Poly(N-isopropylacrylamide) grafted into nanopores: Thermo-responsive behaviour in the presence of different salts

Surface initiated polymerization of N(isopropylacrylamide) (NIPAM) was performed by controlled radical polymerization on PET track-etched membranes presenting two different pore diameters (narrow pores: ∼80 nm and large pores: ∼330 nm). The opening and closing characteristics of the resulting PNIPAM-g-PET membranes were investigated by conductometric measurements carried out at different temperatures below and above the LCST of PNIPAM and in the presence of different salts. Depending on the membrane pore size, two types of permeation control mechanisms are observed. In large pore membranes, expanded PNIPAM chains conformations result in reduced effective pore size and therefore lower permeabilities relative to collapsed macromolecules chain conformations. In contrast, in narrow pore membranes, the expanded PNIPAM brush presents greater degrees of hydration in the surface layer and therefore gives rise to higher permeabilities than the collapsed conformation. In this situation, the overall permeability is thus comparable to that of a hydrogel membrane.     

Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

The nanotribological responses of a series of nonionic polyoxyethylene surfactants (Tween 20, Tween 40, Tween 60, and Tween 80) were investigated after they were adsorbed from aqueous solution onto atomically smooth hydrophobic substrates. The hydrophobic surfaces were composed of a condensed monolayer of octadecyltriethoxysilane (OTE; contact angle theta>110 degrees ). The nanorheological measurements were performed using a modified surface forces apparatus after coating atomically smooth mica with these OTE monolayers, while adsorption measurements were performed using phase-modulated ellipsometry on silicon wafers coated with these same monolayers. The minimum surface-surface separation observed under high load in friction studies agreed quantitatively with the thickness obtained from ellipsometry. For Tweens 20, 40, and 60, the thickness of the adsorbed film increases with increasing alkyl chain length. Systematic investigations of the nanorheological response showed that there is a "solid-like" elastic response from confined surfactant layers, which is the case for the smallest separations to separations up to slightly larger than twice the adsorbed film thickness. In kinetic friction, these confined layers are characterized by a shear stress of approximately 3 MPa with minimal dependence on shear rate. The magnitude of the sliding shear stress is the same as the apparent yield stress at approximately 3 MPa; it is independent of alkyl chain length within the Tween family of surfactants and corresponds to a nominal friction coefficient of mu approximately 1. A similar friction coefficient is observed for boundary lubrication on the macroscopic scale in a tribometer utilizing hydrophobic surfaces and mu approximately 1.1 for Tweens 20, 40, and 60. These results suggest that while Tween molecules adsorb onto hydrophobic surfaces to form a robust separating layer, the lubricating properties of these layers are dominated by a highly dissipative slip plane, the same for all alkyl chain lengths.     

Cloud points and phase separation of aqueous poly(N-vinylacetamide) solutions in the presence of salts

Aqueous poly(N-vinylacetamide) (PNVA) solution was found to exhibit the cloud point in the presence of salt. This cloud point was shown to correspond to a liquid-liquid phase separation, as confirmed when the PNVA-salt solutions were maintained at a temperature above the cloud point. The upper layer had a higher polymer concentration and a lower salt concentration than those in the lower layer. Thus interaction between PNVA and salts are repulsive. The lower critical solution temperatures were estimated to be 18±1°C for 1.25 molal (NH₄)₂SO₄ and 25±1°C for 0.76 molal Na₂SO₄. Divalent anions such as SO _2– ⁴ , SO _2– ³ , HPO _2– ⁴ and CO _2– ³ were effective in causing turbidity when examined at 25°C. Dependence of the effect on the cationic species was similar to but significantly different from that for acetyltetraglycine ethylester. The cloud points of PNVA decreased linearly with the increase of the polymer concentration at a fixed salt concentration or with the increase of the salt concentration at a fixed polymer concentration. A parameter analogous to the salting-out constant was empirically derived from the dependencies of the cloud points on the concentrations of polymer and salt.     

Oxidation of azo dyes with inorganic peroxides in the presence of cationic surfactants

Degradation of Direct Red 23, Rective Red 45, and Orange II azo dyes with hydrogen peroxide and hydroperite in the presence of cationic surfactants alkyldimethylbenzylammonium chloride (Katamin AB) and cetyltrimethylammonium chloride and without them was studied by spectrophotometry. The reactivity of the azo dyes was correlated with their chemical structure.     

Trends of salting out of oxyethylated anionic surfactant potassium bis(alkylpolyoxyethylene)phosphate with inorganic salts

The effect of salt structure on the ability to salt out oxyphos B from aqueous solutions is elucidated using the solubility data in pseudo-ternary inorganic salt–potassium bis(alkylpolyoxyethylene)phosphate (oxyphos B)–water systems in the temperature range of 20–90°С. The results are interpreted according to the Samoilov’s theory of salting-out.     

(1)H NMR and viscometric studies on cationic gemini surfactants in presence of aromatic acids and salts

In this paper, we are reporting the influence of addition of aromatic acids (anthranilic and benzoic acid) and their sodium salts on the micellar morphological changes in three cationic gemini surfactant solutions, viz. 5 mM tetramethylene-1,4-bis(N-hexadecyl-N,N-dimethylammonium bromide), 10 mM pentamethylene-1,5-bis(N-hexadecyl-N,N-dimethylammonium bromide), and 10 mM hexamethylene-1,6-bis(N,-hexadecyl-N,N-dimethylammonium bromide). The solubilization site of the counterions (obtained from the additives) near the micellar surface are inferred by 1H NMR. The behavior is explained in the light of binding of counterions to the micelle as well as the nature of the functional group attached to the additive.     

Field desorption mass spectrometric analysis of mixtures of surfactants and inorganic salts

A field desorption mass spectrometric method is proposed for the analysis of mixtures of surfactant and inorganic salts. Different compounds in the mixture are determined with different heating currents and distinguished on the basis of the exterior character of the mass spectrum. The effectiveness of the method is demonstrated with known mixtures of surfactants and inorganic salts. Two unknown additives for electric tin plating were tested with the method.     

Diffusion of acid dyes in nylon film in the presence of inorganic salts

The concentration-dependence measured by steady-state permeation and unsteady-state dyeing methods for a system of acid dye C.I.Acid Blue 182–nylon 6 film in the presence of inorganic salts such as NaCl, Na₂SO₄, and K₂SO₄, was analyzed in terms of parallel diffusion with simultaneous multimodal adsorption. It would found that the dyeing process with added NaCl was governed by surface diffusion with two kinds of Langmuir adsorption, whereas with added Na₂SO₄ and K₂SO₄ it was governed by surface diffusion with three kinds of Langmuir adsorption.     

Unusual micellar properties of multiheaded cationic surfactants in the presence of strong charge neutralizing salts

The aggregation properties of single-chain surfactants bearing one (H1), two (H2), and three (H3) trimethylammonium head groups have been studied by small-angle neutron scattering (SANS). Growth of aggregates was observed to decrease dramatically with an increase in the number of head groups in the surfactants. The micelles grow progressively smaller with every increase in the number of head groups of the surfactants. Aggregation number (N) continuously decreases and the fractional charge (alpha) gradually increases with the increase in the number of head groups. The semiminor axis (a) and semimajor axis (b=c) of the micelle decrease strongly with the increase in the number of head groups. In the case of H1, dramatic micellar growth is observed on addition of salts such as KBr and sodium salicylate, but this type of micellar growth is not observed in the cases of H2 and H3 when the above salts are added to their micellar solutions. Aggregation number and size of the micelles remain almost the same, even after addition of KBr at a concentration as high as 100 mM. This observation with multiheaded cationic surfactants is unusual. Clearly, the charge density at the head group level of surfactants markedly influences their micellar aggregation properties.     

On acrylonitrile polymerization in the presence of zinc salts

Acrylonitrile polymerization photoinitiated at 365 nm by pyrene and/or azobisisobutyronitrile in the presence of zinc salts in N,N-dimethylformamide solution has been studied by the rotating sector method. It was found that the ratio of the rate constants for propagation and termination (k_p/k_t) increases on addition of zinc salts (chloride, nitrate, acetate). This increase was more pronounced for the azobisisobutyronitrile photoinitiated polymerization of acrylonitrile then for its pyrene photoinitiated polymerization. The results confirm the previously expressed view concerning the dual role of zinc chloride in initiation as well as in propagation steps of acrylonitrile polymerization photoinitiated by aromatic hydrocarbons.

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Interactions of inorganic salts with poly(ethylene oxide)

Evidence is presented for the interaction of metal salts such as potassium iodide with polyethers such as poly(ethylene oxide). This interaction is sufficiently marked that the incorporation of 10–30% of the salt in the bulk polymer markedly reduces crystallinity while retaining compatibility. Examination of electroviscous effects in methanol demonstrates that the salt–polymer adduct behaves as a typical polyelectrolyte at low salt concentrations, while the polymer in absence of salt is essentially insoluble in methanol at room temperature. Measurements of the equilibrium between salt and polymer along with a study of various molecular weight polymers strongly suggest that one salt molecule associates with about nine ethylene oxide units. It is proposed that the association is due to an ion–dipole interaction, and the anion is tentatively postulated as the species directly associating with the polymer. The association of other metal salts and other polymers are interpreted in this light. The significance of these results in interpreting salting-in phenomena is also discussed.     

Dissociation constants of phenol in the presence of polyoxyethylene sorbitan monooleate

Potentiometric titration was used to determine the dissociation constants of di- and trihydroxybenzenes in a 1: 1 deoxygenated water-ethanol system at a temperature of 293 ± 1 K in the presence of a nonionogenic surfactant, polyoxyethylene sorbitan monooleate (Twin-80). The dissociation constants (pK ₁) of phenols without addition of surfactants were evaluated.     

Oxidative destruction of dinitrophenols by Fenton’s reagent in the presence of inorganic salts

The influence of inorganic salts on the kinetics of oxidation of 2,4-and 2,6-dinitrophenols by Fenton’s reagent, hydrogen peroxide, in the presence of iron(II) ions was studied.     

On the feasibility of chemical reactions in the presence of siloxane-based surfactants

Siloxane-containing surfactants have been tested as stabilizers for the preparation of polymer nanoparticles by three types of chemical reactions. Two crosslinking reactions were used to obtain silicone elastomers particles: one involved HO-terminated polydimethylsiloxane and tetraethoxysilane, while the other one was a crosslinking via polyhydrosilylation. The third reaction was a linear polycondensation between a diamine and a siloxane dialdehyde. The monitoring of the reactions has been made by infrared spectroscopy and the resulting particles have been analyzed in dispersion by light scattering and in dry state by electron microscopy and atomic force microscopy. The particles size was of hundreds of nanometers and their spherical shape was generally maintained after drying. The spectral and microscopy data proved efficient stabilization, which allowed the reactions to evolve after the formation of the particles.     

Study of the interfacial tension in model and technical emulsions in the presence of surfactants and inorganic electrolytes

Interfacial tension in the oil/water system in the presence of various ionic surfactants and inorganic electrolytes was studied. Special features of the effect of the surfactant and oil phase natures, of the structure of their molecules, and also of the electrolytes containing ions with various radii, valences, and hydratabilities on the value of the interfacial tension were studied. The criteria and conditions of obtaining model emulsions based on paraffin hydrocarbons and technical emulsions based on vegetable oils were determined.     

The aggregation of nonionic surfactants in the presence of poly(methacrylic acid)

The solution properties of homogeneous hexaethylene and octaethylene glycol mono(n-dodecyl) ethers, C₁₂E₆ and C₁₂E₈, respectively, and octaethylene glycol mono(n-decyl) ether, C₁₀E₈, with poly(methacrylic acid) (PMA) were investigated by dye solubilization, surface tension, fluorescence, viscosity, and pH measurements. The data were discussed regarding non-cooperative and cooperative binding of surfactant to polymer. Whereas in the interaction with poly(acrylic acid) (PAA), the critical aggregation concentrations (cac or T ₁) of these surfactants were lower than the respective critical micelle concentration (cmc), in that with the more hydrophobic PMA, T ₁’s of C₁₂E₆ and C₁₂E₈ were higher than the respective cmc, but that of C₁₀E₈ was lower than its cmc. These may be ascribed to the hydrophobic microdomains (HMD) of the PMA coil in water, probably in its inside. It is considered that some surfactants are bound first to the HMD non-cooperatively and then they are abruptly bound cooperatively at T ₁. This raises T ₁ higher than cmc when the cmc is low, and the amount bound by the HMD is relatively large and vice versa. T ₁ of C₁₂E₆ or C₁₂E₈ is the former case, and that of C₁₀E₈ is the latter. Thus, different from PAA, T ₁ for PMA + nonionic surfactant system consists of the amount of non-cooperative binding and the cac of the cooperative binding in equilibrium. Therefore, this T ₁ has a different meaning from that for PAA and should be called apparent T ₁. As the binding to the HMD is dependent on PMA concentration and cac is not, which is like in the PAA system, separation of apparent T ₁ from the HMD binding was achieved by extrapolating T ₁’s to zero PMA concentration (denoted intrinsic T ₁). This value for C₁₂E₈ was found to be lower than the respective cmc and also lower than the respective T ₁ for PAA. With increase in surfactant concentration, the pH of PMA solution rose and demonstrated a peak. This pH rise and fall may be induced by loosening of the HMD coil due to binding increase and by rearrangement of PMA + surfactant complex in high surfactant concentrations region. By raising the initial pH, the HMD were loosened; consequently, T ₁ rose a little, and at higher pH, no surfactant binding took place.     

Protolytic properties of some bis(dimethylaminomethyl)phenols in the presence of surfactants

Aggregation and protolytic properties of bis(dimethylaminomethyl)phenols containing methyl (HA) and nonyl (HL) substituents at the benzene ring are studied in aqueous solutions of isopropanol and various surfactants with potentiometric titration, tensiometry, and mathematical modeling of equilibria. Monomers, dimers, and tetramers of HA and HL are found. It is shown that the degree of compound aggregation depends on the solution concentration and pH. Sodium dodecyl sulfate and HA form associates, whereas SDS and HL form mixed micelles at the CMC-1 and CMC-2 critical micellization concentrations. In micellar solutions of Triton X-100 and cetyltrimethylammonium bromide, the mixed micelles are not found via tensiometry. Protonated species of tetramer, dimer, and monomer of investigated compounds are revealed, depending on the acidity of the medium. Phenolate forms of HA and HL do not exist under experimental conditions. Apparent protonation constants are determined and it is shown that, for the HA compound that does not form micelles, the protonation constants of the same-type species increase in the presence of the three surfactants used as compared to the water-isopropanol solution. Decreasing constants of analogous HL forms in the solutions of CTAB, nonionic surfactant (C _Tx = 10 mM), and SDS (pH > 7) are attributed to the formation of associates or mixed micelles of this compound and surfactants under experimental conditions.