Identification of Phase Boundaries in Surfactant Solutions via

Article pubs.acs.org/JPCA

Identification of Phase Boundaries in Surfactant Solutions via Compton Spectrum Quenching Denis E. Bergeron* Radiation Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8462, United States S Supporting Information *

ABSTRACT: The critical micelle concentration and the phase boundary between isolated surfactant molecules and aggregates are probed via fluorescence spectroscopy and a Compton spectrum quenching technique for aqueous and toluenic solutions of Triton X-100 (TX-100). The internal fluorophore of TX-100 provides a convenient probe for the fluorescence measurements, and the appearance of redder bands in the fluorescence spectra and their relationship with aggregation (clustering of TX-100) phenomena is addressed. The Compton spectrum quenching approach makes use of quench indicating parameters (QIPs) commonly measured in liquid scintillation counting experiments. Phase boundaries identified by the QIP-based approach are in excellent accord with the fluorescence-based approach. micellar solutions based on TX-100.6 In addition, the phenyl moiety of this surfactant (TX-100 is 4-(1,1,3,3tetramethylbutyl)phenylpoly(ethylene glycol)) provides an intrinsic fluorescence probe for “non-invasive” fluorescence spectroscopy-based cmc determination.10−12 As such, we are able to benchmark our Compton spectrum quenching cmc determinations against fluorescence spectroscopy through a relatively simple set of measurements.

1. INTRODUCTION Our interest in reverse-micellar systems stems from investigations of the micelle size effect on liquid scintillation counting (LSC) efficiencies.1−7 A precise knowledge of the counting efficiency is essential to fundamental measurements of radioactivity, so any loss of energy by a decay product (e.g., a βparticle or Auger electron) that does not produce scintillation light introduces a potential problem in radionuclide metrology. While we recently demonstrated that the micelle size effect is extremely small,7 it is clear that other micelle effects, including changes in transmission of scintillation photons,8 will still be important in LSC experiments. The most drastic changes of this type might occur when crossing a boundary such as a critical micelle concentration (cmc) separating two optically distinct phases. In our previous study,6 we identified what appeared to be such a phase boundary for a commonly used scintillant. In a surfactant solution, aggregation of surfactant molecules into fairly large micelles occurs at the cmc, and above this surfactant concentration additional surfactants form additional micelles with the monomer concentration being preserved at approximately cmc. Techniques for cmc determination abound,9 but for the researcher engaged in a LSC experiment, instrumentation for spectroscopy, light or neutron scattering, viscometry, etc., may not be readily accessible. We describe here a technique for cmc determination that is convenient in the contexts of a LSC experiment since it utilizes the liquid scintillation counter itself for the measurements. The technique relies on the quench indicting parameter (QIP) determined in LSC experiments from measurements of Compton electron spectra generated by an external γ-ray source. In order to establish that Compton spectrum quenching can be used to find cmc, we performed a series of measurements on solutions containing the well-studied nonionic surfactant, Triton X-100 (TX-100). We have some experience with This article not subject to U.S. Copyright. Published 2014 by the American Chemical Society

2. EXPERIMENT 2.1. Sample Preparation. Several series of samples with varying fractions of TX-100 (Sigma-Aldrich, St. Louis, MO, USA),a toluene (Sigma-Aldrich; ≥99.9%), and deionized distilled water were prepared gravimetrically; additions were made via micropipet, and weights of addition were measured on an AT-20 electronic microbalance (Mettler Toledo, Columbus, OH, USA). All reagents were used as-delivered. No special efforts were made to dry the toluenic solutions. Complete details of the sample compositions are given in the Supporting Information, but a summary is provided in Table 1. The concentration ranges were selected based on literature values to include the known phase boundaries.6,10,12−15 The X12 and X25 sample series achieved variation in ω0,T (the molar ratio of water to TX-100) by varying the amount of added water and holding [TX-100] essentially constant. [TX-100] is calculated by dividing the number of moles of TX-100 by the total volume (in liters) of the sample. The TW-S and TW-h series, on the other hand, achieved variation in ω0,T by holding the aqueous fraction essentially constant and varying [TX-100]. Samples were also prepared with the commercial scintillant, Ultima Special Issue: A. W. Castleman, Jr. Festschrift Received: March 13, 2014 Revised: May 14, 2014 Published: May 16, 2014 8563

dx.doi.org/10.1021/jp502540n | J. Phys. Chem. A 2014, 118, 8563−8571

The Journal of Physical Chemistry A

Article

Table 1. Summary of the Samples Used in the Experimentsa series ID

Ns

TX-aq

15

TX-tol

15

X12 X25 TW-S

16 15 10

TW-h

10

[TX-100]/(mol·L−1) 1.1 × 10−4 to 7.5 × 10−4 2.8 × 10−5 to 1.3 × 10−4 1.7(1) × 10−1 4.2(2) × 10−1 4.7 × 10−2 to 2.4 × 10−1 7.8 × 10−2 to 3.2 × 10−1

3. RESULTS 3.1. Fluorescence Spectroscopy. 3.1.1. TX-aq. Aqueous solutions of TX-100 have been studied extensively by a number of methods. Recently, researchers have taken advantage of the fluorescence of TX-100 itself to perform non-invasive cmc determinations.10−12 Anand et al. reported cmc = 2.24 × 10−4 mol·L−1 based on their estimate for the break point in the curve describing the change in fluorescence intensity with increasing [TX-100] in aqueous solutions.12 In our studies, we collected emission spectra for each sample with λEX = 275 and 325 nm. In addition, we collected excitation spectra for each sample with an open emission window and with λEM = 300 and 365 nm. Figure 1A shows the excitation

ω0,T

solvent water

8.3 × 104 to 5.3 × 105

toluene

0

toluene toluene toluene

1.9 × 10−1 to 9.9 1.8 × 10−1 to 8.8 2.1 × 10−1 to 1.1

toluene

3.1 × 10−1 to 1.3

a Each series contains Ns samples. ω0,T is defined as the molar ratio of water to TX-100.

Gold AB (UGAB; PerkinElmer, Waltham, MA, USA), with the range of water fractions selected to cover the previously observed cmc.6 All samples were prepared in 20 mL glass scintillation vials, and LS counting preceded fluorescence measurements. 2.2. Fluorescence Measurements. Fluorescence spectra were collected in excitation and emission modes using an Hitachi F-7000 fluorescence spectrophotometer. Quartz fluorescence cuvettes with 1 cm path length were used, with typical scan rates of 4 nm·s−1 and 1 nm slit widths for excitation and emission. No inner filter corrections were made since observed excitation wavelength dependences of the spectra precluded the application of simpler correction techniques.16 The uncorrected data showed clear indicators of phase transitions. For each sample, excitation spectra were collected at several emission wavelengths (λEM) and emission spectra were collected with several excitation wavelengths (λEX). 2.3. Compton Spectrum Quenching Measurements. Samples were measured on three different liquid scintillation counters: a Wallac Guardian 1414 (PerkinElmer, Wesley, MA, USA), a Packard Tri-Carb A2500TR (PerkinElmer), and a Beckman Coulter LS6500 (Beckman Coulter, Fullerton, CA, USA). Each of these counters is equipped with an internal γradiation source (152Eu, 133Ba, and 137Cs, respectively) to produce Compton electrons in a sample. Measurement of the Compton spectrum reveals information on the degree to which a liquid scintillation cocktail (where a cocktail comprises the scintillant and any added analyte, carrier, quenching agent, etc.) is quenched, and each of the three counters presents data in terms of a quench indicating parameter (QIP). The Wallac counter reports the QIP in terms of SQP(E), which corresponds to the end point of the Compton spectrum.17 The Packard counter reports the QIP in terms of tSIE, which refers to the “special index of the transformed external standard spectrum”; this corresponds to the energy bin which is intersected by the extrapolation of a line drawn between the points corresponding to 20% and 10% of the total counts in the Compton spectrum.18 The Beckman counter reports the QIP in terms of the Horrock’s number (H#), which is the inflection point at the Compton edge.19 The absence or presence of micelles or premicellar aggregates in a sample will affect QIPs if the scintillation efficiency, the transmission of optical (scintillation) photons, or the chemical quenching of fluorescence is affected. We expect that all of these factors will contribute to varying degrees to make QIPs sensitive to micellar phase boundaries.

Figure 1. (A) Excitation (blue) and emission (red and pink) spectra of a 1.1 × 10−4 mol·L−1 aqueous solution of TX-100 (open circles) (where, for example, “2.E-04” represents 2 × 10−4). The excitation spectrum was collected with an open window for the fluorescence. The emission spectra were collected with λEX = 275 nm (red) and λEX = 325 nm (pink). For ease of comparison, the spectra are normalized to a maximum intensity of 1 and the weaker and λEX = 325 nm spectrum has been smoothed with a three-point running average. (B) Total integrated fluorescence emission intensity from excitation spectra taken with λEM = 300 nm (diamonds) and 365 nm (open circles). For ease of comparison, the data are normalized by dividing the integrated emission intensity at each point by the maximum integrated emission intensity in the series.

and emission spectra collected for the TX-aq sample with the lowest [TX-100] (1.1 × 10−4 mol·L−1). In general, the spectral features were consistent with those reported by Anand et al.12 and Om et al.,10 including the appearance of bands to the red of the main features, becoming more prominent with [TX-100] > cmc. These bands can be ascribed to transitions from (in the excitation spectra) or to (in the emission spectra) higher vibrational levels of the ground state. It may be that the Franck−Condon factors (FCFs) for these vibronic transitions are affected as a result of geometric perturbations to the phenyl moieties of TX-100 wrought by aggregation; the local environment of the fluorophore is altered in the formation of premicellar aggregates (clusters of TX-100) or micelles. 8564



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We attempted to exploit the redder bands in our analysis of the fluorescence spectroscopy data. Figure 1B shows the increase in total integrated fluorescence emission intensity from excitation spectra collected with λEM = 300 and 365 nm. As Figure 1A shows, the emission peak is centered near 300 nm. In the emission spectra collected with λEX = 325 nm (to the red of the main band in the excitation spectrum), we observed a definite peak at 365 nm (shown in the pink trace in Figure 1A). Since we expected to excite only complexed (premicellar or micellar) TX-100 with this λEX, we interpreted the peak at 365 nm as owing solely to complexed TX-100. Thus, it is not surprising that the discontinuity at cmc in Figure 1B is much more pronounced with λEM = 365 nm than with λEM = 300 nm. We fit curves such as the ones in Figure 1B for all of our fluorescence data sets, and found that the intersection of linear least-squares fits for the lowest and highest [TX-100] data points to be reasonably robust with respect to the subjective assignment of data points to one curve or the other. We used seven different schemes in which points were assigned to one curve or the other in different combinations, applied them to the five separate intersect determinations (two emission data sets with λEX = 275 and 325 nm and three excitation data sets with λEM = open, 300 nm, and 365 nm), and found that the standard deviation on the average intersect values calculated with the seven different schemes was cmc. At the time, the authors noted that no lower limit on the amount of water required for reliable LSC results had been previously mentioned, and pointed out that, “the danger with this effect is that had the same aqueous fractions been used in all of the experiments...the reported value would have been incorrect by about 1.4 % and there would have been no way to know otherwise.”32 Given the potential for system instability, the ideal LSC experiment should always be performed with cocktail compositions that avoid micellar phase boundaries. For UGAB, this requires f > 0.05.

the materials or equipment identified are necessarily the best available for the purpose.

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(1) Carles, A. G. Micelle, the Micelle Size Effect on the LS Counting Efficiency. Comput. Phys. Commun. 2007, 176 (4), 305−317. (2) Carles, A. G. Analysis and Simulation of the Relative Lethality of Auger-Electron-Emitting Radionuclides with a Liquid-Scintillation Counter. Int. J. Radiat. Biol. 2007, 83 (9), 617−623. (3) Carles, A. G. Simulation of the Relative Damaging Effects of Auger Cascades with Gel Scintillators. Int. J. Radiat. Biol. 2008, 84 (12), 1057−1062. (4) Kossert, K.; Carles, A. G. Study of a Monte Carlo Rearrangement Model for the Activity Determination of Electron-Capture Nuclides by Means of Liquid Scintillation Counting. Appl. Radiat. Isot. 2008, 66 (6−7), 998−1005. (5) Kossert, K.; Carles, A. G. Improved Method for the Calculation of the Counting Efficiency of Electron-Capture Nuclides in Liquid Scintillation Samples. Appl. Radiat. Isot. 2010, 68 (7−8), 1482−1488. (6) Bergeron, D. E. Determination of Micelle Size in Some Commercial Liquid Scintillation Cocktails. Appl. Radiat. Isot. 2012, 70 (9), 2164−2169. (7) Bergeron, D. E.; Laureano-Pérez, L. Micelle Size Effect on Fe-55 Liquid Scintillation Efficiency. Appl. Radiat. Isot. 2014, 87, 282−286. (8) Santiago, L. M.; Bagan, H.; Tarancon, A.; Rauret, G.; Garcia, J. F. Systematic Study of Particle Quenching in Organic Scintillators. Nucl. Instrum. Methods Phys. Res., Sect. A 2013, 698, 26−36. (9) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wile & Sons: New York, 1997. (10) Om, H.; Baker, G. A.; Bright, F. V.; Verma, K. K.; Pandey, S. Noninvasive Probing of Aqueous Triton X-100 with Steady-State and Frequency-Domain Fluorometry. Chem. Phys. Lett. 2007, 450 (1−3), 156−163. (11) Om, H.; Baker, G. A.; Behera, K.; Kumar, V.; Verma, K. K.; Pandey, S. Self-Probing of Micellization within Phenyl-Containing Surfactant Solutions. ChemPhysChem 2010, 11 (12), 2510−2513. (12) Anand, U.; Jash, C.; Mukherjee, S. Spectroscopic Determination of Critical Micelle Concentration in Aqueous and Non-Aqueous Media Using a Non-Invasive Method. J. Colloid Interface Sci. 2011, 364 (2), 400−406. (13) Hinze, W. L. Utilization of Surfactant Systems in Chemical Separations. Ann. Chim. (Rome, Italy) 1987, 77 (1−2), 167−207. (14) Rodríguez, R.; Vargas, S.; Fernandez-Velasco, D. A. Reverse Micelle Systems Composed of Water, Triton X-100, and Phospholipids in Organic Solvents1. Phase Boundary Titrations and Dynamic Light Scattering Analysis. J. Colloid Interface Sci. 1998, 197 (1), 21−28. (15) Hait, S. K.; Moulik, S. P. Determination of Critical Micelle Concentration (Cmc) of Nonionic Surfactants by Donor-Acceptor Interaction with Iodine and Correlation of Cmc with HydrophileLipophile Balance and Other Parameters of the Surfactants. J. Surfactants Deterg. 2001, 4 (3), 303−309. (16) Kubista, M.; Sjoback, R.; Eriksson, S.; Albinsson, B. Experimental Correction for the Inner-Filter Effect in FluorescenceSpectra. Analyst 1994, 119 (3), 417−419. (17) Instrument Manual: Wallac 1414 Winspectral; PerkinElmer: Waltham, MA, USA, 1996. (18) PerkinElmer http://www.perkinelmer.com/Resources/ TechnicalResources/ApplicationSupportKnowledgebase/radiometric/ quench.xhtml (accessed Feb. 20, 2014). (19) L’Annunziata, M. F. Handbook of Radioactivity Analysis; Elsevier Science & Technology Books: Amsterdam, 2012. (20) Taylor, B. N.; Kuyatt, C. E. Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, NIST Technical Note 1297; U.S. Government Printing Office: Washington, D.C., USA, 1994. (21) Kresheck, G. C. The Temperature Dependence of the Heat Capacity Change for Micellization of Nonionic Surfactants. J. Colloid Interface Sci. 2006, 298 (1), 432−440.

5. CONCLUSION We found values of [TX-100] for cmc in aqueous and toluenic solutions using fluorescence spectroscopy and discussed the emergence of vibronic bands to the red of the main feature coinciding with aggregation and micellization. We also studied toluenic TX-100 solutions to which water was added, identifying phase boundaries via fluorescence spectroscopy and a Compton spectrum quenching method. The results were consistent between the methods and consistent with previous determinations.6,14 The QIP-based technique for identifying micellar phase boundaries should be thoroughly convenient for researchers engaged in LSC experiments. We identified such a boundary at f = 0.034(3) for a common scintillant, Ultima Gold AB, using three liquid scintillation counters. This determination is consistent with dynamic light scattering observations6 that indicate a premicellar phase at f < 0.048 and provides some explanation for the observation32 of a measurement bias in the activity standardization of 63Ni.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 lists more compositional details on the samples measured in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks to Diane S. Hunter (Beckman Coulter) for providing information on the photomultiplier tubes in the LS6500 and to Timothy G. Wright (University of Nottingham) and Ron Collé (NIST) for useful discussions. Thanks to Brian E. Zimmerman, Michael P. Unterweger, Jeffrey T. Cessna, and the SGAH NICU group for granting me time to devote to this project.

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ADDITIONAL NOTE Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation by the National Institute of Standards and Technology, nor does it imply that a

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(22) Hamamatsu, http://www.datasheetlib.com/datasheet/815789/ r3048_hamamatsu-photonics.html?page=2#datasheet (accessed Mar. 7, 2014). (23) Herzberg, G. Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules; D. Van Nostrand Co.: Princeton, NJ, USA, 1966. (24) Sharf, B.; Honig, B. Comments on Vibronic Intensity Borrowing. Chem. Phys. Lett. 1970, 7 (1), 132−136. (25) Small, G. J. Herzberg-Teller Vibronic Coupling and Duschinsky Effect. J. Chem. Phys. 1971, 54 (8), 3300−&. (26) Orr, G.; Small, G. J. Vibronic Intensity Borrowing and the Isotope Dependence of Vibronic Structure. Chem. Phys. 1973, 2 (1), 60−68. (27) Menapace, J. A.; Bernstein, E. R. Calculation of the Vibronic Structure of Solute Solvent van der Waals Clusters. J. Phys. Chem. 1987, 91 (10), 2533−2544. (28) Bergeron, D. E.; Ayles, V. L.; Richards, O. J.; Wright, T. G. Electronic Spectroscopy of Para-Fluorotoluene Clusters. Chem. Phys. Lett. 2006, 430 (4−6), 282−286. (29) Grau Malonda, A.; Garcia-Torano, E. Evaluation of Counting Efficiency in Liquid Scintillation-Counting of Pure Beta-Ray Emitters. Int. J. Appl. Radiat. Isot. 1982, 33 (4), 249−253. (30) Coursey, B. M.; Mann, W. B.; Malonda, A. G.; Garcia-Toraño, E.; Arcos, J. M. L.; Gibson, J. A. B.; Reher, D. Standardizatin of C-14 by 4-Pi-Beta-Liquid Scintillation Efficiency Tracing with H-3. Appl. Radiat. Isot. 1986, 37 (5), 403−408. (31) Collé, R. Cocktail Mismatch Effects in 4 Pi Beta Liquid Scintillation Spectrometry: Implications Based on the Systematics of H-3 Detection Efficiency and Quench Indicating Parameter Variations with Total Cocktail Mass (Volume) and H2O Fraction. Appl. Radiat. Isot. 1997, 48 (6), 833−842. (32) Zimmerman, B. E.; Collé, R. Standardization of Ni-63 by 4 Pi Beta Liquid Scintillation Spectrometry with H-3-Standard Efficiency Tracing. J. Res. Natl. Inst. Stand. Technol. 1997, 102 (4), 455−477.

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Identification of Phase Boundaries in Surfactant Solutions via

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