Surfactant Gel Extraction of Metal Ammine Complexes using SDS and KCl at Room Temperature,

and a Small-angle X-ray Diffraction Study of the Surfactant Phase

Shoji TAGASHIRA, Tatsuya ICHIMARU, Kouji NOZAKI and Yoshiko MURAKAMI*

Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan

(Received December 10, 2012; Accepted January 29, 2013)

Micellar solutions of sodium dodecylsulfate (SDS) exhibit the property of being

separated into two phases as a result of a temperature change or the addition of salts.

Upon addition of KCl to an SDS solution, the surfactant phase of Na+DS

- changed to

the potassium dodecylsulfate phase of K+DS

- (KDS) at room temperature around

25°C. The ammine complexes of metal ions such as copper reacted with the

dodecylsulfate anion (DS-) to form the corresponding ion pair, and were separated

from the solution as the surfactant phase. After the phase separation of the solution

containing SDS, KCl, ammonia, and copper, the surfactant phase consisted of KDS

and the ion pair of [Cu(NH3)42＋](DS

-)2 (abbreviated CuDS). The structures of the

surfactant phases were investigated by small-angle X-ray diffraction (SAXRD) and

differential scanning calorimetric measurements. The surfactant phases had lamellar

structures with layer distances (d(001)) of 3.4 nm for KDS and 2.4 nm for CuDS. This

surfactant gel extraction method was applied to the mutual separation of copper and

zinc in a brass alloy. The extractability of the metals was regulated by the initial

metal concentration in the sample solution. The percent extraction was (98.7 ± 0.6)%

and (6.8 ± 1.4)% for copper and zinc, respectively.

1. Introduction

Valuable metals play a significant role in our daily lives and always coexist in environmental wastes

with other metals. Thus, there is a need to find suitable techniques for their simultaneous extraction,

separation, and selective recovery. Removal of heavy metals can be conducted using techniques such as

precipitation, ion exchange, adsorption, and extraction [1]. The most widely employed extraction methods

Solvent Extraction Research and Development, Japan, Vol. 20, 39 – 52 (2013)

- 39 -

use aqueous and organic phases whose stratification occurs as a result of the limited mutual solubility of

water and the organic solvent. Despite their advantages, extraction methods have a number of

shortcomings, the greatest of which is the need for organic solvents, which tend to be flammable and/or

toxic substances. The problem of making extraction safer can be solved by finding less toxic extracting

agents and interest has been shown in the separation of metal ions via extraction methods using ionic

liquids[2] and surfactants[3,4] as the extraction medium.

One of the developed extraction methods is based on the cloud-point phenomenon, where an

aqueous solution of a surfactant becomes turbid and separates into two isotropic phases if a condition such

as temperature is increased or an appropriate substance is added to the solution. Nonionic micelles have

been used extensively for cloud-point extraction (CPE) of metal ions [5-7]. Some CPE procedures have

been developed using mixtures of nonionic and anionic surfactants. However, the use of anionic surfactants

is still rare for the preconcentration of metals, but there are a few methods for the determination of metal

ions based on the use of anionic surfactants [8].

Another effective method is micellar extraction with micellar-enhanced ultrafiltration (MEUF)

[9-12]. When an anionic surfactant such as sodium dodecylsulfate (SDS) is added to water above the

critical micelle concentration (CMC), surfactant monomers aggregate to form negatively charged micelles.

The heavy metal cations can be mostly trapped by the micelle owing to electrostatic interactions. In this

method, separation can be achieved because the micellar aggregates have a size that prevents them from

passing through ultrafiltration membranes. Numerous investigations of the concentration of metals and

metal chelates have been conducted; however, difficulty still lies in the manipulation of highly viscous

concentrated micellar solutions.

Aqueous solutions of anionic surfactants exhibit the phase separation phenomenon when the

solutions are cooled to certain temperatures. The metal ions that were bound to the surface of the

oppositely charged micelles can be efficiently extracted into the surfactant phase, accompanying the phase

separation. Most of the separated surfactant phase exists as a gel-like solid that has the ability to bind a

solute. According to traditional extraction theory, an electrostatic attractive force is the operative

phenomenon for the formation of ion pairs of cationic metal complexes and anionic surfactants, and the

hydrophobic domain of the surfactant is the extraction medium. In order to efficiently remove metal ions

from water, the formation of stable ionic complexes is necessary. The separation of metals is also based on

the stability of the ion pair. Therefore, the extractability of metals is influenced by the concentration of the

ligand that forms the complex and the ionic surfactant that makes the ion pair. This mechanism is the same

- 40 -

as that for ion-pair extraction systems using organic solvents.

The extraction methods based on temperature-induced phase separation of aqueous ionic micellar

solutions may resemble the CPE method using nonionic surfactants [13]. However, we previously reported

that an unexpected decrease in extractability was observed at low metal concentrations for the ion-pair

extraction of dodecylsulfate anions (DS-) and cationic metal ammine complexes. This decrease has been

explained by the solubility of metal ion pairs (MDSs) at low temperature around 0°C [14]. In the present

study, the phase separation and extraction of the metal ammine complexes at room temperature were

examined by the addition of salts such as KCl to anionic surfactant systems of SDS. The solubility of the

MDS was estimated from the dependence of the metal concentration on the percent extraction and the

structure of the surfactant phase was investigated by small-angle X-ray diffraction (SAXRD). In addition,

the developed surfactant gel extraction method was applied to the separation of major base metal

components of a brass alloy. Finally, the solubility of the surfactant gel and related energetic relationships

are discussed.

2. Experimental

2.1 Reagents

All chemicals were analytical grade and were obtained from Kanto Chemical Co., Inc. (Tokyo),

Tokyo Kasei Kogyo Co., Ltd. (Tokyo) or Wako Pure Chemical Industries, Ltd. (Tokyo). The stock

solutions of divalent metal ions were separately prepared by dissolving the appropriate amounts of

copper(II) chloride dihydrate, nickel(II) chloride hexahydrate, zinc(II) acetate dihydrate, and palladium(II)

chloride in dilute hydrochloric acid. Aqueous ammonia (28%) was used as received. Solutions of SDS,

sodium alkylsulfonates(C12, C14, C16), sodium chloride, potassium chloride, and calcium nitrate were

prepared using deionized water.

2.2 Apparatus

Metal concentrations were determined using a Varian Liberty Series II inductively coupled plasma

atomic emission spectrometer (ICP-AES) and a Shimadzu AA-625-11 flame atomic absorption

spectrometer (AAS). All pH measurements were performed using a Horiba F-11 pH meter in combination

with a glass electrode. A Kubota 2800 centrifuge with a cooling circulator was used for phase separation.

SAXRD was performed using a Bruker AXS DIP-220 powder X-ray diffractometer using Cu Kα radiation

between 2= 0 and 12°. The capillary cell was cooled with liquid N2 vapor generated using an Oxford

Cryostream cooler. The melting points of the surfactants and MDSs were measured using a Rigaku DSC

8230. The concentration of DS- was determined using a Shimadzu TOC-5000.

- 41 -

2.3 Procedure

(A) Extraction of metal ammine complexes into the surfactant phase at 0°C

Solutions containing divalent

metals were separately transferred to a

10 cm3 centrifuge tube with a

graduation line. After successively

adding aqueous ammonia and SDS

solution, the resultant solution was

shaken to form the ion pair. Next,

sodium chloride solution was added as

a salting-out reagent and the mixture

was diluted to 10 cm3 with water. The

solution was then cooled in an

ice-water bath and centrifuged (0°C,

4000 rpm, 20 min), and the aqueous and surfactant phases were separated. The aqueous phase was

transferred to another tube and its metal concentration was determined by ICP-AES in order to determine

the percent extraction of the metals. We verified the structure of the MDS in the surfactant phase on the

basis of SAXRD measurements. The surfactant phase, which was located at the bottom of the tube, was

packed into the capillary cell of the SAXRD and recentrifuged at 0°C. The cell containing the surfactant

phase was cooled with liquid N2 vapor during the SAXRD measurements (Scheme 1). Since a small

amount of the aqueous phase remained with the surfactant phase in the cell, the results of the SAXRD

analysis exactly reflect the structure of the surfactant phase in the extraction system.

(B) Extraction of metal ammine complexes at room temperature

Instead of using NaCl as the salting-out reagent, KCl was added to the solution containing the

divalent metal ion, ammonia, and SDS. Then, the mixture was diluted to 10 cm3 with water. After the

phases were centrifuged (25°C, 4000 rpm, 20 min) and separated, the metal concentration of the aqueous

phase was determined. The surfactant phase was stable and was analyzed by SAXRD at room temperature

and DSC. The concentration of DS- in the aqueous phase ([DS

-]aq) was determined by TOC measurement.

2.4 Separation of copper and zinc in a brass alloy

A 0.5567 g sample of brass (Nilaco Corp., brass wire, item no. 91167, Japan) was decomposed in a

small amount of concentrated nitric acid. The solution was heated and evaporated to dryness. The residue

was dissolved in 3 cm3 of concentrated nitric acid and diluted with water to 200 cm

3. The composition of

Scheme1. The setup for SAXRD measurement of

the surfactant phase.

- 42 -

the brass was determined by ICP-AES. Extraction of copper from the sample solution was performed in

the following manner. One cm3 of the sample solution was taken into a 10 cm

3 centrifuge tube, then 1 cm

3

of 0.45 mol dm-3

ammonia solution, 1 cm3 of 0.35 mol dm

-3 SDS solution, and 0.5 cm

3 of 1.3 mol dm

-3

KCl solution or 0.35 cm3 of 1.0 mol dm

-3 Ca(NO3)2 solution were added. After dilution of the mixture to

10 cm3 with water, the solution was shaken and centrifuged (25°C, 4000 rpm, 20 min). The metal content

in the aqueous phase was determined by ICP-AES.

3. Results

3.1 SAXRD analysis of the surfactant phases with NaCl or KCl as salting-out reagents

In concentrated solutions, amphiphilic molecules such as ionic surfactants can form a bilayer, which

consist of a flat phase that includes some solvent in the interior [15]. The micellar solution exhibits the

property of being separated into two phases upon cooling below the Krafft temperature or upon the

addition of salts. One of the phases is a surfactant-depleted phase (aqueous phase) and the other is a

surfactant-rich phase (surfactant phase). The separated SDS phase was the lamella and it had a bilayer

structure. The lamella was made of two parts: a hydrophobic surfactant alkyl chain and a hydrophilic part.

The latter involves the anionic head group of the surfactant, counter ions, and water molecules that hydrate

the ions. The surfactant phase separated at 0°C contained DS-, Na

+, and around 80% (w/w) water, which

was measured by drying the surfactant phase at 110°C for 2 h. Figure 1(a) shows the SAXRD patterns of

the SDS phase. In general, the diffraction peak was fairly broad compared to that of a pure ionic crystal

and its intensity was weak in the extraction system [16]. The layer distance calculated from the Bragg

Figure 1. SAXRD of the surfactant phase precipitated from a solution containing

0.069 mol dm-3

SDS and (a)0, (b) 0.035, (c) 0.14, (d) 0.035, (e) 0.28 mol dm-3

KCl,

at 0°C in (a), (b) and (c), and at 25°C in (d) and (e).

- 43 -

equation (nλ = 2dsin) was the sum of the hydrophobic and hydrophilic parts of the lamella. The

diffraction peak for SDS was observed at 2= 3.02° (d = 2.91 nm). Furthermore, this angle was not

affected by the addition of NaCl as the salting-out reagent.

At room temperature, the surfactant phase was separated by the addition of KCl. This phase

contained DS-, water, and K

+. As shown in Figure 1(e), a new distance of d = 3.35 nm (2= 2.64°) was

obtained upon addition of excess KCl. The increase in the distance is attributed to the formation of the

KDS phase. The melting point of this phase was 35°C and the reported Krafft temperature for KDS is

32–35°C [17,18]. These temperatures are dependent on the conditions; for example, the Krafft temperature

has been reported in the range 7–17°C for SDS [15,19,20]. Since the Krafft temperature of SDS is lower

than room temperature, SDS melted and KDS deposited in the solution. The peak corresponding to the

KDS phase was observed when half an equivalent of KCl relative to SDS was added (Figure 1(d)). As

shown in Figure 1(b), when this solution was cooled to 0°C, the lamellar structure of the surfactant phase

was complicated and two weak peaks corresponding to SDS and KDS were observed.

3.2 The structure of the surfactant phase containing metal ammine complexes

In the presence of ammonia and SDS, copper formed the stable ion-pair, [Cu(NH3)4](DS)2 [21].

Upon the addition of KCl, the SDS surfactant phase changed to the KDS phase at room temperature. Two

mechanisms were assumed for the extraction of copper into this phase. One is the intercalation of

[Cu(NH3)4]2+

by ion exchange with K+ in the hydrophilic part of the KDS phase. In this case, SAXRD

should show only one reflection peak that

moves from low to high angle depending on

the copper concentration. The other

mechanism is independent formation, which

gives two reflection peaks corresponding to

KDS and CuDS.

Figure 2-A shows the SAXRD patterns

of the surfactant phase containing various

amounts of copper extracted at room

temperature. Two peaks were obtained at 2=

2.64° for KDS and 3.68° for CuDS. The latter

is close to the angle of CuDS (3.56°), which

was extracted at 0°C using SDS [22]. It seems

Figure 2-A. SAXRD patterns of the surfactant phase

precipitated from a solution containing 0.069 mol

dm-3

SDS, 0.067 mol dm-3

KCl, 0.45 mol dm-3

ammonia, and (a) 0, (b) 0.007, (c) 0.014, (d) 0.021,

(e) 0.028, (f) 0.035 and (g) 0.042 mol dm-3

Cu(II).

- 44 -

that the intensity of KDS decreases and that

of CuDS increases when the copper

concentration increases. The direct

comparison of these intensities was difficult

because absolute diffraction intensity

changed with the packing conditions of the

gel in the capillary cell. Figure 2-B shows

the relative intensity (ICuDS/(IKDS + ICuDS)),

which increases with increasing copper

concentration. ICuDS is the intensity of CuDS

at 3.68°and IKDS is the intensity of KDS at

2.64°. The mole fraction of copper is defined as mCu(II)/(mCu(II) + mKDS). Above the ratio of mCu(II) : mKDS =

1:2, KDS completely changed to CuDS. For the extraction of copper, the results of the SAXRD analysis

confirmed the independent formation of CuDS, which had a lamellar structure. The same results were

obtained for NiDS and PdDS.

3.3 Extraction of the metal ammine complexes

The deposition of surfactant gels and phase separation are the same phenomena under the Krafft

temperature. The percent extraction was used as an expedient; however, it served to estimate the separation

of metals. The optimal conditions determined for the extraction of copper were 0.0045–0.45 mol dm-3

of

ammonia, and 0.013–0.040 mol dm-3

of SDS. In order to achieve complete phase separation, we used

0.034 to 0.067 mol dm-3

of KCl (Figure 3); the phase separation was incomplete without a salting-out

Figure 2-B. Relationship between the mole fraction

of Cu(II) and the relative intensity.

Figure 3. Effect of KCl concentration on the percent extraction of Cu(II).

[Cu(II)]=1.0×10-3

mol dm-3

[NH3]=0.45 mol dm-3

, [SDS]=0.035 mol dm-3

.

- 45 -

reagent such as KCl. The extractability decreased upon the addition of excess KCl owing to the competing

reaction between the formation of KDS and CuDS.

Most of the copper was extracted without KCl because the melting point of CuDS, 77°C [22], is

higher than room temperature. The melting points, which were measured by DSC, were 45°C and 91°C for

NiDS and PdDS, respectively. The temperatures are indicative of the deposition of MDSs at room

temperature.

If SDS acts as a counter ion for the ammine complex and as an extraction medium, this phenomenon

can be explained with liquid-liquid extraction theory. The extraction model was improved by using the

solubility of the MDSs. As shown in Figure 4, the percent extraction of metal complexes into the surfactant

phase decreased at low metal concentrations. This phenomenon can be explained by the solubility of the

ion pair of DS- and M(NH3)n

2+, where M is palladium, nickel, copper, or zinc [14]. The solubility product

of an MDS, Ksp, is defined as follows:

Ksp = [M(NH3)n2+

]aq [DS-]2aq (1)

where [M(NH3)n2+

]aq and [DS-]aq denote the equilibrium concentrations of the metal ammine complex and

dodecylsulfate anion in the aqueous phase, respectively. In the surfactant solutions involving the solid

surfactant gel, the concentration of the monomer surfactant ion is constant in the aqueous phase. This

phenomenon is similar to that in micellar solutions [23]. Since Ksp and [DS-]aq are constant, the percent

Figure 4. Effect of metal concentration on the percent extraction of metal ions at room temperature.

The solid lines are theoretical. [NH3]=0.45 mol dm-3

, [SDS]=0.035 mol dm-3

, [KCl]= 0.067mol dm-3

.

- 46 -

extraction (E %) is described as follows:

100][][

1(%)2

aqini

spKE

DSM (2)

where [M]ini is the initial concentration of metal ions and the value of [DS-]aq was 6.5 × 10

-4 mol

dm-3

.The values of Ksp at room temperature were 1.1 × 10-12

, 5.6 × 10-11

, and 1.1 × 10-13

for CuDS, NiDS,

and PdDS, respectively. These values almost agree with those obtained at 0°C using SDS [14]. In general,

the previous metal separations have been based on the dependence of pH and/or the ligand concentration

on metal extractability. The dependence of metal concentration was a specific phenomenon in the

surfactant gel extraction and can be made of a combination of general methods.

3.4 Separation of copper and zinc in a brass alloy

In a previous study, a decrease in extractability was observed at low metal concentrations and the

method was applied to the removal of a large excess of palladium from a small amount of platinum in the

cationic surfactant system [24]. In the present study, we examined the separation of the dominant

components of brass, i.e. copper and zinc. The extraction of binary metals using MEUF with SDS was

80.2% for copper and 83.5% for zinc, and MEUF methods have also been used to reject heavy metals in

waste water [25]. Brass is a popular alloy for industrial and household use; thus, copper and zinc coexist in

environmental waste and sludge. The main components of the sample tested in this study were copper

(65%) and zinc (35%), and small amounts of iron, titanium, and scandium (<0.1%) were also present.

Copper and zinc can be separated at initial metal concentrations of between 2.0 × 10-4

and 5.0 × 10-3

mol

dm-3

(Figure 4). The prepared sample solution contained 2.84 × 10-3

mol dm-3

copper and 1.50 × 10-3

mol

dm-3

zinc. Table 1 shows the results of the separation of copper and zinc. More than 99% copper could be

extracted into the surfactant phase and 93% zinc remained in the aqueous phase.

Table 1. Separation of copper(II) and zinc(II) in brass alloy.

Salting-out reagent Temperature Cu(II) Extracted / % Zn(II) Extracted / % Ref

KCl 25 °C 98.7 ± 0.6a)

6.8 ± 1.4a)

This work

Ca(NO3)2 25 °C 97.1 ± 0.2a)

9.5 ± 5.3a)

This work

NaCl 0 °C 98.4 ± 1.5 13.5 ± 5.5 [21]

a) average ± SD (5 samples)

- 47 -

Many anionic surfactants form precipitates with calcium and magnesium ions present in hard water.

The Krafft temperature of 50°C for calcium dodecyl sulfate gave an insoluble precipitate with a solubility

product of only 3.7 × 10-10

at ambient temperature [15]. Therefore, when calcium nitrate (0.035 mol dm-3

)

was used as the salting-out reagent instead of potassium chloride, good results were

obtained for the separation of copper and zinc.

4. Discussion

The Krafft point is defined as the temperature, or more precisely, the narrow temperature range,

above which the solubility of a surfactant rises sharply [26]. At this temperature, the solubility of a

surfactant equals the CMC. The Krafft temperature may vary dramatically with subtle changes in the

surfactant chemical structure and decrease strongly as the alkyl chain length increases. It is determined by

the energy relationships between the solid crystalline state and the micelle solution. It appears that the

micelle solutions vary little between different counter ions; in contrast, the solid crystalline state changes

dramatically owing to packing effects [23].

The MDS formed a water-insoluble gel and was deposited from the aqueous solutions below the

Krafft temperature. The percent extraction of metal was related to the solubility of the gel at low metal

concentrations. To gain a better understanding

of the solubility, we used the energy

relationship shown in Scheme 2. The value of

ΔG is positive for water-insoluble compounds

and is related to the solubility product (Ksp)

according to the equation ΔG = -RT ln Ksp.

Lattice energy (U) is ordinarily treated as the

enthalpy change for a crystalline solid.

However, the lattice energy of the surfactant

gel is defined here for convenience and

involves an enthalpy and entropy term at a

given temperature. GH is the sum of the

hydration energies of the surfactant and

counter ion.

Scheme 2. Energy relation of the solution and gel

containing an ion pair.

- 48 -

We examined the extraction of zinc

using sodium alkylsulfonates (RSO3Na)

in ammonia solutions at 0°C in the same

manner as procedure (A), where R is the

alkyl chain of length C12, C14, or C16. To

obtain the concentration for 50 %

extraction of zinc, zinc extractions were

performed at a concentration of 0.0624

mol dm-3

surfactant, 0.27 mol dm-3

NaCl,

and 1 x 10-6

mol dm-3

- 0.1 mol dm-3

zinc.

Substitution of the sodium ion of RSO3Na

for the cationic complex Zn(NH3)n2+

results in the formation of the gel

[Zn(NH3)n2+

](RSO3-)2; however, the melting temperatures of these zinc ion pairs are unknown. The

horizontal axis in Figure 5 is the Krafft temperature of RSO3Na [27], which parallels the melting

temperature of the gels of the zinc ion pairs. Because the measurement of Ksp was difficult for fairly

soluble gels such as the zinc ion-pair, the vertical axis is the logarithm of the initial concentration when

half of the zinc was extracted ([Zn(II)]E = 50%). The energy U in Scheme 2 increases strongly as the alkyl

chain length of the surfactant increases. The hydration energy (GH), most of which is electrostatic energy,

was considered to be the same for all surfactants because they had the same hydrophilic head group and

counter ion. These gels have different packing conditions of the hydrophobic chains, thus, the remaining

energy, ΔG, increases with increasing alkyl chain length. The larger positive ΔG resulted in lower

solubility and the metal ions began to be extracted at lower concentrations.

The structures of anionic surfactant gels have been studied by SEM, XRD, and optical microscopy in

the presence of metal ions [15,28-31]. The lamellar structure was reported for divalent metal ions such as

copper, which was a counter ion to DS- in the metal extraction method. The solubility product was reported

as 2.4 × 10-10

at 8°C and d = 2.44 nm for Cu2+

(DS-)2 [16]. This layer distance is close to that of the copper

ammine complex (d = 2.4 nm for CuDS). The copper ion is solvated by water molecules and the size of the

hydrated ion is similar to that of the ammonia complex in the hydrophilic part of the lamella.

Figure 5. Relationship between the Krafft temperature

and extractability of Zn(II) for a series of surfactants.

- 49 -

The solubility products (Ksp) and SAXRD patterns were obtained for the gels of NiDS and PdDS.

Figure 6 shows the plot of log Ksp vs. the layer distance (d(001)) measured at 0°C in the SDS system and at

room temperature in the KDS system. The layer distance increased in the order of d = Ni < Cu < Pd which

is the same order for the crystal ionic radii, that is, 69, 73, and 86 pm for Ni, Cu, and Pd, respectively [32].

The structure of the ammine complexes of these metals in the interlayer is not known nor are the ionic radii

of the ammine complexes or the solvated metal ions. However, the size tendency of the ammine complexes

is almost the same as that of the crystalline ionic radii, therefore, it is appropriate to adopt crystal ionic

radii to estimate the size of metal ions. The layer distance increased with increasing ionic radius because

the electrostatic force is inversely proportional to the square of the distance between the surfactant head

group and counter ion. The large attractive force between the anion and cation causes the compression of

lamella. The force is strongest for the small nickel ions; therefore, NiDS had the shortest layer distance.

The order of hydration energies, GH, was estimated from the energies of the counter ions, which are related

to the ionic radii of the metal ions since the same surfactant anion (DS-) was used. The ions were already

surrounded by water molecules in the hydrophilic part of the lamella. The hydration energy of a metal ion

in water [33] is similar to that in the surfactant phase. Therefore, the values of GH were in the order of Ni >

Cu > Pd. The energy U strictly relates to the decomposition energies of the lamella, which consist of the

hydrophobic and hydrophilic parts. We used the same surfactant, except for a minor change in the counter

ions, and most of the energy of U is from the hydrophobic energy of the surfactant alkyl chain C12. The

energy of U was considered constant so that ΔG increased positively with decreasing GH. Therefore, a

large ion such as PdDS is less soluble and was extracted at low metal concentration.

Figure 6. Relationship between the layer distance of the ion pair and the solubility

product at 0°C with SDS(open circles) and 25°C with DS(closed circles).

- 50 -

5. Conclusion

The ionic surfactant SDS and salting-out reagent, KCl, was used for the separation of zinc and

copper by the surfactant gel extraction method using ammonia. This separation method was based on the

dependence of initial metal concentration on the percent extraction. At room temperature, the water-soluble

SDS changed to KDS, which has a high Krafft temperature and forms a solid-like gel. The ion pairs of the

surfactant anion and metal ammine complex also form gels that have different solubilities in water. The

solubility of the ion pairs was an important factor for the separation of the metals. The structure of the

surfactant phase was investigated by SAXRD and DSC measurements. The KDS and the metal ion pair

(MDS), which was formed by the ammine complex and DS-, were gel-like solids. The ammine complexes

could not intercalate into the KDS gel by an ion exchange reaction with K+, therefore, the surfactant phase

consisted of gels of K+DS

- and MDS. The gels of ion pairs and the surfactant had lamellar structures and a

small counter ion compressed the layer distance of the lamella. The large alkyl chain of the surfactant

lowered solubility and metals were extracted at low metal concentration. The investigation of surfactant

phases is useful for improving the separation of metals.

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