J. Phys. Chem. 1991, 95, 6688-6696
6688
Thermal Lens Effect In Electrolyte and Surfactant Medla Mladen Frrrnkot and Chieu D.Tran* Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233 (Received: January 11, 1991; In Final Form: April 8, 1991) A novel method, which is based on the thermal lens effect, has been developed for the simultaneous, sensitive, and accurate determination of the thermal conductivities and temperature coefficients of the refractive indexes of solutions. The k and dn/dT values determined by this method were then used to elucidate the mechanism of the thermal lens enhancement that is induced by electrolytes and/or surfactants. Electrolytes were found to provide up to a 2-fold enhancement in the thermal lens signal. The enhancement is due to the effect of the electrolyte not on the thermal conductivity but rather on the dn/dT of the solution. Higher enhancements (up to 8-fold) were achieved when surfactants were added to the solution. The micellition process does not seem to have any observable effect on this enhancement. Similar to the case of electrolytes, the enhancement is predominantly due to the effect of surfactants on dn/dT of solution. The change in dn/dT upon the addition of electrolytes or surfactants was deconvoluted into two parts. The first, which results from the difference in the specific refractivity and dn/dT of the electrolyte or surfactant solution as compared to that of pure water, has a stronger effect on the enhancement than the second, which is caused by the alteration of the hydrogen-bondingnetwork of water.
Introduction The thermal lens technique is based on the temperature rise that is produced in an illuminated sample by nonradiative relaxation of the energy absorbed from a Specifically, when a laser beam whose intensity is symmetrically distributed (TEM,) is used to excite a sample, the heat generated is strongest at the center of the beam because that is where the beam intensity is strongest. This creates a temperature gradient which in turn produces a refractive index gradient and changes the radial intensity distribution of the beam. When low-concentration, weak absorbing species reach the thermal lens steady-state condition (i.e., the rate of heat generated equals the rate of heat conducted out), the relative change in the beam center intensity in the far field, AIb/&, is given by Ibc
Ak
where P is the laser power, A is the laser wavelength, and A, dn/dT, and k are the absorbance, the temperature coefficient of the refractive index, and the thermal conductivity of the sample,’-) respectively. Examination of this equation reveals that the sensitivity of the thermal lens technique not only is directly proportional to the excitation laser power but also depends on the thermooptical properties of the sample. The former property enables the technique to be used as a highly sensitive method for the determination of chemical species at very low concentrations. In fact, trace chemical species in the form of gas, liquid, or solid whose absorptivities are of the order of lo-’ cm-’ have been determined by using this technique.’-5 The second feature which is perhaps the more interesting one is the prediction that for liquid samples the sensitivity of the thermal lens technique depends on the thermooptical characteristics of the solvent employed.69’ This property can be exploited to determine the thermooptical properties of the sample solutions (i.e., k and dn/dT values) as well as to enhance the sensitivity of the technique; namely, higher sensitivity per unit laser power and sample absorbance is predicted to be achieved if the measurements are performed in solvents having high -dn/dT and low k values.6~~ Water, which is the most widely used solvent, is considered to be the worst medium for thermal lens measurements. This is because its -dn/dT value is low while its k value is high.- As a consequence, considerable efforts have been made in the past few years to ameliorate thermooptical properties of water. Most notable successful efforts include the use of the temperature effect
To whom correspondenceshould be addrwed. ’ Prcsent address: Nuclear Chemistry Department, J. Stefan Institute, Ljubljana, Yugoslavia. OO22-3654/9 1 /2095-6688$02.50/0
and the addition of surfactants to enhance the sensitivity of thermal lens measurements in It was found, in the former method, that up to a 2.4-fold enhancement in the thermal lens signal intensity can be achieved when the temperature of the aqueous solution is increased from +2O.O to +W.O OC.9 This result suggests that it may be possible to enhance the sensitivity of the technique by adding electrolytes to the aqueous solution. This possibility stems from the fact that ( I ) the thermooptical properties of the aqueous solution are collectively improved when it is heated and (2) heating up the aqueous solution has the same effect on the hydrogen bonding of water, as suggested by IR, Raman, and NMR studies, as adding certain electrolytes into the solution.’*’5 Since there are many types of electrolytes, namely, the so-called “water structure former” and “structure breaker”,’*I5 it is important to investigate this possibility and whether it is possible to determine the type of electrolyte which can provide the best enhancement effect. Surfactants have been found to enhance the sensitivity of the thermal lens While it is certain that surfactants as well as heating and possibly electrolytes improve the thermooptical properties of water, it is not clear what effect they have on the individual factors, i.e., k and dn/dT, that affects the intensity of the thermal lens signal. This information is particularly important as it will enable the selection of the most appropriate surfactant, temperature, and electrolyte which can provide the highest enhancement. Such considerations prompted this work which aims to (1) use the thermal lens effect to develop a sensitive and accurate method for the determination of k and dn/dT values of solutions, (2) investigate the possibility of enhancing the sensitivity of the thermal lens technique by adding electrolytes and/or surfactants, and (3) determine the k and dn/dT values of electrolyte and surfactant solutions in order to elucidate the mechanism of the enhancement. Preliminary results will be reported in this communication. (1) Sell, J. A. Photothermal Investigations of Solids and Fluids;Academic: New York, 1989. (2) Harris, J. M.In Analytical Applicationr of Lusers; PiepMeier, E. H., Ed.; Wiley: New York, 1986; pp 451-476. (3) Fang, H. F.; Swdford, R. L. In Ultrasensitive h e r Spectroscopy; Kliger, D. S., Ed.;Academic: New York, 1983; pp 176-232. (4) Tran, C . D. Appl. Specrrosc. 1986, 40, 1108. (5) Tran, C. D.; Franko, M.J. fhys. E Sci. Instrum. 1989, 21, 586. (6) Tran, C. D. Anal. Chem. 1!)88,60, 182. (7) Tran, C. D.; Van Fleet, T. A. Anal. Chem. 1988,60, 2478. (8) Franko, M.;Tran, C. D. Chem. fhys. Lrrt. 1989, I58, 31. (9) Franko, M.;Tran, C. D. Anal. Chem. 1989,61, 1660. (IO) Hartman, K. A., Jr. J . Phys. Chem. 1966, 70, 270. (11) Stangret, J.; Kostrowicki, J. J . Solufion Chem. 1988. 17, 165. (12) Walrafen, G. E.; Hokmabadi, M.S.;Yang, W. H. J . Chem. fhys. 1986,85,6964. (13) Rull. F.; de Saja, J. A. J . Ramon Specrrosc. 1986, 17, 167. (14) Marciacq-Rausclot,M.M.;Lucas. M.J. fhys. Chem. 1973,77,1056. (1 5 ) Bhanumathi, R.;Vijayalakshamma,S.K. J . fhys. Chem. 1986, 90, 4666.
0 1991 American Chemical Society
Thermal Lens Effect in Electrolytes and Surfactants
Experimentnl Seetion Thermal lens signals were measured by using either a singleor dual-beam thermal lens spectrometer which has been previously de~cribed.’.~A small amount (about 1F-10-6 M) of dyes such as nickel phthalocyaninetetrasulfonic acid, tetrasodium salt, iron( 11) 1,lO-phenanthroline perchlorate, ((4-(dimethylamino)phcnyl)am)phenyl4’-maleimide (DABMI, Molecular Robes)(for aqueous solutions and polar solvents), or azulene (for nonpolar solvents) was added into the solution to induce the thermal lens signal. Aqueous solutions were prepared from doubly distilled deionized water. HPLC/spectro grade methanol (Altech) was also distilled twice prior to use. Other high-purity solvents (99.9% D 2 0 (Aldrich), DMF and CC14 (Fisher), DMSO and heptane (B&J), and CHC13(Chempure)) were used without further purification. Solutions of electrolytes were prepared from analytical grade reagents, i.e., sodium chloride (Fisher), tetramethylammonium chloride, sodium carbonate, potassium fluoride, cesium nitrate, sodium nitrate, potassium nitrate (Aldrich), lithium nitrate, sodium chlorate (Coleman & Bell Co.), potassium bromide (Baker Chemical Co.), rubidium nitrate (Fairmount Chemical Co.), and sodium acetate (EM Science). Electrolyte solutions with lower concentrations were prepared daily from corresponding stock solutions which were 5.0,2.5, or 1.0 M, depending on the solubility of the salts. Aqueous and deuterium oxide solutions (2.0 X lo4 M) of iron(I1) 1,lO-phenanthrolineperchlorate (GFSChemicals) were prepared for spectrophotometric measurements in a 2-mm-pathlength cell and also for the use 2s stock solutions for thermal lens measurements (in a 2-mm-pathlength cell). For thermal lens measurements in water and electrolyte solutions, 5.0 X lo-’ M nickel phthalocyaninetetrasulfonic acid, tetrasodium salt (Aldrich), was obtained by injecting 5 pL of 1.0 X M dye stock solution into either 10 mL of water or an electrolyte solution. A 2.5 X lo4 M stock solution of the nickel complex in deuterium oxide was prepared by dissolving 2.4 mg of nickel complex in 10 mL of deuterium oxide. Thermal lens signals were measured after 15 pL of this stock solution was injected into 5 mL of deuterium oxide to give a 7.5 X lo-’ M solution. For measurements in methanol, a saturated solution of nickel phthalocyaninetetrasulfonic acid, tetrasodium salt, in methanol was first prepared. After filtration, it was diluted 1:l with methanol, and 20 pL of such solutions was further diluted M stock to 10 mL. In the case of DMSO and DMF, 1.0 X solution of nickel complex in water was diluted to 2.5 X 10” M, and 10 pL of this solution was injected into 5 mL of solvent to M solution. obtain 5 X Stock solutions of 2.0 X lW3 M azulene in heptane, CCl,, and CHC13were prepared by dissolving 2.6 mg of azulene in 10 mL of solvent. Working solutions were then prepared by injecting 6 pL of stock solution into 5 mL of solvent to yield 2.4 X 10” M solutions. All solutions were filtered through a 0.45-pm membrane filter before measurement. For spectrophotometric measurements the following concentrations of nickel complex were used: 1.0 X lW5 M in water, in electrolyte, and in surfactant solutions and 2.5 X M in deuterium oxide, DMF, and DMSO while in methanol-saturated solution diluted 1:l was used. In the case of nonpolar solvents 2.0 X lo-’ M stock solution of azulene was used to obtain absorption spectra. All the surfactants were used as received from suppliers (sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (CTAC) from Kodak, polyoxyethylene-l,1,3,3-tetramethylbutylphenol (Triton X-I00)and polyoxyethylene(23)-dodecanol (Brij-35) from Aldrich, polyoxyethylene(6)-dodecanol (C,2E6) from Nikkol, and N-dodecylsultaine (SB-12) from Serva). Solutions were prepared with doubly distilled water. For thermal lens measurements in surfactant solutions, 250 p L of the stock solution of 1 .O X lW3 M nickel complex in surfactant solution was injected into 5 mL of the solution having the same surfactant concentration. Special care was taken in the preparation of solutions of DABMI to avoid any errors which may arise due
The Journal of Physical Chemistry, Vol. 95, NO.17, 1991 6689 to its low solubility.16 DABMI was initially dissolved (1.6 mg/5 mL) in methanol to make a 1.O X l(r3M solution. Five microliters each of this DABMI methanolic solution was injected into 25-mL volumetric flasks. These flasks were transferred to a desiccator, and the solvent was evaporated under vacuum by means of an aspirator. Subsequently, the dried flasks were filled with solutions of the appropriate surfactant concentration. Sonication was then performed to facilitate the dissolution of DABMI. The solutions were then filtered through a 0.2-pm membrane filter to remove any undissolved DABMI. The same solutions were used for thermal lens measurements as well as for UV-vis spectrophotometry. Absorption spectra were obtained with IO-cm sample cell, while thermal lens measurements were performed in a 2-mm cell.
Method Since the aberrant model has proven to give better agreement with experimental results than the parabolic model,” it is used to describe the time dependence of the beam center intensity Z(t) 3 + (2 arctan
+ (9 + 9)tC/2t
+ t2+ (9 + t2)rC/2t t2)+ 6tC/tl2+ 16t2 [(2 + t,/r)(3 + E’) (9 + t2)Q + rc/r)2 3
where is related to the distance between the sample and the beam waist Z,and the confocal distance Z,by
5 = ZdZC
(3)
The parameter B is related to the laser wavelength (A), laser power (P), the sample absorbance (A), the temperature coeficient of the refractive index (dn/dT), and the thermal conductivity k of the solvent as shown in the equation B = 2.303PA(-dn/dT)/Xk
(4)
The thermal time constant t, is given by
r, = u2pcp/4k
(5)
where cp is the heat capacity, p is the density, and w is the laser beam spot size. From the recorded time dependence of the beam center intensity, I(t), the parameter B which is related to the thermal lens strength and the thermal time constant tc can be obtained by curve fitting of the data according to eq 2. Using eqs 4 and 5 and the B and t, values, one can calculate the thermal conductivity and dn/dT values if the laser power, sample absorbance, heat capacity, density, and laser beam spot size are known. While the values of P and A can be readily determined and the c and p values can be obtained from the literature, it is relatively dihcult to accurately measure the beam spot size. It is therefore more appropriate to calculate the ratio of tc values for sample and for pure water since density, heat capacity, and thermal conductivity of water are known with a c c ~ r a c y . ’ ~Because - ~ ~ beam spot sizes in water and (16) Patonay, G.; Rallic, M. E.; Warner, I. M. AMI. Chem. 19$5,57,569. (17) Carter, C. A.; Harris, J. M. Appl. Opt. 1984, 23, 476. (18) Kell, G. S. J . Chem. Eng. Data 1967, 12, 66. (19) Ginnings, D. C.; Furukawa, G. T. J . Am. Chem. Soc. 1953,75,522. (20) Steckel, F.; Szapiro, S. Trans. Faraday Soc. 1963,59, 331. (21) Cockett, A. H.; Ferguson, A. Philar. Mag. 1940,29, 185. (22) Riddick, J. A.; Bunger, W. B.; Sabno, T. K. Organic SolventsPhysical Properties and Methods of Purification; Wiley: New York, 1986. (23) International Critical Tables of Numeric Data, Physics, Chemistry and Technology; McGraw-Hill: New York, 1982; Vols. 3, 5.
Franko and Tran
6690 The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 TABLE I: 8 and t c (ms) Values for Iron(I1) 1,lO-PberuathroIine Perchlorate (PFP) in Water and in Deuterium Oxide
[PFPI, M
cell pathlength, cm
5.0 x 10-7
1
A,
nm
514.5
5.0 x 10-7
1
457.9
6.25 X IOd
0.2
514.5
0.2
457.9
6.25 X
lo-"
H20
e = 0.0931 f 0.0002 r, = 43.3 & 0.5 e = 0.0774 f 0.0002 1, = 41.6 f 0.1 e = 0.113 0.0008 r, = 34 f I e = 0.0944 i 0.00oi t,
in the sample can be assumed to be the same, the thermal conductivity of a sample can be calculated as
where the subscripts (w) and (s) denote the values for water and for the sample, respectively. This relation is particularly useful when the ratio of thermal conductivities is desired. In such a case the latter becomes
-k(s)- -
P(S)CP(S)tC(W)
hw)
P(W)CP(W)tC(S)
(7)
Once values of thermal conductivity are obtained, the calculation of the temperature coefficient of refractive index is possible by rearranging eq 4. However, to avoid errors that might result from approximations used in aberrant model and also from the improper position of sample cell, it is better to calculate the ratio (dn/dT)(,,/(dn/dT)(,). When wavelength and laser power are the same for water and for the sample, this ratio is given as WdT)(S) - %)4W)kS) (8) (dn/dT)(w) 4w)A(s)k(w) Equation 8 is particularly suitable for measurements on dual-beam instruments since no model is available that would describe dual-beam thermal lens experiments explicitly. The differences between thermal lens experiments on a mismatched dual beam25and on a single-beam configuration arise exclusively from different spot sizes of excitation beams in these two cases. Normally, when the aberrant model is used for dual-beam experiments, results (e and t, values) obtained should be multiplied by appropriate correction factors. These factors are derived from the ratio of the spot sizes of the excitation and the probe beam. Since correction factors should be the same for pure water and for the sample, they are not needed when the dn/dT and k values are measured relative to those of water as in the method described here. Results and Discussion 1. Determination of k and dn/dTValues. The validity of the method was examined by measuring 0 and rc values of water and deuterium oxide using a dual-beam, dual-wavelength instrument. Preliminary results, obtained using a 1-cm-pathlength sample cell, are summarized in Table I. From these values, ratios of 0 and of t, in deuterium oxide to those in water were calculated. Values of tHs)/tc(w) were found to be 1.10 f 0.01 while ratios 8(s)/8(w)were different at two wavelengths. The values obtained were 0.538f 0.003 at 514.5 nm compared to 0.623 f 0.004 at 457.9 nm. In addition, the measured 6(s)/t9(w)and tc(s)/tc(w) values are not in agreement with the values of 0.782 and 1.163 that were calculated by using experimental parameters (laser power, absorbance, wavelength) and literature values of density, heat capacity, thermal conductivity, and dn/dT.'8-21*24,2628Such discrepancies may be due to the fact that the instrument was aligned only for aqueous (24)Powell, R. W.;Ho,C. Y.;Liley, P. E. Nail. Stand. Rej. Data Ser. 1966,8, 118.
(25) Berthoud, T.;Delorme, N.;Mauchien, P. Anal. Chem. 1985,57, 1216. (26) Challoner, A. R.; Powell, R. W . Proc. R. Soc. London 1956,A238,
90.
(27) Tilton, L. W.;Taylor, K. J . Res. Nail. Bur. Stand. 1983,20, 419. (28) Luten, B. Phys. Reo. 1934,45, 161.
= 33 f 1
D20
e = 0.0500
0.0002
= 47.8 f 0.5 e = 0.0482 0.0006 t, = 45.5 0.5 e = 0.0899 A 0.0001 1, = 38.7 f 0.2 e = 0.0744 0.0003 I, = 38.9 0.2 t,
*
TABLE II: Thermal Conductivity of Water and of Deuterium Oxide at Different Temperatures, Measured by the Thermal Law Technique
T , "C +19.77 +17.98 +16.13 +14.31 +12.52 +10.68 +8.86 +7.13 +5.15 +3.34 + 1.26 -0.76 -2.60 -4.47 -6.78 -8.44
thermal conductivity, mW cm-' K-l H20 D2O 6.1 f 0.1 6.1 f 0.1 6.1 f 0.1 6.1 f 0.1 5.9 f 0.1 5.8 f 0.1 5.9 0.2 5.7 f 0.1 6.0 f 0.2 5.3 f 0.6 7f2 5fl 5.3 f 0.4 5.4 f 0.2 5.6 f 0.1
*
5.8 0.2 5.7 f 0.2 5.8 f 0.1 5.9 f 0.3 5.6 f 0.2 5.2 f 0.2 6fl 7f2 5.6 f 0.6 5.5 0.1 5.5 0.2
TABLE III: dn/dT V d w s of Water at Different Temperatures, Measured by the Thermal Lens Technique dn/dT X IO5, K-I T, OC this work lit.2'
+19.77
+ 17.98
+16.13 +14.31 +12.52 +10.68 +8.86 +7.13 +5.15 +3.34 +1.26 -0.76 -2.60 -4.47 -6.78 -8.44
-8.6 f 0.2 -8.2 f 0.2 -7.4 f 0.2 -6.7 f 0.2 -5.8 f 0.1 -4.8 f 0.1 -4.0 f 0.2 -2.8 f 0.1 -1.7 f 0.1 -0.6 f 0.1 +0.4 f 0.1 +1.3 f 0.3 +2.2 0.2 +3.6 f 0.1 +5.1 f 0.2
-8.8 -8.1 -7.4 -6.6 -6.0 -5.4 -4.5 -3.6 -2.6 -1.6 -0.4
samples. It is therefore possible that the propagation of the laser beams (pump and probe) through the sample would be different in solvents having different refractive indexes. Consequently, inside the sample, the beams are slightly displaced from their original positions, and the displacement is even more pronounced at the detector. Smaller 6 values will therefore be observed. The results obtained which show smaller ratios of 6 than the calculated ones are in agreement with such an explanation. The errors caused by differences in refractive indexes of solvents can be ameliorated by use of a shorter pathlength cell, Le., 2 mm. The results obtained for water and D20by using this cell are also shown in Table I. From these 0 and rc values, the 0(s)/6(w)values were calculated to be 0.796 f 0.006 and 0.788 f 0.003 at 514.5 and 457.9 nm, respectively. The t,(,)/tc(w)values were found to be 1.14 f 0.01 and 1.18 f 0.04 at 514.5 and 457.9 nm, respectively. These 6(s)/0(w) and tc(s)/tc(w),ratio values are in good agreement with the calculated ones, ].e., 0.782 and 1.16. Thus, it is evidently clear that short-pathlength cells are required in thermal lens measurements when results for solvents having different refractive indexes are to be compared. For this reason,
Thermal Lens Effect in Electrolytes and Surfactants
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6691
TABLE IV: dn/dT Values of Deuterium Oxide at Different Tempcntures. Measured by the Tbermrl Lens Techniaw
instrument. Results are presented in Table V. Thermal conductivities were obtained from time constants measured relative to water and from the literature values of density and heat capacity of ~olventsl*-~~ and thermal conductivity of water.24 As demonstrated, agreement with other reported values is very good. The only exemption is DMSO. However, the reported literature values for DMSO and DMF were not obtained experimentally but were calculated29from the equation
T, O C +19.11 +17.98 +I75 +16.13 +15.0 +14.31 +12.52 +11.3 + 10.68
+8.86 +5.15 +4.1 +3.34 +1.6
+ 1.26 -0.76
dn/dT X IOs, K-' this work lit.z8 -6.6 f 0.2 -5.9 f 0.2 -5.6 -5.1 f 0.1 -4.2 -4.3 i 0.2 -2.9 i 0.2 -2.3 -2.0 i 0.1 -1.0 i 0.3 +2.6 i 0.1 +1.0
+2.8 & 0.3 +3.0 +4.1 0.1 +5.9 i 0.2
the 2-mm-pathlength cell was used for all subsequent experiments. These preliminary results suggest that it should be possible to use this method to determine thermal conductivities and temperature coefficients of refractive index (dn/dT) of water and deuterium oxide at different temperatures (-8 to 20 "C). The k and dn/dT values of water at 20 "C, Le., 6.04mW cm-' K-' 24 and -8.8 X K-1,27 respectively, were used as standard references. Results obtained from the thermal lens measurements using 514.5-nmexcitation are summarized in Tables 11-IV. In Table I1 the measured values for thermal conductivity of water and deuterium oxide are presented. Since water at 20 "C was taken as the standard, its value at 19.8"C is not reported. For deuterium oxide the measured value of 5.8 f 0.2is in agreement with the reported value (5.79mW cm-' K-').26 Estimated errors were obtained as standard deviations for several consecutive measurements. For deuterium oxide they were in most cases smaller than 5% and even smaller for water. There were, however, some values at temperatures close to +7.4 "C (for D20) and 0 O C (for H20) which had standard deviations larger than 5%. Since the refractive indexes for these two liquids are known to be maximum at these t e m p e r a t ~ r e s ?the ~ , ~dn/dT values will be zero. As a consequence, thermal lens signals become very small in the vicinity of these temperatures and hence produce larger standard deviations. It was also observed that accuracy within fS% could only be obtained when 8 values are larger than +0.03 and smaller than -0.03. As shown, thermal conductivities of water and deuterium oxide change by less than 10% in the temperature ranges investigated so that most of the differences in k values are less than the experimental error. It is therefore not reasonable to compare the measured and literature values for all temperatures. Only values at the lowest temperatures (Le., 5.6 f 0.1 and 5.5 f 0.2 mW cm-' K-' for water and D 2 0 at -8.44 and -0.76 "C, respectively) will be compared. The corresponding literature values of 5.54mW cm-' K-' (for deuterium oxide at 0 "C) and 5.44mW cm-' K-' (for water at -8 "C) are in good agreement with the values obtained by using the thermal lens technique. The measured and literature values of dn/dT for water are listed in Table 111. Limitations on the accuracy, imposed by the small thermal lens signals in the vicinity of the temperature of maximum refractive index, are similar to those for thermal conductivity. In fact, standard deviations less than 10%were obtained for 8 values larger than +0.03 and smaller than -0.03. Agreement of the measured and literature value is not as good as in the case of thermal conductivity. In general, results obtained were approximately 10% higher. The same conclusions can be made in the case of dn/dT values obtained for deuterium oxide (Table IV). Obviously, more data were required in order to reveal the source of apparent systematic error in the measured dn/dT values and to reconfirm the thermal lens technique as an accurate and reliable method for measurement of thermal conductivity of liquids. Therefore, several organic solvents with different polarity and also solutions of strong electrolytes were studied on a single-beam
k = A~c,,(~/M)'/~
(9)
where A is a constant (4.28X M is the molecular weight of liquid, p is the density, and cp is the heat capacity in cal mol-'. The parameter k is expressed in cal cm-' s-' K-I. The reported literature values for DMSO and DMF might therefore be in error because constant A is not the same for all In Table V, two different sets of dn/dT values are reported in order to elucidate the effects of theoretical approximations and misalignment of the instrument on the results. Values in column A were obtained directly from the measured 8 and t, values of the sample and t, for water. The thermal conductivity of each sample was calculated from the time constant, literature values of cp, and the density (eq 6). This k value was then used to calculate the dn/dT of the sample according to eq 4. Conversely, dn/dTvalues listed in column B were obtained from the measured time constant and 8 values of samples relative to the values in water. Instead of calculating the thermal conductivity of each sample, the ratio of the thermal conductivities was obtained from the ratio of time constants and from literature values of cp and the density (eq 6). Consequently, the ratio of dn/dTvalues was calculated according to eq 8, and dn/dT of the sample was obtained by taking -10.4 X K-' as a reference value for dn/dT of water at 632.8nm and 25 0C.27Therefore, the values in column B are less affected by the aforementioned effects because they are canceled out when the ratios of 8 and of tc values are used. The experimental values were slightly higher than values reported in literature. However, for liquids with -dn/dT higher than 5.0 X lo4 K-' the difference increased drastically and reached over 20% for solvents such as chloroform, carbon tetrachloride, and heptane. Unfortunately, not all of the results could be compared with values from the literature. For instance, data on dn/dT of strong electrolyte solutions are not available except for potassium bromide. Furthermore, based on the description of the method used for the determination of dn/dT of it is expected that this value is not very reliable. A slight improvement was obtained when water was used as a reference (column B). However, this approach does not totally eliminate the discrepancies. For example, while values for deuterium oxide, methanol, DMF, and 2.5 M potassium bromide agree well with literature values, results for less polar solvents are still too high (about 1520%). We are currently investigating effects which cause such discrepancies. Single- and dual-beam thermal lens techniques have proven to be useful and powerful techniques for simultaneous determination of thermal conductivities and temperature coefficients of refractive index. Thermal conductivity values ranging from 1.2to 6 mW cm-I K-'for variety of liquids and strong electrolyte solutions have been measured. Standard deviations were in all cases lower than 5% and hence are considered to be more accurate than the reported values obtained by using pulse laser e x ~ i t a t i o n . ~ ~ J ~ Accuracy of the measured dn/dT values depends on the dn/dT value of the solvent. Improvements are still needed when the dn/dT of solvents are high (Le., nonpolar solvents). For samples having -dn/dT values lower than 4 . 0 X 10" K-I accurate and (29) Tsederberg, N. W. Thermal Conductivity of Gases and Liquids; MIT Press: Cambridge, 1965. (30) Appleby, R.; James, W. D.; Bowie, C. A. Spedrochim. Acra 1984, 4OA. . -.- , 785. . - -. (31) Clever, H. L.; Taylor, M. L., Jr. J . Chem. Eng. Dara 1971,f6,91. (32) Bailey, R. T.; Cruickshank, F. R.; Pugh, D.; Guthrie, S.; McLeod, A. Chem. Phys. 1983,77,243. (33) Bailey, R. T.; Cruickshank,F. R.; Pugh, D.; McLeod, A.; Johnstone, W.Chem. Phys. 1982,68,351.
6692 The Journal of Physical Chemistry, Vol. 95, No. 17, 1991
Franko and Tran
TABLE V d./dl".nd Tbermrl Conductivity of Sdvemts at 25 "C, Measured by the Thermal k Technbue
0.82 f 0.05 4.1 f 0.1 7.5 f 0.3 7.3 f 0.3 4.9 f 0.2 4.6 f 0.2 6.6 f 0.2 1.55 f 0.07 1.84 i 0.09 1.63 f 0.08 1.72 f 0.04
M~OH CCI,
CHCI, DMF DMSO heptane 2.5 M LiNO, 2.5 M NaNO, 2.5 M KNO, 2.5 M KBr TABLE VI: h b " t Chloride Solutions conc, M 0.5 1.o 1.5 2.0 2.5
of the Thermal Lens S i g d in Sodium
eN,Cl/@w
1.27 f 0.04 1.37 f 0.05 1.47 0.06 1.54 f 0.06 1.74 f 0.05
*
0.85 f 0.05 3.85 f 0.1 7.1 f 0.3 7.0 f 0.3 4.8 f 0.2 4.5 f 0.2 6.3 f 0.2 1.90 f 0.07 1.96 f 0.09 2.00 f 0.03 1.76 f 0.04
(dn/dT)N,a/(dn/dT)w 1.25 f 0.07 1.34 f 0.07 1.41 f 0.1 1.55 f 0.1 1.62 f 0.09
(kNaCl/kw)-'
1.02 f 0.05 1.02 f 0.04 1.04 i 0.06 1.00 f 0.07 1.08 f 0.05
precise dn/dT can be obtained by using this method. 2. Effects of Electrolytes. The effect of electrolyte concentration on the thermal lens signal was initially studied by using sodium chloride solutions. This salt was purposely selected because it is one of the most frequently encountered matrices in the analysis of real-time aqueous samples. To date, the importance of the matrix effect has been neglected in the applications of the thermal lens technique to the analysis of ~eawater.M'~Concentrations of sodium chloride used in this study are far beyond those encountered in the seawater, but even in a 0.5 M solution of sodium chloride, which approximates the salinity of oceans, the enhancement in the thermal lens signal is clearly evident as shown in Table VI. Ratios of 8 values have always been corrected for differences in the absorbance of samples. Enhancement in thermal lens signal relative to that in pure water increases almost linearly over the concentration range studied and reaches a value of 1.74 at 2.5 M concentration. The results can be compared with a similar investigation which found that a 0.5 M sodium chloride provided up to about a 28 f 5% increase in the thermal lens signal.3s The increase, unfortunately, did not agree with the calculated value of only 12%, and no explanation was given. This result, together with the 27 f 4% increase obtained from this work for the same sodium chloride concentration, suggests that the calculated value (i.e., 12%) in ref 35 is not very reliable. In fact, this value was obtained by using dn/dT values for water and sodium chloride solution that were extrapolated from those reported for solutions of potassium bromide at 25 0C.30 Careful examination reveals that the extrapolated dn/dT value for 0.5 M solution of sodium chloride (-1.26 X lo4 K-I) also agrees very well with the present value which was found to be (-1.30 f 0.07) X l(r K-I. However, the extrapolated value of dn/dT for water seems to deviate by about 13%. This conclusion is based on a comparison of the extrapolated value (-1.135 X 10" K-') with the most frequently cited value of 1 .OO X IO-" K-'for pure water at 632.8 nm and 25 0C.27936*0 Enhancement relative to pure water, calculated by using the latter (34) Fujiwara, K.; Lei, W.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 54, 2026. (35) Philip, C. M.; Crouch, S.R.; Leroi, G. E. Anal. Chem. 1986, 58, 1710. ..
(36) Olson, J. D.; Horne, F. H. J . Chem. Phys. 1973, 58, 2321. (37) Wexler, R. M.; Weir, L. E.; Schamp, H. W. J . Res. NaL Bur. Stand. 1964, 68A. 489. (38) Andreasson, S.P.; Gustafason, S.E.; Halling, N. 0. Opr. SOC.Am. 1971, 61, 595. (39) Bryngdahl, 0.Ark. Fys. 1961, 21, 289. (40)Abhte, G.; Attanaaio, A.; Bcrnini, U.; Ragozzino, E.; &"a, F. J . Phys. D Appl. Phys. 1976, 9, 1945.
0.84 3.85 5.9 5.9 4.6 3.6 5.1
5.8 1.95 1.01 1.29 1.92 (calcd) 2.22 (calcd) 1.26 5.79 5.90 5.69 5.43
5.9 f 0.1 2.03 0.05 1.20 f 0.07 1.30 f 0.04 1.92 & 0.07 1.93 f 0.07 1.36 f 0.04 5.8 f 0.2 5.9 f 0.2 5.6 f 0.2 5.35 f 0.08
*
1.70
TABLE VII: Enhancement of Thermal Lens Signal by Alkali-Metal Cations solution
eMmple/@w
1M
(dnldT)M,*/(dnldT)w
*
1.45 f 0.03 1.59 f 0.03 1.57 f 0.06 1.53 0.01 1.69 0.05
1.34 0.02 1.57 f 0.04 1.48 f 0.08 1.51 f 0.05 1.6 f 0.1
2.5 M LiN03 1.75 f 0.03 NaNO, 2.08 f 0.05 KNO, 1.91 i 0.04 RbNO3 2.41 f 0.03 CSNO~
1.68 0.07 2.04 f 0.05 1.77 f 0.08 2.11 f 0.08 not soluble
LiNO, NaNO, KNO, RbNO, CSNO~
**
*
(kmm*/kw)-'
*
1.08 0.02 1.02 f 0.02 1.06 0.04 1.01 f 0.03 1.04 0.07
*
1.04 f 0.04 1.02 f 0.04 1.08 0.04 1.14 0.04
**
value and thermal conductivity values of pure and ~eawater,2~*~' agrees well with the experimental enhancement value of 1.28 (Le., 28%). It is thus evident that errors of almost 30% can result when the matrix effect is not taken into account in the preparation of calibration curves for the thermal lens analysis of seawater samples. At higher concentrationsof sodium chloride, the enhancement is considerably higher. This effect can be utilized to enhance the sensitivity of the thermal lens measurements in aqueous solutions. The effect of sodium chloride is similar to the heating effect. However, different electrolytes affect water structure differently. Therefore, depending on the nature of the electrolytes, different enhancements of the thermal lens signal are expected. It is thus necessary to investigate these differences in more detail so that effects induced by different cations and anions can be distinguished. This was systematically performed by measuring thermal lens signals for solutions of electrolytes with a common anion (nitrate) and different cations or electrolytes with a common cation (Na+) and different anions. The effects of alltali-metal cations can be elucidated by using the results listed in Table VII. It is clear that the addition of allyli-metal nitrates also has a similar effect on thermal lens signal as that induced by increasing the temperature of water. Their effect is, however, higher than the effect of sodium chloride. Such behavior is not surprising, since IR and NMR studies have suggested that nitrate ion is a stronger structure breaker than chloride ion.'* The effect of anions is also known to be much larger than the effect of cati0ns.4~ Differences between different cations are small and often difficult to distinguish. The only exception is lithium nitrate, which gives enhancement similar to that of sodium chloride. Lithium is known to be a structure promoter (based on its high surface charge density)?' and its effect partly cancels out the relatively stronger structure-breaking effect of nitrate ion. Consequently, there is no significant difference between the effect of lithium nitrate and sodium chloride. Small differences between the effect of different cations are also evident from the results at lower concentrations. For example, in 1 .O M solutions the enhancements relative to water (41) Jamieson, D. T.; Tudhapc, J. S. Dcscllfnation 1970,8, 393. (42) Hartman, K. A,, Jr. J . Phys. Chem. 1966, 70,270. (43) Verrall, R. E. In W u t e r A Comprehemioe Treafise;Franks, F., Ed.; Plenum: New York, 1974; Vol. 3.
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6693
Thermal Lens Effect in Electrolytes and Surfactants
TABLE VIlI: E.Luccacrt of Tkrmrl Leas !3igd ia 2.5 M Solutiooa of Ekefrolyta
solution NaCl NaNO, NaClO, NaCH,COO Na2C0,
KBr KI (CH,),NCI
L,w/k 1.74 f 0.05 2.08 f 0.05 2.39 0.04 1.96 f 0.06 1.65 f 0.06 1.91 f 0.03 2.60 f 0.01 1.96 0.03
(4/dT)w/(WdT)w 1.62 f 0.09 2.04 f 0.05 1.85 f 0.08 1.50 0.08 1.6 0.1 1.69 f 0.04 2.06 f 0.08 a
*
( k w / W ' 1.08 f 0.05 1.02 f 0.04 1.29 f 0.05 1.23 0.05 1.03 f 0.05 1.13 f 0.02 1.26 0.05 a
*
OData on heat capacity are not available.
are between 1.45 and 1.69 times. Differences become more pronounced at higher concentrations, and the enhancement is increased when the size of the cation increases. The enhancement induced by potassium nitrate seems,however, to deviate from this conclusion. The reason for this deviation is not clear at the moment even though a similar observation was also reported in an IR study, namely, the decrease in structure-breaking effect from sodium to pota~sium.'~ The results obtained also indicate that, in the concentration range studied, the electrolytes have a relatively small effect on the thermal conductivity of the aqueous solution; i.e., the changes in k are small and only about a few percent. Thus, the electrolytes seem to exert more effect on the dn/dT values of the aqueous solutions. In fact, as shown in the table, the measured (dn/dT)J(dn/dT)w ratios are relatively larger and have the same enhancement trend as the O,/ew ratios. The effects of nitrate and chloride ions were further compared to the effects of other anions by measuring the thermal lens signals in 2.5 M solutions of several sodium salts and some other efectrolytes. Results are summarized in Table VIII. Enhancement relative to water increases in the series C1-, Nor, Clop-, which is in agreement with the observations in IR and NMR ~tudies.'~ It is, however, interesting to note that the addition of sodium acetate and sodium carbonate, which are often classified as structure formers,'%" causes an even higher increase in the thermal lens signal than structure breakers such as sodium chloride and potassium bromide. Similar observation was also made for tetramethylammonium chloride which is supposed to have very little or no effect on the structure of water.'2 This demonstrates that the effect of electrolytes on the thermal lens signal cannot be evaluated simply from their structurebreaking or -forming effect on water. Higher specific refractivity and higher -dn/dT of solutes may also play a major role in the effect of increasing -dn/dT values of solution. Enhancement of thermal lens signal through the effect of the solute on the change in dn/dT of the solution seems to have more influence than its effect on the change in the structure of water. This deduction can be confirmed by utilizing the so-called "structural temperature"." This is the temperature at which the extent of hydrogen bonding in pure water is the same as in the solution of electrolyte at a certain temperature. On the basis of infrared spectra," it can be calculated that the average strengths of hydrogen bonding in 1 .O M solutions of sodium chloride and of potassium nitrate at 25 OC correspond to those in pure water at about 28.6 and 30.8 OC, respectively. According to our previous work on the effect of the temperature on the thermal lens signal, 3.6 and 5.8 OC increases in the temperature of pure water correspond to enhancements in thermal lens signals of only 1.08 and 1.12, relative to those in pure water at 25 OC. However, the actual enhancements in thermal lens signals for solutions of the two electrolytes were found to be much higher, Le., 1.37 and 1.57. Therefore, it is not unreasonable to propose that, for 1.0 M solutions of electrolytes, about 80% of enhancement in thermal lens signal arises from the change in average specific refractivity and dn/dT of solution that is caused by the different specific refractivity and dn/dT of electrolyte. Only about 20% of the enhancement in thermal lens signal results from the electrolyte ions' (44)KIM, M.;Naberuchin, J. U. Water-struktur und dynamik; Akademie Verlag: Berlin, 1986.
1 1
.
1
.
-
1
.
0
1
I
200
time,
ma
Figure 1. Thermal lens signals of 5.0 X IO-' M Ni-PC in water and in 1.0 M SDS solution. SCHEME I: Structwea of Nkkd P b t k y ~ t e ~ Acid, d c Tetrasodium Salt (Ni-PC),rad ((e( M l m t h y h m i n o ) p k a y l ) r Z O ) ~ y ~ 4 ' -(DABMI) ~~~
Ni-PC
DABMI
induced hydrogen bonds breaking. This study has confirmed the importance of the matrix effect in thermal lens measurements of aqueous solutions. This effect has to be taken into account when preparing calibration curves for routine analysis. Electrolytes have also been demonstrated to be useful additives to improve the sensitivity of thermal lens measurements of aqueous samples. In 2.5 M solutions, improvements of higher than 2-fold have been observed. Where the solubility of electrolyte is not the limiting factor, improvements can be even higher. The contribution of the electrolyte to the dn/dT value of solution has more effect on the enhancement of the thermal lens signal than the change in thermal conductivity. The latter was found to be on the order of only a few percent for the concentration range studied. Furthermore, the change in the dn/dT upon the addition of electrolyte was deconvoluted into two parts. The first one results from the difference in the specific refractivity and in the dn/dT of electrolyte (as compared to that of pure water) and has a stronger influence than the second part which is caused by the alteration of the hydrogen-bonding network of liquid water. 3. Effects of Surfactants. Nickel phthalocyaninetetrasulfonic acid, tetrasodium salt, and ((4-(dimethy1amino)phenyl)azo)phenyl-4'-maleimide (DABMI) are two dyes which were used to induce thermal lens signals in surfactant solutions (Scheme I). The former dye has four negative charges and is very soluble in water while the latter can only be dissolved in nonpolar solvents. They are, thus, expected to locate in different regions of micelles; namely, the former is mostly in the water bulk and the latter is in the micellar core." Their thermal lens signals will, therefore, (45) Fendler, J. H.; Fendler, E.J. Catalysis in Micellar and Macrome lecular Systems; Academic: New York, 1975.
6694 The Journal of Physical Chemistry, Vol. 95, No. 17, 1991
Franko and Tran
3
r
n 0
5z
3 3 B
h
\
0
UF
J
2
CmC -4
-3
-2
-1
0
-3
-4
log C w a
Figure 2. Ratios of 6 in SDS solutions to those in water for Ni-PC (e) and for DABMI (0).
help to distinguish the contribution of the surfactant and the micelle to the enhancement in thermal lens signal relative to pure water and to gain insight into the enhancement mechanism. The increase in thermal lens signal was immediately evident when pure water was replaced with a 1 .O M SDS micellar solution. The magnitude of the increase in thermal lens signal can be estimated from Figure 1, and it was found to be correlated to the surfactant concentration. The concentration range of surfactant extended from premicellar region through critical micellar concentration (cmc) and up to concentration of about 1 M. When results for two dyes in SDS were compared (Figure 2), no significant difference in the enhancement was observed. Actually, at all concentrations of surfactant, signals never differ by more than the experimental error. It seems that the thermal lens measurement cannot distinguish the difference between the signal of DABMI which is incorporated inside micelles to that of the nickel complex which is repulsed away from micelles into the bulk water. Therefore, micellization cannot be considered the main factor for the surfactant-enhanced thermal lens effect. In addition, there is no significant increase in the enhancement of thermal lens signal relative to water when the concentration of surfactant exceeds the cmc. If micellization would be the only reason for the surfactant-enhanced thermal lens signal, this cmc transition should have produced a pronounced change in the enhancement. The first significant increases were however observed at concentrations of surfactants that are about 30 times higher than the cmc and 1000 times higher than the concentration of DABMI. At higher concentrations of surfactant, thermal lens signals were enhanced even more. Enhancement (relative to water) increased more or less linearly with surfactant concentnations as in the case of electrolytes. Similar but less dramatic increases were also observed for time constants (Figure 3). It is evident that the main reason for the surfactant-enhanced thermal lens effect is the change in thermooptical properties of solution and not the micellization. Micellization does not affect the thermal lens signal the same way as it does fluorescence.'sm Fluorescence kinetics is faster than the exchange rate of monomers between micelles and bulk solution which occurs on a microsecond to millisecond time Conversely, the thermal lens effect which depends on the heat diffusion is much slower than the mentioned dynamic processes in micelles. Consequently, molecules that are presumably incorporated inside micelles experience only (46) Fendler, J. H. Membrane Mimetic Chemistry; Wilcy: New York,
1982.
(47) Hinze, W.L. In Solution Chemistry oJSurfactanrs; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 1, pp 79-127. (48) Hinze, W. L.;Armstrong, D. W.Ordered Media in ChemicalSeparations; American Chemical Society: Washington, DC, 1987. (49) Armstrong, D.W.Anal. Chem. 1987,59, 84A. (SO) Andersson, B.;Olofsson, G . J . Chem. Soc., Faraday Trans. I 1988, 84,4087.
-2
0
-1
log csos
Figure 3. Ratios of 1, in SDS solutionsto those in pure water for Ni-PC ( 0 )and for DABMI (0).
I
I
I
-8
if
c
-4
Ol
I
-4
I
-3
I
-2
If
I
I
-1
0
log C a w .
Figure 4. Effect of surfactants and their concentration on the relative values of B: Brij-35 (---),Triton X-lo0(--), CI2Eb(---), SB-12 (- -), CTAC and SDS (-). (..a),
the average environment of the solution on the time scale of thermal lens formation. On the other hand, on the time scale of the relaxation through emission the environment of such molecules can be taken as that inside the micelles. Consequently, the fluorescence of surfactant solutions is enhanced by micellization, but the thermal lens effect is enhanced because the addition of surfactant to water changes thermooptical properties of the solutions. The effect of surfactants on the thermal lens signal in aqueous solutions is therefore similar to the effect of electrolytes, but much more pronounced. The improvement in the thermal lens signal by surfactants is higher than those by electrolytes. For example, the increase in the thermal lens signal (relative to that in pure water) by a 1.0 M solution of SDS was more than 2-fold higher than the highest enhancement induced by electrolyte at the same concentration, Le., cesium nitrate in Table VII. A variety of surfactants ranging from nonionic (C,,E,, Triton X-100,Brij-35) to cationic (CTAC) and zwitterionic (SB-12) were used in subsequent investigation to fully exploit the effect of Surfactants. The effect of these surfactants on the thermal lens signals is compared to the effect of SDS (Figures 4 and 5 ) . Basically, the effects of surfactants on the enhancement of thermal lens signal and on the time constant (relative to water) are similar, regardless of the nature of surfactant. There is no enhancement in the thermal lens signal in the vicinity of the cmc, and in all cases the enhancement increases with concentration. The highest
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6695
Thermal Lens Effect in Electrolytes and Surfactants
4
3
n 0
< -
n ! ! ' 0 v
.S
1 %r.
-3
-4
-2
-1
log Csur.
Figure 5. Effect of surfactants and their concentration on the relative values of t,. Symbols used are the same as in Figure 4.
I
,
,
,
,
, , .s csur.
,
,
,
,
,
,
(MI
Figure 7. Effect of surfactants and their concentration on the relative values of thermal conductivity (data are presented as k,/k,). Symbols used are the same as in Figure 6.
0
I
1 (M)
Figure 6. Effect of surfactants and their concentration on the relative values of dn/dT: Triton X-100 (--), CI2E6(-.-), CTAC (-), and SDS
(-1. enhancement relative to water was obtained with Triton X- 100 (Le., 8.8 f 0.2 in 1.13 M solution). At this concentration, the ratio of time constants was 1.88 f 0.07. Due to the limited solubility of some surfactants (Brij-35, C12E6),the effects were compared only at the concentrations where all of them are soluble. As illustrated, for a particular concentration, the enhancement in the thermal lens signal increases with the increasing molecular weight of surfactant. For example, the order of enhancements by 0.25 M solutions-SDS, CTAC, SB-12, CI2E6,Triton X-100, Brij-35-is proportional to the molecular weights of these surfactants which are 288.4, 320.0,335.5,450,624, and 1200 g mol-(, respectively. The same order was also found for ratios of time constants. For solutions of surfactants whose heat capacities and densitiesss2 are known, the thermal conductivity and dn/dT relative to those in water were calculated. Results are summarized in Figures 6 and 7. The trends for the increase in ratios of dn/dT (Le., (dn/dr),,,l,/(dn/dr)w) and of the reciprocal of thermal conductivity (i.e., (kmm le/kw)-l) are similar to those for water. again The increases in (ksa,,,b/kw)-land in (dn/d'T),*/(dn/dT), (51) Quirion, F.; Desnoyen, J. E. J. Colloid In?erfuce Sci. 1986, 112, 565. (52) Shvets, V. F.; Makarov, M.G.; Suchkov, Y.P.;Tsivinskii, D. N. Deposited Doc., VlNlTl 2589-81, 12 pp, Avail. VINITI: Moscow, 1981.
increase with concentration. At any surfactant concentration, the increase is higher for surfactants with higher molecular weight. Similar to the case of electrolytes, the change in dn/dT has more effect on the enhancement in the thermal lens signal than the change in the thermal conductivity. Data on dn/dT and thermal conductivity of surfactant solutions are not widely available, and therefore the thermal lens strength cannot be calculated for every solution. The results obtained, however, provide useful information to improve and/or to predict the sensitivityof the thermal lens technique in surfactant solutions. In general, higher molecular weight surfactants should be used to obtain higher sensitivity. Their solubility must however be considered since in general high molecular weight surfactants have relatively lower solubility. The comparison between the effects of surfactants and of electrolytes on thermal lens signal is not complete without considering the effect induced by changes in hydrogen bonding. Unfortunately, for micellar solutions this cannot be evaluated quantitatively as it has been done for solutions of electrolytes. This is because no data are available on the structural temperature of micellar solutions. However, the effect of surfactants on thermooptical properties of the solution (i.e., higher specific refractivity and -dn/dT) seems to be more important than their effect on the hydrogen bonds of water. This deduction can be confirmed by the results obtained for solutions of nonionic surfactants such as Triton X-100. This surfactant is known to be a structure former. In these micelles, some of the CHI groups are not well incorporated inside the micellar core but rather are exposed outside. As a consequence, hydrogen bonds of water around the exposed CH2 groups increase significantly.s3-ss It is anticipated that the increased hydrogen bonding in solutions of Triton X- 100 should lower the thermal lens signal. However, solutions of this surfactant were found to provide much higher thermal lens signals than pure water. Furthermore, the enhancement produced by Triton X-100 is higher than, for example, CTAC, which has chloride anions and is known to be a structure breaker. It is clear that, similar to the case of electrolyte solutions, the change in the structure of water is only a minor factor in the enhancement of the thermal lens effect. It is noteworthy to add that at surfactant concentrations higher than 0.5 M another effect was observed. The effect is characterized by a long time constant (several seconds) and is similar to the effects observed in binary mixtures.s658 We became aware (53) Sharma, V. K.;Bhat, R.; Ahluwalia, J. C. J. Colloid Interfuce Sci. 1986, 112, 195. (54) Frank, H . S.;Evans, M.W . J. Chem. Phys. 1945, 13, 507. (55) Frank, F.; Reid, D. S.In Wuter-A Comprehensive Treutise;Franks, F., Ed.; Plenum Press: New York, 1973; Vol. 2, Chapter 5. (56) Giglio, M.;Vendramini, A. Appl. Phys. Let?. 1974, 25, 555.
66%
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 2.2
> b-
k in z
2.1
k
ll
!\
kJ
2.0
a
1.9
z
w LD
w
I\ f
/
\
I
0
‘
1
100.0
200 0
TIME, seconds
Figure 8. Changes in the probe beam center intensity during and after
the excitation of highly concentrated micellar solutions. of this effect after we observed a continuous decrease in the initial intensities of probe beam for several consecutive thermal lens signals over a period of about 30 s. This effect was fairly reproducible, and it was observed only at higher concentrations of surfactants. Careful investigation provided insight into the origin of this apparent drift in probe laser intensity. The time dependence of probe beam intensity as shown in Figure 8 can easily be divided into several fractions that correspond to different time constants. The initial portion of the curve is a typical decrease in beam center intensity characteristic of the thermal lens effect and is due to dn/dTwith a time constant on the order of milliseconds. However, even at times longer than several time constants, where the thermal lens signal should reach a steady state, the beam center intensity kept decreasing. This decrease is now characterized by a time constant of several seconds. This thermal lens signal originates from the change in refractive index due to the concentration gradient created within the sample by thermal diffusion, i.e., dn/dc. This effect is actually present in every solutionIs6but normally thermal diffusion is canceled out by mass diffusion in the solution of low viscosity and the effect is not seen. In very viscous solutions, such as glycerol” or highly concentrated surfactant solutions, and in binary liquid mixtures near their consolute points,Jbs8 mass diffusion is slow enough to allow the concentration gradient and consequently dn/dc to form. By blocking the excitation beam, it was demonstrated that the resulting additional decrease in beam center intensity is not a drift in probe laser power. A typical thermal lens decay due to dissipation of heat to the environment (r, of few milliseconds) is obsetved immediately after the excitation beam was blocked. This was followed by a much slower decay of concentration gradient until the beam center intensity reached the same value a s before the excitation. (57) Giglio, M.;Vendramini, A. Phys. Reo. Lett. 1975, 31, 561. (58) Hardcastle, F. D.;Harris, J. M. Appl. Spectrose. 1986, 10, 606. (59) Tran, C. D.Unpublished results.
Franko and Tran The observed effect was found to increase with concentration of surfactant (and increase of the viscosity) and provides additional increase in the thermal lens signal. This is, however, impractical for routine analysis because of the relatively long time required for measurements. In addition, fluctuations in laser power, which are difficult to avoid, can be the source of additional experimental error in the application of these phenomena. However, it is important to pay attention to these effects when measuring thermal conductivities and dn/dT in order to avoid any errors even if they are small. Actually, the thermal diffusion could contribute an additional increase in thermal lens signal and alter the time constant on the millisecond time scale. To eliminate this contribution, all solutions were irradiated by the excitation beam until the concentration gradient in the solution reached steady state. This was reflected as a constant value of the initial beam center intensity for several consecutive thermal lens signals. At this point the averaging of thermal lens signals was started. The frequency of excitation (2 Hz)was found to be high enough to maintain a constant contribution to the thermal lens signal from concentration gradient. Therefore, the measured thermal lens signal resulted primarily from the change in the refractive index due to the temperature gradient, Le., dn/dT. Taken together, surfactants were found to be useful additives to enhance the thermal lens signal of aqueous solutions. The improvement in sensitivity of the thermal lens technique obtained by surfactants was found to be several times higher than those obtained by electrolytes and by other additives such as acetone.60 The micellization process was found to have no effect on the enhancement of the thermal lens signal. This is because the formation of the thermal lens is a much slower process than the dynamic processes in micelles.
Summary The thermal lens technique, demonstrated in the present work, provides a sensitive means for the accurate and precise determination of the thermal conductivity and temperature coefficient of the refractive index. The k and dn/dTvalues determined by this method were then used to elucidate the mechanism of the thermal lens enhancement which is induced by electrolytesand/or surfactants. It was found that the contribution of electrolytes and/or surfactants to the dn/dT value of solution has more of an effect on the enhancement in the thermal lens signal than does the change in the thermal conductivity. The latter was found to be on the order of only a few percent. The change in the dn/dT upon the addition of electrolyte or surfactant was deconvoluted into two parts. The first, resulting from the difference in the specific refractivity and dn/dT of the electrolyte or surfactant solution as compared to that of pure water, has a stronger influence than the second part, which is caused by the alteration of the hydrogen-bonding network of liquid water. Acknowledgment. The apthors are grateful to the National Institutes of Health for financial support of this research. (60) Dovichi, N. J.; Harris, J. M.Anal. G e m . 1w9, 51, 728.
