Cross-linked poly(dimethylsiloxane) (PDMS) was irradiated with a Xe2*-excimer lamp (172 nm) under ambient conditions. The irradiation in combination with the formed ozone results in an oxidation of PDMS to SiO2 at the polymer-air interface. The surface properties of the irradiated surfaces were studied by means of contact angle measurements, infrared spectroscopy, and X-ray photoelectron spectroscopy. The photochemical conversion of surface methylsilane groups to silanol groups is responsible for the large increase in surface free energy. Subsequent degradation of the polymer and formation of SiOx was monitored by infrared spectroscopy. As determined by X-ray photoelectron spectroscopy, the binding energy shifts reach values corresponding to SiO2. The atomic ratio concentration O:Si changes from about 1:1 (PDMS) to about 2:1 (SiO2). On the basis of the XPS and IR results, the photochemical reaction pathway from PDMS to silicon oxide via surface silanol groups is discussed. The strict linearity of the contact angle versus irradiation time and the clear dependence from irradiation intensity allows the tuning of the chemical surface functionalities.

http://materials.web.psi.ch/Publications/Publ_MatDev_files/2004/Macromolecules_Vera_final.pdf

Photochemical Modification of Cross-Linked Poly(dimethylsiloxane) by Irradiation at 172 nm

A series of silica xerogels that support Rhodamine B as a template were synthesized using distinct sol-gel routes, namely, acid-catalyzed routes, a base-catalyzed route, acid-catalyzed with base-catalyzed (two steps) hydrolytic routes, and a FeCl3 -catalyzed nonhydrolytic route. The extraction methods (thermal, Soxhlet, water washing, and ultrasound) were also evaluated. The resulting xerogels were characterized through porosimetry using nitrogen adsorption/desorption. The samples were further analyzed through small-angle X-ray scattering, Fourier transform infrared spectroscopy, and SEM. The preparation route affected the materials' textural properties. Extraction was optimized using acid and two-step routes. The acid route from Rhodamine B to Rhodamine 6G generated the highest selectivity factor (2.5). The nonhydrolytic route produced the best imprinting factor. Competitive adsorption was also used, from which the approximate imprinting factor was 2. The cavity shape generated during the production of the imprinted silica dictates the adsorption behavior, not the magnitude of the surface area.

Effect of the sol–gel route on the textural characteristics of silica imprinted with Rhodamine B

Saturated alkanes adsorption on synthetic well-crystallized hydroxyapatite was studied by the inverse gas chromatography method, at zero coverage. Hydroxyapatite was prepared from Ca(OH)2 and H3PO4 aqueous solutions by a slow titration technique. The powder was characterized by XR diffraction, surface area measurement, and chemical analysis. Adsorption of saturated alkanes was investigated in the temperature range 338–353 K, using a flame ionization detector, and nitrogen as a carrier gas. The gas/solid partition coefficients were calculated from the retention data as well as the standard thermodynamic functions of adsorption, ΔH0d, ΔS0a and ΔG0a. Linear relationships between lnK and 1/T and between ΔG0a and the number of carbon atoms in saturated alkanes were observed. The dispersive component of the surface free energy was calculated for the investigated temperature region. A possible analytical application of hydroxyapatite to n-alkanes separation is given.

An inverse gas chromatographic study of the adsorption of alkanes on hydroxyapatite

Inverse gas chromatography (IGC) was applied to characterize the surface energy of organically modified silicates (ormosils) by measuring the interaction of molecular organic probes with the ormosil surface. Ormosils were prepared by the sol-gel method by the reaction of TEOS (tetraethoxysilane), PDMS (polydimethylsiloxane) and different types of silica (Aerosil 130, Aerosil 200 and Aerosil 380). The isosteric heat of adsorption, q
st, and the dispersive component of the surface energy, γs
D, were estimated by using the retention volume of different nonpolar and polar probes at infinite dilution. The dispersive component shows an increase as the specific surface area of the silica is increased from 29.6 mJ/m2 to 51.4 mJ/m2 at 60°C. Such values are lower than that obtained for aerosil particles meaning that PDMS chains impede the interaction with silanol groups located on the silica surface. The specific interaction parameter, ISP, and the enthalpy of specific adsorption, ΔH
a
SP, of polar probes on the ormosil surface were also measured in order to obtain the acid-base character of ormosil surface. The ΔH
a
SP, was correlated with the donor, DN, and the acceptor, AN, numbers of the probes to quantify the acidic, K
A, and the basic, K
B, parameters of the substrate surface. The obtained results suggest that the silica particles were covered by PDMS chains in a different way depending on the type of silica used. The values of K
A and K
B suggest that the ormosil surface is amphoteric, with predominantly acceptor electron sites.

https://link.springer.com/content/pdf/10.1023/A:1008759708396.pdf

Surface Energy of Silica-TEOS-PDMS Ormosils

Chlorobenzene, chloroform, and carbon tetrachloride adsorption on undoped and metal-doped sol–gel substrates (SiO2, Ag/SiO2, Cu/SiO2 and Fe/SiO2)

Inverse gas chromatography has been used to measure the surface adsorption of alkane probes on cleaned silicoaluminate glass fibres. The thermodynamic variables (enthalpy, δH

 A
 ℴ
 ; entropy, δS

 A
 ℴ
 ; free energy, δG

 A
 ℴ
 ) and the dispersive component of the surface free energy (γ
 S
 D
 ) have been evaluated by the measurement of specific retention volumes of the alkanes on the silicoaluminate glass fibres. The effectiveness of the acetone extraction procedure and the thermal treatment for the cleaning of the samples was also evaluated. The London (dispersive) component of the surface free energy has been used as a measurement of surface contamination by comparing the results for the washed samples and thermal treated samples.

Surface thermodynamic analysis of cleaned silicoaluminate glass fibres

Changes in the microstructure and texture of a silica xerogel have been studied through out the gel to glass conversion temperature. A molar ratio H 2 O/TEOS=4 was used to obtain silica xerogel. Different samples were treated at 400, 600, 800, 1000°C. Infrared spectroscopy, XRD and nitrogen adsorption have been used in this study. Infrared spectra showed a reorganization of the silica xerogel from the amorphous state to the glassy state. XRD profiles showed that the gel to glass conversion was carried out without formation of any crystalline silica form. Nitrogen isotherms were obtained by means of nitrogen adsorption-desorption in the whole range of partial pressures. The obtained isotherms were type IV of BDDT classification being characterized by a hysteresis loop which indicates the presence of porosity in all samples. Pore size distribution is changed with treating temperature in accordance with the specific surface area. The three types of porosity (micro, meso and macropores) have been found in two samples. Their contribution is different depending on the heating temperature. We have studied the contribution of the pore area to the total specific surface area. Samples treated at low temperature showed a greater macropore contribution than samples treated at higher temperature.

Effect of heating on surface area and pore size distribution of monolithic silica gels.

Gas chromatography is used as a useful and important tool for obtaining physicochemical information. Surface properties of aluminosilicate glass fiber have been studied by adsorption of vapors (alkanes C 5 -C 9 ) at a gas fiber interface. Unlike conventional adsorption techniques, gas chromatography allows the measurement of adsorption data down to very low vapor concentrations where the surface coverage approaches zero and only adsorbate (alkane)-glass fibers interactions are important. The zero coverage on Henry's law regions is considered. The fundamental parameter measured in gas chromatography is the retention volume, which characterizes the equilibrium partitioning of the solute vapor (alkane) between the mobile gas (helium) and stationary phases (aluminosilicate glass fibers). The standard free energy of adsorption (ΔG° A ), the differential heat of adsorption (ΔH A ) and the standard entropy of adsorption (ΔS° A ) are obtained from the retention volumen data

Adsorción de n-alcanos sobre fibras de vidrio silicoalumínosas (tratadas térmicamente) en la zona de recubrimiento cero

A silicoaluminate glass fibre has been treated with HCl aqueous solutions for reaction times of 1, 3, 6 and 24 hours. The pH of these solutions has been 1, 2 and 3. The acid leached silicoaluminate glass fibres have been characterized by means of nitrogen adsorption, FT-IR spectroscopy and Scanning Electron Microscopy. Leached solutions were also analyzed chemically. The extraction of silicon and aluminum ions is higher at low pH and increased with the time of attack. At pH=3 the extraction of silicon and aluminum ions is the same. The lowering of the pH increase the aluminum extraction giving a microporous surface. FT-IR spectroscopy has only shown the major concentration of OH and molecular water in the leached fibres. These surface changes are not detected by SEM. The fibre as received has a specific surface area of 0.26 m 2 /g. This value increase to 16 m 2 /g for that fibre treated at pH=l and 24 hours. Specific surface areas increase with the concentration of leached ions but no relationship has been found. Leached glass fibre of 16 m 2 /g was also studied by volumetric adsorption obtaining an isotherm of type I of the BET classification corresponding to a microporous surface. This microporosity is interpreted according to a selective extraction of aluminum ions from the surface to the bulk of the glass fibre.

M. C. Bautista

Papers

Surface thermodynamic analysis of cleaned silicoaluminate glass fibres

Effect of heating on surface area and pore size distribution of monolithic silica gels.

Adsorción de n-alcanos sobre fibras de vidrio silicoalumínosas (tratadas térmicamente) en la zona de recubrimiento cero

Characterization of the porosity of an acid leached silicoaluminate glass fibre

Surface characterization of the polypropylene matrix used in composites