Site-selective Probing of Surfactant Assemblies using Excited State Proton Transfer of Pyranine

Site Selective Investigation of Surfactant Compounds Using Excited State Proton Transfer of Pyranine” is an authentic record of the results obtained from the research work carried out under my supervision in the Department of Chemistry, Indian Institute of Technology Guwahati, Assam, India, was carried out. The effect of cosurfactant on the hydration at the interfacial region of a quaternary water/CTAB/octanol/cyclohexane reverse micelle is discussed in chapter 5.

Introduction

In Chapter 3, we investigated the effect of confinement on the water pool solubility behavior of the water/AOT/n-heptane reverse micelle toward a so-called n-octanol-insoluble alcohol. The interfacial hydration modification associated with the rod-to-sphere structural transition of the water/DDAB/cyclohexane reverse micelle is discussed in Chapter 6.

Experimental methods

We also confirmed the presence of HPTS at the interface of the reverse micelle by 2D NOESY NMR and fluorescence anisotropy. Our results indicate penetration of the water molecules into the interface of cationic tertiary reverse micelles with increase in hydration level.

Figure 3.1: Schematic representation of ESPT modulation in confined and quasi- quasi-confined water with incorporation of n-octanol in the water/AOT/n-heptane reverse micelle

Confinement Induced variable solubility of n-octanol inside the aqueous core of

Is the interface of the reverse micelle dry or wet? Insights from ESPT of HPTS

This was attributed to a less hydrated interface of the zwitterionic micelles compared to that of the cationic one. The interface of zwitterionic sulfobetaine micelle is found to be more hydrophobic and rigid than that of the cationic micelle.

Figure 5.1: Schematic representation of the interface of water/CTAB/n-octanol /cyclohexane quaternary RM and the ESPT and fluorescence anisotropy results

The Impact of co-surfactant on the interfacial hydration of cationic quaternary

How does interfacial hydration alter during rod to sphere transition in

In this chapter, we extended our ESPT study to correlate interfacial packing and reverse micelle hydration of didodecyldimethylammonium bromide (DDAB/water/cyclohexane). This is an interesting entity that undergoes a structural transition from rod to spherical shape above a certain value (w0 ~ 8).

How do the interfacial properties of zwitterionic sulfobetaine micelles differ

Excited State Proton Transfer
HPTS as multifunctional probe

ESPT of HPTS in Water
ESPT of HPTS in solvent mixture
ESPT of HPTS inside Confined Assemblies

Macrocyclic hosts: Cyclodextrin and Curcubit[n]uril
Protein and protein-surfactant complex
Membranes, surfaces and porous materials
Micelles and Reverse Micelles
Objectives of the Thesis

K.; Sahu, K.; Ghosh, S.; Sen, P.; Bhattacharyya, K.: Excited-State Proton Transfer from Pyranine to Acetate in γ-Cyclodextrin and Hydroxypropyl γ-Cyclodextrin. Singha, D.; Barman, N.; Phukon, A.; Sahu, K.: Selective investigation of reverse micelle interfacial layer on silver nanoparticle formation using dynamic Stokes shift measurements.

Figure 7.1: Schematic comparison of the interfacial properties between zwitterionic (SB12 and SB16) and cationic (DTAB and CTAB) micelles

Steady-state measurements

Time correlated single photon counting (TCSPC)

Single wavelength decay of the protonated and deprotonated emission
Bimodal distribution of HPTS: Matching decay of the protonated form with
Time Resolved Emission Spectra (TRES) and Time Resolved Area
Time Resolved Fluorescence Anisotropy Decay

Wobbling in Cone Model (WIC)

Nuclear magnetic resonance spectroscopy

Dynamic light scattering (DLS) measurements

Materials used

Preparation of Reverse micelle and Micelle solutions

Introduction

Although the effect of different alcohols on the structure and properties of reverse micelles has been studied for a long time, 232-234 the distribution of alcohols into the water pool or at the interface has only been analyzed in terms of their bulk miscibility properties. Water soluble short chain alcohols (methanol, ethanol, propanol) assumed to be mixed in the RM water pool. However, long-chain alcohols that are immiscible with water (e.g., n-heptanol, n-octanol, and higher alkanols) would avoid the water pool and would mainly act as cosurfactants that partition to the interface.235 Furthermore, n-octanol was also used as a continuous solvent in a water/AOT/n-octanol reverse micelle due to its immiscibility with water236.

To focus our study on the water core, we used 8-hydroxypyrene-1, 3, 6-trisulfonate (HPTS or pyranine) as a molecular probe due to its high negative charge (-3 and -4, respectively, in the protonated and deprotonated. forms) and thus, sits in the center of the water pool surrounded by negative AOT headgroups (Scheme 3.1). Here we observe the breakdown of the usual proposition of the water immiscibility of n-octanol at a certain degree of isolation in the reverse micelle-enclosed pool of water. The highly negative HPTS is forced to stay in the center of the water pool due to the repulsion from the negative AOT interface and is suitable to selectively probe the water pool microenvironment.

Results

Steady-State Absorption and Emission Spectroscopy
Time-resolved fluorescence
Dynamic light scattering

At lower pH, the absorption of the protonated form dominates (absorption maximum ~ 403 nm), and at higher pH, the absorption of the deprotonated form (absorption maximum ~ 450 nm) becomes gradually prominent. The ratio between protonated emission (ROH, 440 nm) and deprotonated emission (RO−, 510 nm) varies linearly with alcohol. The emission spectra clearly show an increase in the intensity of the protonated emission band (at 440 nm) with a simultaneous decrease of the deprotonated emission (at 510 nm); gradually by increasing the molar ratio of alcohol to water both for water encapsulation (w0 = 5 and w0 = 10 reverse micelle) by incorporating a medium-chain alcohol (ethanol) into the reverse micelle (Figure 3.4, left panel).

Another possibility is that n-octanol, which is immiscible with water, actually becomes miscible in the highly confined water of the water pool (at w0 = 5) but remains immiscible with quasi-immiscible water (at w0 = 10). In this case, the distribution of n-octanol into the nucleus directly disrupts the H-bonding network of water present in the water pool, similar to the case of ethanol (highly soluble in water). We can estimate the approximate dimension of the water pool and more precisely how far the HPTS molecule is from.

Figure 3.1: Effect of pH variation on the absorption spectrum of HPTS. At lower pH the absorption of the protonated form (absorption maximum ~ 403 nm) dominates but at higher pH the absorption of the de-protonated form (absorption m

Summary and Conclusions

Introduction

Results

Steady-state Absorbance and Emission Spectroscopy
Time-resolved emission measurements
Kinetic isotope effect
Dynamic light scattering
Fluorescence anisotropy decay
NMR spectroscopy

Introduction

Results

Time-Resolved Fluorescence
Fluorescence Anisotropy Decay

Summary and Conclusion

Introduction

A micelle or reverse micelle has a different shape and structure depending on the nature and concentration of surfactant molecules.206 For example, it is well established that the AOT molecule acquires a spherical orientation and swells with increasing water content.255 On the other hand, sodium bis-(2 -ethylhexyl) phosphate (NaDEHP) readily forms a giant rod assembly. 256,257 Considered atom simulation studies indicate conformational changes of the AOT reverse micelle from their original spherical to an elliptical shape. 258,259 Interestingly, didodecyldimethylammonium bromide (DDAB) provides an attractive reverse micelle assembly, which undergoes a structural transition from a rod-like to a spherical shape when the amount of water loading, w0= ([water]/ [surfactant]) exceeds a certain value (w0 ~ 8)260 Many studies on the involvement of structural influence on the control hydration properties of these assemblies are explored, as any change in these properties results in a modulation of the structure or nature of a molecule such as a protein. It is also reported that the microemulsion structure of DDAB is critically dependent on composition or volume fraction (ϕ) = (V𝑠𝑢𝑟𝑓𝑎𝑐𝑡𝑎𝑛𝑡+V𝑤𝑎𝑡𝑒𝑟)/ (V𝑢𝑟𝑓𝑎𝑐𝑡𝑎𝑛𝑡 + V𝑤𝑎𝑡 𝑒𝑟 +V𝑛𝑜𝑛−𝑝𝑜𝑙𝑎𝑟), where V𝑖 denotes the volume of the polar component type in question (Table 6.2). . For this purpose, anionic HPTS is a suitable ESPT probe, as it selectively localizes in the interface. Thus, this study can provide valuable insight into the interlayer hydration behavior of different waters.

As shown in Scheme 6.1, in the smaller reverse micelle (w0 ≤ 8), water molecules, like our selected probe, distribute the HPTS into two distinct regions, i.e. Structures of DDAB/water/cyclohexane reverse micelle at low hydration (w0 ≤ 8) with two possible interfacial regions viz. less hydrated cylindrical and more hydrated hemispherical regions. In the higher (eg, w0 = 12) water content, spherical micelles are present and the two regions can be mixed together.

Results

There is no absorption band at ∼450 nm indicating the absence of the deprotonated form in the ground state. Emission spectrum of HPTS in water (dashed line) is also included for comparison. b) The ratio metric plot of the deprotonated to the protonated emission intensity vs. The lifetime of the deprotonated form (τ𝑓′) is given by the decay component (5.4 ns) of the deprotonated emission (Table 6.1).

The two different populations of the protonated species can be realized in the following way. Solvation dynamics fits to parameters of the protonated form of HPTS in water/DDAB/cyclohexane reverse micelle system at different w0 values. The plots of the variation of the anisotropy parameters (amplitude, anisotropy decay time constants, semi-cone angles and diffusion coefficients) with w0, inside the DDAB RM.

Figure 6.2: (a) Emission spectra of HPTS inside DDAB RM at various w 0 values (λ 𝑒𝑥 = 390 nm)

Introduction

Zitterionic surfactant molecules possess two different opposite charges, separated by a spacer in their hydrophilic head group. For example, they exhibit excellent biocompatibility and biodegradability, good water solubility and can withstand higher temperatures compared to common surfactants.213-215 Consequently, zwitterionic surfactants are preferred in cosmetic, biomedical and pharmaceutical applications and in chemical reactions.275-278 of zwitterionic micelle possesses cationic nature despite the presence of both cationic and anionic hydrophilic head group. Because of this special arrangement, the interface has a greater tendency to preferentially adsorb external anions.214 Excited state proton transfer (ESPT) can provide useful information about the nature and dynamics of water within this interface.

The zwitterionic sulfo-betaine wetting molecules possess two oppositely charged head groups linked by alkyl chain (C3 in this case) arranged in L-shaped pattern, exposing the cationic head group to the interface similar to cationic alkyl-ammonium wetting composition. In this chapter we discussed our study on the nature of zwitterionic 3-(dodecyldimethylammonio)-propanesulfonate (SB12) and 3-(hexadecyldimethylammonio)-. The emission nature of HPTS is sensitive to local hydration inside micelles and reverse micelles and may be a better alternative to Nome ESPT probe279 due to its stronger photoacidity than 2-naphthol. Furthermore, HPTS with three and four negative charges in the protonated and deprotonated forms, respectively, serves as a prototype of large anion that can be accommodated at the interface.

Results

Fluorescence Anisotropy

ESPT is also significantly suppressed with the increase in the tail length of the surfactant (Figure 7.4). The fluorescence decay inside the SB micelles is relatively slower compared to that of the cationic micelles of identical chain length (Figure 7.5). A clear ultraslow rise component is observed in the deprotonated emission transients indicative of the ESPT phenomena.

Thus, the ESPT dynamics in the SB micelles are more delayed than those of the corresponding cationic micelles. Notably, the relative contributions of the protonated and deprotonated emission bands differ for different micelles. The results indicate that the interface of the sulfobetaine micelles is more compact compared to the cationic micelles with identical tail length.

Figure 7.2. Emission spectra of HPTS (8 µM) in the presence of different surfactants (a) DTAB, (b) SB-12, (c) CTAB and (d) SB-16 (𝜆 𝑒𝑥 = 390 nm)

Effect of Counter Ions

Summary and Conclusions

Proton transfer to a soft base. M.: Conformational-locked chromophores as models of excited-state proton transfer in fluorescent proteins. Lourderaj, U.: Time-Dependent Density Functional Theoretical Investigation of Photoinduced Excited-State Intramolecular Double Proton Transfer in Diformyldipyrromethane. Excited state proton transfer in the lysosome of living lung cells: normal and cancer cells.

110) Simkovič, R.; Huppert, D.: Excited-state proton transfer of weak photoacids adsorbed on biomaterials: Proton transfer in glucosamine chitosan. 250) Phukon, A.; Ray, S.; Sahu, K.: Effect of cosurfactants on the interfacial hydration of CTAB quaternary reverse micelles probed using excited-state proton transfer. Chu, T.-S.: Effect of water cluster size on excited-state proton transfer in aqueous solvent.