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Soil Science Society of America Journal 65:1129-1135 (2001)
© 2001 Soil Science Society of America

DIVISION S-2—SOIL CHEMISTRY

Effects of Amphiphilic Amines on Moisture Characteristics of Alluvial and Volcanic Soils

Atsushi Suetsugu*, Tsuyoshi Miyazaki and Masashi Nakano

Laboratory of Environmental Soil Physics and Soil Hydrology, Department of Biological and Environmental Engineering, The Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

* Corresponding author (suetsugu{at}par.odn.ne.jp)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
If organic matter (OM) has amphiphilicity, the properties of hydration of the OM-coated soil particles would vary drastically in dry conditions. Therefore, soil-moisture characteristics within the relatively low water potential should be understood in relation to the conformation of the sorbed amphiphilic OM. In the present study, the effects of amphiphilic behavior of OM on the moisture characteristics of alluvial soil (AS) and volcanic soil (VS) were investigated using three simple amphiphilic amines. The psychrometry of AS showed the decrease of sorbed water around 1.8 nm of statistical thickness because of the sorption of hexadecyltrimethyl ammonium (HDTMA). Fourier transform infrared spectroscopy (FTIR) of soils showed an increase in the wavenumber of the antisymmetric stretching of -OH and -CH2, suggesting the formation of hydrophobic outer surfaces by n–hexadecyl function. The peaks in the -CH2 rocking band suggest that the HDTMA sorbed at AS had the residual amorphous moieties. In the case of VS–HDTMA, the peak was hidden by a broad peak due to hydrated aluminosilicates in VS. These findings imply that the amorphous moieties of HDTMA sorbed at both soils were affected by residual water in the air dried samples. According to the clay mineralogy of the soils, the greater hydrophobicity in AS–HDTMA was attributed to intercalation of HDTMA into the expansible phyllosilicates in AS. In contrast, the relatively moderate hydrophobicity in VS–HDTMA indicated that the hydrophilic micropores (D < 2 nm) in VS restrict the sorption of HDTMA but enable it to exchange water.

Abbreviations: AS, alluvial soil • AA, amphiphilic amine • CEC, cation-exchange capacity • CMC, critical micelle concentration • ESR, electron spin resonance spectroscopy • FTIR, Fourier transform infrared spectroscopy • HDTMA, hexadecyltrimethyl ammonium • HDTMAC, hexadecyltrimethyl ammonium Cl • NMR, nuclear magnetic resonance spectroscopy • OM, organic matter • SOM, soil organic matter • TMA, tetramethyl ammonium • TMPA, trimethylphenyl ammonium • VS, volcanic soil


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
AMPHIPHILICITY IS A distinctive trait of soil organic matter (SOM) among soil components, and it sometimes causes serious changes in physical or physicochemical properties of soils, such as moisture characteristics, water infiltration, aggregate stability, and almost all other properties that play a role in controlling water distribution within the soil environment. The amphiphilicity of SOM has been regarded as the property that represents the heterogeneous nature of sources or humification paths. In some cases, it has been indicated that the aliphatic species of SOM are important participants in amphiphilic behavior of SOM (Guetzloff and Rice, 1996; Milanovskiy et al., 1995; Holmgren et al., 1990). Capriel et al. (1995) found a relatively large amount of aliphatics in sandy soils from a large number of FTIR measurements of various textured arable soils. Their study indicated that the selective degradation of hydrophilic protein and hydrocarbon occurs when the soil has a lower specific surface and a lower ability to retain hydrophilic moieties. The relatively small amount of aliphatics would not play a constitutional role in the humic substances (Orlov, 1995), but there have been some observations of amphiphilic behavior in humic substances, which implies the participation of hydrophobic species in the microorganization of dissolved OM (Guetzloff and Rice, 1996; Engebretson and von Wandruszka, 1994). Furthermore, 13C-nuclear magnetic resonance spectroscopy (13C-NMR) on bulk soils, and humic materials samples have shown predominantly aliphatic nature (Hatcher et al., 1985; Kögel-Knabner et al., 1989, 1997; Zech et al., 1997).

When solid surfaces exist, typically in the soil–water system, it has been predicted that the distribution and arrangement of microorganization of dissolved OM are changeable through interaction with the surfaces (Israelachvili, 1992; Mukerjee et al., 1995). This phenomenon is sometimes known as surface aggregation or hemi-micellization in light of the simultaneous progression of aggregation and sorption. If OM forms the surface coating of soil particles, the contribution of surface aggregation is not negligible, because the macroscopically observed coating of the OM is expected to be stabilized by long-range interactions such as hydrophobic interactions (Xu and Boyd, 1995). In the study of surfactant sorption at soils, cationic surfactant sorption above the cation-exchange capacity (CEC) of soils is usually attributed to the hydrophobic interaction between hydrophobic moieties of sorbed and dissolved species (Xu and Boyd, 1995). These effects are typically understood by reference to a hemi–micellization model (Somasundaran and Fuerstenau, 1966).

In relation to the soil moisture characteristics, the surface coating of OM causes strong hysteretic behavior of the soil (Nakaya, 1981). In the water imbibition process, the energy required for hydration of the surface coating causes water repellency (Nakaya et al., 1977). Water repellency was observed with a surface coating of humic substances on sand (Nakaya, 1981). The effects of OM on water repellency are usually significant in sandy soils because of their relatively low surface area, which will make the thickness of the surface coating larger than it is in clayey soils (DeBano et al., 1970). Although such explanations are usually consistent, the relationship between the thickness of the sorbed-OM layers (solloids) and the hydration property is still not clear. For example, it has been indicated that some siloxane-based synthetic polymers cause maximum water repellency at monolayer sorption (Fink, 1970). However, Ma'shum et al. (1988) observed the maximum water repellency when 16 times of the requirement for the monolayer formation of 1–hexadecanol was sorbed on the soil particles. These observations suggest that a number of factors affect water repellency other than the thickness of solloids. Boyd et al. (1988) made further observation of hydrophobicity development in some swelling phyllosilicates by the interlayer sorption of alkyl ammonium ions. These studies (Boyd et al., 1988; Jaynes and Boyd, 1991) showed that the amphiphilicity of solloids is affected by the swelling characteristics of clay minerals and the length of the hydrophobic moieties of OM.

The amphiphilicity of solloids has been studied in relation to the colloidal behavior of clay minerals or clayey soils. However, there is less information about the solid-state characteristics of the sorbed amphiphilic OM layer as observed in water-repellent soils. According to the hemi–micellization model (Somasundaran and Fuerstenau, 1966), the amount or the conformation of amphiphilic OM sorption does not vary with the loading of the OM above the critical micelle concentration, (CMC). This is inconsistent with the current understanding of hydrophobic interactions, which are characteristic of the liquid–crystalline region (Israelachvili, 1992) because a decrease in the content of water relative to the OM means a corresponding decrease in interactions among the hydrophobic moieties of the OM. Somasundaran and Krishnakumar (1994) indicated the major contribution of a small amount of water to the lateral diffusion of sorbed surfactant molecules in nonaqueous systems using various spectroscopic methods, such as electron spin resonance spectroscopy (ESR) or FTIR. These spectroscopic methods have provided information on OM–water interactions, but they have usually been conducted without reference to a precise measurement of water potential. If SOM contains amphiphilicity similar to that of surfactant molecules, the hydration properties of the OM-coated soil particles should vary drastically in dry conditions. Therefore, soil moisture characteristics at relatively low water potential should be understood in relation to the conformation of the sorbed amphiphilic OM.

In the present study, we aimed to clarify the effects of amphiphilic behavior of OM on moisture characteristics of soils using simple amphiphilic amines (AAs), emphasizing the changes in moisture characteristics because of the sorption of AAs in the different types of soils; and the relationship between the hydration properties of soil surfaces and the stability of the hydrophobic moieties of the sorbed AAs.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Samples
Volcanic soil was sampled from a B horizon of Pachic Melanudand in the Field Production Science Center of The University of Tokyo (Nishi-Tokyo, Japan). Volcanic soil contains principally allophane (up to 10%), imogolite and gibbsite. Allophane is an amorphous aluminosilicate that forms microporous hollow spherules 3.5 to 5.0 nm in diameter (Wada and Wada, 1977). Alluvial soil was sampled from a B horizon of a Fluventic Dystrochrept in a leek (Allium fistulosum L.) farm in Fukaya City, Japan. Alluvial soil contains principally vermiculite (up to 44%), smectite, kaolinite, and mica. Both soils were treated with H2O2, saturated by Ca2+ using CaCl2 solution, dried at 105°C, and then cooled in a desiccator. In the H2O2 treatment, each soil sample was placed in a 500-ml beaker up to one-third of the height, and ponded with distilled water. Twenty-milliliters of an aqueous solution of hydrogen peroxide (30%) was added to the sample. The beaker was covered with a watch glass. Then, the beaker was heated with a steam bath. The sample was stirred well, intermittently. When the bubbles in the sample vanished, H2O2 was supplied. When the color of the soil sample changed into gray or yellow-brown (indicating the reduction of a dark-colored organic fraction), and the bubbles could not be observed in spite of the addition of H2O2, the oxidation of the organic fraction of the samples was considered complete. The beaker was continuously heated to remove the residual H2O2 in the sample. Then, the sample was cooled and air-dried.

Sorption of Amphiphilic Amines
About 10 g of each soil sample was mixed with 90 g of 13 mM aqueous solutions of HDTMA, tetraethyl ammonium (TMA), and trimethylphenyl ammonium (TMPA), respectively. The mixture ratio of dry soil/solution was precisely adjusted to 1:10 (w/w). Each mixture was shaken for 72 h in a 200-ml Pyrex glass bottle (Corning Glass Works, Science Products, Corning, NY) at 240 rpm. Shaking for 72 h was sufficient for equilibrium sorption (based on the results of shaking trials of 12 to 144 h). Each of the suspensions was filtered through a 0.2-µm pore membrane filter (Millipore Co., Bedford, MA). Total C of the obtained solution was analyzed by a NC-ANALYZER (Sumika Chemical Analysis Service, Japan). The amount of the sorption was calculated from the residual C content in the solution. The results of the three prepared blank solutions were averaged and used to calculate the amount of sorption. The pH value of the equilibrium solution was measured by glass electrode.

Psychrometry
Moisture characteristics of the untreated and AA-sorbed soils were measured by psychrometry, a method that is considered suitable for measuring total water potential in a relatively dry region (Rawlins and Campbell, 1986). Six 1-g portions of the sample were treated with 0.5, 0.3, 0.1, 0.05, 0.03, or 0.01 g of distilled water, respectively. For moisture equilibration, the samples were kept at 25°C overnight. The psychrometer (SC-10A, Decagon Devices, Inc., Pullman, WA) was calibrated with saturated solutions of LiCl, MgCl2, Mg(NO3)2, KCl, KNO3 and distilled water. The water potential of each sample was measured according to the SC-10A operator's manual (Decagon Devices Inc., 1980). Briefly, each sample was placed in a sample cup. The cup was set in the psychrometer chamber, and allowed at least 20 min for temperature and moisture equilibration. Then, the humidity in equilibration with the sample was calculated by measuring the wet bulb depression as the decrease of electrical potential of the wetted thermocouple.

Each of the moisture characteristic curves obtained by the psychrometer was translated into t-plots that indicated the relationship between the amount of sorbed water (y-axis) and the statistical thickness of the sorbed water layer (x-axis, referred to as the t-value). If the sample consists mostly of macropores (D > 50 nm), the shape of the t-plots is almost linear, but if the sample has some micropores (D < 2 nm) or mesopores (2 < D < 50 nm), the gradient of the t-plots in the region t < 2 nm will be greater than that of in the region t > 2 nm. The gradient of t-plots also represents the density of the hydrophilic sites of the pore surfaces.

The reference data for nonporous silica that predicts the exact thickness of sorbed water were used to transform the obtained water potential by psychrometry into t-values (Naono and Hakuman, 1991). The details of the procedure were originally described by Linsen (1970). Precise transformation requires reference data from the same materials as the components of the soil. We do not discuss here the absolute difference between hydration properties of silica and soil samples, since preparation of nonporous soil components is usually not possible. Therefore, we used the obtained data for the approximate estimation of pore-size distribution, and for the comparison of hydration properties among the measured soil samples.

Fourier Transform Infrared Spectroscopy
The surface hydration properties and the stability of the solloid at soils were analyzed using FTIR spectroscopy. In this experiment, both HDTMA-sorbed and HDTMA-mixed soils were used. HDTMA-mixed soils were made by adding and mixing the powder of hexadecyltrimethyl ammonium chloride (HDTMAC). The amount mixed was equal to the amount sorbed. These HDTMA-treated samples were expected to become hydrophobic when the HDTMA formed hydrophobic surfaces because of its n-hexadecyl functional group. One milligram of each of the air-dried samples was mixed with 200 mg of KBr and pressed under a vacuum to make a transparent pellet. The pellet was installed into the FTIR apparatus (FTIR-8000, Shimadzu Scientific Instruments, Inc., Columbia, MD) and measured in transmission mode. The resolution of the spectra was 2 cm-1. The obtained transmission spectra were used for the wavenumber analysis. The spectra were converted into absorbance spectra using Kramers–Krönig's relation for the estimation of signal intensity.

The wavenumber of the hydroxyl function because of the antisymmetric stretching of the sorbed water shifts from that of liquid water (3430 cm-1) to vapor water (3756 cm-1) when the sample becomes hydrophobic (i.e., Holmgren et al., 1990). A shift in wavenumber because of symmetric and antisymmetric stretching of -CH2 (2860 and 2920 cm-1) will also occur, if there is some conformational change in the attracted n-hexadecyl group. The crystallinity of the macromolecules that contain long alkyl-chains is usually evaluated by comparing -CH2 rocking bands in 720 (amorphous + crystalline) and 730 cm-1 (crystalline) (Lin-Vien et al., 1991).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Moisture Characteristics of Untreated Soils
Psychrometry measurement of the untreated AS and VS was conducted to compare the moisture characteristics of the soils before AA sorption. Figures 1 and 2 show the t-plots of the untreated AS (circle) and VS (circle). The difference between AS and VS in the gradient of their t-plots within the low water content was remarkable. In case of VS, a sharp gradient appeared in 0 to 10% water content, which indicates the contribution of hydrophilic micropores to the moisture retention. Rousseaux and Warkentin (1976) reported that allophane soil contains a high concentration of micropores with a hydraulic radius <0.7 nm. In the case of AS, the shape of the t-plots is almost linear, which indicates that AS contains principally macropores (Linsen, 1970). This difference between the two soils suggests that the small cavity of allophane or the other hydrophilic microporous components in VS had a greater impact on water sorption than the larger voids of inter- or intraparticles of phyllosilicates in AS. The effects of residual OM in the soil samples also should be considered, because the H2O2 treatment of the soils does not completely remove OM. The residual C content of AS and VS after H2O2 treatment was 0.6 and 2.0%, respectively (by dry combustion). These residual C contents correspond to 1.4% and 4.0% of the OM content, respectively, if the empirical relation of C content and OM content (Nelson and Sommers, 1996) is employed. A previous study indicated that the specific surface area of SOM by monolayer adsorption of polar molecules (ethylene glycol) falls in the range of 65 to 481 m2 g-1 (Pennel et al., 1995). The specific surface area of the VS sample was 289 m2 g-1. From these results, we calculate that the surface area per 1 g of VS sample includes 270 to 283 m2 of the mineral surface and 2.6 to 19 m2 of the OM surface. The surface area per 1 g of AS sample includes 85.7 to 91.7 m2 of mineral surface and 0.93 to 6.9 m2 of OM surface. Therefore, the water sorption was considered as most significantly affected by the mineral fraction of the soil samples. However, the OM contents of soil samples were not negligible in the case they have a uniform orientation at the mineral surfaces. We considered the effect of residual OM of the soils by the results of the wavenumber analysis in FTIR experiment.



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  		Fig. 1. The t-plots of Amphiphilic Amine-sorbed Alluvial Soil (AS). TMA represents tetramethyl ammonium. TMPA represents trimethyl phenyl ammonium. HDTMA represents hexadecyltrimethyl ammonium.

 


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  		Fig. 2. The t-plots of Amphiphilic Amine-sorbed Volcanic Soil (VS). TMA represents tetramethyl ammonium. TMPA represents trimethyl phenyl ammonium. HDTMA represents hexadecyltrimethyl ammonium.

 
Sorption of Amphiphilic Amines at Soils
Table 1 shows the amount of the AAs sorption at the soils. Among the AAs measured, the amount of HDTMA sorption was the greatest for both soils, although the difference in the final pH value was not significant compared to the difference in the amount of sorption. Therefore, it would appear that the long alkyl chain (n-hexadecyl) of HDTMA seems to have significant effects on the amount of sorption because of the hydrophobic interaction among HDTMA molecules. It may also be true that the surface affinity of the soils for the longer alkyl chains contributes to the higher sorption of HDTMA. The amount of AAs sorption was greater in AS, which indicates that the macro- or mesopores of AS have a greater ability to sorb these OMs than do the micropores of VS. The specific surface area of AS was smaller than that of VS (Table 1), while the amount of AA sorption was greater in AS. Therefore, uniform monolayer sorption is not likely. According to a previous study using expansible phyllosilicates (Xu and Boyd, 1995; Boyd et al., 1988; Jaynes and Boyd, 1991), the swelling characteristics of phyllosilicates increased the amount of AAs sorption. Therefore, the phyllosilicates in AS were considered to have contributed to the surface aggregation of the AAs, while the micropores of VS restricted the sorption of the AAs. The differences in the amount and in the conformation of surface aggregation between AS and VS were expected to cause different hydration properties of these soils.


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  		Table 1. Specific surface area and Amphiphic Amines sorption of soils.

 
Moisture Characteristics of Amphiphilic Amine-Sorbed Soils
Psychrometry measurement of the AA-sorbed AS and VS was conducted to compare the effects of the AA sorption on the moisture characteristics of these soils. Figures 1 and 2 show the t-plots of the AA-sorbed soils with the reference data of the untreated soils. The effect of AA sorption on the shape of the t-plots was significant in AS, corresponding to the relatively large amount of sorption by AS. Among the samples measured, the sorption of HDTMA caused a significant decrease of water content in AS at t equals 1.2 to 1.8 nm. However, this decrease recovered at t > 2.0 nm. The decrease in the gradient of the t-plots of AS–HDTMA indicates the inhibition of water sorption on the pore surfaces by the n–hexadecyl function of HDTMA. Kung and Hayes (1993) indicated that HDTMA sorbed on silica surfaces reverts from a flat form to an aggregated form during wetting when the coverage is low. Therefore, the inhibition of water sorption that occurred in AS–HDMTA is also attributed to the conformational change of sorbed-HDTMA. The recovery of the gradient also indicates the progress of condensation of water within the pores in AS, because the retained water changes from interparticles to intraparticles of the phyllosilicates in AS around t equals 2.0 nm. The decrease because of the sorption of AAs was not significant in VS. Only the sorption of HDTMA resulted in a slightly decreased water content in VS. For both soils, the maximum decrease by HDTMA sorption occurred around t equals 1.8 nm, which implies that about six times the statistical monolayer thickness of sorbed water was required for the complete hydration of the HDTMA–coated surfaces of the soil pores. In the cases of TMA and TMPA, the changes in gradient by the sorption of these AAs were negligibly small through the obtained t value. These differences in the effects of AAs can be attributed to (i) the irregularity of the sorbed species due to interactions with a small amount of water; and (ii) the high lateral diffusion of TMA and TMPA molecules because of the fact that their molecular weight is smaller than HDTMA.

Fourier Transform Infrared Spectra of HDTMA–Sorbed and–Mixed Soils
The obtained FTIR spectra of soils and HDTMA modified soils were shown in Figure 3. Figure 4a shows the peak wavenumber of hydroxyl absorption due to the antisymmetric stretching of retained water in HDTMA–sorbed/mixed soils. Each spectrum of the samples showed a peak in the range of 3420 to 3460 cm-1; the peak was broad yet distinguishable. The peaks due to N–H stretching were small; they did not hide the tips of the peaks of hydroxyl function.



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  		Fig. 3. Fourier transform infrared spectra of hexadexyltrimethyl ammonium (HDTMA)-sorbed/mixed soils. HDTMAC represents hexadecyltrimethyl ammonium chloride (a), AS represents alluvial soil (b), AS–HDTMA sorbed (c), AS–HDTMA mixed (d), VS represents volcanic soil (e), VS-HDTMA sorbed (f), and VS–HDTMA mixed (g).

 


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  		Fig. 4. The change of the wavenumber of methyl/methylene peaks by sorbing/mixing HDTMA to soils. The (a) wavenumber of -OH antisymmetric stretching of sorbed water and (b) the wavenumber of C–H stretching of HDTMA and SOM. Volcanic soil is represented by VS. Hexadecyltrimethyl ammonium is represented by HDTMA. Alluvial soil is represented by AS. Hexadecyltrimethyl ammonium chloride is represented by HDTMAC.

 
In the case of VS, the wavenumber increased by 9 cm-1 after sorbing HDTMA. In the case of AS, this increase was up to 15 cm-1. The increase indicated that the sorbed water became unstable and that the hydrophobicity of the surfaces increased by sorbing HDTMA. Since the increase in wavenumber indicates an increase in the hydrophobicity of the soil, the sorbed HDTMA at AS and VS probably exerted the effects of a hydrophobic material because of the formation of surface aggregates having the hydrophobic outer surfaces of n–hexadecyl functions. The conformation of HDTMA at VS was considered to be mainly single layer with hydrophobic outer surfaces, because the amount of the HDTMA sorption was small compared to the specific surface. This is also supported by the fact that the opposite side of HDTMA (trimethyl ammonium) tends to be bound to the negatively charged hydrophilic sites on the meso- or macropores of VS. In contrast, AS probably has a higher partition of surface aggregates of HDTMA that also have hydrophobic outer surfaces, because the sorption density per unit surface area [by ethylene glycol monoethyl ether (EGME)] is much larger in AS-HDTMA (1404 nmol m-2) than VS-HDTMA (173 nmol m-2). In both soils, mixing HDTMAC caused a relatively small increase in wavenumber. These peaks due to the methylene chain of the mixed HDTMAC were sharp compared with those due to the sorbed HDTMA. The latter peaks were hidden by the peaks of hydroxyl function (Fig. 4b). The difference between sorbing and mixing indicates the contribution of conformational change (from a hydrophilic to a hydrophobic state of HDTMA during dissolution) to the formation of bonding between soil minerals and HDTMA. Therefore, it is suggested that soil wetting is necessary to form hydrophobic soils by amphiphilic OMs. The residual OM of the soil samples was considered unstable compared to the sorbed HDTMA, because the wavenumber of the peak from symmetric stretching of methylenes (Fig. 4b) was the highest in the residual SOM. However, the wavenumber of the peak from antisymmetric stretching of hydroxides (Fig. 4a) indicated that the residual OM has not contributed significantly to the hydrophobicity of the soil samples. Therefore, the effect of hydrophobidization of the residual OM of the soil samples was considered not significant.

The change in crystallinity of the methylene chains of HDTMA because of the sorption at soils was estimated by analyzing –CH2 rocking bands. Figure 5a shows the FTIR spectra of the hydrated powder of HDTMAC. The peak intensities both around 720 (crystalline + amorphous) and 730 (crystalline) cm-1, were almost the same, while a small decrease in the crystal band was observed in the dry powder of HDTMA–Cl (Fig. 5b) and the sorbed HDTMA at AS (Fig. 5c). In aqueous solution of HDTMA (Fig. 5d), the crystal band could not be observed even above the CMC. In the case of VS-HDTMA, these peaks were hidden by a broad peak due to the amorphous aluminosilicates in VS within 1050 to 680 cm-1. The peak intensity increased with that of hydroxyl bands. These facts imply that the amorphous moieties of the solloids of HDTMA at both soils were affected by residual water in the air-dried samples. It is also supposed that water molecules within the micro- or mesopores of the soils contribute to the soil–AA interaction by their ability to form hydrogen bonds especially in VS–HDTMA.



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  		Fig. 5. Fourier transform infrared spectra of hexadecyltrimethyl ammonium (HDTMA) in the region of -CH2 rocking bands. The peaks are represented by hydrated powder (a), dry powder (b), sorbed at alluvial soil (c), and aqueous solution above the critical micelle concentration (d).

 
Mechanisms of the Effects of Amphiphilic Amines on Moisture Characteristics of Soils
According to the FTIR spectra of HDTMA-sorbed soils at air-dry moisture contents, the solloids formed at both AS and VS were equally hydrophobic. However, the observed decrease of the t-plots below t = 1.8 nm was significant only in AS. The results of the sorption experiment for both soils suggest that the interlayer of expansible phyllosilicates is large enough to sorb HDTMA monomers or aggregates while the cavity of allophane is too small to sorb HDTMA into their inner surfaces. Therefore, the HDTMA solloids and its hydration phenomena could be illustrated by Fig. 6. In Fig. 6, HDTMA at the expansible phyllosilicates will cause relatively greater hydrophobicity as compared to that of allophane, because the hydration of the phyllosilicates intercalated by HDTMA will require conformational changes of the intercalated HDTMA and interlayer expansion. In contrast, the solloids at allophane will not hinder the intrusion of water molecules into their micropores. Therefore, the decrease in the gradient of the t-plots of AS–HDTMA within t = 1.8 nm could be attributed to inhibition of water intrusion into the interlayer of the phyllosilicates. The irregularity (amount of amorphous moiety) of the solloid at soils also confirmed the potential to hydrate the intercalated species in the phyllosilicates.



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  		Fig. 6. Schematic diagram of relationship between hydrophobicity of sorbed hexadecyltrimethyl ammonium (HDTMA) (solloid) and residual water.

 

   ACKNOWLEDGMENTS
 
Psychrometry measurement was supported by H. Imoto, Technical officer of Environmental Soil Physics and Soil Hydrology Lab. in The University of Tokyo. FTIR experiment was conducted skillfully by H. Shintani, assistant professor of Wood Chemistry Lab. in The University of Tokyo.

Received for publication May 5, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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