Revealing New Structural Insights from Surfactant Micelles through DLS, Microrheology and Raman Spectroscopy

Influence of tail group length, amide functionality and added salt ion identity on the behaviour of betaine surfactants

Hypothesis The behaviour of surfactants in solution and at interfaces is governed by a combination of steric and electrostatic effects experienced by surfactant molecules as they interact with solvent, other species in solution, and each other. It would therefore be anticipated that highly interacting groups would significantly influence surfactant behaviour. The widely used amide functionality has polar H-bond donor/acceptor properties, and therefore its inclusion into a surfactant structure should have a profound effect on surface activity and self-assembly of that surfactant when compared to the equivalent molecule without an amide linker. Further, chaotropic or kosmotropic salt ions that affect water structuring and hydrogen bonding may provide opportunities for further tuning surfactant interactions in such cases. Experiments A library of betaine surfactant with tail lengths n=14-22 both with and without an amidopropyl linker were synthesised to study the effect of the amide functionality on surfactant properties. Characterisation of the molecules interfacial properties were performed using pendant drop tensiometry and their solution state formulation properties were probed using small-angle neutron scattering (SANS) and rheological measurements. Findings Presence of an amidopropyl linker had little effect on aggregation propensity (as evidenced by critical micelle concentration) and aggregate morphology of betaine surfactants, but did increase the Krafft temperature of these surfactants. SANS analysis indicated that aggregate morphology of alkyl betaine surfactants could be influenced by the addition of sodium salts with chaotropic counterions (I- and SCN-), but they were insensitive to more kosmotropic anions (SO42-, F- and Cl-), providing unique and novel solution control methods for this (supposedly salt-insensitive) class of surfactants.

Is Micellar Catalysis Green Chemistry?

Many years ago, twelve principles were defined for carrying out chemical reactions and processes from a green chemistry perspective. It is everyone's endeavor to take these points into account as far as possible when developing new processes or improving existing ones. Especially in the field of organic synthesis, a new area of research has thus been established: micellar catalysis. This review article addresses the question of whether micellar catalysis is green chemistry by applying the twelve principles to micellar reaction media. The review shows that many reactions can be transferred from an organic solvent to a micellar medium, but that the surfactant also has a crucial role as a solubilizer. Thus, the reactions can be carried out in a much more environmentally friendly manner and with less risk. Moreover, surfactants are being reformulated in their design, synthesis, and degradation to add extra advantages to micellar catalysis to match all the twelve principles of green chemistry.

Investigation of the Associations between a Nanomaterial's Microrheology and Toxicology

With the advent of Nanotechnology, the use of nanomaterials in consumer products is increasing on a daily basis, due to which a deep understanding and proper investigation regarding their safety and risk assessment should be a major priority. To date, there is no investigation regarding the microrheological properties of nanomaterials (NMs) in biological media. In our study, we utilized in silico models to select the suitable NMs based on their physicochemical properties such as solubility and lipophilicity. Then, we established a new method based on dynamic light scattering (DLS) microrheology to get the mean square displacement (MSD) and viscoelastic property of two model NMs that are dendrimers and cerium dioxide nanoparticles in Dulbecco's Modified Eagle Medium (DMEM) complete media at three different concentrations for both NMs. Subsequently, we established the cytotoxicological profiling using water-soluble tetrazolium salt-1 (WST-1) and a reactive oxygen species (ROS) assay. To take one step forward, we further looked into the tight junction properties of the cells using immunostaining with Zonula occluden-1 (ZO-1) antibodies and found that the tight junction function or transepithelial resistance (TEER) was affected in response to the microrheology and cytotoxicity. The quantitative polymerase chain reaction (q-PCR) results in the gene expression of ZO-1 after the 24 h treatment with NPs further validates the findings of immunostaining results. This new method that we established will be a reference point for other NM studies which are used in our day-to-day consumer products.

Microrheology to Understand the Viscosity Behavior of a Sophorolipid Biosurfactant

The microstructure of the aqueous solutions of purified acidic Sophorolipid (SL) has previously been studied using highly sophisticated methods such as SANS and Cryo-TEM. We were interested in whether (a) the main findings also apply to commercially available SL (which is a mixture of acidic and lactonic SL) and (b) more readily available methods such as DLS can be used to gain insight into the molecular aggregation of SL. Our work was motivated by the increasing interest in biosurfactants for applications in personal and household care. Moreover, the origin behind the more or less lack of rheological response to changes in pH is of practical relevance, as it is somewhat unusual for a carboxylate-group containing surfactant. By using DLS microrheology, we could elucidate the aggregation structure and dynamics of the surfactant on a microscopic scale. Surprisingly, the different degrees of protonation only impacted the microscopic properties such as exchange kinetics and the plateau values of the storage moduli.

Rheology Control Using Nonionic Cosurfactants and pH Titration in an Amino Acid-Derived Surfactant Composition

Sulfate-based formulations can be easily thickened by adding salt or amphoteric cosurfactants. However, sulfate-free and amino acid-based surfactants cannot. We explored an alternative thickening mechanism by studying the thickening effect of adding nonionic cosurfactants to a mixture of an amino acid-based surfactant, sodium lauroyl sarcosinate (SLSar), and a zwitterionic cosurfactant, cocamidopropyl hydroxysultaine (CAHS) at a 6:9 weight ratio. To characterize the formulations, we combined traditional rheometry with a state-of-the-art mesoscopic analysis of micelle dynamics obtained via diffusing wave spectroscopy. In addition, the formulations were characterized by cross-polarized light microscopy and dynamic light scattering. The cosurfactants studied included fatty alcohols, alkanediols, a fatty acid, and fatty alcohol ethoxylates (C_nE₃ and C_nE₆). Adding the nonionic cosurfactants increased the zero-shear viscosity up to 350 times the viscosity of the no-additive system at neutral pH. When pH titration was incorporated as a second thickening mechanism, the viscosity maximum was lower than the no-additive mixture. Furthermore, the pH of the viscosity maximum was shifted to higher pH for all systems except for C_nE₆, which shifted the maximum to lower pH. The nonionic amphiphiles also broadened the viscosity maximum, particularly in the C₁₀OH system. Consequently, the C₁₀OH system had a more favorable profile for development as a practical thickening system for an amino acid-based cleanser. Analysis according to the Zou and Larson micelle dynamics model revealed that the broadening effect was associated with substantially longer breakage times for the C₁₀OH system (4-208 ms) compared to the no-additive system (4-38 ms).

High-performance sulphate-free cleansers: Surface activity, foaming and rheology

OBJECTIVE

The main objective of this paper is to analyse the composition of a sulphate-free binary or ternary surfactant system with alkyl olefin sulfonate (AOS), alkyl polyglucoside (APG) and lauryl hydroxysultaine (Sultaine). The composition was optimized by observing critical parameters such as surface activity and rheological properties while varying the concentration of APG in a ternary system, varying the ratio of AOS and Sultaine in a binary system and studying the effect of sodium chloride addition. The experimental results can provide an alternative, sulphate-free surfactant system to replace the common system containing sodium laureth sulfate (SLES) and cocamidopropyl betaine (CapB), without compromising on the parameters previously mentioned.

METHODS

A DuNouy Ring was utilized on a tensiometer to measure the surface tension of the samples. To observe foaming abilities of samples, a visual foaming study was conducted and recorded by taking pictures. A TA instruments mechanical rheometer was used to measure the viscosity.

RESULTS

Studying the effect of APG concentration on surface tension illustrated that as APG concentration decreases, surface tension decreases as well. The minimum surface tension was found to be 26.587 for 7.5 wt.% AOS and 7.5 wt.% Sultaine. When the ratio between AOS and Sultaine changed, the 1:1 system produced the lowest surface tension value again. As the concentration of AOS decreased in the ratio, the surface tension increased. When the ratio was held constant, and APG was introduced into the system, the systems containing APG had higher surface tension values compared with the systems with the same ratios but did not have APG added. As the concentration of salt increased, the surface tension decreased for AOS, increased for the binary mixture and had no effect on Sultaine. Foaming has a direct correlation with surface tension so a decrease in surface tension led to better foaming abilities; therefore, the 1:1 ratio of AOS:Sultaine had the best foaming qualities. The AOS/Sultaine system exhibited Maxwellian behaviour, suggesting the presence of worm-like micellar structures. As the concentration of APG increased, the viscosity decreased and at a concentration of 2.5 wt.% AOS, 5 wt.% APG and 7.5 wt.% Sultaine, the system changed from shear thinning non-Newtonian fluid to Newtonian. Varying the ratio of AOS:Sultaine in the binary system, showed as the ratio changed, the viscosity decreased, and at a ratio of 1:3 AOS:Sultaine, the system was primarily Newtonian. As the ratio remained constant and APG was added, the viscosity continued to decrease and the 1:3 ratio was completely Newtonian. The addition of salt had no effect on the viscosity of Sultaine, but the viscosity of AOS increased at a salt concentration of 2 wt.%. The binary mixture saw as the concentration of salt increased, the viscosity profile decreased, even though the system remained non-Newtonian and shear thinning.

CONCLUSION

The surface activity and rheological study of the sulfate-free surfactant systems unveil a strong synergistic interaction between AOS and Sultaine specifically at a ratio of 1:1 resulting in high surface activity and corresponding good foaming and formation of entangled wormlike micelles resulting in excellent viscosity build in the system. If introducing a biobased surfactant into this system is desired without completely compromising the performance properties, the ternary surfactant system 4.5 wt.% AOS, 3 wt.% APG and 7.5 wt.% Sultaine should be considered optimal. Any further addition of APG or changing the ratio results in detrimental reductions of all performance controlling physio-chemical parameters.

High Performance Conditioning Shampoo with Hyaluronic Acid and Sustainable Surfactants

Recently, consumers have become invested in more natural and sustainable ingredients contained in personal care products. Unfortunately, cationic surfactants are still heavily relied on as primary conditioning agents in products such as conditioning shampoos because of their ability to cling well to the negatively charged surface of hair follicles. Additionally, sulfates are utilized as cleansing agents because they are highly effective and low cost. The objective of this study is to find a more sustainable formulation for a conditioning shampoo without compromising the desired wet combing, rheological, and surface activity properties. The systems which were investigated contained hyaluronic acid (HA) at a variety of molecular weights and concentrations, in combination with a surfactant, either acidic sophorolipid (ASL) or alkyl polyglucoside (APG), and varying the presence of sodium chloride. A Dia-stron was utilized to test the wet combing force, a rheometer recorded the viscosity at various shear rates, and a tensiometer measured the surface tension of the samples before a visual foaming study was conducted. Molecular weight and concentration seemed to have a large impact on wet combing force, as well as rheology, with the largest molecular weight and concentration producing the lowest friction coefficient and desired rheological profile. The addition of a surfactant significantly aids in the reduction in surface tension and increased foamability. Therefore, the optimal system to achieve the largest reduction in wet combing force, large viscosity with shear-thinning behavior, and relatively low surface tension with decent foaming is composed of 1% HA at 800 kDa, 10% ASL and 1% NaCl. This system shows a viable sulfate-free and silicone-free option that can achieve both conditioning and cleansing.

Effect of pH on the Structure and Dynamics of Wormlike Micelles in an Amino Acid-Derived Surfactant Composition

We studied the impact of pH as a thickening mechanism on the structure and dynamics of wormlike micelles in a mixture of sodium lauroyl sarcosinate (SLSar) and cocamidopropyl hydroxysultaine (CAHS). The viscoelastic properties were obtained using mechanical rheometry and diffusing wave spectroscopy, which provided access to a wide range of frequencies. By using a mesoscopic simulation method [Zou; Larson. J. Rheol. 2014, 58 (3), 681-721], characteristic micelle lengths and times were extracted including contour length, persistence length, entanglement length, reptation time, breakage time, breakage rate, and breakage rate constant. The interplay of pH-dependent reptation times (10-1000 ms) and breakage times (4-38 ms) leads to a minimum in the ratio of reptation time to breakage time of about 0.02 at pH 4.8. This minimum was closely associated with the sharp increase and decrease of the observed viscosity maximum at pH 4.8 in this system. These values may be contrasted with much longer breakage times (20-300 ms) that have been measured in more easily thickened sulfate-based systems. The low breakage times of the SLSar/CAHS system were attributed to the high and pH-sensitive breakage rate constants (0.01-0.17 ms^-1 μm^-1).

Impact of polyelectrolyte-surfactant interactions on the rheology and wet lubrication performance of conditioning shampoo

OBJECTIVE

The purpose of this study is to understand the impact of the cationic polymer merquat on the rheological behavior of the mixed surfactant system of sodium lauryl ether sulfate (SLES) and cocamidopropyl betaine (CapB) as well as the impact of varying formulation conditions on the wet lubrication performance of the SLES-CapB-Merquat system.

METHODS

Rotation mechanical Rheometry was used to study the rheological response of the SLES-CapB-Merquat systems. Frequency sweeps were conducted to analyze the rheological properties of the system at low frequency ranges and bulk viscosity of the system was studied at high shear rates at varying salt and polymer concentrations. Wet combing tests and hair friction tests were run on the Dia-stron MTT175 flexible miniature tensile tester in order to evaluate the conditioning effects of the different formulations.

RESULTS

The SLES-CapB system on its own exhibits very little viscosity. The bulk rheology results show that the addition of Merquat enhances the viscosity and viscoelastic properties of the SLES-CapB-Merquat system indicating the presence of electrostatic interactions between the surfactant and polyelectrolyte. Addition of salt had a significant impact on the system's rheological response due to the charge screening effect of salt. Wet combing force data indicate that the charged polyelectrolyte binds to the hair substrate resulting in reduced combing force values after the product was applied. The addition of silicone oil to the formulation seemed to greatly enhance the conditioning effect of the formulation.

CONCLUSION

The charge interactions between SLES, CapB and Merquat results in the formation of an integrated gel like network, thus building the viscosity of the system. Variation of parameters like polymer and salt concentration has the potential to modify the bulk rheological properties and consequently the wet lubrication properties of the system.

Research Progress on the Collaborative Drag Reduction Effect of Polymers and Surfactants

Polymer additives and surfactants as drag reduction agents have been widely used in the field of fluid drag reduction. Polymer additives can reduce drag effectively with only a small amount, but they degrade easily. Surfactants have an anti-degradation ability. This paper categorizes the mechanism of drag reducing agents and the influencing factors of drag reduction characteristics. The factors affecting the degradation of polymer additives and the anti-degradation properties of surfactants are discussed. A mixture of polymer additive and surfactant has the characteristics of high shear resistance, a lower critical micelle concentration (CMC), and a good drag reduction effect at higher Reynolds numbers. Therefore, this paper focuses more on a drag reducing agent mixed with a polymer and a surfactant, including the mechanism model, drag reduction characteristics, and anti-degradation ability.

Rheology of mixed solutions of sulfonated methyl esters and betaine in relation to the growth of giant micelles and shampoo applications

This is a review article on the rheological properties of mixed solutions of sulfonated methyl esters (SME) and cocamidopropyl betaine (CAPB), which are related to the synergistic growth of giant micelles. Effects of additives, such as fatty alcohols, cocamide monoethanolamine (CMEA) and salt, which are expected to boost the growth of wormlike micelles, are studied. We report and systematize the most significant observed effects with an emphasis on the interpretation at molecular level and understanding the rheological behavior of these systems. The experiments show that the mixing of SME and CAPB produces a significant rise of viscosity, which is greater than in the mixed solutions of sodium dodecyl sulfate and CAPB. The addition of fatty alcohols, CMEA and cationic polymer, leads to broadening of the synergistic peak in viscosity without any pronounced effect on its height. The addition of NaCl leads to a typical salt curve with high maximum, but in the presence of dodecanol this maximum is much lower. At lower salt concentrations, the fatty alcohol acts as a thickener, whereas at higher salt concentrations - as a thinning agent. Depending on the shape of the frequency dependences of the measured storage and loss moduli, G' and G", the investigated micellar solutions behave as systems of standard or nonstandard rheological behavior. The systems with standard behavior obey the Maxwell viscoelastic model (at least) up to the crossover point (G' = G") and can be analyzed in terms of the Cates reptation-reaction model. The systems with nonstandard rheological behavior obey the Maxwell model only in a restricted domain below the crossover frequency; they can be analyzed in the framework of an augmented version of the Maxwell model. The methodology for data analysis and interpretation could be applied to any other viscoelastic micellar system.

Ecotoxicity and micellization behavior of anionic surfactant sodium dodecylbenzene sulfonate (SDBS) and its mixtures with nonionic surfactant fatty alcohol-polyoxyethylene ether (AEO)

Surfactant mixtures have extensive industrial applications due to their ideal properties and low ecotoxicity. However, the ecotoxicity of surfactant mixtures with different proportions and their correlation with surface properties have remained poorly investigated. In this study, the ecotoxicity and surface activity of the composites of anionic surfactant sodium dodecylbenzene sulfonate (SDBS) and nonionic surfactant fatty alcohol-polyoxyethylene ether (AEO) in various mass ratios were assessed, and the correlation between ideal application properties and safe ecological perspective of the composites was explored. The ecotoxicity of individual SDBS, AEO, and SDBS/AEO mixtures was determined using the bioluminescence inhibition assay with Photobacterium phosphoreum, and the critical micelle concentrations (CMC) were measured by surface tension method and steady-state fluorescence spectroscopy. Sodium dodecylbenzene sulfonate (SDBS) showed a considerably higher toxicity than individual AEO and SDBS/AEO mixtures. Scanning electron microscope images illustrated the rupture of bacteria membrane induced by SDBS, and the addition of AEO alleviated the damage. According to the dose-response relationship on luminous bacteria, SDBS/AEO mixtures were divided into three groups (group I with a high proportion of SDBS, SDBS:AEO = 4:1 and 3:2; group II, SDBS:AEO = 1:1; group III with a high proportion of AEO, SDBS:AEO = 2:3 and 1:4). The sequence of toxicity of the SDBS/AEO mixtures was group II > group III > group I, demonstrating that the toxicity of the composites was related to the mixture proportion instead of the amount of AEO added. The CMC order of SDBS/AEO mixtures was group II > group I > group III, and it was proportion dependent. Furthermore, ΔC^M was defined as the difference of the experimental (C^M) and ideal CMC (C^Mi_deal) of the mixed system, indicating the interaction between the two kinds of surfactants. The order of the ΔC^M was group II > group III > group I, which was consistent with the sequence of the toxicity. Therefore, ΔC^M can be a potential indicator for the hazardous assessment of surfactant mixtures involving high ionic strength.

Colloidal Stability & Conformational Changes in β-Lactoglobulin: Unfolding to Self-Assembly

A detailed understanding of the mechanism of unfolding, aggregation, and associated rheological changes is developed in this study for β-Lactoglobulin at different pH values through concomitant measurements utilizing dynamic light scattering (DLS), optical microrheology, Raman spectroscopy, and differential scanning calorimetry (DSC). The diffusion interaction parameter kD emerges as an accurate predictor of colloidal stability for this protein consistent with observed aggregation trends and rheology. Drastic aggregation and gelation were observed at pH 5.5. Under this condition, the protein's secondary and tertiary structures changed simultaneously. At higher pH (7.0 and 8.5), oligomerizaton with no gel formation occurred. For these solutions, tertiary structure and secondary structure transitions were sequential. The low frequency Raman data, which is a good indicator of hydrogen bonding and structuring in water, has been shown to exhibit a strong correlation with the rheological evolution with temperature. This study has, for the first time, demonstrated that this low frequency Raman data, in conjunction with the DSC endotherm, can be been utilized to deconvolve protein unfolding and aggregation/gelation. These findings can have important implications for the development of protein-based biotherapeutics, where the formulation viscosity, aggregation, and stability strongly affects efficacy or in foods where protein structuring is critical for functional and sensory performance.