Methorics of the Precipitation Processes. XXV. Effects of Non-Ionic Surface-Active Agents on Growth and Aggregation of Silver Iodide 801s

Precipitation kinetics and adsorption behaviour in AgI colloid systems prepared \u27in statu nascendi\u27 in the presence of non-ionic surface-active agents of octaphenylethoxylate series were investigated by light scattering method at 20 CC. The rate of precipitation was followed directly by using light scattering photometel\u27 with recorder. The adsorption isotherms exhibiting monelayer coverage in the regi on of maximum floceulation and multilayer adsorption in the stabilization region have been obtained. The mechanism of floeeulation and stabilization can be assumed considering several aggregation steps in the dependence of concentration and time

Effect of “Cooking” Method on the Hulled and Unhulled Barley Grain Composition

Barley is one of the oldest cultivated crops in the world and in the past represented a significant grain in human nutrition. The main purpose of barley cultivation today is for the livestock needs and the brewing industry, where it is used in the form of malt, but also is increasingly used in human nutrition as barley porridge. During the barley grain processing for human consumption, to provide better digestibility, barley is hulled and such grain has a reduced nutritional value. To improve the nutritional properties of barley after hulling, the grains are subjected to a thermal cooking treatment due to the starch gelatinization. This investigation includes two varieties of hulled and unhulled barley Vedran and Titan. In all samples, the nutritional composition according to standard methods was investigated, namely the content of moisture, ash, fat, protein and starch were determined. After the cooking method (adapted "cooking" method), at a pressure of 0.5 bar and a time of 10 and 15 minutes and convection drying of samples with air temperatures of 50 °C, 60 °C and 70 °C, the nutritional composition of all samples was also determined. The research showed that the cooking method and drying affected the change in the nutritional composition of hulled and unhulled barley, and in all investigated nutritional composition parameters there were changes compared to the initial samples

Studies of Thiophene and Substituted Thiophenes at Platinum(111) Electrodes by Vibrational Spectroscopy and Auger Spectroscopy: Monomers, Dimers, and Polymers

[[abstract]]The adsorption behavior of various thiophenes from organic as well as aqueous solutions at well-defined Pt( 111) surfaces is examined in this study. The adsorbates studied include 3-thiophenecarboxylic acid (3TCA), 2-thiophenecarboxylic acid (XTCA), 3-thiopheneacetic acid (STAA), 2-thiopheneacetic acid (ZTAA), thiophene (TPE), 3-methylthiophene (3MT), 3,3’-dimethyL2,2’-bithiophene(3 3’DMBT), and 4,4’-dimethyL2,2’- bithiophene (44’DMBT). Packing densities (moles adsorbed per unit area) were measured for each compound by Auger spectroscopy. Surface vibrational spectra were obtained by electron energy loss spectroscopy (EELS) and were assigned by comparison with the IR spectra of the pure compounds. The Pt(ll1) surfaces used in this study were characterized by LEED. All the thiophenes studied are adsorbed with the ring plane nearly perpendicular to the platinum surface. Vibrational spectra of thiophenecarboxylic acids, and the dependence of adsorption on electrode potential, give valuable information about the adsorbate structure. The compounds 3TCA and 2TAA adsorb with pendant carboxylic acid moieties and give vibrational spectra that are noticeably dependent on the electrode potential. Adsorption at relatively positive potentials (+0.4 V vs Ag/AgCl) shows increased interaction of the carboxylic acid moieties with the metal surface compared with relatively negative potentials (-0.1 V vs Ag/AgCl), as evidenced by diminution of the intensities of bands due to 0-H and C=O stretching as well as shifts in the frequency and intensity of aromatic CC modes. Further evidence for the pendant nature of the carboxylic acid moieties in 3TCA and 2TAA is their reactivity with KOH. Auger spectroscopy shows that the pendant carboxylic acid functionality in 3TCA and 2TAA reacts with KOH to a larger extent than does STCA or 3TAA. The geometry of STCA and 3TAA allows the carboxylic acid to interact with the metal surface, even at relatively negative (-0.1 V vs Ag/AgCl) electrode potentials. This is borne out by low-intensity 0-H and C=O stretching vibrations, as well as relatively little reactivity with KOH (compared with STCA and 2TAA). The C/S elemental ratios based on Auger spectra are smaller than those expected on the basis of molecular formulas for all the thiophene derivatives studied. This result is due to desulfurization of the thiophenes at the Pt surface, occurring while the Pt is immersed in the thiophene solution, and is more pronounced on surfaces immersed at relatively positive potentials. The amount of desulfurization increases continuously with thiophene concentration in acetonitrile but reaches a maximum at about lo4 M in water. This study includes the first reported surface vibrational spectra of poly(3-methylthiophene) (P-3MT), as well as the first electropolymerized polymer fiis on well-characterized single-crystal metal surfaces. P-3W vibrational spectra are compared with the EELS spectra of the corresponding monomer and dimers (33’DMBT and 44’DMBT), and spectral evidence is presented for oxidative C-C bond formation in the electropolymerization of 3MT

Structure and Comparison of Platinum(111) and Platinum(100) Surfaces as a Function of Electrode Potential in Aqueous Sulfide Solutions

[[abstract]]Studies are reported in which surface layers formed by immersion of well-defined Pt(ll1) and Pt(100) electrode surfaces into aqueous NazS solutions were characterized with regard to structure, composition, and reactivity by means of low-energy electron diffraction (LEED), Auger electron spectroscopy, electron energy-loss spectroscopy (EELS), linear scan voltammetry, and coulometry. Voltammetry reveals that only oxidative desorption of S occurs on the Pt surfaces; no S reductive desorption is observed over the useful potential range. Combined surface analysis data (Auger), vibrational spectra (EELS), and structural data (LEED) permit identification of adsorbed layer composition and structure on the Pt(ll1) and Pt(100) surfaces as a function of potential. At potentials between -0.6 and 0.0 V (vs Ag/AgCI), LEED reveals that stable ordered adsorbed sulfur layers are formed on both surfaces: Pt(lll)(d3Xd3)R3O0-S and Pt- (100)(d2Xd2)R45�-S. The best clarity of the LEED patterns is found at pH 9. Potentials more positive than 0.0 V give rise to increasingly diffuse intensity related to oxidative desorption of S. Voltammograms for oxidative desorption of S from both surfaces are markedly different, indicating different mechanisms of S oxidation at the two surfaces: at pH 9, four voltammetric peaks are present for S at the Pt(ll1) surface, compared with only one peak for the Pt(100) surface. Coulometric data reveal that approximately six electrons are transferred in oxidation of adsorbed S at both surfaces at pH less than 10. Voltammetric behavior of the sulfur layer is sharply dependent upon pH

Oriented Adsorption at Well-defined Electrode Surfaces Studied by Auger, LEED, and EELS Spectroscopy

[[abstract]]Quantitation by use of Auger spectroscopy and cyclic voltammetry of molecular layers adsorbed at Pt(111) and Pt(100) surfaces from aqueous electrolytes is examined in this work for the following compounds: hydroquinone (HQ); phenol (PL); catechol (CT); 3,4-dihydroxyphenylacetic acid (DOPAC); L-3,4-dihydroxyphenylalanine (DOPA); l-tyrosine (TYR); l-phenylalanine (PHE); nicotinic acid (NA); 2,5-dihydroxy-4-methyl-benzyl mercaptan (DMBM); thiophenol (TP); benzylmercaptan (BM); 3-thiophene carboxylic acid; and 2,5,2′,5′-tetrahydroxybiphenyl (THBP). Two independent methods of measurement of packing density based upon Auger spectroscopy, and one based upon cyclic voltammetry are employed and the results compared. Voltammetric oxidation/reduction of adsorbed layers formed from these compounds at Pt surfaces in aqueous electrolyte is found to be essentially the same whether carried out before or after lengthy evacuation. Therefore, the results of surface spectroscopy in UHV are directly applicable to the liquid—solid chemistry and electrochemistry of these adsorbed compounds. Packing densities measured by means of two Auger spectroscopic methods were in good agreement with each other and with the voltammetric measurements

Comparison of Voltammetric Behavior of Adsorbed or Dissolved Unsaturated Alcohols at Vacuum-Annealed and Electrochemically Cycled Pt(111) and Pt(polycrystlline) Electrodes

[[abstract]]These studies compare the voltammetric behavior of solutions and adsorbed layers of the following unsaturated alcohols: benzyl alcohol (BZA), 4-pyridylcarbinol (4PC), allyl alcohol (AA), propargyl alcohol (PGA), cis-2-butene-1,4-diol (CBED), and 2-butyne-1,4-diol (BYD). They were undertaken for well-characterized Pt(111) electrode surfaces which were either annealed in an ultrahigh vacuum (UHV) or electrochemically cycled, as well as for annealed (UHV) Pt(poly). Electrochemical oxidation of BZA, AA, PGA, CBED, and BYD in aqueous fluoride electrolyte proceeds in two stages: first the alcohol moiety is oxidized to CO2 (0.4 V), followed by oxidation of the resulting adsorbed hydrocarbon (alkene, alkyne, or phenyl, 1.0 V); 4PC is relatively inert. The alcohol moiety in 4PC, which is located opposite the pyridine ring from the surface, does not undergo oxidation at an appreciable rate. in fact, 4PC effectively passivates the surface and poisons the electrochemical activity. Electrochemical cycling of the annealed Pt(111) single-crystal surface greatly increases the oxidation rates of the aliphatic alcohols and, to a lesser extent, increases the oxidation rate of BZA. The oxidation rate enhancement is somewhat smaller if the electrode is electrochemically cycled in the presence of the alcohol. The majority of the difference effected by cycling can be observed after only one cycle. Voltammetry on polycrystalline Pt surfaces resembles that on cycled Pt(111) surfaces for PGA and AA, but resembles that on cycled Pt(111) surfaces for PGA and AA, but resembles that on annealed Pt(111) more closely for the diols CBED and BYD