl butyrate evaporates off at ∼70 h, while parts of other compounds remain in the reservoir. At ∼ 720 h, there was 84 wt % of the least volatile compound, cyclamen aldehyde, remaining in the reservoir. The individual perfume evaporation rates were also calculated from their concentrations in the perfume reservoir and their weight losses. Figure S3 shows the results. The evaporation rate of ethyl-2-methyl butyrate decreases from 4.6 to 0.1 mg/cm2-h in 2 days, while the evaporation rates of benzyl acetate and phenyl ethyl alcohol decline from 0.9 and 0.7 to 0.5 and 0.2 in 2 days, respectively. As a result, the cumulative evaporation rate continuously reduces as the more volatile compounds in the perfume evaporate off. To sustain the product’s lifetime of air fresheners, future research and development directions should focus on the design of new membranes32 and other freshener configurations33,34 with a relatively constant perfume release rate.

Figure 10. Release rates of the model perfume (a perfume comprised 4 compounds, each 25 wt %) from the membrane air freshener (testing conditions: air velocity: 0.1 m/s, temperature: 25 °C).

the overall evaporation rates as a function of time. Under the wet mode, the evaporation rate decreases from 6 to 0.03 mg/ cm2-h over the 720 h test duration. The release rate may be too high initially, but it drops to below 0.05 mg/cm2-h after 600 h. This is a little lower than the required perfume release rate for a small space which typically needs a release rate of 0.051−1.02 mg/cm2-h to scent a 28-m3 room.30 Interestingly, the wet mode F

DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 11. Variations of model perfume compositions in the air freshener as a function of time under (a) wet mode and (b) evaporation-only (testing conditions: air velocity: 0.1 m/s, temperature: 25 °C).

3.4. Transport of Perfume Compounds in the Membrane Air Freshener. The main functions of conventional air freshener membranes are to (1) allow a small amount of perfume flow in the membrane pores for evaporation at the surface and (2) prevent the liquid perfume from spilling out from the perfume reservoir. To prevent the perfume spillage from the reservoir, the liquid perfume must be withheld in the membrane pores as shown in Figure 12. In other words, the

N=

(6)

where ε is the membrane porosity and r is the pore radius in m. When the calculated liquid flux, 1.97 mg/cm2-h, is higher than the experimental release rate at 0.73 mg/cm2-h, the liquid flux cannot be completely evaporated off. As a result, liquid droplets appear on the membrane surface. In addition, the less volatile compounds require a lower liquid head for the liquid to spill out of the membrane because of the lower evaporation rate. To prevent the liquid from spilling out of the air freshener reservoir, the maximum perfume static head in the perfume reservoir must be considered in addition to the membrane properties such as porosity, pore size, and thickness together with the evaporation rate to design a membrane air freshener. Once the liquid perfume is near the membrane surface, evaporation of the perfume takes place. The perfume release rate, JA, may be estimated from eq 7:36

Figure 12. Perfume transport through a membrane pore in the air freshener.

N=

transmembrane pressure has to be less than or equal to the head loss from the membrane pore. The head loss and the liquid flow are governed by the Darcy−Weisbach equation, which is an empirical equation used to determine the frictional head loss in a pipe or a very fine channel.35 For a laminar flow, eq 5 is used to calculate the head loss, Δp, in Pa: Δp 128 μQ = L π D4

ε πr 2

4aD M v Pvs(T ) πd RT

(7)

where a is a numerical factor for a circular liquid disc, Mv is the molar mass of the vapor in g/mol, D is the perfume diffusivity of A in air in cm2/s, d is the diameter of the liquid disc in cm. Pvs is the perfume vapor pressure in kPa. R is the gas constant, and T is the temperature in K. Given the most dominant terms for perfume evaporation are related to the perfume physical properties such as diffusivity, molecular weight, and vapor pressure, the perfume composition has a dominant role on the release rate of the membrane air freshener as discussed in Section 3.3. In addition, the evaporation rate from the membrane pores may be reduced by the perfume condensation in the membrane pores,37,38 as reflected in the lower perfume release rates of the membrane air freshener compared to those of the evaporation-only (Figure 10). Therefore, the evaporation may be adjusted by manipulating the membrane properties and the design of the perfume reservoir.

(5)

where L is the length of the membrane pore in m, D is the pore diameter in m, μ is the dynamic viscosity in Pa·s, and Q is the volumetric flow in m3/s. Table S5 shows the volumetric flow for a single pore. The membrane flux can be calculated for the required static head if there is a liquid spillage from the membrane. The static heads of all perfume compounds can be determined except the most volatile compound, ethyl-2-methyl butyrate, which has a high evaporation rate. For example, when the static head of benzyl acetate in a reservoir (shown in a diagram in Table S5) is filled up to 11 cm, liquid droplets are observed at the bottom part of the membrane. The volumetric flow through a single pore as 3.89 × 10−24 m3/s can be then calculated with eq 5. Then, the flux through the membrane can be estimated by multiplying the number of pores, N, which is calculated by the following equation:

4. CONCLUSIONS We thoroughly studied the commercial membrane air freshener. Extensive characterizations of the membrane and intrinsic interactions between the selected perfume compounds and the membrane were carried out. In addition, the perfume transport through the membrane was also investigated. The following conclusions can be drawn: (1) Fillers such as SiO2, TiO2, and CaCO3 are detected inside the membrane with a total loading of around 51 wt %. (2) The membrane has a G

DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research homogeneous pore size of around 0.02 μm and symmetric structure at μm-level. (3) The liquid uptake in the membrane follows the trend: benzyl acetate > phenyl ethyl alcohol > cyclamen aldehyde > ethyl-2-methyl butyrate. The uptake amount is not affected by the temperature change. (4) Capillary tests show that the membrane wetting follows the same trend as the vapor pressure. The higher the vapor pressure, the quicker the wetting. (5) The membrane air freshener has a release rates ranging from 5.4 to 0.03 mg/cm2-h for a model perfume. The inconsistent release rate is primarily due to the change of perfume compositions throughout the lifetime of an air freshener. (6) Darcy−Weisbach equation can be used to predict if a membrane can prevent the spillage of liquid perfume.

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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.iecr.7b04134. Physicochemical properties of the four selected perfume compounds; experimental and calculated liquid sorption amounts of the air freshener membranes; contact angles and the liquid sorption behaviors of the perfume compounds; compounds absorbed through the capillary; and the calculated release rate that the liquid perfume spills out of the membranes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 65 6516 6645; Fax: 65 6779 1936; E-mail: chencts@nus. edu.sg. ORCID

Tai Shung Chung: 0000-0002-6156-0170 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank P&G (Grants R-279-000-455597 and R-279-000-455-720) and A-STAR (Grant R-279-000455-305) for funding this work. The authors would also like to thank Dr. Kuo-Sung Liao for the PALS analysis of the membrane; Miss Jianqiao Cui, Mr. Xiao Han, and Miss Shiyu Zhang for conducting some experiments. Miss Shiyu Zhang is also appreciated for proof reading of the manuscript. Thanks are also due to Dr. Susilo Japip for simulating the chemical structures of the perfume compounds and valuable discussion. Special thanks are due to Dr. Bee Ting Low for her suggestions to this work.

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DOI: 10.1021/acs.iecr.7b04134 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX