Spectral Absorption of Unpolarized Light Through Nano- Materials in the Absence of a Magnetic Field

А study of optical properties, such as light absorption, of a colloidal nano-material, provides information on the biphasic, solid – liquid system microstructure. The nano-material under study is a magnetic liquid (ferrofluid). The disperser agent is petroleum mineral oil and the dispersed material is a brown spar powder (nano-particles). The stabilizer is oleic acid. Light absorption through ferrofluid samples reveals the tendency of solid particles in а colloidal solution to form aggregates. The paper emphasizes the linear dependence between the spectral absorption coefficient, concentration and wavelength. The aggregates cause deviations of the extinction coefficient from values according to the Bouger-Lambert-Beer law. Fe3O4 aggregates sized 58.76 nm are formed in the system. The average number of nanoparticles forming aggregates is 6. The magnetic liquid to be studied is secure stable and, thus, trustful in technological and biological applications.


Introduction
Nano-materials are physical systems whose characteristics are determined by dimensions and structure of nano-particles they include.
The interactions between nano-particles, as well as between nano-particles and disperser liquid agent decide the development process of the system, and its structural transformation into a homogeneous and continuous medium.
In the specific case described bellow, the magnetic liquids (ferrofluids) are biphasic solid -liquid systems, behaving like homogenous mediums, both in the presence and absence of a magnetic field.The present paper observes a system's behavior in the absence of a magnetic field.
The properties of nano-materials depend on the preparation technique (for instance, a sintering procedure), which directly influences the microstructure and the dielectric properties of a nano-material [1].Properties of some types of nano-material (PbS, PbS/PVA and polyvinyl alcohol) have already been examined [2].By means of the optical absorption spectrum method, it was determined that the size of nano-particles was 26 Å.
Research regarding absorption of solar radiation through ferrofluids revealed the possibility to use these nano-materials, as thin layers, in thermo-solar conversion [3,4,5].
In biology, nanomaterials are used to select and transport organic cells or viruses.In this case, the dimension of the magnetic nano-particles is within the range of (5 nm to 100 μm).Magnetic liquids suited for biological applications contain nano-particles similar in size with the biological entities: cells (10 -100 μm, viruses (20 -450 nm), proteins (5 -50 nm), and genes (2 nm width and 10 -100 nm length) [6,7,8,9].
In the present paper, the biphasic systems contain nano-particles of Fe 3 O 4, with an average size of 11.354 nm, floating into the carrying liquid, which is petroleum mineral oil.Stabilization of the biphasic system needed fat acid use, namely oleic acid, .The polar group,

COOH −
, is adsorbed by the brown spar particles.The hydrocarbon string dissolves in oil, insuring the system's stability.
Between the brown spar particles in the ferrofluid London and van der Waals interactions settle.Fig. 1 shows the nature of forces existing inside the ferrofluid, at rest, under a gravitational field: F 1 -London force, F 2 -van der Waals force, F 3 -Archimedes force, F 4 -hydrocarbon -hydrocarbon interaction force, P -gravity force.Number 1 denotes magnetic nucleus and number 2 denotes the oleic acid pellicle.

Fig. 1 Forces acting on the brown spar particles in the ferrofluid
As a result of these interactions, especially the London and van der Waals forces, the magnetite particles tend to cluster into aggregates or agglomerates [10].Single nano-particles, small aggregates with 4-20 nano-particles, and big aggregates with more than 1000 nanoparticles co-exist in the colloidal solution [11].
Our previous paper regarding much diluted ferrofluids revealed that the average number of nano-particles in the aggregates is 4 [12].
The structure of magnetic liquids can be studied, by means of different methods, such as optical methods, electronic microscopy and magneto-optical methods.In interaction with light, the magnetic fluid behaves like a continuous medium if the wavelength of the monochromatic radiation is much bigger than the average diameter of the solid particles.So, the magnetic colloid behaves like a homogenous medium and obeys the Bouguer -Lambert -Beer law if the average diameter of the solid particles is less than the tenth part of the radiation wavelength [13,14].
Previous research revealed that ferrofluid's transparency in a magnetic field depends on magnetic induction [15].This phenomenon can be used for light signal modulation in the information transmission technique.
The quality of applications involving nano-materials requires preservation of the nano-material, in order to avoid the production of new nano-particle agglomerations.Agglomeration size, established during the elaboration process, must preserve the same value.
Large dimensions of aggregates change the physical properties of the nano-material and make their use difficult for practical purposes.
In the present paper we have examined some optical properties of the magnetic fluid.The work investigates the spectral absorption of unpolarized light in an average concentrated ferrofluid, in the absence of a magnetic field.The entire study is based on the fact that the spectral natural coefficient of extinction for a sample i, related to the natural coefficient of extinction for the reference sample, is equal to the unit for all wavelengths and for all concentrations if the dimensions of the nano-particles or aggregates are smaller than the 10 th part of the wavelength.
The aim of this work is to calculate the natural coefficient of spectral absorption of the average-concentrated nano-material and to determine the average number of nanoparticles from the aggregates that form within this ferrofluid.
It is estimated that in the near future, ferrofluids will be used to develop some new techniques for preparing sintered materials.These sintering technologies will use two properties of ferrofluids [1]: 1.In a non-uniform magnetic field, the apparent density ap ρ of the colloid medium is variable, depending on the gradient of the external magnetic field.If the gradient of the external magnetic field is oriented vertically down, for a non-magnetic particle there are three possibilities: 2. In a rotational external magnetic field, the ferrofluid also rotates.The angular frequency of ferrofluid is linearly dependent on the frequency multiplied with the intensity of the magnetic field.

Sample preparation
The ferrofluid was prepared using the chemical precipitation method.Ferrous and ferric hydroxides result according to the following chemical reaction: Brown spar, at the temperature of 373 K, is obtained on the basis of the reaction: Brown spar density is higher than petroleum's, so dipol-dipol and van der Waals forces occur between particles.In order to prevent sedimentation and formation of very large aggregats, the brown spar particles are covered with a pellicle of oleic acid, by stirring.The pellicle ( ) plays the role of a stabilizer.The polar group of the oleic acid is adsorbed by the brown spar particles and the hydrocarbon string dissolves in petroleum, to which it is very much alike in composition.

COOH −
The salts in the solution and porosity in particles were washed away by water at 60 -80 o C. The exceeding oleic acid breaking of brown spar grains needs an acetone addition.The reaction products disperse in oil heated at 120 o C. Fig. 2 shows the micrography of brown spar nano particles, obtained with an electronic microscope (magnification 6•10 4 X).Fig. 2 shows that nano-particles in the ferrofluid obtained using the method described above, are of an irregular shape.For further computations, the brown spar particles are assumed to be spherical.Diameters for 100 nanoparticles were measured using optical microscopy and electronic micrographies.The main geometrical feature of nano-particles is proposed to be the average diameter of 100 nanoparticles, 354 .11 = d nm [17,18].The physical characteristics of the magnetic liquid are [16,17,18,19

Experimental Method
The monochromatic light absorption when passing through a homogenous solution is described by the Bouguer-Lambert-Beer law [10,11,16]: (3) where (see, Fig. 3): 0 J is the intensity of the light passing through the solution t J -intensity of the emergent light l -thickness of the absorbent layer K -natural absorption coefficient.
The studied liquid is held in a transparent container.The transparent walls absorb and reflect a part of the incident light.In order to eliminate the effect of light absorption and reflection by the container's walls, the light intensities and J are compared for two containers in which the solution has the same concentration but the thickness is and .Therefore, the absorption coefficient can be computed using the formula: The absorption coefficient is characteristic to the medium and depends on the light's wavelength.The value of coefficient K is proportional with the solution's concentration : where ε is the natural coefficient of extinction.It is independent on the concentration, but it depends on the wavelength.
In order to establish if the magnetic liquid behaves like a homogenous solution, the following method was used: -one of the samples was considered to be the reference sample, (in the present research sample No.1 is the reference sample) -the extinction coefficient, j i, ε , was explained, with Eqs.(4 and 5), for all concentrations and all wavelengths -ratios, The meaning of parameters in Eq. ( 6) is: i -index of the sample j -index of the monochromatic radiation r -reference sample.The j value is the spectral natural coefficient of extinction of the i sample, related to the natural coefficient of extinction for the reference sample.

Experimental Installation
The absorption of light by the biphasic system was determined with the monochromator PU2.Fig. 3 presents the installation.The components of the installation are: the light source -1; entrance slit -2; spherical mirror -3 (the entrance slit is situated in the focus of the mirror); total reflexion prism -4 (face AB is silver coated); spherical mirror -5; exit slit -6; convergent lens -7; ferrofluid recipient -8; photodetector -9; measuring instrument (microampermeter) -10; micrometric screw for monochromatic wavelength selection -11 (by means of prism's rotation).A microampermeter indicates the electric intensity of the current generated by the photodetector, I.The intensity of the current at the microampermeter is directly proportional with the intensity of the incident light on the sensor, I≡J t .

Experimental Results
In the visible range, the natural coefficient of absorption for the oil is constant in respect with the radiation's wavelength and has the value [12].

Natural coefficient of absorption
The measurements of the absorption of unpolarized monochromatic radiations in the optical spectrum indicated in Tab.II, achieved on the ferrofluid samples indicated in Tab.I were accomplished using the installation presented in Fig. 3.The absorption due to the brown spar particles and agglomerates is equal to the difference between the absorption of the biphasic system and the absorption of the disperser.The wavelengths are given in the box in the left part of Fig. 4. Fig. 4 shows that the natural coefficient of absorption increases in respect with the concentration for all wavelengths.Experimental point groups depicted for each wavelength were processed and trendlines were computed.The slope of lines varies between 0.385 and 0.395 (1/cm).On the other hand, according to equation ( 5), the slopes of lines represent the mean values of extinction coefficients.For the ferrofluid under study in the paper, the above values correspond to the ones presented in [12,16].Future developments of this field of research will, therefore, require a careful examination of the conditions that can lead to apparition of a relatively pronounced absorbtion effect in the wavelength range 486.1 -546.1 nm for high concentrations.was chosen as the reference sample.Variation of the value, in respect to the wavelength for the concentrations mentioned above is presented in fig.6, The dispersion of points is bigger for smaller wavelengths and higher concentrations.The points regroup around the unitary value when the wavelength increases and the brown spar concentration in the sample decreases.Maximum deviation from the unit for the points representing the size Zij occurs at wavelengths within the range of (486.1-546.1)nm.As the concentration raises, the point groups are in the region of the value 1, until the concentration reaches 10 r.u.Then, the higher the concentration gets, the more spread the points become.This means that the aggregate dimensions grow proportionally to the colloidal solution concentration.
The dependence of the arithmetical mean values j Z (for all concentrations) in respect to the wavelengths (Fig. 8) shows that we have constant behavior for small wavelengths of the visible spectrum.It also indicates that the dependence  One can notice that small concentrations are favorable to values close to unity, opposite to the cases when the concentrations are big.was rounded up to the closest entire number.The paper [18] indicates the minimum diameter (d 1 = 6.2 nm) and the maximum diameter (d 2 = 17.8 nm) of the brown spar particles in the studied forrofluid.Therefore, the maximum number of particles joining to form aggrgates is

Conclusion
The concentration of the brown spar particles in the magnetic liquid, in which aggregate formation was studied, varied between 0.311 kg/m 3 and 6.220 kg/m 3 .In the magnetic liquid aggregates formed for all concentrations comprised between 0.311 kg/m 3 and 6.220 kg/m 3 .The diameter of the aggregates increases as the brown spar concentration grew. .The small number of particles in an aggregate indicated good stability of the studied ferrofluid.
The stability of the studied ferrofluid indicates the opportunity of using it in technological applications such as: measurement devices (manometer with variable inductivity), solar collectors, magneto-fluidic valves, magneto-optical transmission of information and others.The magnetic liquid can be used in biological applications such as selection and transportation of proteins, whose dimensions are between 5 and 50 nm.
Further research in this domain should focus on a detailed examination of the ferrofluid's spectral absorption at wavelengths ranging from 486.1 to 546.1 nm.At the same time, future papers should reveal ferrofluid's behavior exposed to light both under a uniform and variable magnetic field.
Once solar energy is implemented, thorough studies get more and more important regarding the behavior of installations using ferrofluid and working in the natural environment.

Fig. 2
Fig. 2 Micrography of the brown spar nano-particles, achieved with the electronic microscope

,λ.ZZ
the solution is homogenous and the aggregate's diameter for the transmitted radiation is smaller than 10 / j λ , then for the radiation with j λ as spectral coefficient of extinction, j i, ε will not change in respect with the variation of the concentration, .For another wavelength, the spectral coefficient of extinction may have another value, with Eq. (6), must be equal to unity for a certain fixed value of i and all j values.the Bouguer-Lambert-Beer law is not obeyed anymore, and the value is too far from the unit.Knowing the minimum wavelength, from which the Bouguer-Lambert-Beer law is obeyed, so that moves towards the unit, the maximum diameter of the aggregates can be estimated toThe values of are dispersed around the value of 1.For values, two means can be calculated: as the arithmetical mean of j i Z , , values (for all concentrations) associated to each wavelength, j, of the monochromatic radiation: , values ( for all wavelengths) associated to each i concentration of the biphasic nano-structured system: selected radiations are presented in Tab.II

Fig. 4
shows the parametric variation of the spectral coefficient of absorption in relation to the ferrofluid's concentration.The parameter is the monochromatic radiation wavelength.

Fig. 4 λ
Fig. 4 Dependence of the absorption coefficient in respect to concentration for different wavelengths

Fig. 5
Fig. 5 Dependence of the absorption coefficient in respect to the wavelength for different concentrations

Fig. 6 Fig. 7
Fig. 6 Variation of Z i,j value in respect to the wavelength for different concentrations

.
has an inflexion point in the region of the green spectrum, For the big wavelengths of the visible spectrum (from yellow to red monochromatic radiation) the value of j Z is around unity, according to the Bouguer-Lambert-Beer law.

Fig. 9
Fig. 9 Variation of i Z value in respect to the concentration of the nano-material

, explicitly n 2 = 4 .
The relatively small number of Fe 3 O 4 particles in the aggregates and the small dimensions of the former, indicate a remarkable stability of the ferrofluid and, thus, a safe use in different technological applications.