Well-defined oxide core-polymer shell nanoparticles: interfacial interactions, peculiar dynamics, and transitions in polymer nanolayers.

Interfacial interactions, chain dynamics, and glass and melting transitions were studied in well-defined core-shell nanoparticles with amorphous silica or crystalline alumina cores and noncrystallizable poly(vinyl pyrrolidone) (PVP) or crystallizable poly(ethylene glycol) (PEG) shells. Varying particle composition caused regular changes in the shell thickness from 1 to 2 nm (monomolecular layer) up to 90 nm. Far- and mid-IR spectroscopy allowed us to register hydrogen bonding and, tentatively, Lewis/Brønsted (LB) interfacial interactions as well as changes in the dynamics and conformational state of the polymer chains as a function of the nanoshell thickness. Their most pronounced peculiarities were found for the monomolecular polymer layers. The LB interactions were stronger with the alumina substrate than silica. DSC analysis was performed, and the data obtained were in agreement with the spectroscopic data. Unlike the bulk polymer, the PVP monolayer was characterized with an extraordinarily large dynamic heterogeneity within the glass transition while broadening the transition range and varying the activation energy by an order of magnitude. The PEG monolayer adsorbed on silica was totally amorphous, whereas a highly crystalline one with the anomalously thin lamellae, down to 3 nm thick, was adsorbed on an alumina surface, presumably as a result of the quasi-heteroepitaxial crystallization process.

Successive interaction of pairs of soluble organics with nanosilica in aqueous media.

Successive interaction of different pairs of water-soluble polymers (poly(ethylene glycol) (PEG), poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA)), proteins (bovine serum albumin (BSA), ovalbumin, gelatin, and ossein), and smaller organics such as lecithin (1-stearoyl-2-oleoyl phosphatidylcholine, SOPC) and Aethonium (1,2-ethylene-bis(N-dimethyl carbodecyl oxymethyl) ammonium dichloride) with nanosilicas A-300 (S(BET)=232 and 297 m(2) g(-1)) and A-50 (S(BET)=52 m(2)g(-1)) was studied using dynamic light scattering, adsorption, and infrared (FTIR) spectroscopy methods. Time-dependent rearrangement of particle size distributions (PSDs) depicts appearance of both smaller and larger aggregates for silica/PEG(I-first adsorbate)/BSA(II-second adsorbate) and silica/PVP(I)/BSA(II) (i.e., BSA adsorbs onto PEG/silica or PVP/silica) than that for silica/organic compound I. However, in the cases of PVA(I)-BSA(II) and PVA(I)-SOPC(II) a similar effect is not observed because only increased aggregation occurs. The successive equilibrium adsorption of similar pairs shows a diminution of the adsorption of the second compound (gelatin, ovalbumin) with increasing amount of the first adsorbed polymer (PEG or PVP).

Interaction of poly(ethylene oxide) with fumed silica.

Interaction of poly(ethylene oxide) (PEO, 600 kDa) with fumed silica A-300 (SBET = 316 m2/g) was investigated under different conditions using adsorption, infrared (IR), thermal analysis (TG-DTA), AFM, and quantum chemical methods. The studied dried silica/PEO samples were also carbonized in a flow reactor at 773 K. The structural characteristics of fumed silica, PEO/silica, and pyrocarbon/fumed silica were investigated using nitrogen adsorption-desorption at 77.4 K. PEO adsorption isotherm depicts a high affinity of PEO to the fumed silica surface in aqueous medium. PEO adsorbed in the amount of 50 mg per gram of silica (PEO monolayer corresponds to CPEO approximately 190 mg/g) can disturb approximately 70% of isolated surface silanols. However, at the monolayer coverage, only 20% of oxygen atoms of PEO molecules take part in the hydrogen bonding with the surface silanols. An increase in the PEO amount adsorbed on fumed silica leads to a diminution of the specific surface area and contributions of micro- (pore radius R < 1 nm) and mesopores (1 < R < 25 nm) to the pore volume but contribution of macropores (R > 25 nm) increases with CPEO. Quantum chemical calculations of a complex of a PEO fragment with a tripple bond SiOH group of a silica cluster in the gas phase and with consideration for the solvent (water) effect show a reduction of interaction energy in the aqueous medium. However, the complex remains strong enough to provide durability of the PEO adsorption complexes on fumed silica; i.e., PEO/fumed silica nanocomposites could be stable in both gaseous and liquid media.

Study of interaction of proteins with fumed silica in aqueous suspensions by adsorption and photon correlation spectroscopy methods.

The interaction of fumed silica A-300 (S(BET) = 297 m2 g(-1)) with bovine serum albumin (prepared by different methods), ovalbumin, human hemoglobin, and gelatin as a function of pH, salinity, and concentrations of components in aqueous medium was studied by adsorption and photon correlation spectroscopy (PCS) methods. Comparison of equilibrium (incubation time t(i) approximately 1 h) adsorption of proteins on A-300, minute (t(i) approximately 1 min) flocculation rate, and the particle size distributions measured by the PCS method shows different rearrangement of particle swarms depending on pH, salinity, and concentration of proteins, especially at pH close to IEP of silica or proteins. The electrokinetic mobility of protein/silica swarms is greater than that of individual components at pH far from the IEP of proteins. Changes in the Gibbs free energy (DeltaG) on protein adsorption depend on pH (-DeltaG is minimal at pH 2, close to the IEP of silica, and maximal at pH between the IEP of protein and silica), concentration (-DeltaG is maximal at C(p) between 1 and 6 mg/ml), type of proteins, and their preparation technique.