Teoría Cognitivo Social del Aprendizaje

AQ is a neutral analogue to the AQDS sensitizer. It has not been as commonly employed

as the BP sensitizer discussed in Chapter 2 because of its much lower solubility, and therefore,

there is less data on its photophysical properties available in the literature. The complete

insolubility of AQ in room temperature water and its low solubility in surfactant solutions make it

highly likely that the AQ molecule, once solubilized, exists almost exclusively in the micellar

phase. The TREPR of AQ in SDS, DTAC, and CTAC collected at 500 ns delay are shown in

Figure 3.5. There is a strong similarity between all three spectra, which have pronounced first

derivative-like line shapes. The poor S/N ratio relative to the previous spectra is due to the

substantially lower concentration of AQ that could be dissolved in the surfactant solutions. The

S/N ratio on the DTAC spectrum is, in fact, too poor to allow for quantitative simulations, although

it appears similar in line shape to the SDS-AQ spectra.

Comparison of the SDS-AQ and CTAC-AQ simulations accounts for some of the subtle

differences between the two spectra (Figure 3.6). A simulation of the SDS-AQ spectrum (Figure

3.6, top) can be obtained with almost identical set of parameters as the simulations of SDS-BP

presented in Chapter 2, but a greater contribution of TM polarization is observed in the AQ

spectrum. This can be accounted for by simulation results that indicate a slightly faster relaxation

rate for the populations of the triplet excited states of the anthraquinone (krel = 1.28 x 108 s-1 for

AQ vs krel = 2-3 x 10-8 s-1 for BP around room temperature) and an escape rate for the RP from the

micelle of 2.5 x 105 _s-1_{. The presence of a cationic versus and anionic head group seems to make}

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DTAC

CTAC

SDS

20G

Figure 3.5. X-band TREPR spectra acquired 500 ns after a 308 nm laser flash in aqueous solutions of 50 mM DTAC (top), CTAC (middle), and SDS (bottom) with 1.1 mM AQ. The central line in the spectrum is due to the AQ radical and has been cut off to vertically expand the signal of the surfactant alkyl radicals.

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Figure 3.6. X-band TREPR spectra acquired after a 248 nm laser flash in aqueous solutions of SDS (grey) and CTAC (blue) at a delay time of 500 ns using the AQ sensitizer (1.1mM). The solid black line is the best fit simulation of the experimental data using the microreactor model.

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observed most clearly in the signal from the AQ radical. The simulation results for AQ in CTAC

micelles (Figure 3.6, bottom) return a lower limit on the value of krel = 0 s-1 and an escape rate of

the RP from the micelle of 5.1 x 105 s-1. These differences allow for greater accumulation of the semianthraquinone radical in the CTAC spectrum and are most likely related to differences in the

micelle properties.

Based strictly on a comparison the AQDS and AQ sensitizers, introduction of an

electrostatic charge on the sensitizer molecule may be expected to lead to Coulombic attraction or

repulsion that will have profound effects on the diffusive behavior of the RP and the observed

CIDEP in the TREPR spectra. However, the TREPR results of the singly charged, AQS sensitizer

do not exhibit the same clear-cut electrostatic control of diffusion that is observed in for the AQDS

sensitizer. The opposite charges of the AQS sensitizer and the DTAC micelles naturally lead to

the prediction of strong APS in the TREPR spectra, which is indeed observed (Figure 3.7). Minor

differences between line shape of the spectra of DTAC and CTAC with AQS mirror the differences

observed between CTAC and DTAC with AQDS, including a more pronounced contribution of

escape radicals in the CTAC spectrum and broader line widths for the DTAC alkyl radicals. These

are likely due to differences in the micelle aggregation number and size. However, a relatively

strong contribution of SCRP polarization is also observed in the SDS-AQS spectra. In fact, both

the SDS-AQS and CTAC-AQS spectra can be fit with identical simulation parameters (Figure

3.8).

The observation of strong SCRP polarization in both anionic and cationic micellar systems

requires some explanation. The AQS sensitizer is more hydrophilic than the AQ sensitizer, but is

solubilized better in the presence of surfactants than in pure water. The strong similarity between

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DTAC

CTAC

SDS

20G

Figure 3.7. X-band TREPR spectra acquired 500 ns after a 308 nm laser flash in aqueous solutions of 50 mM DTAC (top), CTAC (middle), and SDS (bottom) with 1.1 mM AQS. The central line in the spectrum is due to the AQS radical and has been cut off to vertically expand the signal of the surfactant alkyl radicals.

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Figure 3.8. X-band TREPR spectra acquired after a 308 nm laser flash in aqueous solutions of SDS at a delay time of 500 ns (gray) or CTAC at a delay time of 300 ns (blue) using the AQS sensitizer (1.1mM). The solid black line is the best fit simulation of the experimental data using the microreactor model.

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the observed CIDEP. Despite its charged, AQS must still strongly and preferentially associate with

the hydrophobic interior of the micelle, and this hydrophobic interaction must be strong enough to

overcome the effects of electrostatic repulsion between the SDS micelle surface and the AQS

radical.

An explanation of the similarities between CTAC and SDS must then have its source in

the physical properties of the interior of the micelle. A stable nitroxide spin probe, 2,2,4,4-

tetramethyl-1,2,3,4-tetrahydro-γ-carboline-3-oxyl was incorporated into solutions of the three

surfactants. This particular spin probe was chosen for a comparison because it has a similar

molecular volume to the triplet sensitizers used in the TREPR experiment. The nitroxide group is

marginally hydrophilic, but molecule on the whole is better solubilized in organic solvents or

surfactant solutions. The rotational correlation time of the spin probe was measured by SSEPR and

extracted from the line shape of the SSEPR spectra using a program by Freed et al.44 _{The results}

of the spin probe study are reported in Table 3.3. It is clear that for this particular spin probe, which

has a molar volume closes to that of the sensitizer, the rotational correlation times in SDS and CTAC are very close to one another (τc = 6.4 x 10-10 s and τc = 6.2 x 10-10 s), despite the difference

in the length of the alkyl tail and the size of the micelles of the two surfactants. This implies that,

for molecules of a similar structure, the rotational diffusion inside the SDS and CTAC micelles is

essentially identical. A similarity in the diffusive behavior of AQS in SDS and CTAC micelles

most likely accounts for the striking similarities between the TREPR spectra.

Spin probe studies of three other stable nitroxide molecules were conducted in the SDS,

DTAC, and CTAC systems. In all cases, the rotational correlation times of these spin probes were

more similar for SDS and DTAC micelles than for CTAC micelles. It is important to note,

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Table 3.3 SSEPR results for the rotational correlation times of four spin probes incorporated into micellar solutions of SDS, DTAC, and CTAC

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structure is substantially different from that of Probe 1. Probe 1 has a relatively ridged, three ring

structure. Both of the doxyl-stearic acid probes (probe 2 and 3) are very commonly used to

characterize micelle viscosity by SSEPR. The length of the stearic acid chain, and the attachment

of the nitroxide at different positions along that alkyl chain, make these spin probes particularly

sensitive to the motion of long alkyl chains in the micelle core. The ubiquitous use of these spin

probes to characterize micelles is a result of the expectation that the motion of the stearic acid

chain will be sensitive to or even mimic the motion of the alkyl chains of the surfactant tails when

incorporated into a micelle. These structures, and the diffusive behavior of an alkyl chain in the

micelle, are very different than the diffusive behavior of the much more rigid Probe 1. Probe 4 is

also often used to explore the properties of micelle via SSEPR, but it is important to note that

Probe 4 is relatively hydrophilic. It is often observed to partition between the aqueous phase and

the micelle interior. This partitioning was also observed for this particular spin probe in the SDS,

DTAC, and CTAC solutions. Spectral decomposition into a fast motion component, which

represents the diffusional behavior of the spin probe in the bulk aqueous phase, and a slower motion “micellar” component that reflects the properties of the micellar phase, is necessary. Only the rotational correlation time obtained from the slow motion component is reported in Table 3.3 – but for obvious reasons, this superposition issue renders Probe 4 a less reliable measure of the rotational diffusion or solubilization behavior of AQS in these surfactant solutions.