Differential Capacitance Measurements Research Articles

On the Parasitic Surface Adsorption of Pyrrolidinium and Phosphonium-based Ionic Liquids Preventing Accurate Differential Capacitance Measurements

Differential capacitance measurements are known to provide vital information regarding electrical double layer charging as well as interfacial structuring of ionic liquids and ionic liquid-based electrolytes. Several hurdles have prevented these types of measurements from becoming widely used, including the fact that there exists no real consensus as to how the measurement needs to be performed and the results analyzed. To add to the difficulty, some ionic liquids are known to display a hysteresis process, thus inducing measurement variabilities. In this report, we study pyrrolidinium and phosphonium-based ionic liquid electrolytes and show that hysteresis processes indeed exist and that these are mostly the consequence of cationic adsorption on the surface. Atomic force microscopy experiments reveal that pyrrolidinium-based systems display a much denser degree of ionic compaction at the interface, compared to the phosphonium-based systems, a fact that we correlate with the much more intense hysteresis measured in pyrrolidinium-based systems. We further propose a new method for the measurement of differential capacitance and compare it with other methods in use. It is found that the proposed method allows to minimize hysteresis phenomena, thereby leading to better accuracy.

Impact of acetazolamide on the double layer parameters at the R-AgLAFe/chlorates(VII) interface in the presence of ionic surfactants

Adsorption of acetazolamide (ACT) and formation of a mixed adsorption layers of acetazolamide (ACT)—sodium 1-decanesulfonate (SDS) and acetazolamide—hexadecyltrimethylammonium bromide (CTAB) at the R-AgLAFe/chlorates(VII) interface are described. The systems were characterized by the measurements of differential capacity, potential of zero charge, and surface tension at this potential. The adsorption parameters determined in the studied systems indicate the SDS domination in the adsorption equilibria and the competitive adsorption between the ACT—SDS or mixed micelles. However, acetazolamide dominates at adsorption equilibria of the ACT—CTAB mixture.

Correlating Structural and Electrical Characteristics of Threading Dislocations in GaN -on- Si Heterostructures and p - n Diodes by Multiple Microscopy Techniques

Structural and electrical properties of a- and a + c-type threading dislocations in metal-organic vapor-phase epitaxy-grown $\mathrm{Si}$-doped $\mathrm{Ga}\mathrm{N}$ (0001) are determined by combining multiple scanning probe microscopy approaches. The analysis examines the space-charge region (SCR) formed around dislocation cores and clarifies the role it plays in influencing the local recombination, the surface-potential characteristic, and even the conductivity. Direct evidence of the SCR is obtained from a differential capacitance (dC/dV) measurement at dislocation sites on the (0001) surface. Experimental dC/dV (V) measurement results supported by technology computer-aided design calculations of capacitance, accounting for the theoretical deep levels related to different atom-core structures of the identified dislocations, reveal quantitative differences in their respective trap densities. This study is extended to p-n $\mathrm{Ga}\mathrm{N}$ vertical diodes to analyze the doping distribution across active layers. Through a correlated cross-section investigation combining electron-channeling contrast-imaging microscopy, the underlying dislocations within the depth limited by the screening of their SCRs are found to severely impact the homogeneity of the cross-section dC/dV contrast in the low-doped n-type drift layer of the diodes. This raises serious concerns regarding any quantification efforts by differential capacitance measurements in $\mathrm{Ga}\mathrm{N}$ on $\mathrm{Si}$, where the dislocation density is in the order of ${10}^{9}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$.

The Effect of Anodizing Bath Composition on the Electronic Properties of Anodic Ta-Nb Mixed Oxides.

Anodic oxides were grown to 50 V on Ta-Nb sputtering deposited alloys, with high Nb content, in acetate ions containing an aqueous solution to study the effect of the anodizing bath composition on anodic layers' dielectric properties. Photoelectrochemical measurements proved the presence of a photocurrent in the band gap of photon energy lower than oxides, due to optical transitions involving localized electronic states as a consequence of acetate ions incorporation. Flat band potential value estimates assessed the insulating nature of the anodic oxides grown in the acetate buffer solution. Differential capacitance measurements showed that the highest capacitance value was measured for the sample grown on Ta-66%Nb. This capacitance value was higher with respect to those estimated for pure Ta and pure Nb anodic layers and with respect to the same alloy anodized in NaOH solution, i.e., acetate-free anodizing bath.

Nonwoven Mats Based on Segmented Biopolyurethanes Filled with MWCNT Prepared by Solution Blow Spinning.

To prepare nonwoven mats constituted by submicrometric fibers of thermally responsive biopolyurethanes (TPU) modified with multiwalled carbon nanotubes (MWCNT), solution blow spinning (SBS) was used. The TPU was the product of synthesis using poly(butylene sebacate)diol, PBSD, ethyl ester L-lysine diisocyanate (LDI), and 1,3-propanediol (PD) (PBSe:LDI:PD) as reactants. TPU was modified by adding different amounts of MWCNT (0, 0.5, 1, 2, and 3 wt.%). The effect of the presence and amount of MWCNT on the morphology and structure of the materials was studied using field-emission scanning electron microscopy (FESEM) and Fourier-transform infrared spectroscopy (FTIR), respectively, while their influence on the thermal and electric behaviors was studied using differential scanning calorimetry (DSC) and capacitance measurements, respectively. The addition of MWCNT by SBS induced morphological changes in the fibrous materials, affecting the relative amount and size of submicrometric fibers and, therefore, the porosity. As the MWCNT content increased, the diameter of the fibers increased and their relative amount with respect to all morphological microfeatures increased, leading to a more compact microstructure with lower porosity. The highly porous fibrous morphology of TPU-based materials achieved by SBS allowed turning a hydrophilic material to a highly hydrophobic one. Percolation of MWCNT was attained between 2 and 3 wt.%, affecting not only the electric properties of the materials but also their thermal behavior.

Drift Rejection Differential Frontend for Single Plate Capacitive Sensors

Capacitive sensing at long ranges (10x the plate diameter) for long-term environmental monitoring can be limited by slow but significant measurement drifts from charge induction, which can exceed the small capacitance variations of interest, below 0.01%. This paper proposes a slope modulation differential capacitance measurement method for single-plate capacitive sensors operating in loading mode that effectively rejects noise from quasi-constant drift currents. It is based on periodical plate capacitance charge-discharge, like the period modulation frontends (astable multivibrators) that were used in past work, but it uses a constant oscillation period and compares the slopes of adjacent plate voltage ramps. Plate capacitance is calculated by averaging the slope magnitudes, which rejects the noise from quasi-constant drift currents. The sensitivity and noise rejection are compared with the period modulation techniques analytically, and the results are validated using simulations and experiments. They show that human body sensing range doubles and the noise floor is substantially lower compared to the state-of-the-art (period modulation).

On the Influence of Ball Milling Time on the Structure and Electrochemical Performance of (Sn71Co29)50C50 wt% Anodes for Li-Ion Battery Applications

Abstract Ball milled SnCoC composites are an attractive commercial anode material to conventional graphite due to their higher specific capacity and low temperature performance. The effect of ball milling time on the structure and electrochemical properties of the (Sn71Co29)50C50 wt% composite anodes are studied to understand the reasons for the non-realization of the theoretical capacity. Structural analysis reveals the damage of graphite structure with increasing ball milling time from 10 h to 60 h. The cyclic voltammetry and differential capacity measurements indicate the decreasing contribution to capacity from graphite and increasing contribution from Sn with increase in the milling time. The charge-discharge cycling of the anodes at different C rates indicates that though the specific capacity does not improve with longer milling time, the rate capability improves significantly. The damage in the graphite structure during high energy ball milling is found to reduce the capacity of the SnCoC anodes. Based on the investigations, it can be concluded that 10 h of milling time is optimum to realize high specific capacity, whereas longer durations of milling are desirable for high rate discharge characteristics.

Self-Assembled Monolayer of Monomercaptoundecahydro-closo-dodecaborate on a Polycrystalline Gold Surface.

In this work, we present an electrochemical study of the boron cage monomercaptoundecahydro-closo-dodecaborate [B12H11SH]2− in solution and in a self-assembled monolayer over a polycrystalline gold electrode. Cyclic voltammetry of the anion [B12H11SH]2− in solution showed a shift in the peak potentials related to the redox processes of gold hydroxides, which evidences the interaction between the boron cage and the gold surface. For an Au electrode modified with the anion [B12H11SH]2−, cyclic voltammetry response of the probe Fe(CN)63−/Fe(CN)64− showed a ΔEp value typical for a surface modification. Electrochemical impedance spectroscopy presented Rtc and Cdl values related to the formation of a self-assembled monolayer (SAM). A comparison of electrochemical responses of a modified electrode with thioglycolic acid (TGA) reveals that the boron cage [B12H11SH]2− diminishes the actives sites over the Au surface due to the steric effects. Differential capacitance measurements for bare gold electrode and those modified with [B12H11SH]2− and (TGA), indicate that bulky thiols enhance charge accumulation at the electrode–solution interface. In addition to electrochemical experiments, DFT calculations and surface plasmon resonance measurements (SPR) were carried out to obtain quantum chemical descriptors and to evaluate the molecular length and the dielectric constant of the Boron cage. From SPR experiments, the adsorption kinetics of [B12H11SH]2− were studied. The data fit for a Langmuir kinetic equation, typical for the formation of a monolayer.

Electric Double Layer Action of High-quality Ionic Liquid Crystal Thin Films

Abstract We successfully prepared high-quality vacuum-deposited thin films of ionic liquid crystal 1-hexadecyl-3-methylimidazolium hexafluorophosphate ([C16mim][PF6]). The key point is to anneal the films in vacuum at the liquid crystal temperature of [C16mim][PF6], thereby much improving their crystallinity, surface smoothness, and air-exposure stability. The differential capacitance measurement of such a high-quality thin film revealed that an electric double layer was formed in the liquid crystalline phase, while it behaves like a normal dielectric in the crystal phase.

Electrochemical Intercalation of Magnesium into Graphite Via an Ethylenediamine-Based Electrolyte

The formation of dimeric or larger cluster metal intercalates within layered hosts may reveal new chemistries for charge storage electrodes. In previous work, we reported a series of novel ternary stage-1 graphite intercalation compounds (GICs) containing alkaline earth metal cations (M = Mg, Ca, Sr, or Ba) and ethylenediamine (en). Pair distribution function (PDF) analyses indicated the presence M-M bonding within the intercalates in the monolayer galleries.In this work, we present the results of electrochemical intercalation reactions for Mg-en complexes within graphite. Using Mg(SO3CF3)2 in en as the electrolyte, galvanostatic charge and discharge profiles show capacities of 810 and 455 mAh g-1 during the first intercalation and deintercalation steps, respectively. After this formation cycle, a capacity of ~120 mAh g-1 can be retained for over 100 cycles with a columbic efficiency of >93%. Ex-situ X-ray diffraction confirms that stage-1 GICs are produced in the fully reduced state.Cyclic voltammetry (CV) and first-cycle differential capacity measurements (dQ/dV) both show that four redox couples arise in the first charge/discharge process. Very large polarizations (2.2 V) are observed, suggesting that there may be changes in Mg-N bonding in this redox chemistry. Galvanostatic intermittent titration technique (GITT) studies as well as X-ray Raman spectroscopy are currently underway to further examine reaction kinetics and confirm the presence of Mg-Mg bonding. This work may provide a new paradigm toward charge storage in intercalation electrodes.

Effect of Molecular Structure on Electrochemical Phase Behavior of Phospholipid Bilayers on Au(111).

Lipid bilayers form the basis of biological cell membranes, selective and responsive barriers vital to the function of the cell. The structure and function of the bilayer are controlled by interactions between the constituent molecules and so vary with the composition of the membrane. These interactions also influence how a membrane behaves in the presence of electric fields they frequently experience in nature. In this study, we characterize the electrochemical phase behavior of dipalmitoylphosphatidylcholine (DPPC), a glycerophospholipid prevalent in nature and often used in model systems and healthcare applications. DPPC bilayers were formed on Au(111) electrodes using Langmuir-Blodgett and Langmuir-Schaefer deposition and studied with electrochemical methods, atomic force microscopy (AFM) and in situ polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The coverage of the substrate determined with AFM is in accord with that estimated from differential capacitance measurements, and the bilayer thickness is slightly higher than for bilayers of the similar but shorter-chained lipid, dimyristoylphosphatidylcholine (DMPC). DPPC bilayers exhibit similar electrochemical response to DMPC bilayers, but the organization of molecules differs, particularly at negative charge densities. Infrared spectra show that DPPC chains tilt as the charge density on the metal is increased in the negative direction, but, unlike in DMPC, the chains then return to their original tilt angle at the most negative potentials. The onset of the increase in the chain tilt angle coincides with a decrease in solvation around the ester carbonyl groups, and the conformation around the acyl chain linkage differs from that in DMPC. We interpret the differences in behavior between bilayers formed from these structurally similar lipids in terms of stronger dispersion forces between DPPC chains and conclude that relatively subtle changes in molecular structure may have a significant impact on a membrane's response to its environment.

Thermal stress, cooling-rate and fictive temperature of silicate melts

The unknown cooling-rate history of natural silicate melts can be investigated using differential scanning heat capacity measurements together with the limiting fictive temperature analysis calculation. There are a range of processes occurring during cooling and re-heating of natural samples which influence the calculation of the limiting fictive temperature and, therefore, the calculated cooling-rate of the sample. These processes occur at the extremes of slow cooling and fast quenching. The annealing of a sample at a temperature below the glass transition temperature upon cooling results in the subsequent determination of cooling-rates which are up to orders of magnitude too low. In contrast, the internal stresses associated with the faster cooling of obsidian in air result in an added exothermic signal in the heat capacity trace which results in an overestimation of cooling-rate. To calculate cooling-rate of glass using the fictive temperature method, it is necessary to create a calibration curve determined using known cooling- and heating-rates. The calculated unknown cooling-rate of the sample is affected by the magnitude of mismatch between the original cooling-rate and the laboratory heating-rate when using the matched cooling-/heating-rate method to derive a fictive temperature/cooling-rate calibration curve. Cooling-rates slower than the laboratory heating-rate will be overestimated, while cooling-rates faster than the laboratory heating-rate are underestimated. Each of these sources of error in the calculation of cooling-rate of glass materials—annealing, stress release and matched cooling/heating-rate calibration—can affect the calculated cooling-rate by factor of 10 or more.

Adsorption of Halides on Au (111) Electrode in Aprotic Solvents: AC- Voltammetry, EIS, XPS and Surface-Enhanced Infrared Spectroscopy

Specific adsorption of ions on the electrode surface has a significant impact on the electric double layer (EDL) structure and the kinetics of electrode processes[1]. The ion adsorption at aqueous solution interfaces[2] on various metals has been examined intensively, but much fewer studies from non-aqueous solvents have been conducted[3]. Such studies are important because the metal-solvent [4] and ion-solvent interaction energy [5] depend on the chemical nature of the solvent.In this study, three organic solvents (propylene carbonate (PC), diglyme (DG), and dimethyl sulfoxide (DMSO)) were investigated for the adsorption of iodide and bromide ions on Au(111) single crystal electrode. This analysis was conducted by using differential capacity measurements, cyclic voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and Surface-Enhanced Infrared Spectroscopy (SEIRAS).The magnitude of anion adsorption on Au(111) electrode increases in the series of halides I- ˃ Br- due to better solvation [6] and in the solvent order PC ˃ DG ˃ DMSO[7]. As well, the influence of water content on the adsorption rate of iodide on Au(111) electrode in PC and DMSO was studied. This effect, and the much higher adsorption rate in water is explained by the closer approach of the solvated halogen ion to the surface and the resulting stronger interaction of the transition state with the electrode. Quantification of the adsorbed iodide amount from XPS spectra confirms the coverage estimated from the adsorption charge in CVs. During SEIRAS measurements, a bipolar band is observed at around 1805 cm−1 which correlates to the C-O-vibration of PC as reported on silver electrodes from SERS measurements[8]. A stronger electrical field with decreasing potential is observed in absence of iodine leads to higher absorbance of PC molecules on the surface. Acknowledgments Financial support by the German Federal Ministry of Education and Research (BMBF) of the “LuCaMag” project (Fkz: 03EK3051A) under the framework of “Vom Material zur Innovation” program is acknowledged.

Validation of An Elbow Position Super-Twisting Sliding-Mode Controller for Upper-Limb Exosuit Using A Soft Position Sensor

Flexible exoskeletons, also known as exosuits, are robotic wearable devices intended to help healthy and unhealthy subjects in different tasks, such as daily life activities, load lifting or rehabilitation. A position control is required to assure stability and compliance in all assisted movements. In this paper, the authors propose the use of a flexible position sensor based on differential capacitance measurement as position feedback for the elbow joint in the flexible exoskeleton LUXBIT. This exosuit is controlled in position by a super-twisting sliding mode controller (SMC), which is robust against disturbances. Different movements involved in rehabilitation therapies are performed over three healthy subjects in order to evaluate the exosuit performance using this motion sensor. The measures given by the flexible sensor are compared to the ones obtained by OptiTrack motion capture system, which are used as ground truth.

Adsorption of bromide and iodide anions on a renewable liquid (Cd–Ga) electrode from acetonitrile solutions. Effect of (Cd–Ga)–solvent chemisorption interaction on the anion adsorption energy

The specific adsorption of bromide and iodide anions was studied at a (Cd–Ga)/acetonitrile (AN) interface from mixed solutions of [0.1 m M LiBr + 0.1 (1–m) M LiBF4] and [0.1 m M LiI + 0.1 (1–m) M LiBF4] at the following values of molar fraction m of the surface–active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The main experimental method was measurement of differential capacitance on a renewable liquid (Cd–Ga) electrode. The obtained experimental results can be described quantitatively by the Frumkin isotherm in a wide range of charge densities (σ). The adsorption parameters of halide anions (Hal−) were determined using regression analysis of two-dimensional pressure isotherms. It is shown that the adsorption energies (ΔGads) of Br− and I− at the (Cd–Ga)/AN interface practically coincide. It follows that the difference in the interaction energies of Br− and I− with (Cd–Ga) is equal to the difference of the solvation energies of these anions: (ΔG(Cd‐Ga)‐Br‐ − ΔG(Cd‐Ga)‐I‐)≈(ΔGAN‐Br‐ − ΔGAN‐I‐) and therefore ΔG(Cd-Ga)-I− < ΔG(Cd-Ga)-Br−. The adsorption parameters of Hal− obtained on the (Cd–Ga)/AN interface are compared with the similar data for the (Cd–Ga)/DMF interface. The adsorption energy of Br− and I− on a (Cd–Ga) electrode increases significantly in AN as compared to DMF. This result can be unambiguously explained by a significant decrease in the energy of chemisorption interaction between a (Cd–Ga) electrode and solvent molecules in the Helmholtz layer upon transition from DMF to AN. The ΔGads,σ-dependence obtained at the (Cd–Ga)/AN interface in the first approximation characterizes the change of the free energy of (Cd–Ga)–Hal− interaction with σ. It is shown that the slope of the ΔGads, σ-dependence decreases upon transition from the (Cd–Ga)/AN interface to the (Cd–Ga)/DMF interface due to a significant increase in the energy of the chemisorption interaction of (Cd–Ga) with DMF molecules in the Helmholtz layer during the transition from negative to positive values of charge density.

Adsorption of Iodide and Bromide on Au(111) Electrodes from Aprotic Electrolytes: Role of the Solvent

AbstractThe adsorption of iodide and bromide ions on Au(111) single‐crystal electrodes in three organic solvents [propylene carbonate (PC), diglyme (DG), and dimethyl sulfoxide (DMSO)] was studied by differential capacity measurements, cyclic voltammetry (CV), X‐ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The anion adsorption charge in DMSO is approximately half as large as in propylene carbonate and that in water. Quantification of the adsorbed iodide amount from XPS spectra confirms the coverage estimated from the adsorption charge in CVs. As expected, the rate of anion adsorption on Au(111) is higher for I− than for Br− due to better solvation of the latter. The extent of anion adsorption on Au(111) electrode decreases and the corresponding adsorption rate decreases in the solvent order H2O&gt;PC&gt;DG&gt;DMSO. Since the donor number (DN) of the solvent increases in the same order (PC&lt;DG&lt;DMSO), we assume that it plays a decisive role since it determines the chemical interaction of the solvent with the metal surface. Moreover, the adsorption rate of iodide and the extent of its adsorption is increased with increasing water content in PC and DMSO. This effect, and the much higher adsorption rate in water is explained by the closer approach of the solvated halogen ion to the surface and the resulting stronger interaction of the transition state with the electrode.

Optimization of anodizing process of tantalum for Ta2O5-based capacitors

Anodic oxides were grown to 50 V on Ta in several organic ions containing anodizing baths. Their properties were compared with anodic Ta oxide film grown to the same formation voltage in 0.1 M NaOH. Anodizing process carried out in sodium citrate led to the growth of the anodic oxide with the best blocking properties whilst, when Ta is anodized in sodium adipate, a significant part of the circulated charge is wasted in side reactions, such as oxygen evolution. Photoelectrochemical measurements showed the presence of optical transitions at energy lower than the band gap for the anodic films grown in citrate and tartrate electrolytes, attributed to localized electronic states located close to the valence band mobility edge of the films generated by anions incorporation into the oxide. Differential capacitance measurements proved an increase by 17% in capacitance value for the oxide grown in citrate-containing solution with respect to that grown in NaOH electrolyte. A sketch of the energetic of the metal/oxide interface is provided.

Effect of surface structure of Au(1 0 0) electrode on phase transitions within adsorbed adlayer of coumarin in halide electrolytes in the presence of tetrabutylammonium cations

Effect of surface structure of Au(1 0 0) electrode on the phase transitions within adsorbed adlayer of coumarin (CUM) in halide electrolytes in the presence of tetrabutylammonium cations (TBAc) was studied by means of cyclic voltammetry and differential capacity measurements. At the nearly unreconstructed surface of Au(1 0 0) electrode in chloride and bromide solutions, TBAc induced this transition, while at the reconstructed surface these cations caused a shift of it to more negative potentials. In KI as the supporting electrolyte, such phase transition was not detected at the reconstructed surface, but it was induced at the unreconstructed one. Additionally, new phase transitions in CUM adlayer have been found at the unreconstructed surface in bromide electrolyte, which are, however, completely inhibited by TBAc.

The Determination of Electrochemical Active Surface Area and Specific Capacity Revisited for the System MnOx as an Oxygen Evolution Catalyst

Abstract In the last decades several different catalysts for the electrochemical water splitting reaction have been designed and tested. In so-called benchmark papers they are compared with respect to their efficiency and activity. In order to relate the different catalyst to each other the definition of well-defined procedures is required. Two different methods are mainly used: Either the normalization with respect to the geometric surface area or to the catalyst loading. Most often only one of these values is available for a sample and the other one cannot be estimated easily. One approach in electrocatalysis is to determine the Helmholtz double layer capacitance (DLC) and deduce the electrochemical active surface area (ECSA). The DLC can be obtained from two different methods, either using differential capacitance measurement (DCM) or impedance spectroscopy (EIS). The second value needed for the calculation of the ECSA is the specific capacitance, which is the capacitance for a perfectly flat surface of given catalyst material. Here, we present the determination of the different capacitance values using manganese oxide as the exemplary model for the oxygen evolution reaction (OER). We determine the capacitance by DCM and EIS to calculate the ECSA using literature values for the specific capacitance. The obtained values are comparable from the two methods, but are much larger than the surface areas obtained by atomic force microscopy. Therefore, we consider the possibility of using the measured AFM area together with the Helmholtz capacitance to determine the specific capacitances for this material class. The comparison of these results with literature values illustrates the actual limits of the ECSA method, which will be discussed in this paper.

Nanoparticle's size, surfactant and concentration effects on stability and isotropic-nematic transition in ferronematic liquid crystal

Isotropic-nematic phase transition in composites based on 4-cyano-4-hexylbiphenyl (6CB) liquid crystal and spherical magnetic iron oxide nanoparticles with various nanoparticle sizes and various concentrations were investigated. The iron oxide nanoparticles were covered by oleic acid surfactant that prevents their aggregation. Dependences of the phase transition temperature shift on the nanoparticle size and the nanoparticle volume fraction were acquired by optical measurements, differential scanning calorimetry measurements and capacitance measurements. TEM and SEM examination showed a nonuniform thickness of oleic acid layers for different nanoparticle sizes. At the same nanoparticle volume fraction, therefore, the volume fraction of oleic acid varies with nanoparticle size. Although oleic acid surfactant prevents the magnetic aggregation, it modifies isotropic-nematic phase transition temperature significantly. The theoretical considerations of oleic acid provided in the present paper give the interpretation of obtained experimental results.