論文., March 2012, Vol. 120, 45 ‒ 54

Original
Paper
45

Adsorption of Sodium Dodecylbenzene Sulfonate on Highly
Humic Non‒allophanic Andisol at High‒Electrolyte
Concentration
Farook AHMED1, Munehide ISHIGURO2, Kazuki MORIGUCHI1 † and Takeo AKAE1

Abstract : To clarify the adsorption characteristics ofsurfactants, it is important to understand the surfactant behavior in the soil and water environments.
However, there are few adsorption studies for highly
humic soil. In this study, the adsorption characteristics
of dodecylbenzene sulfonates in a highly humic soil
were investigated. A non‒allophanic Andisol was usedsince this soil contains a large amount of humic substances and is only negatively charged. Thus, electrically, only repulsive force is generated between thesoil and the surfactant. The adsorption amount wasmeasured using the batch method at an electrolyte
concentration of 100 mmol L−1 NaCl in order to shield
the electric ﬁeld near the soil particle surface and emphasize hydrophobic reactions. The adsorption isotherm was examined using the Langmuir‒Freundlich‒
Hill equation. The adsorbed amount increased sharply
with increasing concentrations in the low‒concentration range. The sharp increase indicated cooperative
adsorption caused by hydrophobic interaction among
the carbon chains of adsorbing surfactants. The adsorption amount became higher at a lower pH because
electrostatic repulsion between the soil and the surfactant decreased. The adsorption of dodecylbenzene sulfonate with a linear carbon chain was larger than thatwith a branched chain due to the diﬀerence of the carbon chain structure.
Key words : anionic surfactant, electrostatic repulsive
force, dodecylbenzene sulfonate（DBS），adsorption
isotherm, volcanic ash soil.

2007）as active ingredients of laundry detergent andmany domestic and industrial materials including
chemical fertilizers as an anti‒caking agent. It is also
used in agricultural chemicals as an emulsifying agent,because of its ability to change the physicochemical
nature of surfaces（Koopal et al., 1999）． Linear alkylbenzene sulfonate（LAS），one of the major constituents of synthetic detergent, is the most widely used
SDBS for domestic and industrial purposes, in both
percentage and absolute amount（Inoue et al., 1978 ; Heet al., 1991）． It is also the most common pollutant
found in almost all environmental settings（Sullivan
and Swisher, 1969 ; McEvoy and Giger, 1986 ; Takada
and Ishiwatari, 1987; Yediler et al., 1989 ; Papaport andEckhoﬀ, 1990 ; McAvoy et al., 1993）．LAS is the mostwidely used anionic surfactant in detergent formulations and surface cleaners（Marcomini and Giger,
1988）．Overall, SDBSs are used in large quantities andenter the environment primarily through waste waterand sludge.
When these compounds are adsorbed in soils, they
appear to degrade slowly（Tabor and Barber, 1996）
and to impede microbial processes in soil（Kristiansen
et al., 2003）．Adsorption of LAS by soils and sediments is an important process aﬀecting its transport,
and toxicity（Wolf and Feijtel, 1998 ; Knabel et al., 1996 ;Fleld et al., 1992; Garcia et al., 2005 ; Rubio et al., 1996）

1. Introduction

and, thus, its application in remediation of contaminated soil and groundwater（West and Harwell, 1992 ;Sodium dodecylbenzene sulfonates（SDBS）are very

Rouse and Sabatini, 1993 ; Ko et al., 1998a ; DiVincenzo

common, efficient anionic surfactants（Fachini et al.,and Dentel, 1996 ; Allred and Brown, 1996）． The adsorption of LAS may also aﬀect the environmental be-

1

Graduate School of Environmental Science, Okayama University,
Okayama 700 ‒ 8530, Japan
2
Research Faculty of Agriculture, Hokkaido University, Sapporo
060 ‒ 8589, Japan
Corresponding author : F. AHMED, Graduate School of Environmental Science, Okayama University, 3 ‒ 1 ‒ 1 Tsushima-naka, Okayama
700 ‒ 8530, Japan. E ‒ mail: farookahmed12@yahoo.com
Tel. & Fax 086 ‒ 251 ‒ 8874.
Present address : †Kinki Regional Development Bureau, Ministryof Land, Infrastructure and Transport, 1 ‒ 5 ‒ 44, Otemae, Chuo ‒ ku,
Osaka 540 ‒ 8586, Japan
2011 年 10 月 4 日受稿，2011 年 12 月 19 日受理
土壌の物理性 120 号，45 ‒ 54（2012）

havior of the toxic substances（Jones‒Hughes andTurner, 2005 ; Ko et al., 1998a ; DiVincenzo and Dentel,
1996）and nanoparticles（Lacoanet et al., 2004）．In
past decades, adsorption studies have been conducted
to understand the fate of discharged LAS（Wolf and
Feijtel, 1998）．Since the surfactant concentrations
used in soil and groundwater remediation（Zhao et al.,2006 ; SDBS concentration from 0.2 to 10 g L‒1）and
nanomaterial preparation are always higher than critical micelle concentrations（CMCs）（West and Harwell,

土壌の物理性 第 120 号（2012）46

Table 1. Physical and chemical characteristics of the soil used in the experiment.

Soil Name

Sand

Silt

Clay

TexturepH（%）
Andisol

43 . 631 . 824 . 6Clay loamc

5 . 0, d4 . 2

Carbon content

a

CEC

b

AEC

（%）

（mmolc kg−1）

13 . 812 . 30a

CEC was measured at pH 6 with a 1 mmolc L−1 KNO3 solution.
AEC was measured at pH 6 with a 1 mmolc L−1 K2SO4 solution.
c
pH was measured with distilled water（soil : distilled water at the ratio of 1 : 5）dpH was measured with 0.1M KCl（soil : 0 . 1M KCl at the ratio of 1 : 2 . 5）b1992 ; Tanaka et al., 1997 ; Ko et al., 1998b ; Lecoanet etsurface and emphasize hydrophobic interaction. The

al., 2004），adsorption of LAS in a wide concentration

adsorption isotherm was evaluated using the Lang-

range needs to be investigated. Diﬀerent mechanisms

muir‒Freundlich‒Hill equation. The inﬂuences of car-such as hydrophobic adsorption（Hand and Williams,

bon chain structure on adsorption were also evaluated.

1987 ; Di Toro et al., 1990 ; Westall et al., 1999），speciﬁc

2. Materials and methods

adsorption（Ou et al., 1996 ; Westall et al., 1999），hydrogen bonding（Ou et al., 1996），electrostatic interac-

2. 1 Soiltions（Westall et al., 1999）and precipitation（West andHighly humic, non‒allophanic Andisol（volcanic ash

Harwell, 1992 ; Rouse and Sabatini, 1993）have beensoil）of A horizon from Daisen grazing ground, Tottori

suggested as determining the adsorption of LAS on

Prefecture, Japan was used in this experiment. The

soils, but there is no consensus on this issue. The inﬂu-

physical and chemical characteristics of the soil areence of sediment on adsorption of LAS has been re-given in Table 1. Moist ﬁeld soil sieved with a 2‒mmviewed by Wolf and Feijtel（1998）．As diﬀerent soil

sieve was used in the experiment.

and sediment constituents may have diﬀerent adsorp-

2. 2 Surfactants

tion characteristics（Knaebel et al., 1994），adsorption

Anionic surfactants, SDBS, with branched and linearof LAS would depend not only on the types of thesecarbon chains having the same chemical composition

constituents but also on their relative contents（Zhu et（C12 H25 C6 H4 SO3 Na）and molecular weight of 348.48 gal., 2003 ; Wolf and Feijtel, 1998）．

mol ‒ 1 were purchased from Tokyo Kasei Kogyo Co.

Branched‒carbon‒chain DBS has low bio‒degrad-

The two types of SDBS had purities of about 95% andability（Alexander M 1973 ; Cain RB 1987 ; Cain RB

were used without further purification. The former

1994 ; Greek BF 1991）．In some Latin American coun-

mixture of hydrophobic branched chains consisted oftries, branched‒chain SDBSs in diﬀerent formulations

12 carbon molecules. The branched structures wereare used due to their low cost, to the detriment of the

not constant, but their structures were not linear. The

environment and especially water quality（Campos‒

latter contained hydrophobic 12‒carbon linear‒chain

Garcia et al., 1999）．Few studies exist aboutmolecules.

branched‒chain DBS adsorption on soils. Adsorption

2. 3 Adsorption experimentstudies comparing linear and branched‒carbon‒chain

A batch experiment was conducted to obtain the
DBS adsorption isotherm of the soil. The experimentstructures are also rare.
In this study, the adsorption characteristics of DBS

was conducted to investigate the inﬂuence of pH, time,in a highly humic soil were investigated in the surfac-

and carbon chain structure at room temperature（25tant concentration range from very low to around crit-± 1℃）．The soil（2 . 5 g dry weight basis）was placed

ical micelle concentration by using an anionic surfac-

in a 50‒cm3 centrifuge tube. It was equilibrated with atant selective electrode that we ourselves devised and100 mmol L‒1 NaCl solution, and the solution pH wasbuilt. A non‒allophanic Andisol was used because theadjusted to 4 . 5, 5 . 5 or 6 . 5 by adding dilute HCl or

soil contains a large amount of humic substances andNaOH. After discarding the supernatant from theis only negatively charged（Ishiguro et al., 2003）．The

tube, 25 ml of SDBS solution（ranging from 0 . 0008 toadsorption amount was measured using a batch meth-

10 . 0 mmol L‒1）at 100 mmol L‒1 NaCl was mixed in‒1

the soil and shaken well for 3 or 24 hours. After the

NaCl in order to shield the electric ﬁeld near the soil

elapsed time the soil solution was centrifuged for 10

od at an electrolyte concentration of 100 mmol L

論文：Adsorption of Sodium Dodecylbenzene Sulfonate on Highly Humic Non-allophanic Andisol at High-Electrolyte Concentration 47
was carefully washed before each measurement andalways checked with standard solution in order to get
the proper result.
The adsorbed amount of surfactant in the soil was
obtained using the following equation,
DBS adsorption（mmol kg‒1）= {added DBS conc.（mmolL‒1）×0 . 025（L）−DBS conc. in supernatant（mmol L‒1）
×（0.025+V）（L）}/Dry weight of the soil
（kg），

（2）

where V（L）is the water volume remaining in the soil
after discarding the supernatant and before adding the
SDBS solution. The experiment was conducted underthe 100 mmol L‒1 NaCl solution condition, in order to decrease the inﬂuence of the negative charge of the soil.
Fig. 1 The schematic diagram of the anionic surfactant
selective electrode used in the experiment. a：Electrometer（Advantest R8240），b：AgCl/Ag electrode（DKK,TOA HS ‒305D）
，c：NH4NO3（Agar bridge），d：Ion selective electrode, e：
Magnet, f：1M NH4NO3, g：Stirrer, h：Measured/test solution, i：Reference solution, j：Beaker.

2. 4 Critical micelle concentration（CMC）
SDBS CMC at 100 mmol L‒1 NaCl was obtained using the anionic surfactant selective electrode. The
EMF was measured at each SDBS concentration. Under the CMC, a straight calibration line（EMF versus
the logarithm of the SDBS concentration）was ob-

min at 8000 rpm. The supernatant of the tube was col-tained according to equation（1）．Above the CMC, thelected and the surfactant concentration was measured

EMF became almost constant. The CMC was obtainedby an anionic surfactant selective electrode（Fukui etat the kink of the calibration line（Ishiguro et al., 2007 ;al., 2003）that we ourselves devised and built. The con-Ishiguro and Koopal, 2009）．

A g /AgCl electrode Agar bridge

reference solution＿＿＿（ 0） functional membrane

2. 5 Theoretical analysis of adsorption isotherm＿＿

centration cell was constructed as follows :test solution（ 1） AgarTo evaluate the adsorption isotherm, the following
equation was used:＿bridge Ag/AgCl electrode,
where

0

and

1

are the concentrations of the surfac-＝Qtant in the reference solution and that in the collected

（

）

1 +（

（3）

）

supernatant. The schematic diagrams of the anionicsurfactant selective electrode used in the experiment

=

Δ

log ,

（4）

are given in Fig. 1.
is the adsorption amount, Q is the maximum

The electromotive force（EMF）was read using a digi-

wheretal voltmeter with high input impedance at 25 ± 1℃.

adsorption amount,

The EMF,, can be expressed with the following‒1（mol L ），is the equilibrium concentrationis the adsorption constant,ity（deviation from Langmuir equation,

equation :is non‒ideal=1），Δ

isthe diﬀerence in free energy before and after adsorp=log（ 1/ 0），（1）tion（J mol‒1），

is the gas constant andis the ab-solute temperature. Equation（3）can be re‒written as
where

is the experimental slope. The theoretical val-ue ofis 59 . 2 mV at 25℃（Nernstian slope）．The mea-

follows（Ooi et al., 2000）:

sured values for the standard solutions ranged fromθ

54 . 35 mV to 59 . 00 mV. When the concentration was

1 −θ

＝（

（5）

）

higher than the critical micelle concentration, the concentration was measured after dilution. The electrolyte

where θ is /Q. When θ is ≪ 1, the equation（5）canconcentrations for the standard solutions, test solutions

be re‒written as follows:

and reference solutions were all 100 mmol L‒1 NaCl.
Because humic substances dissolved in the solution af-

log ＝ log

+ log（Q）．

（6）

fect the measurement when the electrode is dipped
longer in the solution without washing, the electrode

Therefore, when the isotherm is plotted in double loga-

土壌の物理性 第 120 号（2012）48

rithmic scale, the slope indicates theWhenvalue for q≪Q.

is larger than unity, this equation is called

3. Results and Discussion

the Hill equation（Hill, 1910 ; Ooi et al., 2000）; implying

3. 1 Eﬀect of pH on DBS adsorptionthat adsorbates attract each other and adsorb coopera-

The adsorption isotherms for branched‒carbon‒

tively. The cooperativity occurs when adsorbed mole-chain DBS and linear‒carbon‒chain DBS are shown incules attract each other by lateral interaction; adsorp-

Fig. 2. The DBS concentrations for the adsorption iso-tion becomes stronger with that lateral interaction.

therms in the ﬁgures show the solution concentrations

Whenis smaller than unity, this equation is calledin the supernatants after mixing and centrifugation.

the Langmuir‒Freundlich equation（Jaroniec, 1983, Ooi

The highest adsorption was observed at pH 4.5 andet al., 2000）; this indicates heterogeneity of adsorbing

the lowest at pH 6.5 for both branched and linear DBS

sites. For surfactant adsorption on the soil both thein this experiment（Fig. 2）．This result clearly showsheterogeneity of the adsorbing sites on the soil and athat adsorption is higher at a lower pH and lower at a

positive cooperativity（ ＞ 1）due to hydrophobic at-

higher pH. The electrostatic force is repulsive because

traction will play a role. To express this, eq.（3）isboth DBS and the soil are negatively charged and nocalled the Langmuir‒Freundlich‒Hill（LFH）equation,

positive charge was detected by AEC measurement

and（Ishiguro et al., 2003）．Therefore, the electrostatic in-

can be smaller or larger than unity. The LFH

eq. with＜ 1 implies that the heterogeneity eﬀects

are larger than the hydrophobic eﬀects, andteraction lowers the adsorption. When the soil pH in-＞1 im-

creases, the negative pH‒dependent charge of the soil

plies that the hydrophobic attraction is stronger thanincreases, and the increased electrostatic repulsive

the heterogeneity effects. Pseudo‒ideality（ = 1）oc-

force causes the adsorption to decrease. A similarcurs when the eﬀects of heterogeneity and hydropho-

trend was reported by Fytianos et al.（1998）in soil ofbicity compensate each other（Ishiguro and Koopal,

diﬀerent organic matter content. Our result shows that2011）．

pH has a strong inﬂuence on the adsorption isotherm.

Fig. 2 Eﬀect of pH on（a）branched DBS and（b）linear DBS
adsorption isotherm.（after 24 ‒ hour mixing）

Fig. 3 Adsorption isotherm of DBS at different time（a）
branched DBS and（b）linear DBS（after 3 ‒ and 24 ‒ hour
mixing）．

論文：Adsorption of Sodium Dodecylbenzene Sulfonate on Highly Humic Non-allophanic Andisol at High-Electrolyte Concentration 49
Because both DBS and the soil have hydrophobic partsequation（2），the decomposed amount is included inand the electrostatic interaction is repulsive, DBS is

the adsorption amount. However, because the differ-mainly adsorbed by hydrophobic interactions.

ence between those after 3 hours and 24 hours was

3. 2 Eﬀect of time on DBS adsorption

not signiﬁcant, the adsorbed linear‒carbon‒chain DBS

The adsorption isotherms after 3‒hour and 24‒hourdecomposes slowly, as Tabor and Barber（1996）sug-

mixing are compared in Fig. 3. No significant differ-gested. Further experimental research is needed toence was detected at all pH conditions. This result in-

identify the restrictive eﬀect of adsorption on the de-dicates that the soil is equilibrated within 3 hours. Li

composition.

et al.（2008）reported that DBS required 100 minutes

3. 3 Evaluation of the adsorption isotherms

for adsorption equilibrium in marine sediments. For

The experimental result showed that the adsorption

linear‒carbon‒chain DBS, the adsorptions after 24of DBS on the soil increases with the increment of con-

hours were slightly larger than those after 3 hours.

centration for both branched and linear DBS irrespec-

This result indicates the inﬂuence of DBS decomposi-tive of pH（Figs. 2 and 3）．At much lower concentrations

tion, because linear‒carbon‒chain DBS decomposesof DBS（＜0 . 001 mmol L‒1）the adsorption isotherms weremore readily than branched‒carbon‒chain DBS whenvery steep, indicating strong cooperativity（ ≫ 1）; theit is dissolved in water（Sekiguchi et al., 1975）．As thesurfactant molecules aggregate due to hydrophobic in-adsorption amount was calculated with the subtraction

teraction among their carbon chains and are adsorbed

Fig. 4 Comparison of the calculated adsorption values using the Hill equation（3）and measured adsorption isotherm.（a）Branched DBS at pH 6 . 5,（b）branched DBS at pH 4 . 5,（c）linear DBS at pH 6 . 5 and（d）linear DBS at pH 4 . 5.

土壌の物理性 第 120 号（2012）50
Table 2. Obtained

value andvalue by using Hill equation（3）for branched and linear DBS.

pH

pH 4 . 5

Concentrationlog

pH 6 . 5

Low

High

Low

High

Branched

11

0.9

11

1.1

Linear

11

0.8

11

0.9

Branched

6 . 04

3.6

5 . 78

3.9

Linear

6.2

3 . 95

6 . 044.1

on the soil surface together. Above these lower con-tant and the soil surface. DBS is adsorbed separately

centrations, the adsorption isotherms show a gradualon each remaining adsorption site of the soil by hydro-‒1increase to around CMC（0 . 33 mmol L and 0 . 09 mmol

phobic interaction.

L‒1 for branched and linear DBS, respectively）．The

The steep increase is followed by a gradual increaseelectrostatic repulsive force between the soil and thein all adsorption isotherms. Similar isotherms havesurfactant is supposed to decrease the slope of the iso-been observed for a cationic surfactant‒anionic poly-

therm. The surfactant adsorption leads to an increase

mer system（Satake and Yang, 1976 ; Shirahama, 1998;

in the negative charge. This aﬀects the adsorption and

Ishiguro and Koopal, 2009）．In their cases, the adsorb-

contributes to the decrease of the slope of the iso-ing force between the surfactant and polymer was due

therm.

to electrostatic interactions. In our study, the electro-

The adsorption equation（3）is applied to the ad-static force was repulsive, diﬀerent from their condi-

sorption isotherms as shown in Fig. 4. The adsorption

tions. The cooperative interaction was generated byisotherms are plotted on a log‒log scale in order to dis-the carbon chains of the surfactants, and the gradualtinguish the isotherms. The

value in equation（3）isincrease of the isotherm at higher concentration is re-equal to the slope of the isotherm in the log‒log scalelated to the electrostatic repulsion caused by theas explained by equation（6）．The obtained

valuescharge of adsorbed surfactant in both systems. Coop-andvalues by ﬁtting with the obtained data are list-

erative adsorption has been observed also in an anionic

ed in Table 2. Curve‒ﬁtting calculations were conduct-

surfactant‒neutral polymer system（Arai et al., 1971;

ed at two regions separately for each adsorption iso-

Shirahama, 1974）．On the other hand, in an anionictherm: a region at lower concentration and that at asurfactant‒humic acid system, distinct adsorption was

higher concentration around the CMC. The

valuesnot observed（Koopal et al., 2004 ; Yee et al., 2009）．

are larger at lower concentrations, indicating strong

The high adsorption on the soil indicates that part ofcooperative adsorption as mentioned before. At pH 4.5

humic substances in the soil are much more hydropho-

for linear DBS, the adsorption amounts at very low

bic than humic acids.

concentrations corresponding to strong cooperative adsorption could not be measured because the adsorption

3. 4 Comparison between branched and linear DBS
adsorption isotherms

increased at too low a concentration. Therefore, theFrom the experimental results it was observed that

calculated isotherm at pH 4.5 at lower concentration

the linear DBS adsorption is higher than that offor linear DBS was given by using=11 as shown inbranched DBS adsorption in the soil in all pH condi-

value denotes the number of ag-

tions（Fig 5）．The results indicate that the linear DBS

gregated surfactant molecules when adsorbed onto one

adsorbs and aggregates together at a lower concentra-

surface site（Hayakawa and Kwak, 1991; Ooi et al.,tion compared to the branched DBS. That is conﬁrmed2000）．

by the fact that the CMC of the linear SDBS was low-

Figure 4（d）．The

The

values at higher concentrations around theer than the branched SDBS. The former was 0.09

= 1 in-mmol L‒1 while the latter was 0. 33 mmol L‒1, measured

dicates no cooperativity. In this region, cooperative ad-by the anionic surfactant selective electrode. The con-sorption caused by the carbon chains is cancelled withformation of linear DBS is more advantageous for ag-

the electrostatic repulsive force between the surfac-

gregation on the soil and micellization in water than

CMC show roughly 1.0 for all conditions. The

論文：Adsorption of Sodium Dodecylbenzene Sulfonate on Highly Humic Non-allophanic Andisol at High-Electrolyte Concentration 51
that of branched DBS.

4. Conclusions

than our results.
The adsorption amount decreased with increasing
pH because electrostatic repulsion increased at higher

The adsorption characteristics of DBS on a highly

pH. Because the adsorption is strongly aﬀected by pH,

humic non‒allophanic Andisol were systematically in-

the pH must be carefully considered when the fate ofvestigated. The adsorption was measured using theionic surfactants in soils is discussed.

batch method at the electrolyte concentration of 100

The adsorption of linear‒carbon‒chain DBS wasmmol L‒1 NaCl. The adsorption isotherm was evaluatedlarger than that of branched‒carbon‒chain due to theusing the Langmuir‒Freundlich‒Hill equation.

diﬀerence of the chain structure. There was no signiﬁ-

The adsorption amount on the soil increased sharply

cant diﬀerence in adsorption amounts after 3 and 24

with increasing concentrations at lower concentrations

hours; hence, the reaction must reach equilibrium

for both the branched and linear DBS. The sharp in-within 3 hours while decomposition is not signiﬁcantcrease indicates cooperative adsorption caused by hy-

or small within 24 hours. Puriﬁcation of water contami-

drophobic interaction among the carbon chains of ad-nated with SDBS by highly humic non‒allophanic An-

sorbing surfactants. This cooperative adsorption

disol will therefore take more than 24 hours. In per-phenomenon is a novel observation for soils. This ﬁnd-spective, further research is needed on the decomposition

ing and other characteristics could be detected be-of the surfactant.

cause（i）the electric potential near the soil surface
was shielded by the high electrolyte concentration,（ii）Acknowledgementsa highly humic soil with many hydrophobic sites was

We thank Prof. K. Shirahama（Saga Univ.）for his

used, and（iii）a highly sensitive method of surfactant‒

instructions regarding the preparation of surfactant‒selective electrode measurement was applied. The ob-

selective membrane and Du Pont for supplying Elvar-

tained adsorption characteristics can be fundamentallyoy 742. This research was supported by Grants‒in‒Aidapplied to any soil containing humic substances al-

for Scientiﬁc Research（No. 22380130）from the Japan

though those adsorption amounts would be smaller

Society for the Promotion of Science

List of Abbreviations
SDBS : sodium dodecylbenzene sulfonate
CMC : critical micelle concentration
EMF : electromotive force
LAS : linear alkylbenzene sulfonate
CEC : cation exchange capacity
AEC : anion exchange capacity

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土壌の物理性 第 120 号（2012）

54

要

旨

界面活性剤の吸着特性を明らかにするためには，土壌と水環境中での界面活性剤の挙動の理解が重要である．しかしながら，多腐植質土壌の吸着に関する研究は少ない．本研究では，アニオン性界面活性剤
であるドデシルベンゼンスルホン酸の吸着を検討した．非アロフェン質黒ボク土を用いたのは，この土が多量の腐植質を含み，負荷電のみを有するためである．その結果，土粒子と界面活性剤の間には，電気的には反発力だけが生じる．吸着量は，土粒子表面近傍の電場を遮蔽し疎水的反応を強調するため，
100 mmol L−１NaCl の電解質濃度下で測定した．吸着等温線には Langmuir‒Freundlich‒Hill の式を用いて分析した．ドデシルベンゼンスルホン酸の吸着量は，低濃度領域において濃度の上昇とともに急激に増大した．この急激な増大は，吸着された界面活性剤の炭素鎖間の疎水性相互作用による協同吸着を示唆した．吸着量は土粒子と界面活性剤の間の静電的反発が減少するために，
低 pH でより高い値となった．直鎖型炭素鎖を持つドデシルベンゼンスルホン酸の吸着は炭素鎖構造の違いにより，分岐型のそれよりも高い値を示した．
キーワード：アニオン性界面活性剤，静電的反発力，ドデシルベンゼンスルフォン酸ナトリウム，吸着
等温線，黒ぼく土