Water Adsorption on Hydrophilic Fibers and Porous and Deliquescent Materials: Cellulose, Polysaccharide, Silica, Inorganic Salt, Sugar Alcohol, and Amino Acid

Water adsorption isotherms are systematically summarized by using celluloses and polysaccharides as hydrophilic crystal/amorphous materials with functional groups, silicas as hydrophilic porous materials, and inorganic salts, sugar alcohols, and amino acids as hygroscopic deliquescent materials. For hydrophilic fibers such as celluloses and polysaccharides, water was adsorbed on amorphous solids, and water clusters were formed around functional groups. For porous materials such as silicas, capillary condensation occurred in the micropores of silicas. For deliquescent materials such as inorganic salts, sugar alcohols, and amino acids, water adsorption rapidly increased stepwise at a specific threshold relative humidity, accompanied with a structure transformation to a liquid state. In addition, the water activity (Aw) of materials used in packed products was able to be estimated from the water adsorption isotherms of the pure component. This indicated that the deliquescent materials have a great effect on the depression of Aw for the suppression of microbial growth at an extremely high water content. The deliquescent materials could be useful to develop new environmentally and sustainable products and technologies with the mediation of water vapor and/or hydration.


INTRODUCTION
In recent years, hydrophilic materials have been gaining considerable interest for various water-related technologies in the fields of food, water, energy, biotechnology, environment, and medicine industries. 1,2irst, product quality depends on the water content of materials used in packed products.−5 Undesirable migrations of water have caused serious deterioration in the quality.Water adsorption includes the adsorption on solid surfaces or into amorphous solids, the capillary condensation into micropores, the deliquescence, and the crystal hydration. 6The relationship between the water adsorption behavior and material structural properties for practical/commercial applications has been summarized. 2That is, the hygroscopic behavior is characterized by the profile of a water adsorption isotherm.Different types of materials exhibit different water capacities and shapes of water sorption isotherms.Therefore, systematic water adsorption isotherms of materials used in products are crucial to predicting the water content and quality of products during storage, transport, and marketing.
Second, the hygroscopic characteristics of materials used in packed products play an important role in the product preservation and hygiene of products during storage, transport, and marketing. 7,8Moist foods are liable to be contaminated by certain microorganisms, and their microbial growth requires nutrient and temperature to a certain extent as well as free water available to bacteria.Hence, FDA sets the water activity (Aw) of foods to 0.85 or less as a criterion for preventing food spoilage and microbial growth. 9In addition, salt preservation and sugar preservation methods are well-known to depress water activity. 7,8Herein, water activity is a water vapor pressure (p) in equilibrium with materials having a given water content divided by a saturated vapor pressure (p o ) of water at the same temperature 10,11 p p Aw / (0 Aw 1) o

=
The water activity at a given water content can be determined by adsorption thermodynamics or solution thermodynamics. 11herefore, to prevent microbial contamination and store products for a long time, the water activity should be set to 0.85 or less by adding inorganic salts and sugars.
Third, scientists have been interested in deliquescent materials.Studies on the hygroscopic behavior of deliquescent atmospheric aerosol particles such as NaCl have shown that an adsorbed water layer in core−shell form on the surface of airborne atmospheric aerosols lead to impacts on global climate change. 12,13Furthermore, applying deliquescence to products from a different point of view, a formulation has been filed in which an active ingredient is retained in a glassy transparent film formed by a deliquescent sugar alcohol and is gradually released in a sustained manner upon use as a product for transmucosal oral administration of the active ingredient. 14Qualitative and quantitative information regarding deliquescence may also be helpful for improving product qualities and adding a new feature with an associated self-organizing surface layer.
The objectives of this study are to summarize the water absorption−desorption isotherms for various hydrophilic materials such as celluloses and polysaccharides as hydrophilic crystal/amorphous materials with functional groups, silicas as hydrophilic porous materials, and inorganic salts, sugar alcohols, and amino acids as hygroscopic deliquescent materials, to discuss the hygroscopic characteristics of water adsorption behavior, and to provide useful information on the aspects of water-related utilizations with the mediation of water vapor and/ or hydration.

EXPERIMENTAL SECTION
2.1.Materials.For celluloses, Vitacel powdered cellulose samples of L600-30 with a particle size of 30 μm, L00 with a particle size of 120 μm, and LC200 with a particle size of 300 μm and a Vivapur microcrystalline cellulose sample of 200 with a particle size of 190 μm were supplied by J. Rettenmaier (USA).Viscopearl porous cellulose was supplied by Rengo Co., Ltd.(Japan).Spherical microcrystalline cellulose (MCC) was supplied by Asahi Kasei Corporation (Japan).
For salts, sodium dihydrogen phosphate, potassium carbonate and sodium carbonate, and sodium chloride were supplied by Univar (UK) and trisodium phosphate was supplied by Israel Chemical Limited (Israel).Calcium lactate was supplied by Taihei Chemical Industrial Co., Ltd.(Japan).
For sugar alcohols, xylitol was supplied by Roquette (France) and maltitol was supplied by Caldic Nordics (Sweden).
For sweeteners, acesulfame K was supplied by Univar and sucralose was purchased from FUJIFILM Wako Pure Chemical Corporation (Japan).
2.2.Water Adsorption Studies.Water vapor adsorption isotherms (at 25 °C) of samples were measured volumetrically by using an adsorption apparatus (Belsorp, MicrotracBEL Corp., Osaka, Japan).To remove any organic residues and moisture from the sample surface without surface reactions, the silica and others were evacuated below 0.1 Pa at 250 °C for 2 h and 105 °C for 1 h before adsorption, respectively.The preliminary tests confirmed that these desorption conditions were adequate to ensure that the adsorbed water could be measured with good reproducibility.The tolerance ranging from 0 to 90% R.H. p/p o is within 0.3% of the reading variation per 300 s, and that ranging from 90% R.H. to ca. 95% R.H. is within 0.3 Pa of the reading variation per 300 s.

RESULTS AND DISCUSSION
The water adsorption isotherms of celluloses, polysaccharides, silicas, salts, sugar alcohols, amino acids, and others are shown in Figures 1−7.3.1.Cellulose.As shown in the water adsorption isotherms of celluloses in Figure 1, they are increased in the area of zero relative humidity and the surface is hydrophilic. 3−17 However, the water content of celluloses was less than those of others and was not related to the fiber length.−20 Although Tammelin et al. 21reported that a more crystalline cellulose film possessed nanoporosity, more surface area (more binding sites for water), and higher adsorption capacity of water when compared to an amorphous cellulose film, the lower water content of Vivapur and MCC with higher crystallinity 22 has proven that they have less porosity, as shown in Figure 1.As shown in the macropore size distribution (i.e., greater than 50 nm) obtained by mercury intrusion porosimetry in Figure 8, Viscopearl, which is composed of regenerated celluloses made from viscose, is a porous powder with an average macropore volume equal to or greater than 8 times that of the other celluloses.However, the water content of Viscopearl is about 1.5 times more than that of the other celluloses and is less dependent on the macropore volume.In general, smaller molecules are more easily adsorbed into smaller pore spaces and are more strongly adsorbed due to micropore filling. 23,24This means that water capacity depends on the small pore volume ranging from micropores (i.e., less than 2 nm) to mesopores (i.e., 2−50 nm) along with the larger surface area.
Moreover, water adsorption isotherms of citrus fiber are also shown in Figure 1, and its water content was higher than that of cellulose, indicating that polysaccharides have a higher adsorption capacity of water than celluloses, as will be discussed later.
The macropore size distribution of the samples was determined by mercury intrusion porosimetry using a Pore Master 60-GT (Quantachrome Instruments, Boynton Beach, FL, USA); it can cover pores ranging from 6.5 nm (at a usual high pressure of 33000 psia) to 10 μm (at 20 psia).The sample weight is 0.2 g.The cell size is 10φ × 30 mm, and its volume is 0.5 cc.The contact angle of mercury is 140°, and the surface tension of mercury is 480 dyn/cm.The samples were evacuated for 105 °C and 2 h before measurement.
3.2.Polysaccharide.The potential uses of natural polysaccharides have recently attracted interest in water purification, drug delivery, tissue engineering, agriculture, and antimicrobial and biomedical applications. 1The water adsorption isotherms of the polysaccharides are shown in Figure 2. The water adsorption isotherms are typically sigmoidal in shape, have been classified as type II, and are more than those of cellulose. 16,17,22Polysaccharides have plenty of functional groups, such as a carboxyl group and amino group.−27 Thus, it can be seen that the water content of the polysaccharide was relatively high, and for example, at the condition of an Aw of 0.85, the water content of the polysaccharide is 2 to 5 times greater than that of celluloses.The water adsorption isotherms of polysaccharide increased in the following order: gellan < tragacanth gum < pectin, gum arabic, κ-carrageenan, agar < sodium alginate, xanthan gum.

Silica.
Wet process silica has several types, such as precipitated and gel-type silica.The precipitated or gel-type silica was synthesized by reacting a sodium silicate solution with  sulfuric acid under an alkaline or an acid condition, respectively.The precipitated silica is produced by neutralizing silica at reaction conditions of a relatively high temperature and an alkaline pH range.The primary particles grow at a faster rate and are highly condensed in flock-like form, which are then washed with water, filtered, and dried before consolidation.Alternatively, the gel-like silica is produced by consolidating the primary particles under the reaction conditions where the growth of the primary particles is suppressed by allowing the neutralization reaction to proceed in an acidic pH range, and the primary particles are transformed into a three-dimensional network structure.−30 Recently, the precipitated silica having a structure close to that of gel-type silica has also been produced by using a synthesis method in which primary particles are grown in an aggregated state after causing aggregation while controlling the reaction temperature, pH, and salt concentration of the synthesis of silica primary particles and suppressing the growth of primary particles. 29,30The gel-type silica is featured by a high pore volume of micropores and mesopores without macropores (i.e., μm-sized voids). 30The pore properties and silanol group numbers of precipitated silica and gel-type silica are listed in Table 1, and the pore size distribution is shown in Figure 9.As shown in Table 1, M.S.GEL, Carplex BS-305, and Sylopage,   which have a feature of gel-type silica, have plenty of micropores and mesopores without macropores, and these surface functional groups (i.e., silanol groups) are less than those of Sipernat, which has a feature of precipitated silica.In addition, as shown in Figure 3, the water adsorption isotherms of silicas exhibited type IV isotherms, and the water adsorption isotherms of M.S.GEL, Carplex BS-305, and Sylopage are higher than those of Sipernat at the same relative humidity.The equilibrium water contents of the gel-type silicas reached as much as 2−7 times as those of precipitated silica at a relative humidity of 85%.Thus, the high performance of water adsorption with the gel-type silica resulted from the capillary condensation into the abundance of micropores and mesopores with a hydrophilic surface.Also, since the hysteresis loop of the adsorption and desorption route was remarkably observed, it was proven to be difficult to desorb when water was adsorbed once.
The specific surface area (S BET ) of the samples was determined from N 2 adsorption−desorption isotherms obtained at 77 K using a BELSORP-max (BEL Japan, Osaka, Japan); the specific surface areas were calculated using the Brunauer− Emmett−Teller (BET) method.The pore volume and pore size distribution were determined using the Barrett−Joyner− Halenda (BJH) for mesopores and macropores and Horvath− Kawazoe (HK) methods for micropores. 40A micropore, mesopore, and macropore are defined as a pore with a diameter of ≤2 nm, 2−50 nm, and ≥50 nm, respectively.The total pore volume was assessed to be the N 2 adsorption volume at a relative pressure (p/p o ) of 0.99.The number of silanol groups (Sears number, ρ(pcs/nm 2 )) on the surface of silica by titration was determined by a method as developed by Sears. 41The Sears number was given as where B is the consumed volume of sodium hydroxide between pH 4 and pH 9 (mL), N A is Avogadro's number, A is the sample weight (g), and S BET is the specific surface areas (nm 2 /g).

Inorganic Salt.
The adsorption isotherms of salts are shown in Figure 4 and exhibited type IV isotherms.Water adsorbs significantly more on the inorganic salts than on the others in the range of over 80% R.H.The adsorption capacity of water can reach as high as 10 times the sample weight for sodium chloride, 7 times the amount for potassium carbonate, 6 times the amount for sodium carbonate and sodium dihydrogen phosphate, and 5 times the amount for trisodium phosphate.These findings show that the inorganic salts are hygroscopic deliquescent materials and have the highest adsorption capacity of water.The water adsorption isotherm gradually rises to a threshold relative humidity, rapidly increases stepwise when the relative humidity reaches a threshold level, and then rises gradually until it becomes saturated, while the water desorption isotherm decreases monotonically with decreasing relative humidity even when the relative humidity reaches the threshold level. 31As shown in Figure 4, the threshold relative humidity is 75% for sodium chloride, in agreement with the previous reports. 32,33Moreover, the threshold relative humidity is 76% for sodium carbonate, 40% for potassium carbonate, 68% for sodium dihydrogen phosphate, and 32% for trisodium phosphate.It is reported that a phase change of the inorganic salts occurs from solid to liquid by the absorption of water vapor at the threshold relative humidity. 34Additionally, it has been reported that in metal/metal oxides, silicon/silicon oxides, fluorides, and two-dimensional materials, the liquid water structure is superior to the solid-like structure, also denoted as  the ice or ordered structure, over 30−60% R.H. 35 and the solid network swelling would occur due to electrical repulsion between the functional groups or electrostatic interaction at high water contents. 11The findings indicated that at the threshold relative humidity, in the adsorption route, the structure swells and transformation to a liquid state occurs, but even at the threshold relative humidity in the desorption route, the inorganic salts do not transform to the initial solid state and keep the liquid state for a while, which is responsible for the adsorption hysteresis loop in the isotherms.
3.5.Sugar Alcohol.The water adsorption isotherms of sugar alcohols are shown in Figure 5.Although the water adsorption isotherms of xylitol are reported to be nonlinear, typically sigmoidal in shape, and have been classified as type II, 17 the water adsorption isotherms of xylitol and maltitol exhibited combined isothermal adsorption isotherms (between type II and type IV) and rapidly increased stepwise at the threshold relative humidity, in a similar behavior to the salts, as shown in Figure 5. Sugar alcohols are also hygroscopic deliquescent materials.The adsorption capacity of water can reach as high as 3 times the sample weight for xylitol and 1.4 times the amount for maltitol.The threshold relative humidity is 77% for xylitol and 87% for maltitol.Sugar alcohols have a high adsorption capacity of water.
3.6.Amino Acid.To understand the deliquescence characteristics of amino acids at an extremely high relative humidity ranging from 80 to 99.5%, the water adsorption isotherms were measured even when liquefied, as will be mentioned later.The water adsorption isotherms of amino acids are shown in Figure 6.The hygroscopicity of amino acids including hysteresis depends on the interactions between polar groups such as coordination into the crystal structure. 36The hygroscopicity of amino acids in atmospheric aerosols was also reported to be influenced by whether or not crystallization and deliquescence occur. 37irst, the water adsorption isotherms of γ-aminobutyric acid and monosodium glutamate exhibited the combined isothermal adsorption isotherms (between type II and type IV) and increased stepwise at the threshold relative humidity.The adsorption capacity of water can reach as high as 1.7 times the sample weight for γ-aminobutyric acid and 2.5 times the sample weight for monosodium glutamate.The threshold relative humidity is 74% for γ-aminobutyric acid and 86% for monosodium glutamate.γ-Aminobutyric acid and monosodium glutamate are deliquescents and have a high adsorption capacity of water.It is reported that a deliquescent material exhibits a solid to solution phase transformation when the relative humidity reaches its threshold, with kinetics increasing as the relative humidity further increases. 38The findings showed that their water uptake is attributed to liquefaction over the threshold relative humidity.Both γ-aminobutyric acid and monosodium glutamate changed from white powder to viscous transparent fluid when deliquescence occurs at a high relative humidity (see also Figure S1 in the SI).When evaluated using a spectrophotometer (CM-5, Konica Minolta, Japan), γ-aminobutyric acid and monosodium glutamate after the water adsorption experiments are shown as transparent.After a while, γ-aminobutyric acid remains a viscous transparent film, while monosodium glutamate becomes an opaque solid (see also Figure S1 in the SI).The stability of deliquescence appears to depend on the stability of a quasi-stable state of the liquid state.
Second, although the water adsorption isotherm of arginine increases stepwise at the threshold relative humidity of 40%, the adsorption capacity of water is just 0.2 times the sample weight and has a low adsorption capacity of water.3.7.Artificial Sweetener.As shown in the water adsorption isotherms of sweeteners in Figure 7, water hardly adsorbed on the artificial sweeteners (sucralose and acesulfame K) and the water adsorption isotherms of sweeteners exhibited type II isotherms.Artificial sweeteners have low adsorption capacity of water.
3.8.Application to Water Activity Estimation of Products.For the suppression of microbial growth, a mass fraction of mixture with an Aw of 0.85 or less at a given water content is essential to be fixed experimentally by adjusting deliquescent materials such as salt, sugar, and the like.When the water activity can be calculated by using the water adsorption isotherm from various mass fractions, the mass fraction having the Aw of 0.85 or less can be efficiently determined.Kumagai et al. 11 reported the measurement of water sorption isotherms of superabsorbent polymers, and their water activity was evaluated by solution thermodynamics to prevent microbial contamination during transportation or storage.However, since it is difficult to apply solution thermodynamics to solids that do not dissolve in water, the water activity was determined by adsorption thermodynamics.In this study, the total water content of multicomponents at a given Aw is equal to a weighted average of water contents of pure components at a given Aw, as reported in a previous study. 39That is, the water activity of the mixture is calculated by summing the multiplication of each water content by its mass fraction in the pure component.
The estimation of Aw was proven as follows.First, a mass fraction of the mixture was determined in equilibrium with the Aw of 0.85 under the condition of a water content of 50% wet basis (w.b.) by experimentally repeating the mixing and the measurement of Aw with a LabMaster-aw NEO (Novasina, Lachen, Schweiz).Second, the water content of a pure component in equilibrium with the relative humidity of 85% was obtained from the water adsorption isotherm, and the total water mass of the mixture was calculated by summing the water mass in each component, as listed in Table 2.The estimated water content (% w.b.) becomes 51% w.b. and 54% w.b. in the adsorption and desorption routes, respectively, and almost agrees with the experimental value (50% w.b.).
Ghorab et al. 31 reported that synergistic interactions, among the deliquescent components partially or completely dissolved, occur among the blending of NaCl and maltodextrins when the deliquescent components such as sodium chloride were in contact with or in close proximity to the amorphous maltodextrin particles.However, as the water content of the mixture can be calculated by the pure adsorption isotherm, the blending of the deliquescent solids and the others has little synergistic effect.This indicated that plenty of water adsorbs on the deliquescent solids around the Aw of 0.85, except for maltitol with a threshold relative humidity of 87% and sodium glutamate with a threshold relative humidity of 86%, and a further increase in the equilibrium water content by blending does not occur.
The water contents (% w.b.) in equilibrium with the Aw of 0.85 in the adsorption and desorption routes are listed in Table 3.By utilizing these values, it is possible to calculate the mass fraction of mixture depressing the Aw equal to or less than 0.85 to suppress microbial growth during transportation, storage, and marketing.This demonstrated that the deliquescent materials, i.e., the inorganic salts, sugar alcohols, and amino acids, can have the greatest effect on the depression of the Aw.

CONCLUSIONS
The water adsorption−desorption isotherms of celluloses, polysaccharides, silicas, salts, sugar alcohols, amino acids, and others were measured.Celluloses and polysaccharides were used as the hydrophilic crystal or amorphous materials with functional groups, silicas were used as the hydrophilic porous materials, and the inorganic salts, sugar alcohols, and amino acids were used as the deliquescent materials.
For the celluloses and polysaccharides, polysaccharides with plenty of functional groups have higher adsorption capacity of water than celluloses, with associated cluster formation.For the silicas, the adsorption capacity of water depends on their pore volume based on capillary condensation into the micropore.The gel-like silicas that were synthesized under acidic conditions have a greater pore volume of micropores and mesopores without macropores than the precipitated silica that was synthesized under alkaline conditions.The equilibrium water contents of the gel-type silicas showed 2−7 times more than those of the precipitated silicas at a relative humidity of 85%.For the hygroscopic deliquescent materials, the inorganic salts induced the water content uptake above the threshold relative humidity, water is absorbed on them from 5 to 10 times their own mass, and the threshold relative humidity ranges from 32 to 75%, with associated structural transformation.Sugar alcohols and some amino acids also induced the water content uptake and have a water adsorption capacity several times their own mass, but their threshold relative humidity of over 77% is relatively high compared with the inorganic salts.Maltitol with a threshold relative humidity of 87% and monosodium glutamate with a threshold relative humidity of 86% have little effect on depressing the Aw below 0.85.
The water activity of products can be estimated using the water adsorption isotherms of pure material used in products and the effect on depressing the Aw for the suppression of microbial growth increased in the following order: celluloses < silicas < polysaccharides < sugar alcohols < amino acids < inorganic salts.In addition, γ-aminobutyric acid and monosodium glutamate can form a self-organizing surface layer on the product with the mediation of water vapor and/or hydration, which makes it possible to improve product qualities and add a new feature.
Appearance of amino acids immediately and preserved for 9 days after the water adsorption experiment (PDF)

■ ACKNOWLEDGMENTS
The author thanks UBE Scientific Analysis Laboratory, Inc. for supporting the mercury intrusion porosimetry and titration analysis.

Figure 9 .
Figure 9. Micropore and mesopore pore size distribution of silica obtained using the (a) HK method and (b) BJH method.

Table 1 .
Specific Surface Area, Micropore, Mesopore, and Macropore Volume, and Number of Silanol Functional Groups

Table 2 .
Estimated Water Content at a Water Activity of 0.85

Table 3 .
Estimated Water Content at a Water Activity of 0.85