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Öğe Drying Behavior of Mushroom Slices(Marcel Dekker Inc, 1990) Yapar, S; Helvaci, Ss; Peker, SÖğe Effect of electrolytes on the surface behavior of rhamnolipids R1 and R2(Elsevier Science Bv, 2004) Helvaci, SS; Peker, S; Ozdemir, GThe surface behavior of solutions of the rhamnolipids, R1 and R2, were investigated in the absence and presence of an electrolyte (NaCl) through surface tension measurements and optical microscopy at pH 6.8. The NaCl concentrations studied are 0.05, 0.5 and 1 M. Electrolytes directly affect the carboxylate groups of the rharmolipids. The solution/air interface has a net negative charge due to the dissociated carboxylate ions at pH 6.8 with strong repulsive electrostatic forces between the rhamnolipid molecules. This negative charge is shielded by the Na+ ions in the electrical double layer in the presence of NaCl, causing the formation of a close-packed monolayer, and a decrease in CMC, and surface tension values. The maximum compaction is observed at 0.5 M NaCl concentrations for R1 and R2 monolayers, with the R1 monolayer more compact than R2. The larger spaces left below the hydrophobic tails of R1 with respect to that of R2, due to the missing second rhamnosyl groups are thought to be responsible for the higher compaction. The rigidity of both R1 and R2 monolayers increases with the electrolyte concentration. The rigidity of the R1 monolayer is greater than that of R2 at all NaCl concentrations due to the lower hydrophilic character of R1. The variation of CMC values as a function of NaCl concentration obtained from the surface tension measurements and critical packing parameter (CPP) calculations show that spherical micelles, bilayer and rod like micelles are formed in the rhamnolipid solutions as a function of the NaCl concentration. The results of optical microscopy supported these aggregation states indicating lamellar nematic liquid crystal, Cubic lamellar and hexagonal liquid crystal phases in R1 and R2 solutions depending on the NaCl concentration. (C) 2004 Elsevier B.V. All rights reserved.Öğe Effect of Interfacial Instabilities and Hydrodynamic Interaction on Liquid-Liquid Mass-Transfer(Pergamon-Elsevier Science Ltd, 1980) Peker, S; Comden, M; Atagunduz, GÖğe Effect of interfacial properties on the drop size distribution of high internal phase ratio emulsions(Elsevier Science Bv, 2001) Peker, S; Bora, K; Over, YInterfacial properties of polyoxyethylene(2) oleyl ether and drop size distributions of emulsions prepared with this surfactant are investigated experimentally in this work; 80, 88 and 94% (v) internal phase ratio W/O type emulsions were prepared, where the aqueous phase was 1% sorbitol solution and the oil phase, a 1:1 v/v mixture of surfactant and paraffin oil. Experimental evidence indicated an interaction between the hydrocarbon chains of surfactant molecules and the solvent paraffin oil through van der Waals forces and between the ethoxylate groups and sorbitol, a polyalcohol, in the aqueous phase through hydrogen bonds, notably below the transition temperature. Continuous phase liquid present in the plateau borders was found to act as a reservoir of surface active agents, minimizing the effect of stretching of the film under shear. In cases where the amount of liquid hold up in plateau borders is negligible, the drop size distributions were found to be very sensitive to shearing rate. Interplay between the interfacial shear viscosity and the interfacial tension was found to control the rate of coalescence of drops less than 0.48 mum only in the case of 94% emulsions, corresponding to maximum stretching of interfacial area. In other cases, interfacial film stability seems to control the rate of coalescence. (C) 2001 Elsevier Science B.V. All rights reserved.Öğe Effect of pH on the surface and interfacial behavior of rhamnolipids R1 and R2(Elsevier Science Bv, 2004) Ozdemir, G; Peker, S; Helvaci, SSSurface and interfacial tensions of pure R1 and R2 solutions at the pH values of 6.8 and 5 were comparatively studied in this work, with decane and hexadecane as the oil phase. Conductivities were measured as a function of concentration to supplement the surface tension results. Photographs of these solutions under cross-polarized light were taken to show the existence of liquid crystals. Foaming ability and stability of pure rhamnolipid solutions were determined using a modified Bikerman device. Results of the measurements showed that R1 and R2 molecules form compact phases at the surface beginning from very low concentrations. R1 molecules are more surface active at concentrations below CMC independent of the bulk phase pH. Nevertheless, neither the value of CMC nor the minimum surface tension at CMC is affected by the type of rhamnolipid significantly. These depend only on the pH of the solution, since greater interactive forces exist between the undissociated rhamnolipid molecules at pH = 5.0 resulting in a greater compaction at the surface monolayer. The interfacial behavior of R1 and R2 molecules at concentrations below CMC differ considerably with the structure of the molecules with which they interact at the interface: R1 molecules are more capable of reducing the interfacial tension than R2 when using decane as the oil phase instead of hexadecane. Strong intermolecular interactions between rhamnolipid molecules allow foam formation to occur only in a narrow range of air flow rates. Stability of the foam depends on the rhamnolipid type with R1 forming more stable foams. Maximum foam volume depends on the pH of the solution and not on the type of rhamnolipid. (C) 2003 Published by Elsevier B.V.Öğe Flow of high internal phase ratio emulsions through pipes, orifices and nozzles(Begell House, Inc, 1998) Peker, S; Tanilmis, T; Ozsaygi, R; Tekden, E; Maron, DMThe flow behavior of high internal phase ratio emulsions through pipes of 3.5, 5.6 and 21 mm internal diameters, orifices with D-o / D-i ratios of 0.2, 0.4, 0.6 and 0.8 and nozzles with uniform decrease in diameter is investigated in this work. W/O type emulsions were formed with 1% sorbitol solution as the aqueous dispersed phase and light mineral oil as the continuous oil phase and polyoxyethelene(2) oleyl ether as the surface active agent. Slip flow is observed in.21 mm internal diameter stainless steel pipes where the shear stress at the wall is not excessive. The emulsion undergoes a structural change through which the viscosity increases sharply in the small diameter pipes, where the wall shear stress is considerable. 88% emulsions, liable to changes in drop configuration are affected the most by variations in the wall shear stress. Elongational viscosities can be correlated with the rate of extension only if all of the surface active agent is present in the films separating the dispersed phase and not in the plateau borders which can act as reservoirs. The phase ratio becomes a factor only in orifices with D-o/D-l ratio of 0.4, where maximum pressure differences are observed.Öğe Interface-subphase interactions of rhamnolipids in aqueous rhamnose solutions(Amer Chemical Soc, 2003) Peker, S; Helvaci, S; Ozdemir, GEffects of the presence of rhamnose (RH) sugar in the bulk phase on the interfacial properties of rhamnolipids (RL) are investigated in this work. Rhamnolipid compositions chosen to be studied were R1, R2, and their mixtures in the ratios of R2/R1 = 1.2 and R2/R1 = 1.07. Rhamnose in the bulk phase had a constant ratio (RH/RL) of 0.5, 1, 2, or 3 to the rhamnolipids. The results obtained from surface tension measurements were supported by surface shear viscosity and conductivity measurements. The interactions between rhamnolipids were confirmed with photographs taken under cross-polarized light. The overall effect of these properties was observed in the foam ability and stability of the solutions. The effect of rhamnose in the bulk phase is dependent on the composition of the monolayer; significant variations are observed with pure R1 and R2. In the rhamnolipid mixtures, rhamnose molecules penetrate into the monolayer at very low concentrations. At higher rhamnolipid concentrations, rhamnose molecules form bridges between the rhamnolipid molecules, lending rigidity to the monolayer.Öğe Interface-subphase interactions of rhamnolipids in aqueous rhamnose solutions(Amer Chemical Soc, 2003) Peker, S; Helvaci, S; Ozdemir, GEffects of the presence of rhamnose (RH) sugar in the bulk phase on the interfacial properties of rhamnolipids (RL) are investigated in this work. Rhamnolipid compositions chosen to be studied were R1, R2, and their mixtures in the ratios of R2/R1 = 1.2 and R2/R1 = 1.07. Rhamnose in the bulk phase had a constant ratio (RH/RL) of 0.5, 1, 2, or 3 to the rhamnolipids. The results obtained from surface tension measurements were supported by surface shear viscosity and conductivity measurements. The interactions between rhamnolipids were confirmed with photographs taken under cross-polarized light. The overall effect of these properties was observed in the foam ability and stability of the solutions. The effect of rhamnose in the bulk phase is dependent on the composition of the monolayer; significant variations are observed with pure R1 and R2. In the rhamnolipid mixtures, rhamnose molecules penetrate into the monolayer at very low concentrations. At higher rhamnolipid concentrations, rhamnose molecules form bridges between the rhamnolipid molecules, lending rigidity to the monolayer.Öğe Resistance of water and oil soluble surfactant mixtures to oxygen transfer(Elsevier Science Bv, 1999) Yapar, S; Peker, SEffectiveness of the following mixed surfactant system in minimising oxygen transfer at surfaces was investigated: oil soluble ethoxylated (6) nonylphenol (NP6EO), water soluble ethoxylated (9) nonylphenol (NP9EO) and the alkylbenzyldimethylammonium chloride, (ABDAC). The surfactants were incorporated with a liquid paraffin and water to form an O/W type emulsion. The type of emulsion formed was characterized by optical microscopy and drying tests. In this study the effect of synergism in interfacial behavior was investigated for the following surfactant pairs: NP9EO/ABDAC in the aqueous phase; NP6EO/NP9EO and NP6EO/ABDAC in the oil/water binary phase system. The results were used to interpret the transfer rate of oxygen through the emulsion and the oil phase of the dehydrated emulsion. Typical S shaped profiles with an initial lag time is observed in the transfer of oxygen through the oil phase. Duration of the lag depends on the structure of the air/oil surface. Corrosion tests performed on carbon steel according to G60-79 ASTM method showed that there is an inverse relationship between the lag time and the rate of corrosion. (C) 1999 Elsevier Science B.V. All rights reserved.Öğe Rheological properties of high internal phase ratio emulsions(Begell House, Inc, 1998) Peker, S; Kizildemir, M; Maron, DMW/O high internal phase ratio emulsions (HIPRE) with three different dispersed phase volumes of 80, 88 and 94% were prepared with 1% sorbitol solution as the aqueous phase, light mineral oil as the oil phase and polyoxyethylene (2) oleylether as the surfactant. Viscosities of the emulsions were measured as a function of the rate of premixing before the measurements(200 or 1000 rpm), shear rate (0.02-2.0 l/s), and relaxation time over a period of several hours. Assuming Herschel - Bulkley model to be valid, yield stresses, tau (0), consistencies, K, and flow indices, n. of the emulsions were determined. tau (0) was found to increase with volume percent of the dispersed phase and rate of pre-mixing, K decreased with relaxation time at a rate dependent on the dispersed phase drop configurations. n decreased with an increase in the dispersed phase volume percent and rate of pre-mixing. Complex modulus of viscoelasticity of the emulsions did not follow the same trends as viscosity parameters: the largest moduli were observed with 88% emulsions pre-mixed at 1000 rpm. This was attributed to changes in drop shape and configuration under shear.Öğe Shear-dependent rheological properties of starch/bentonite composite gels(Dr Dietrich Steinkopff Verlag, 1997) Besun, N; Ozguclu, B; Peker, SRheological properties of starch/bentonite gels (5.3-8.2% solids, 0-100% starch) were investigated at sheer rates 0.0083-0.33 s(-1) (Brookfield viscometer). Prior to these measurements the strain introduced during preparation of the gel was kept as low as possible. Under these conditions six different types of structural units could be identified in the gel: bentonite particles associated in a band-type structure; bands coated with starch polymers; bundles of bands interlaced and enveloped by starch polymers (strands), individual bentonite platelets dispersed in a polymer matrix; starch polymer networks; and swollen granules. A power-law model was fitted to the experimental viscosity data: mu(app) = K gamma(n-1). In all cases n was found to be less than 0.5. Its value decreased with the ability of the structural components to reorient under applied shear. K was found to be proportional to the compaction and/or entanglement of the structural units. These trends in K and n were further confirmed by the index of thixotropy (IT) and complex modulus of shear elasticity (G*) measurements.Öğe Solvent Effect on the Thermal Performance of Cholesteric Liquid-Crystals(Scientific Technical Research Council Turkey, 1995) Tokay, H; Ilyinsky, A; Yapar, S; Helvaci, S; Peker, SSpectral distributions of selective reflection from liquid crystal layers were investigated as a function of the dipole moment of the chlorinated hydrocarbon molecules used as solvent in casting the film. It was found that the use of non-polar solvents such as CCl4 does not affect the interactions between liquid crystal molecules. As the polarity of the solvent increases, the rotation of the helical arrays are hindered with a resultant decrease in the sensitivity. The solvent molecules remain as an impurity with very low concentrations in liquid crystal films. Their effect is not as great as the effect of an L.C. additive. Yet even at these low concentrations they may change the sensitivity as much as 100%.Öğe Structure of starch-bentonite gels(Dr Dietrich Steinkopff Verlag, 1997) Besun, N; Peker, S; Kokturk, U; Yilmaz, HMixed gels of starch and bentonite are investigated in the interval 0.056-0.089 of total solids/water ratio and 0-100% starch in the solids. The bentonite was a sodium calcium bentonite with a Na/Ca ratio of 1.76. In water it forms gels consisting of a network of band-type structures. Starch forms gels through hydrogen bonds between granules and/or amylose and amylopectin present on the external surfaces of granules and/or in fully stretched form. Mixed gels of bentonite and starch were obtained by adding corn starch granules to the already formed bentonite gels and heating the mixture above the Kofler gelatinization temperature. Amylose and amylopectin were adsorbed on strands of band-type structures of montmorillonite lamellae. Starch gellation, e.g. diffusion of amylose out of the granule, was facilitated in the presence of bentonite, On the other hand. the presence of starch favored delamination of the montmorillonite particle into thinner lamellae, Maximum gelatinization and polymer adsorption were observed for gels with 20% starch and 80% bentonite. Montmorillonite networks formed the continuous phase for 0-80% starch, At higher starch concentrations. montmorillonite flakes were dispersed within the polymer network. increase in the water content of the gels caused segregation of the bentonite and starch.Öğe Synergistic Behavior of the Nonylphenol(9) Ethoxylate and Alkylbenzyldimethylammonium Chloride Binary Mixture(Amer Chemical Soc, 1994) Yapar, S; Peker, SIn this investigation, the interaction between the cationic alkylbenzyldimethylammonium chloride (ABDAC) and nonionic nonylphenol (9) ethoxylate (NP9EO), expected to interact very weakly according to criteria proposed by Rosen, was investigated. The criteria for synergism were found to be in the order of magnitude of an anionic-cationic surfactant pair. The reasons for this strong interaction were explained with the help of the shifts in the peaks of characteristic IR absorption bands. Disappearance of the peaks corresponding to C-N stretches and appearance of the peaks of C-C stretching in the wavelength interval of the 1250-1000-cm(-1) region were interpreted as the seclusion of the positively charged amine group of ABDAC by the electrophilic ethoxylate group of NP9EO.Öğe Temperature-Variations In Drying of Sulfuric-Acid-Solutions In Sand Beds .1. Temperature-Variations In the Bed During the Drying of Water(Pergamon-Elsevier Science Ltd, 1995) Yapar, S; Helvaci, Ss; Peker, SLocal temperature variations were determined in sand beds during drying. Two different particle sizes of 200 mu m and 550 mu m's were used in the experiments. Drying was conducted at 80 degrees C and 150 degrees C. Highest temperature were observed at the bottom of the bed due to capillary lift during drying at 80 degrees C. Rapid evaporation at 150 degrees C ambient temperature caused heat to be transferred from both the surface and bottom of the bed, so the coolest section was at the center. Isothermal conditions were approached as the capillary size increased as in 550 mu m particle beds.Öğe Temperature-Variations In Drying of Sulfuric-Acid-Solutions In Sand Beds .2. Temperature-Variations In the Bed During the Drying of Water-Sulfuric Acid Mixtures(Pergamon-Elsevier Science Ltd, 1995) Helvaci, Ss; Yapar, S; Peker, STemperature distributions were used to explain the drying behavior of sulfuric acid solutions in packed sand beds in this part of the investigation. Temperature of the acid solutions in free evaporation are expected to increase during drying due to the decrease in heal capacity with increase in concentration. Deviations of concentration from those expected from the solution temperatures were attributed to the effects of the convective motion induced by the increase of density and surface tension of the acid solutions. These effects were found to be superimposed on the temperature variations in porous beds determined in the first part of this paper[1].Öğe Urea permeation through complex coacervate membranes(Plenum Press Div Plenum Publishing Corp, 1998) Peker, S; Helvaci, S; Esen, H; Hincal, AA; Kas, HSÖğe Variation of droplet sizes during the formation of microcapsules from emulsions(Taylor & Francis Ltd, 1997) Ovez, B; Citak, B; Oztemel, D; Balbas, A; Peker, S; Cakir, SThe size distributions of microcapsules are important in determining the surface area over which the contents of the microcapsules are released. The size distribution is generally not normal arournd the mean, but biased toward smaller capsule sizes, forming a mode around some characteristic diameter. In order to assess the factors effective in determining this characteristic drop size, size distribution analyses were carried out photographically at each stage of the complex coacervation process. In addition, the emulsion drop size distributions were investigated as a function of stirring rate, stirring time and phase ratio. The results agreed qualitatively with the theory of isotropy but were an order of magnitude smaller. This theory is valid for Newtonian liquids. Elongational stresses could be operative with large-sized macromolecules and a mechanism was proposed based on variation of elongational stresses with velocity and rates of extension. Elongational viscosities measured for the gelatin solutions supported the proposed mechanism.