Absorption Spectroscopy in Homogeneous and Micellar S. Sadiq Shah and Leonard G. Henscheid Washington State University, Pullman, WA 99164
In view of the growing interest of chemists in carrying out a variety of theoretical and experimental studies of systems in micellar media, we introduced the following experiment in second semester undergraduate physical chemistry laboratory. The experiment has successfully helped students learn the principles of absorption spectroscopy, the effect of solvent polarity on absorption spectra and some micellar chemistry. The dye selected for the procedure is a merocyanine dye l-methyl-4-[(oxocyclohexadienylidene)ethylidene]1,4-dihydropyridine hereafter referred to as MOED. This dye can he synthesized easily ( 1 ) with good yields and is stable for an extended period of time. A handout describing the experiment, the theory of ahsorption spectroscopy, micellar chemistry, and the effect of solvent polarity on the spectrum, is available from the authors. Surfactant molecules undergo self-association resulting in a colloidal aggregate of high molecular weight called a micelle. Micelles exist in a dvnamic eouilibrium with surfactant monomers. Self-association of surfactants results from hydrophobic interaction between the nonpolar hydrocarbon chains of the surfactant molecules. The concentration of the snrfactant solution at which the molecules self aggregate to form spherically shaped micelles is called the critical micelle concentration (CMC). Also it is believed that water penetrates the micelles up to at least seven carbon atoms. The discussion of various aspects of the micellar structure and the dynamics is given in detail in references under (2). The hydrophohic molecules can reside anywhere starting from the purely nonpolar inner-core out to the partially pol& surface hydrophobic sites. Several techniques have been successfullv used to de-
mation of the micelles. Almost no solubility increase is ohserved until the CMC of the surfactant is reached, hut above the CMC the increase in solubility of the substrate is directly proportional to the concentration of the snrfactant over a certain range. This increase of soluhilitv of the solute in micellar medium is due to the soluhilization of the substrate molecules into the micelle. This Drocess of the oenetration of the solute molecule from a pureiy aqueous enGironment to a less aqueous environment, or nonaqueous environment in the micelle can he detected by the use of UV-VIS absorption soectroscoov. Chances in the wavelength of maximum ah-
s,,
1
u (gar1
(cl
The effect of solvation stabilization of the(a)excited state. (blground state, and (c)both ground and excited States and S' > S'.
a wav that the around state of the solute molecules undergoes solv&ion s t a b i b t i o n relative to thegas phaseground state. The excited state is shown to undergo a similar solvation effect. These effects would either decrease or increase the transition enerm, Er.This change in the transition energy is dut! to change* in cithrr the sulviitiun tmergy o t the ground t i l w or the errilcd .ztalt.,ur hulh it' the idvaticm irvhilization of the excited state is increased, a decrease of Er,would occur and produce a red shift [Fig. ](a)] of the absorption band. Stabilization of the ground state would lead to an increase of ET, or a blue shift [Fig. l(h)]. Whether a blue or a red shift occurs depends ultimately on the relative shift of the ground and excited states [Fig. I(c)j. Brooker (4) suggested that the MOED molecule has a strongly polar ground state and a nonpolar excited state. From a consideration of the two resonance forms of MOED (I),a strongly polar ground state (Ih) is predictable on the hasis of the additional stabilization acquired due to the presence of the henzenoidal rings. Lack of aromaticity of the rings in (Ia)
-
garding the mode of soluhflization bf the substrate molecule. Absorption spectra of merocyanine dyes are known to show shifts with changes in the solvent polarity characteristic A,, (1,4,5). These shifts in the absorption maxima of the spectra of MOED are known to he due to the interactions of both the le with pdar ground and 1 . x ~ited i ~ ; u e a, r i rhr h o l ~ ~mdecule nonoular , ~ I V E I I I m d v i u l r . F r e q u r m . ~shills trft he .;ohtion spe&um are presented in the figure. 1n solution the solvent molecules are oriented around the solute molecules in such
causes this resonance form to undergo destabilization relative to (Ih), and this form can he associated with the nonpolar excited state (4). Recent literature support.? the assertion that the ground state is more polar than the excited state, since upon excitation, the dipole moment of MOED decreases by about 5.5D (6). Experimental The visible absorption spectrum of MOED in pure pyridine, ethanol, and 0.1 M aqueous Sodium Dodecyl Sulfate (SDS)was recorded
Volume 60
Number 8
August 1983
685
Transition Energies of MOED Soiutlons in Various Solvents Solvent
Color
water ethanol pytidine SDS
yellow red blue yellow
A-
(nm) 442 510 603 466
E, X lo'* (J) 4.46 3.90 3.28 4.27
students are required to calculate the transition energies from the experimentally determined A,. values and to explain the shifts in ,A in terms of the interaction between the ground state and excited state of the dye and the solvent molecules as a function of the solvent polarity. By using the information from the pyridine and ethanol solutions they are asked to predict the approximate site of solubilization of MOED in a SDS micelle.
Literature Cited using a Beekman DB Spectrophotometer. Stock solutions of MOED (5 X 10V M )in ethanol and ovridine were f i t h e r diluted to 5 X M for mnkine the nhsorntion measurements. To aoueous stack solution f 1 " 1 - i f , I i UaUH w n i ad& u n r ~ lthe lrnrlr
