Bo Fang,Tingyang Lu,Jinshuang Wang,Lianghao He,Haiwei Lu,Lichen Yu,Jie Chen

Shanghai Key Laboratory of Multiphase Materials Chemical Engineering,Lab of Chemical Engineering Rheology,East China University of Science and Technology,Shanghai 200237,China

Keywords:Photosensitive micellar system Cetyl trimethylammonium bromide trans-chlorocinnamic acid Rheokinetic equation Photorheological property

ABSTRACT The photosensitive micellar system with prospect in drag reduction and heattransfer enhancementhas been the research focus ofscience and industry.In this paper,a new type ofphotosensitive micellarsystemformed by cetyl trimethylammonium bromide(CTAB)and trans-chlorocinnamic acid(trans-ClCA)was investigated.The effects of counter-ion concentration,surfactant concentration,UV irradiation time,and the position of substituent groups of counter-ion on photorheological properties were discussed.The results indicate that after UV irradiation the relative viscosity and thixotropy ofmicellar systems reduce obviously.At the same time,the relative viscosity of micelle systems and concentration of trans-ClCA decline during UV irradiation time.The flow curves of micellar solutions before and after UV irradiation could be described by non-linear co-rotational Jeffreys model correctly.The rheokinetic equation was firstly established to describe the relationship between relative viscosity of micelle systems and conversion of trans-ClCA.

1.Introduction

The fluids whose rheological properties could be varied with extrinsic stimuli(such as light,pH,temperature,etc.)attracted much more attention in recent years[1-4].Wolff T and co-workers[5,6]studied one kind of photorheological(PR) fluids whose structures and rheological properties could be changed by UV irradiation at certain wavelength[7].Because light is easy to control and can be directed at a precise spot with resolution of a few micrometers,PR fluids have potential values in microscale applications,such as micro fluidic devises,microvalves or flow sensors[8].Recently PR fluids have been used in drag reduction and heat transfer enhancement[9],control of surface tension[10,11],microemulsion phase separation[12],and biological systems[13].

There are three categories of PR fluids:photoisomerizations,dimerization,and photoscission reactions[14-17].The main mechanism isintroducing photosensitive groups(such as azobenzene groups,stilbene groups or cinnamate groups)to various types of molecules[18].Such light(＜400 nm)can be used to cause cis-trans-isomerizations,bond breakings,molecular cyclization polymerizations[19-23].Obviously,such dramatic changes could lead to molecular conformations and properties ofmicelles.Butthe complex synthesis ofphotosensitive molecules hampers the applicability of PR fluids[24-26].Currently,some researchers studied a series of mixtures to simplify experiments and reduce costs,such as mixtures of cetyl trimethylammonium bromide(CTAB)and trans-ortho-methoxycinnamic acid(OMCA),mixtures of cationic surfactant Ethoquad O/12 PG(EO12)and OMCA[27-30].For CTAB/OMCA(CCTAB=60 mmol·L-1,COMCA=50 mmol·L-1)sample,before irradiation,the sample is highly viscous and strongly shearthinning,after 20 min UV irradiation,the viscosity is reduced obviously and the sample behaves as a Newtonian fluid[29].

In this paper,we report a new class of PR fluids that are based on simple,inexpensive chemicals.The fluids are composed of the cationic surfactant CTAB with a photosensitive counter-ion molecule,trans-chlorocinnamic acid.The results show that surfactant/counter-ion mixtures could form good viscoelastic micelles.The relative viscosity of micelle solution decreased by more than 35%after UV irradiation and the photoisomerization is irreversible,i.e.,relative viscosity could not be increased by visible light irradiation.

UV spectrum of trans-ClCA samples before and after UV(365 nm)irradiation were analyzed.It shows a slight shift in the peak of trans-ClCA to a lower wavelength after UV irradiation.It indicates that trans-structure could be transfered to cis-structure after UV irradiation,which is the main reason for decreasing viscosity of micelle solutions.After UV irradiation,the micellar systems showed obvious photorheological property in reducing viscosity and thixotropy,indicating that UV irradiation can damage stability of the micellar system.Thixotropic loop areas of micelles with different CTAB concentrations were compared before and after UV irradiation,showing that mixing ratio of CTAB/trans-pClCA could in fluence thixotropy.

2.Experimental Sections

2.1.Materials

Trans-para chlorinated cinnamic acid(trans-pClCA)and trans-ortho chlorinated cinnamic acid(trans-oClCA)(industrial grade)were purchased from Wuhan Yuan Cheng Gong Chuang Science and Technology Co.Ltd.Cetyl trimethyl ammonium bromide(CTAB,analytical grade)was purchased from Shanghai Lingfeng Chemical Reagent Co.Solutions containing trans-pClCA/trans-oClCA dissolved in hot water were prepared with a slight excess of base(NaOH),and CTAB was then added to these solutions to reach the final composition.Samples were stirred continuously under mild heat to be homogeneous.The solutions were then left to equilibrate overnight at room temperature before any experiments were conducted.The pH in the samples was 13.

2.2.Surface tension

Surface tension measurements were made on a full-automatic surface tension meter plate method test apparatus which trigger tension was 5 mN·m-1,platinum sheet with a width of 24.1 mmwas selected.CTAB and CTAB/trans-ClCA samples were studied at 25°C.

2.3.Sample response before and after UV irradiation

CTAB/trans-ClCA samples were irradiated with UV light from a 365 nm UVlamp(centralintensity at4800μW·cm-2),which was purchased from Spectorline Company.Samples were placed in a Petri dish(inner diameter of 6 cm)and irradiation was done under stirring.To measure the viscosity and rheologicalproperties,trans-ClCA/CTAB samples were irradiated for different time with UV light(365 nm).

2.4.Relative viscosity measurement

When the viscosity of dilute solution(η)is greater than the viscosity of pure solvent(η0)at the same experimental condition,we could use the relative viscosity to describe viscosity.The relative viscosity ηris calculated according to Eq.(1).

where t and t0are the time of dilute solution and solvent flowing through the same Ubbelohde viscometer(inner diameter of 1 mm)respectively,which was purchased from Shanghai Experimental Instrument Factory.

2.5.Concentration determination of micelle

The absorbance of 10 trans-ClCA standard solution samples with a concentration range of 0.01 to 0.1 mmol·L-1was measured under the wavelength of 273 nm,with de-ionized water as a reference.An absorbance-concentration of trans-ClCA standard curve was plotted by the resulting data.The absorbance of CTAB/trans-ClCA samples after UV irradiation was measured and then converted to a trans-ClCA concentration value using the absorbance determined from the standard curve.

2.6.Rheological experiments

Steady and dynamic rheological experiments were performed on a Physica MCR 101 rheometer(Anton Paar Company).Samples were run at 25°C on a cone-and-plate geometry(24.955 mm diameter,1.001°cone angle).Frequency sweep experiment(strain=1.0%,ω =0.01-100 rad·s-1)and thixotropy experiment(share rate increases from 0 to 100 s-1in 40 s,and then share rate decreases from 100 to 0 s-1in 40s)were performed.

3.Results and Discussion

3.1.The effect of substituent position of trans-ClCA isomers on the surface activity

We firststudied mixtures ofCTAB and the isomers ofchlorocinnamic acid,i.e.,para-chlorocinnamic acid(pClCA),meta-chlorocinnamic acid(mClCA)and ortho-chlorocinnamic acid(oClCA).All three compounds could increase the viscosity of CTAB solution and corresponding samples exhibited all the hallmarks of wormlike micelles.Effects of the trans-ClCA substituent position on reducing the surface tension of CTAB were shown in Fig.1.Fig.1 shows that the addition of trans-ClCA does cause a reduction in the surface tension of CTAB.The substituent positions of counter ions trans-ClCA would in fluence the effectiveness of reducing surface tension.The trans-pClCA has the highest effectiveness in reducing surface tension,followed by the trans-mClCA and the trans-oClCA has the weakest effectiveness that may due to the strong synergistic effect of hydrophobic interaction between transpClCA and CTAB.

Fig.1.Surface tension varies with substituent positions of trans-ClCA.

Trans-ortho-methoxy-cinnamic acid(OMCA),whose structure is similar with trans-ClCA,is known to undergo photoisomerization.To con firmthat trans-ClCAcould isomerize to cis-ClCA upon UV irradiation,we measured UV-vis spectra for oClCA before and after UV irradiation(Figs.2 and 3).As shown in Figs.2 and 3,UV irradiation causes a blue shift in the UV spectrums of trans-oClCA with the peaks shifting to 267 nm from 273 nm before the irradiation.The blue shift indicates that there occurs photoisomerization from trans-o ClCA to ciso ClCA(Fig.3).CTAB solution can form long cylindrical micelles with trans-pClCA or trans-oClCA,while form short cylindrical micelles with cis-p ClCA or cis-oClCA.In short,the viscosity and the rheological properties of the micelles of trans-ClCA/CTAB change obviously after UV irradiation.

Fig.2.UV spectra of oClCA before and after UV irradiation.

Fig.3.Scheme of cis-trans-isomerization of oClCA before and after UV irradiation.

The effects of substituent positions of trans-ClCA on micelle formation and photosensitive properties are discussed in Figs.4 and 5.

Fig.4.The relative viscosity vary with concentration of trans-ClCA(C CTAB=4 mmol·L-1)at 25°C.

Fig.5.The relative viscosity reduction percentage vary with concentration of trans-ClCA after 1 h UV irradiation(C trans-ClCA=7 mmol·L-1)at 25 °C.

Fig.4 shows that,at the same concentration,the maximum relative viscosity is investigated in the system of trans-pClCA/CTAB,while the least is appeared in trans-oClCA/CTAB.Because transpClCA shows a smaller steric hindrance compared with transoClCA,which causes longer cylindrical micelles formed with CTAB,then micelles undergo physical entanglement resulting in the bigger viscosity of the system.Fig.5 shows that trans-pClCA is more sensitive to UV light for its biggest decreased rate of relative viscosity upon UV irradiation,indicating that its micelle structure is easily to be destroyed.The results are coincided well with the findings of Baglioni's work[28].

3.2.Thixotropy before and after UV irradiation

We examined the thixotropy of the micellar systems and the results are shown in Figs.6 and 7.Fig.6 showsthatthe sizesofthixotropic loops are obviously narrowed after UVirradiation.The thixotropic loop area of CTAB/pClCA (CCTAB= 4 mmol·L-1,CpClCA= 7 mmol·L-1)is 45.26 Pa·s-1before UV irradiation,and 11.25 Pa·s-1after UV irradiation.From Fig.7,the thixotropic loop areas of CTAB/mClCA(CCTAB=4 mmol·L-1,CmClCA=7 mmol·L-1)and CTAB/oClCA(CCTAB=4 mmol·L-1,CoClCA=7mmol·L-1)are much less than that of CTAB/pClCA(CCTAB=4mmol·L-1,CpClCA=7 mmol·L-1),and also decrease after UV irradiation.Therefore,it is proved that UV irradiation can damage the network structure ofmicellar systems,leading to decrease of thixotropy.The drastic thixotropic changes reported here is likely to be attributed to a change in the steric hindrance effect.Trans-pClCA and CTAB are easily assembled into long wormlike micelles due to weak steric hindrance effect.Therefore,network structure of micellar systems depends on the mixing ratio of CTAB and trans-ClCA.The thixotropy of micelles with different CTAB/trans-ClCA concentration ratio shows a quite different picture.On the other hand,ClCA undergoes a photoisomerization from its trans to cis form after UV irradiation and the cis-ClCA is likely to destroy the network structure of wormlike micelles due to its strong steric hindrance effect,resulting in decreases of thixotropy of micelles drastically[14,30].

Fig.6.UV effect on the thixotropy for CTAB/pClCA(C CTAB=4 mmol·L-1,CpClCA=7 mmol·L-1).

3.3.Constitutive equation for photosensitive micelles

In order to study the constitutive equation for photo sensitive micelle,we chose the non-linear co-rotational Jeffreys model to describe the flow curves of PR micelle systems.For shear thinning fluid, flow curve can be predicted by the non-linear co-rotational Jeffreys model as Eq.(2):

where η0is zero-shear viscosity and λ1and λ2are the characteristic times of the system.

Using Jeffreys model,we describe the flow curves of CTAB/transoClCA(CCTAB=4 mmol·L-1,Ctrans-oClCA=7 mmol·L-1)and CTAB/trans-mClCA(CCTAB=4 mmol·L-1,Ctrans-mClCA=7 mmol·L-1)before and after UV irradiation,the results are shown in Figs.8 and 9.From Figs.8 and 9 and Table 1,it indicates that the non-linearco-rotational Jeffreys model can be applied to describe the viscosity curves of surfactant/counter-ion correctly,the calculated values are in good agreement with the experimental data.

Fig.7.Thixotropy of systems before and after UV irradiation at 25 °C.C CTAB=4 mmol·L-1,C oClCA=7 mmol·L-1,C mClCA=7 mmol·L-1.

3.4.Photorheokinetics for CTAB/trans-ClCA micelle systems

The effect of irradiation time on relative viscosity of micelle systems is investigated.The result shows the relative viscosity of surfactant/counter-ion decreases with increase of irradiation time.In first 30 min of irradiation,relative viscosity of micelle systems decreases considerably.The relative viscosity reduction ratio is smaller during 30-60 min,and the equilibrium value of the relative viscosity can be attained after the irradiation of 60 min,as shown in Fig.10.

How to describe the relationship between relative viscosity of micelle systems and UV irradiation time?The photorheokinetics for CTAB/trans-ClCA micelle systems is established as follows.

For photosensitive viscoelastic micellar systems,long wormlike micelles could transfer to short spherical like micelles over UV irradiation time.The prediction of the process is:

Fig.8.The flow curve of CTAB/trans-oClCA(C CTAB=4 mmol·L-1,Ctrans-oClCA=7 mmol·L-1)before and after UV irradiation described by nonlinear co-rotational Jeffreys model.

Fig.9.The flow curve of CTAB/trans-mClCA(C CTAB=4 mmol·L-1,Ctrans-mClCA=7 mmol·L-1)before and after UV irradiation described by nonlinear co-rotational Jeffreys model.

Table 1 Parameters for nonlinear co-rotational Jeffreys model for viscosity curves of different micellar systems

Fig.10.Relative viscosity curves of viscoelastic micellar systems described by rheological dynamics equation.C CTAB=4 mmol·L-1,C oClCA=7 mmol·L-1,C pClCA=7 mmol·L-1.

Here,A is the wormlike micelle and B is the spherical like micelle.We all know that the viscosity of spherical like micelle systems is equal to the water's.The amount of wormlike micelle is related to the viscosity of systems.Assuming that viscosity of system varies directly with concentration of wormlike micelle A.

When t=0,concentration of A is CA0;when UV irradiation time is t,concentration of A is CA.Assuming that the transfer of A to B is corresponding with the first order reaction as follows:

where,ηr0is the initialrelative viscosity,and k is the rate constantofthe structural changes.

Eq.(2)could be integrated as follows:

Assuming that the reduced amount of micelle system's relative viscosity is proportional to the reduced amount of A,which is given by:

where,K is the proportionality coef ficient.

When A is converted to B completely,CA0is equivalent to the maximum concentration ofB,and KCAcorresponds to the reduced amountof system's relative viscosity(ηr0-ηr,min).Then the rheological kinetic equation of systems could be shown by:

Here,contains 3 parameters,ηr0,ηr,min,and k.

Eq.(5)that was used to describe relative viscosity of CTAB/ClCA systems varies with UV irradiation time.The results and the model parameters are shown in Fig.10.and Table.2 respectively.

Table 2 Model parameters for rheological dynamics equation for relative viscosity curves

The effects of irradiation time on concentration of trans-ClCA were studied.The results show that conversion rate of trans-ClCA isomerize to cis-ClCA at the first 30 min irradiation is greater than that of 60 min irradiation.In order to obtain the kinetic equation of CTAB/trans-ClCA micelle systems,the first-order kinetic Eq.(3)is used to describe that concentration of trans-ClCA curve varies with time.The results are shown in Fig.11 and the model parameters of reaction kinetics equation are shown in Table 3.

The relationship of viscoelastic micellar systems'viscosity and conversion rate could be described by rheokinetic equations.In order to obtain the rheokinetic equation of CTAB/trans-ClCA micellar systems,assuming that the reduced amount of relative viscosity is proportional to the amount of a reduced concentration of trans-ClCA.Such as

Fig.11.Concentration curves of trans-ClCA in micelle systems described by reaction kinetics equation.

Table 3Model parameters of reaction kinetics equation for CTAB/pClCA micelle systems

where ηr,minis the minimum relative viscosity,and Cequis the minimum concentration of trans-ClCA in the experiments.

The conversion rate of trans-ClCA transfer to cis-ClCA is as follows:

According to Eqs.(6)and(7),photorheokinetic equation of micelle systems is shown by:

According to Eq.(8),relative viscosity curves that vary with conversion rate of CTAB/trans-ClCA were described as Fig.12.

Table 4 Model parameters for rheokinetics equation for relative viscosity curves of CTAB/pClCA micelle systems

From Fig.12 and Table 4,it is proved that the relative viscosity curves that vary with conversion rate of CTAB/trans-ClCA micellar systems can be described by the established rheokinetic equation.

4.Conclusions

(1)In the appropriate concentration and proportion ofCTAB and ClCA,the relative viscosity of micellar solution can be affected by UV irradiation and decreased by almost 35%.Because of stereohindrance effect hydrophobic interaction,the micelle system of pClCA and CTAB is more stable and sensitive to the UV irradiation.

(2)The relative viscosity of micelle systems and concentration of trans-ClCA declines with UV irradiation time.The established rheological kinetic equation could well describe the relationship between relative viscosity of micelle systems and conversion of trans-ClCA.

(3)The thixotropy loops ofthe micellarsystem CTAB/ClCAdecrease signi ficantly after UV irradiation,indicating that UV irradiation can damage stability of the micellar system.The non-linear corotational Jeffreys model can be applied correctly to describe the flow curves before and after UV irradiation.

Fig.12.Relative viscosity curves vary with conversion rate of CTAB/trans-ClCA described by rheokinetic equation.