Experimental study of solvent effect on the heat transfer characteristics of 4-[(4-aminophenyl)- (4-imino-1-cyclohexa-2,5-dienylidene) methyl] aniline hydrochloride – A concentration based study

the heat transfer characteristics of


Introduction
The thermal conductivity and thermal diffusivity (T D ) are two important parameters that characterize the heat transfer properties of a material.The speed at which a material heats up or cools down during thermal processing is influenced by the substance's thermal conductivity and thermal diffusivity.Conduction, convection, and radia-tion are the three ways through which heat is transferred.Conduction is the primary mechanism for heat transfer in solid materials whereas convection plays a major role in liquids.In a heat exchanger both conduction and convection are involved.Different materials have different thermal conductivities, with metals typically being significantly greater conductors than non-metals.Thermal conductivity is a term used to describe how well a sub-stance can conduct heat.There can be two types of heat transfer processes: steady-state and unsteady-state heat transfer.When a process is in a steady state, all the heat energy goes through the material without being heating it.Unsteady state heat transfer process is the process in which the temperature of a system or an object change as a function of time.
The thermal diffusivity is defined as the ratio of thermal conductivity to the product of specific heat and density (Eq.( 1)). (1) Thermal diffusivity, provides a measurement of how quickly the temperature will vary when it is heated or cooled.High thermal diffusivity materials will heat up or cool down quickly, whereas substances with a low thermal diffusivity will heat or cool slowly.Therefore, determining thermal diffusivity is crucial when thinking about unsteady-state heat transmission scenarios.Thermodynamic diffusivity can be measured experimentally or assessed based on an understanding of the individual properties.
Molecules are significantly more tightly packed in liquids than in gases.The molecular diffusion effect, or the random movement of molecules, is what mostly determines the heat conductivity of liquids.The flow of heat through liquids is impeded by the increased random movement of molecules.Molecular motions become more random as temperature rises which prevents heat diffusion through liquids.Therefore, thermal conductivity of liquids decreases with increasing temperature except in the case of pure water.Pure water's thermal conductivity first rises with increasing temperature and then falls.A substance with a high thermal diffusivity will also have a high thermal conductivity.The rate at which heat diffuses into the medium, increases with increasing thermal diffusivity.A low thermal diffusivity rating indicates that most heat is absorbed by the material and only a little quantity is transported further.Due to the numerous industrial heating and cooling issues, nonequilibrium heat transport is crucial.
In this technique, the sample is excited by a Gaussianprofiled intensity modulated laser beam.The sample absorbs the intensity of the incident light, causing internal temperature changes that cause a gradient in the refractive index and the development of an induced lens.The induced lenses that are created are divergent ones since the majority of materials have negative temperature coefficients of resistance.To detect the created induced lens, a second laser beam-the probe beam-is used.The laser beam is diverged and extended in the far field as a result of this generated lens.This phenomenon is known as thermal blooming and is depicted in the Fig. 1.
Depending on the liquid environment of the sample under study, thermal blooming will vary and hence the T D .Nature and properties of the solvent, intensity of the Gaussian beam, aggregation of the sample etc., are the crucial parameters determining the T D of the sample.Thermal diffusivity value determines the applicability of a material as heat sinks or coolants.In this scenario the present work on calculation of T D of the dye in different solvents requires attention.The impact of solvent on the dye's quantum efficiency has already been reported [22].To the best of our knowledge, the influence of solvent and the concentration on the T D of the dye under study is not yet carried out.

Experimental details
4-[(4-Aminophenyl)-(4-imino-1-cyclohexa-2,5-dienylidene) methyl] aniline hydrochloride, a magenta-coloured dye (molecular weight 337.85, molecular formula C 20 H 20 ClN 3 ) [7] was purchased from M/S Sigma Aldrich.This dye is commonly used for dyeing in textile industry and for the Staining of Bactria in medical field.Figure 2 illustrates the molecular structure of the dye.The experimental set up is shown in Fig. 3 and is already reported [7].With the use of a neutral density filter placed between the laser and the chopper, the output from the laser source in the current investigation was adjusted to 80 mW and is employed for the studies.
For making the stock solution, the powdered dye is weighed out and is then dissolved in the proper solvents.Different diluted concentrations were made from this stock solution.The solvents were purchased from M/S Sigma Aldrich.The sample names and concentrations are listed in Table 1.

Theory
The thermo-optic characteristics of the sample are found to be proportional to the measured probe beam intensity using the following expression. ( Here t c is the characteristic time constant and is defined as where ω is the beam radius, ρ is the density, c is the specific heat and k is the thermal conductivity.The parameter θ is related to the thermal power P th and pump laser wavelength, λ by (4) where (dn/dt) is the temperature coefficient of refractive index of the sample.The t c and θ can be obtained by fitting the experimental data to the Eq. ( 2).The T D can then be calculated by (5)

Results and discussions
The absorption spectrum of the dye in three solvents is measured with the corresponding solvent as the reference.The absorption peak is at 550 nm.It has previously been reported that the solvent has no effect on the dye's absorption peak wavelength but the absorption intensity (for a particular concentration in the three solvents) is varied because of the solvatochromic effects.The absorption spectra for the dye in ethanol for four different concentrations is shown in Fig. 4a.Similar is the concentration dependent variations for the other two solvents.The normalised absorption spectra for the dye in the solvents  Although the solvent has little effect on the absorption spectrum, the kind of solvent has a substantial effect on the fluorescence emission spectrum.The dye's fluorescence emission wavelength changes depending on its concentration.The solvent properties are listed in Table 2.
Because of the solvatochromic effects, the dye's fluorescence emission spectra in the three solvents vary depending on intensity, much like the absorption spectrum did. Figure 6a depicts the dye's fluorescence emission It is owing to the energy misfortunes because of dissemination of vibrational energy, reallocation of electrons in the encompassing dissolvable atoms, reorientation of the dissolvable particles, and associations between the fluorophore and the dissolvable or the solute [16].Table 3 lists the wavelength-dependent changes for the dye in the solvents at various concentrations.
The solvent dependent emission spectra can be related to refractive index, dipole moment and dielectric constant by the Lippert equation, The terms h, a corresponds to constant (= 6.6256 • 10 -27 ergs), radius of the cavity the fluorophore resides and speed of light (= 2.9979 • 10 10 cm/s).( ) and ( ) are wavenumbers (cm -1 ) corresponding to the absorption and emission.The term (∆f) is the term for orientation polarizability.The energy difference between the ground and excited states decreases as the electrical and molecular polarizability increases [22].Temperature and viscosity also plays a crucial part in the rate of solvent relaxation [22].Greater orientation polarizability, which is brought about by an increase in the dipole moment, results from increased solvent polarity and causes a larger Stokes shift.[23].4. The decay curve fitted plot for the dye in ethanol is shown in Fig. 7b.
It is because of the solvation characteristics of the dye in different solvents, the diffusivity is varied.The dye is having maximum solubility in acetone and least solubility in distilled water.In the case of distilled water and ethanol, more aggregates will be created, which lowers the diffusivity in these solvents.The increased number of aggregates result in slower diffusion.Irrespective of the solvent, for higher concentrations, more aggregates will be formed.These increased number of aggregates not only produces extra random collisions but also produces a temperature rise.Because of this, heat diffusion will be slower and a reduced thermal diffusivity value is observed.There can also formation on higher concentrations.These clusters will further reduce the heat dissipation.
For the lifetime measurements, M/S Horiba Scientific's DeltaPro fluorescent lifetime system based on the TCSPC approach was utilised.Fluorescence intensity decay over time is given by where τ is the fluorescence lifetime, t is the time after the absorption and I 0 is the intensity at t = 0.
The fluorescence decay curve fitted using least square fitting algorithm is depicted in Fig. 8. Using the slope of the decay curve, the fluorescence lifetime was calculated.The measured average lifetime was shown Table 5.

Conclusions
We have studied solvent and concentration dependency on the thermal diffusivity of the dye in three solvents using laser induced dual beam thermal lens technique.It was observed that the thermal diffusivity was decreased with increasing concentration irrespective of the solvent.This is attributed to the increased number of aggregates on higher concentrations and the solvatochromic properties of solvents.The Brownian motion of the dye particles also plays a vital role in the thermal diffusivity.It was concluded that the dye studied could be effectively used as insulators or heat sinks with proper choice of concentration and solvent.

Figure 2 .
Figure 2. Molecular structure of the dye

Figure 1 .
Figure 1.Thermal blooming of the probe beam

Figure 3 .
Figure 3. Experimental set up for the dual beam thermal lens approach

Figure 7 .Figure 8 .
Figure 7. Observed thermal lens signal for the dye in ethanol (a) and thermal lens decay curve (of the dye in ethanol) fitted (b)

Table 1 .
Molarity and sample names

Table 2 .
Properties of solvents

Table 3 .
Fluorescence emission wavelength of the dye vs solvent and molarity

Table 4 .
Thermal diffusivity and t c of the dye vs solvent type and molarity

Table 5 .
Fluorescence lifetime of the dye in solvents