Adsorption of Anionic Surfactant Sodium Dodecyl Sulfate Onto Alpha Alumina with Small
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:
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) 
where ζ is the zeta potential (mV), the difference of potential (mV), the pressure difference (mbar), the viscosity of the solution (mPa.s), the conductivity of the solution (mS/cm), the relative dielectric constant of the liquid and is the electric permittivity of vacuum (8.85410-12 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 .
Figure S1 shows that the present isoelectric point (IEP) of α-Al2O3 without SDS adsorption (open triangles) is around 6.7 . 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 810-3 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.
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 . 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 . The band appeared at around 1216 cm-1 due to the FTIR-ATR spectra of pure α-Al2O3 coating deposited on Ge crystal in 0.01M NaCl at pH 3.3 . In addition, the characteristic peak of 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.
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
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