ZHANGZichaoCHENLuyiHEJianleiZHIYifanWEIYujuanCHANGYuqiXUJia(許佳)

1 College of Textile and Garments,Hebei University of Science &Technology,Shijiazhuang 050018,China 2 School of Public Policy and Management,Tsinghua University,Beijing 100084,China 3 College of Chemistry,Chemical Engineering and Biotechnology,Donghua University，Shanghai 201620,China

Abstract:Deep understanding has been achieved for nanotechnology,which has greatly advanced the development of technology. The adsorption property,as one of conventional properties of nanomaterials,is well known but has not been fully applied. In this paper,nanomaterials were used as adsorbents,and further their adsorption properties on substrates were studied. Spherical nano-silica was selected as a representative,which was labeled with a dye. The chemical bonding between the dye and nano-silica was confirmed by Fourier transform infrared (FTIR) spectroscopy. It was proved that the adsorption was a spontaneous,endothermic and entropy increase process. The adsorption model conformed to the Langmuir-type adsorption model. Reaction kinetic studies showed that the number of reaction stages in the adsorption process was about 1.45. The adsorption state was studied to verify the existence of nanomaterials as an adsorbent on the surface of the polymer,and then provided a demonstration for feasibility of the general polymer surface modification method.

Key words:nanomaterial;adsorption;nano-silica;polymer;surface modification

Introduction

With decades of research on nanotechnology,physical and chemical properties of nanomaterials have been well understood.The technology has been transformed into practice,which advances social and economic development for the benefit of mankind.As described in many references,more and more properties of nanoparticles have been exploited.For example,thermal properties of nanoparticles have a great influence on energy[1],photoelectric properties of nanoparticles have a huge impact on the field of electronic information[2],and other properties are used in the field of catalyst and protection[3-6].The full utilization of these properties of nanoparticles makes new materials provide strong support for human daily life and happiness.

Though some of the properties of nanoparticles have been known,they have not been fully utilized.Among them,the adsorption performance is a nanomaterial property that everyone knows,but is not fully exploited.Previous studies have found that the high specific surface area of nanoparticles greatly increases the adsorption properties of nanoparticles.Therefore,this property is easily utilized in the preparation and the use of adsorbents,and a series of research results have been achieved[7-14].This success has further promoted the researchers to invest a lot of energy in order to consider how to make molecular designs for these nanoparticles and improve their performance[15-20].

It is quite interesting that if we consider the more fundamental adsorption mechanism,there is no essential difference between adsorbents and adsorbates,which is the same process as adsorption.In other words,adsorbent materials can also be used as adsorbates.Therefore,it is conceivable that the nanomaterials are similar to the adsorbent and the adsorbate,but they lead to two different technological trajectories.A four-quadrant diagram is given in order to illustrate the research direction (shown in Fig.1).The nano-adsorbed materials are divided into two categories,namely adsorbates and adsorbents,and further,classification is proceeded according to their mechanisms.It can be found that current nanomaterial research is well produced in nano-adsorbents,and the research on adsorbates is still limited[21-22].This means that many current pieces of research focus on the study of nanoparticle adsorbents,which inadvertently weakens the investment of nanomaterials as adsorbates.

Fig.1 Categories of usage of nanomaterials in engineering

Obviously,when the nanomaterial is used as the adsorbate,it can be directly used for surface modification,and it can act as a new reaction and adsorption site on the surface of the material by acting as a bridge[23].As shown in Fig.1,it is found that a small number of studies use nanomaterials as adsorbates,mainly in silver nanoparticles,carbon-based nanoparticles,or some inorganic non-metallic nanomaterials[24-27].However,the exploration of silicon-based organic nanomaterials is still very limited.Chemical engineering researchers can simultaneously utilize scalability and adsorption advantages of nanomaterials,develop more environmentally friendly and highly efficient organic nano-adsorbed substances,and promote the development of surface modification of polymers.

In this paper,we selected spherical nano-silica as a representative,labeled it with dye,studied the adsorption characteristics of the colored nanosphere on the cellulose substrate,verified the existence of the nanomaterial as the adsorbate on the surface of the polymer,and explored general-purpose polymer surface modification methods.

1 Experiments

1.1 Material preparation

Reactive brilliant red K-2G (C.I.18200) was obtained from Jinan Yongxing Dye Chemical Co.,Ltd.(Jinan,China).Tetraethyl orthosilicate and other chemicals were purchased from Tianjin Damao Chemical Reagent Co.,Ltd.(Tianjin,China).

The amino-modified nano-silica was prepared by the method in Refs.[28-31].In a typical preparation,63 mL absolute ethanol was placed in a three-necked flask,and 4.5 mL tetraethyl orthosilicate and a certain amount of aminopropyl ethoxy silane (0-1 mL) were added.The mixture was stirred for 30 min to make it evenly mixed at 50 ℃.Then a mixture of an appropriate amount of anhydrous ethanol,7.2 mL water,and 6.0 mL ammonia water were slowly added.After being stirred for 8 h,the product was centrifuged and washed with ethanol and water.

We labeled the nano-silica according to the procedure shown in Fig.2,and reactive brilliant red K-2G was selected as the marker.Amino nano-silica (0.5 g) was dispersed into 50 mLN,N-dimethylformamide by ultrasonicated for 20 min.The mixture was magnetically stirred at 75 ℃ and then reacted at 75 ℃ for 5 h after adding 0.02 g reactive brilliant red K-2G andN,N-diisopropylethylamine (DIPEA) with an appropriate amount.At the end of the reaction,the product was obtained by centrifugation,washed with water,and dried at 70 ℃.Reactive brilliant red K-2G reacts with a large number of isolated hydroxyl groups and dihydroxy groups on the surface of nano-silica under basic conditions to obtain colored nano-silica.

Fig.2 Schematic preparation of colored nano-silica

1.2 Characterization

Fourier transform infrared (FTIR) spectra were measured with an FTIR Nicolet 6 700/FT-Raman modules (Thermo Fisher) using KBr pellets at a range of 4 500-400 cm-1.The surface morphologies of silica nanoparticles were observed by a field-emission scanning electron microscope (FESEM) (S-4800-Ⅰ,Hitachi Ltd.,Japan).

1.2.1Drawingofadsorptionratecurve

Seven parts of (0.500 ± 0.002)g pretreated cellulose fibers were placed in a numbered conical flask with stoppers respectively.Colored nano-silica emulsion (0.3 g/L) was added to the above seven conical flasks at a solid to liquid ratio of 1∶100.After the solution was shaken at 80 ℃ for a preset time,the concentration of the residue solution was tested.According to the concentration difference before and after adsorption,the adsorption amountQf(mg/g) of the nano-silica per gram of fiber was calculated,and the adsorption rate curve was plotted to determine the time required to reach the adsorption equilibrium.

1.2.2Drawingofadsorptionisotherms

Seven parts of (0.500 ± 0.003)g pretreated cellulose fibers were placed in a numbered conical flask with stoppers respectively.Then the above prepared 0.1,0.2,0.3,0.4,0.5,0.6 and 0.7 g/L colored nano-silica emulsions at a solid to liquid ratio of 1∶100 were added.After the adsorption at 80 ℃ for a long time,the absorbance of the residual liquid was measured by an ultraviolet spectrophotometer,and the concentration of the residual liquid after adsorption was converted according to the absorbance-concentration working line.According to the concentration difference in the conical flask before and after adsorption,the number of nanospheres adsorbed per gram of cloth was calculated.The adsorption isotherm of cellulose nanofibers with colored nano-silica at 80 ℃ was gained by plotting the concentration of colored nano-silica on the fiber versus the concentration of the emulsion in the conical flask after adsorption.

2 Results and Discussion

2.1 Coloring of modified SiO2

In order to determine exactly how the reactive brilliant red K-2G and the silicon sphere are bonded,the FTIR spectrum is characterized by the colored and white nano-silica,as shown in Fig.3(a).It can be seen that there is no significant difference in the FTIR spectra before and after the nano-silica grafting reactive brilliant red K-2G.The possible reason is that the added amount of the grafted dye is relatively low,and the change before and after the grafting cannot be detected.To better characterize the reaction mechanism,thin layer chromatography (TLC) analysis was carried out.If the reactive brilliant red K-2G was bonded to the nano-silica by the way of physical deposition,after sufficient elution of the dye developing agent,the adsorption site between the dye and the nano-silica was replaced by the developing agent,and the color on the nano-silica should be washed away,thereby changing to the white state.

Four kinds of commonly used developing agents for reactive dyes are prepared,and the developing agent formula is as follows:(1)butanol,acetic acid and water at a molar rate of 16∶5∶15;(2)butanol,water and DMF at a molar rate of 11∶11∶3;(3)butanol,pyridine,water and ammonium hydroxide (25%) at a molar rate of 5∶5∶3∶2;(4)water.

TLC analysis results were shown in Fig.3(b).The reactive brilliant red K-2G can be developed in all four developing agents.The retention factorRfusing the Nos.(1),(2),(3) and (4) developing agents is 0.73,0.92,0.67 and 0.93,respectively.

The obtained colored nanoparticles were sufficiently ultrasonically eluted by four kinds of developing agents,and then centrifuged.It is found that the color is not eluted,as shown in Figs.3(c) and (d).The experimental results show that the color of the nano-silica does not fade after the full elution of the developing agent,and indicate that the color cannot be washed away by the developing agent.So,it can be proved that the chemical bond is formed between the dye and the nano-silica,and the reaction process shown in Fig.2 is reasonable.

The picture of the resulting red nano-silica is inset in Fig.3(a).The labeled dye and the nano-silica are covalently bonded and can be stably present during adsorption with the cellulose fibers.Scanning electron microscope (SEM) images of nano-silica,amino modified nano-silica,and colored nano-silica are shown in Fig.4.It can be seen from Fig.4 that the sizes of the nano-silica before and after labeling do not show significant change,and the particle size after labeling is about (130 ± 38) nm.

In order to expand the application of nanomaterials,chemical modification is a commonly used method in engineering applications.Therefore,in this study,the colored nano-silica can be more favorable for studying its adsorption characteristics on the cellulose substrate,and the adsorption characteristics can more accurately describe the actual modification process of the nanomaterial on the polymer surface.

Fig.3 Characterization of colored nano-silica:(a) FTIR spectra;(b) spreading phenomena of reactive brilliant red K-2G in different expanded matters;(c) dispersion of colored nano-silica in different expanded matters before centrifugation;(d) dispersion of colored nano-silica in different expanded matters after centrifugation

Fig.4 SEM images:(a) and (b) nano-silica;(c) and (d) amino modified nano-silica;(e) and (f) colored nano-silica

2.2 Adsorption of colored nano-silica on cellulosic fibers

In order to better study the adsorption characteristics of nano-silica on cellulose substrates,the cellulose substrate was first pretreated with sodium hydroxide solution.After the treatment,the adsorption height of the 30 cm × 5 cm sample to water was 11 cm in 30 min in the vertical direction.

For the measurement of the concentration of nano-silica sol,there is an absorbance method after color development using a reagent such as ammonium molybdate.In this paper,the nano-silica is colored with a dye,and the adsorption on the cellulose substrate can be more intuitively observed.At the same time,it is possible to determine the concentration of the nano-silica sol by spectrophotometry.Therefore,we first verified the correctness of the sol concentration by spectrophotometry.Colored nano-silica was measured by the UV spectrophotometer,and it was found that the colored nano-silica emulsion absorbed in the whole wavelength range,and the maximum absorption wavelength was 305.5 nm.The reason for the absorbance in the whole wavelength range may be that the nano-silica emulsion system introduces partial reflection of light.The design experiment verified the correctness of the method for determining the emulsion concentration by measuring the absorbance of the colored nano-silica emulsion at a wavelength of 305.5 nm.The absorbance of different concentrations of colored nano-silica emulsion was determined,and the absorbance-concentration curve of colored nano-silica was drawn.There is a good linear relationship between absorbance and concentration,and the relationship between the two can be described:y=1.269 1x,as shown in Fig.5.In addition,standard nano-silica emulsions with known concentrations of 0.15,0.25 and 0.35 g/L were used to verify the standard curve with an error lower than 1.57‰.

Fig.5 Absorbance-concentration curve of colored nano-silica

Therefore it is considered that there is a good linear relationship between the absorbance and the concentration of the colored nano-silica emulsion,and the standard curve can be used to convert the concentration of the emulsion with the absorbance number.

Taking the concentration on the cellulose fiberQfversus time,the adsorption rate curve is obtained,as shown in Fig.6(a).The adsorption process was carried out at 80 ℃,and the liquor ratio was 100∶1 with 0.3 g/L colored nano-silica emulsion.With the extension of time,the adsorption amount of colored nano-silica on the fiber gradually increases until 136 min,reaching the adsorption equilibrium,and the maximum adsorption capacity is 28.9 mg/g.

The isothermal adsorption character of colored nano-silica emulsion (0.1-0.7 g/L) on cellulose fibers was studied at 80 ℃.The adsorption isotherm is shown in Fig.6(b).The adsorption isotherm is used to describe the relationship between the liquid phase and the adsorbate concentration on the adsorbent when the adsorption reaches equilibrium under given conditions.

Fig.6 Adsorption curve of colored nano-silica on cellulose:(a) adsorption rate curve of 0.3 g/L colored nano-silica emulsion on cellulose fibers;(b) adsorption isotherms of colored nano-silica on cellulose fibers

2.2.1Adsorptionmodel

The Langmuir-type adsorption model is widely used to characterize the liquid phase adsorption model,which is effective for single-layer adsorption with the same adsorption site.If the adsorption of cellulose fibers by colored nano-silica is assumed to be consistent with Langmuir-type adsorption model,the adsorption amount of colored nano-silica on the fiberQf(mg/g) and the concentration of residual liquidCs(mg/L) should be consistent with the following formula when the adsorption is balanced:

(1)

namely,

(2)

where,Vm(mg/g) is the maximum adsorption amount of colored nano-silica on cellulosic fibers;b(L/mg) is the adsorption equilibrium coefficient.

The 1/Qf-1/Csplot should be a straight line.By examining the linear relationship between 1/Qfand 1/Cs,it can be used to judge whether the adsorption of cellulosic fibers by colored nano-silica conforms to the Langmuir-type adsorption model.bandVmcan be obtained by calculating the intercept and the slope.Based on the experimental data for the corresponding adsorption equilibrium at different initial concentrations,plot 1/Qfversus 1/Cs,as shown in Fig.7(a).

The Freundlich-type adsorption model assumes that there are different adsorption sites on the adsorbent,and the surface of the adsorbent is not uniform.It is assumed that the adsorption model of the colored nano-silica on the cellulosic fiber conforms to the Freundlich-type adsorption model.When the adsorption reaches equilibrium,the amount of nano-silica adsorptionQfand the concentration of residual liquidCsshould be consistent with the following formula

(3)

namely,

lgQf=lgp+qlgCs.

(4)

This model is obtained by summarizing the experimental rules.According to the above formulae,in whichpandqare empirical constants,lgQfand lgCsshould have a good linear relationship if they are in accordance with the Freundlich-type adsorption model.qandpcan be obtained from the intercept and the slope.According to the above-mentioned adsorption equilibrium test data at different initial concentrations used to draw the adsorption isotherms,lgQfis plotted versus lgCs,as shown in Fig.7(b).

Fig.7 Fitting curve of adsorption model of colored nano-silica on cellulose:(a) Langmuir-type adsorption model;(b) Freundlich-type adsorption model

It can be seen from Fig.7(b) that there is certain linearity between lgQfand lgCs,but the linear relationship is poor as the correlation coefficient is only 0.850 6.The linear relationship can be expressed as

lgQf=0.458 3 lgCs+0.658 6.

(5)

Through the intercept and the slope calculation,p=4.556 2 andq=0.458 3 can be obtained.Therefore,the adsorption isotherm obtained by simulating the Freundlich-type adsorption model can be expressed as

(6)

It can be seen from Fig.7(a) that there is a good linear relationship between 1/Qfand 1/Cs.The simulated linear varianceR2=0.947 6,and its linear relationship can be expressed as

(7)

The maximum adsorption amount on the fiberVm=74.074 1 mg/g and the adsorption coefficientb=0.015 49 L/mg were obtained by the intercept and the slope.Therefore,the adsorption isotherm obtained by the Langmuir-type simulation can be expressed as

(8)

Through the simulation of the above two adsorption models,the adsorption model of colored nano-silica on cotton fabric is more consistent with the Langmuir-type adsorption model,and the fitting variance is 0.947 6,which can be considered as single-layer adsorption.

2.2.2Thermodynamicparameters

The Gibbs free energy change ΔGof cellulose fibers adsorbed at different temperatures was calculated.The parameters used are shown in Table 1.According to Table 1,lnKdis plotted versus 1/T,and Fig.8 is obtained.It can be seen from Fig.8 that there is a good linear relationship between lnKdand 1/T,and the variance is 0.975 4.According to the relationship of the slope,the intercept,ΔHand ΔS,the calculation can be obtained:ΔH=75.117 kJ/mol,ΔS=289.78 J/(mol·K).

Table 1 Thermodynamic parameters for the adsorption of colored nano-silica on cellulosic fibers

The calculated ΔGat 323.15,333.15,343.15 and 353.15 K is negative,which indicates that the adsorption process of cotton fabrics by the colored nano-silica prepared is spontaneous.In addition,as the temperature of the adsorption system increases,the absolute value of ΔGincreases,which indicates that the adsorption behavior is more likely to occur under high-temperature conditions.The free enthalpy becomes a positive value,indicating that the colored nano-silica absorbs heat when nano-silica is adsorbed on the cellulosic fiber.Theoretically,the adsorption equilibrium of the colored nano-silica on the cellulosic fiber should move toward the adsorption direction as the temperature increases during the adsorption process,which is confirmed by the experimental data that as the temperature increases,the amount of adsorption on the fiber does increase with temperature increase.The increase of temperature will make the cotton fabric swell more fully in water,increase the kinetic energy of the nanosphere,and increase the Brownian motion,which increases the collision probability of the colored nano-silica and the fiber surface,and increases the adsorption amount.The value of the free enthalpy is also an important reference data for distinguishing between chemical adsorption and physical adsorption.When the enthalpy change ΔHof the adsorption process is 2.1-20.9 kJ/mol,the adsorption is considered to be physical adsorption.Therefore,the adsorption of cellulosic fibers by colored nano-silica is chemical monolayer adsorption,which is consistent with the conclusion obtained by fitting the adsorption model.By calculation,the entropy change ΔSis also positive,indicating that the adsorption process is a process of increasing the entropy value,i.e.the adsorption process increases the disorder of the fiber surface.

Fig.8 Plot of ln Kd versus 1/T

2.2.3Kineticparameters

The larger the reaction order is,the greater the reaction rate constant is affected by the concentration.The reaction rate equation containing two reactants is

(9)

(10)

k=kAk0,

(11)

wherekAis the reaction rate constant.

Then taking the natural logarithm of the formula gets:

(12)

The initial rate method was used to calculate the reaction order,and the average rate in the first ten minutes was used to represent the initial ratev0.The reaction order was gained by calculating the slope of the lnv0-lnC0curve as shown in Fig.9.Therefore,the adsorption order of the colored nano-silica prepared in this research on the cellulosic fiber was 1.448 6.

Fig.9 ln v0-ln C0 curve

3 Conclusions

In this paper,nano-silica was labeled with reactive brilliant red K-2G to obtain nanoparticles with a particle size of about (130 ± 38)nm.Adsorption thermodynamics studies show that the adsorption of colored nano-silica on cellulose fibers is a spontaneous,endothermic and entropy increasing process,which is demonstrated by ΔG<0,ΔH>0 and ΔS>0.The colored nano-silica on cotton fabrics is more consistent with the Langmuir-type adsorption model,and the fitting variance is 0.947 6,which can be considered as monolayer adsorption.The reaction order of the adsorption process calculated by the initial rate method is about 1.45.The spontaneity of the adsorption process is verified,which provides a feasibility demonstration for the general polymer surface modification method.