Adsorption of anionic surfactant sodium dodecyl sulfate onto alpha alumina with small
surface area
Tien Duc Pham, Motoyoshi Kobayashi*, Yasuhisa Adachi
Graduate School of Life and Environmental Sciences, University of Tsukuba,
Tennodai 1-1-1, Tsukuba-shi, Ibaraki 305-8572, Japan
*:
Corresponding author: kobayashi.moto.fp@u.tsukuba.ac.jp
Tel.: +81 29 853 4861; Fax: +81 29 853 4861.
S1. Streaming potential measurement
A streaming potential technique was applied to characterize the zeta potential of α-Al2O3 beads. The ζ potential is
related to the slope in the streaming potential versus pressure line using Helmholtz – Smoluchowski’s equation (HS) [1]
Ustr ηK L
ζ=
×
(Eq. S1)
ΔP
εε0
where ζ is the zeta potential (mV), Ustr the difference of potential (mV), ΔP the pressure difference (mbar), η the
viscosity of the solution (mPa.s), K L the conductivity of the solution (mS/cm), ε the relative dielectric constant of the
liquid and ε0 is the electric permittivity of vacuum (8.854×10-12 F/m).
A Zeta CAD (CAD Instrument, France) was used in this study. The detail of experimental procedure of
streaming potential with Zeta CAD was described in our previously published paper [2].
Figure S1 shows that the present isoelectric point (IEP) of α-Al2O3 without SDS adsorption (open triangles) is
around 6.7 [2]. Open square points in Fig. S1 demonstrate that the ζ potential of α-Al2O3 increases dramatically in
the pH range of 4 – 9 after SDS adsorption. Adsorption of SDS was conducted at the concentration of 8×10-3 M
(higher CMC) in 0.01M NaCl (pH 4.0). The IEP of α-Al2O3 after SDS adsorption shifts to 8.7, indicating that after
adsorption of SDS, the surface charge of α-Al2O3 increases by proton adsorption on surface site of α-alumina.
Surfactant adsorption at low salt concentration in the presence of hemimicelles induces an increase of surface charge
although admicelles can be formed after CMC. While desorption of SDS is possible in equilibrium step, the higher
positively charged α-Al2O3 surface seems to remain during the measurement of streaming potential. It should be
noted that the adsorption of anionic dyes on the alumina, data not shown, shifts the IEP to lower pH.
1
60.00
ζ potential (mV)
40.00
20.00
0.00
-20.00
0.01M NaCl
-40.00
0.01M NaCl + SDS
-60.00
3
5
7
9
pH
Fig. S1 The ζ potential of α-Al2O3 without adsorption (open triangles) and after SDS adsorption (open squares) as
a function of pH in 0.01 M NaCl.
S2. FTIR-ATR spectroscopy
Infrared spectra were taken to examine the structure of adsorbed SDS on the surface of α-Al2O3. They were recorded
on a Perkin Elmer GX FT-IR spectrometer equipped with a deuterated glycine sulphate (DTGS) detector. An
attenuated total reflection (ATR) attachment with a micro germanium (Ge) crystal was used. All spectra were
recorded at room temperature and at a resolution of 4 cm-1.
In Fig. S2, a weak peak of Al2O3-SDS at 1470 cm-1 is assigned to the alkyl bending modes of SDS molecules [3].
The asymmetrical deformation vibration of CH3 at 1468 cm-1 can be seen in the spectra of SDS powder (Fig. S3).
Furthermore, the relative intensity of asymmetrical and symmetrical stretching of – CH2 –presented at 2919 and
2850 cm-1 (Fig. S3) decreases dramatically in the spectra of Al2O3-SDS in the wavenumber range of 400 – 4000 cm-1
(data not shown). These results suggest that the hydrophobic interaction can work on the surface of α-Al2O3. The
adsorption of SDS onto α-Al2O3 was conducted in 0.01 M NaCl at pH 4 and higher concentration of surfactant than
CMC so that admicelles can be formed on surface of alumina after the formation of hemimicelles [4]. The band
appeared at around 1216 cm-1 due to the FTIR-ATR spectra of pure α-Al2O3 coating deposited on Ge crystal in
2-
0.01M NaCl at pH 3.3 [5]. In addition, the characteristic peak of SO4 at about 1219 cm-1 [3, 4] of Al2O3-SDS is
similar to this peak of Al2O3 while the band of SDS powder appears very strong in Fig. S3. It is demonstrated that
SDS with the sulfate head groups contact to the surface of α-Al2O3 via the electrostatic attraction. The results of
FTIR-ATR spectra of α-Al2O3 after SDS adsorption compared with another of SDS powder and α-Al2O3 beads
suggest that SDS molecules mainly adsorb on the surface of α-Al2O3 by electrostatic attraction as well as probably
by hydrophobic interaction.
2
Fig. S2 Ex-situ FTIR-ATR spectra in the wavenumber range of 1000 – 2200 cm-1 for α-Al2O3 without adsorption
and after SDS adsorption.
Fig. S3 FTIR-ATR spectra of SDS powder in the wavenumber range of 400 – 4000 cm-1
3
References
1. Delgado AV, González-Caballero F, Hunter RJ, Koopal LK, Lyklema J (2007) J Colloid Interface Sci 309:194224.
2. Pham TD, Kobayashi M, Adachi Y (2013) Colloids Surf A 436:148-157.
3. Singh MK, Agarwal A, Gopal R, Swarnkar RK, Kotnala RK (2011) J Materials Chem 21:11074-11079.
4. Sperline RP, Song Y, Freiser H (1992) Langmuir 8:2183-2191.
5. Del Nero M, Galindo C, Barillon R, Halter E, Madé B (2010) J Colloid Interface Sci 342:437-444.
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