flscc membership time to renew!€¦ · peter toth [email protected] chair-elect rebeca pupo...

February 2020Issue #1www.flscc.org

FLSCC 2020-2021 OfficersChairPeter [email protected]

Chair-electRebeca [email protected]

TreasurerArthur Joseph [email protected]

Treasurer-electShawn [email protected]

SecretaryKrupa [email protected]

Area IV DirectorMichelle [email protected]

Area IV DirectorAngela [email protected]

Newsletter [email protected]

AdvertisingPeter [email protected]

RSVP to Chapter [email protected]

ContactsWebsitehttp://www.flscc.org

SCC National Office120 Wall StreetSte 2400New York, NY 10005-4088(212) 668-1500FAX (212) 668-1504email: [email protected]

Contents

Pg 2/3 - OfficersPg 4/5 - Upcoming Eventspg. 6 - Message from Outgoing ChairPg. 7/8 - New Officer Installation PicsPg. 10-16 - Technical Article

Pg. 17/18 - 2019 SSS Sponsors Pg. 19/20 - SCC Info

Welcome all new and existing chapter members. We look forward to seeing you at our upcoming

meetings this year!

FLSCC MembershipTime to Renew!

It is time to renew your membership for 2020 if you have not done so already! You must be a member to attend 2020 Chapter meetings and

annual CEP Course for FREE! Visit www.scconline.org

We want you to continue to be an active part of the Society of Cosmetic Chemists Florida Chapter!


2020/2021 Florida Chapter Officers

Peter TothChair

Arthur Joseph VallejoTreasurer

Michelle LinscottArea IV Director

Rebeca PupoChair-Elect

Shawn LovelyTreasurer Elect

Krupa KoestlineSecretary

Angela EpplerArea IV Director


2020 Chapter Meeting Dates:March 13th, 2020: Andretti Indoor Karting - 9299 Universal Blvd, Orlando, FL 32819April 9th, 2020: Columbia Restaurant - 2117 E 7th Ave, Tampa, FL 33605September 17th, 2020: Location TBDNovember 12th, 2020: Location TBDDecember 6th, 2020: Holiday Social, Location TBD

FLSCC Upcoming Events

2020 FLSCC Newsletter Advertising CampaignIt’s time to update or create your ad for the 2020 FLSCC Newsletters. There will be at least 6 newsletters published in 2020, with the goal of reaching at least 10 for the year. That means a Newsletter for every single month going forward to keep our chemists well informed with current technologies and ingredientsOur goal is to obtain revised or new ads by March 13th, 2020. We realize that isn’t a lot of time, but we’re counting on everyone to help support the FLSCC!As of now, this issue of the newsletter features all paid and unpaid advertisements. Next month’s March Newsletter will feature the paid ads only. As a courtesy to all ad-vertisers from 2019, your ad has been published in this newsletter as a thank you for your continued support. We hope that you continue to support the Florida Chapter with your renewal of the advertisement for the remainder of the year.If you are interested to advertise in the upcoming 2020 newsletters, please email us at:[email protected], [email protected] & [email protected] Line : 2020 Newsletter Advertising

VOLUNTEER OPPORTUNITIES - The Florida Chapter is seeking Volunteers with a good understanding of social media and how to utilize active live Twitter feeds for upcoming events. Please contact Rebeca Pupo at [email protected] if you are interested.

SPEAKER OPPORTUNITIES - The Florida Chapter SCC is now accepting Speaker and topic submissions for our last 2020 general Meeting in November, and also for 2021. Talks are generally 30-40 minutes and geared towards education and advancement of our members.


Message from the New Chair, Peter Toth

First, as your new FLSCC Chair, I would first like to thank all of the SCC members who contributed to the success of the FLSCC these past two years. To all the members, vendors, and anyone who is active in the chapter, thank you. I would also like to thank my employer, McCullough & Associates, for allowing me the time to contribute to the FLSCC.

We also need to extend a very grateful thank you and high-five to our exiting board members who really made a huge impact on the FLSCC. This team really knocked the ball out of the park and the FLSCC would not have been as successful without their hard work and dedication. Marisa Bailey-Furlonge served as Chair for the last 4 years, Stephen Dawes served as Treasurer for 8 years, and Vanessa Thomas served as the Secretary for 2 years. Thank you team. We hope to continue to strive for the success of the chapter. May you enjoy your downtime, freedom, and retirement.

Second, we have three more meetings this year. Please see the meeting flyers below for more details.March 13th in Orlando, FL @ Andretti Indoor KartingApril 9th in Tampa, FL @ Columbia RestaurantNovember 12th TBD

The CEP course will be offered for free to all FLSCC members will be held on September 17th. The location will be finalized soon as we explore various options. We are playing with the idea of potentially having 2-4 different speakers, all offering various degrees of technology and information. So please save the date!

Finally, the board is considering hosting a holiday social to be held during the first week of December 2020. This will not be a typical meeting, rather a holiday themed social event for members to come out and be amongst friends and colleagues in a fun social setting. In order for this to happen, we need to hear from our FLSCC members to know if there is interest in attending. As for the venue, we are looking at several options. We are looking for your feedback on the location. You will receive an email from us that will take you to a quick 3-question survey to help us gauge where to have the event. Please fill this out at your convenience.

Thank you to all of the members. I look forward to making the next two years fun and eventful for all FLSCC members.


FLSCC MARCH MEETINGFriday, March 13th, 2020

Location: Andretti Indoor Karting & Games9299 Universal Blvd, Orlando, FL 32819

Cocktail hour: 5:30p-6:30pDinner/Presentation: 6:30p-8:30p

Starch Chemistry for Personal CareSarah Thiewes

Glenn Corp., part of Azelis Americas

Abstract

More consumers today are seeking more natural sounding INCI names on their Personal Care Products pushing formulators to look at alternatives for tried and true ingredients. While starch is not new to the industry, many formulators are either not familiar with how to work with starches in their formulas, what benefits starch can bring, or both. Starch can be a great natu-ral alternative to some of the synthetic ingredients some consumers and brands are looking to avoid. This presentation is designed to provide a background on starch chemistry and highlight the benefits of native and modified starches in a variety of Personal Care formulations.

Speaker BioSarah Thiewes is a graduate of The College of New Jersey with a Bachelor of Science in Chemistry. She began her career as a formulation chemist for personal and pet care products at a contract manufacturer in southern NJ in 2002. From there she has worked for two major ingredients suppliers in a variety of roles including Technical Service, Marketing, and Product Management. In 2016 she joined Glenn Corp, part of Azelis Americas as the Technical Development Manager. and Tristate CACS.

RSVP by Monday, March 9th

[email protected]://tinyurl.com/flsccmarch2020

Florida Chapter Members FREE!!! Non-Florida Chapter Members & Non-SCC Members - $45.00

Cocktail Hour Sponsored by


FLSCC APRIL MEETINGThursday, April 9th, 2020

Location: Columbia Restaurant2117 E 7th Ave, Tampa, FL 33605

Cocktail hour: 5:30p-6:30pDinner/Presentation: 6:30p-8:30p

An Introduction into Oleosomes and its Applications in CosmeticsDr. Kristin Brancato & Mr. John Chase of Botaneco

AbstractBotaneco focuses on seed technology, specifically isolating oil bodies, or oleosomes which are the storage units of natural oil inside the seed. Oleosomes are comprised of the natural oil, triglyceride core, coated by a phospho-lipid monolayer and stabilized by unique proteins called oleosin. This unique structure is isolated as it appears in nature through a sustainable process and is used in personal care and food and feed applications. This structure is excellent at preserving the integrity of the natural oil core and lends itself to be an excellent delivery system for personal care products as well.

Speaker BiosDr. Kristin Brancato has over 25 years of experience in technical and executive positions in the consumer products space primarily focusing on new product development for home care, personal care, women’s health, and the nutraceutical markets. Bringing invaluable knowledge and expertise, she plays a key role in estab-lishing and delivering Botaneco®’s innovation and technology strategy. Kristin has a Ph.D. in Bioinorganic Chemistry from the University of Notre Dame, a Bachelor of Science in Chemistry from Pennsylvania State University, and is the holder of six patents.

John Chase is Senior Formulations Lab Manager at Botaneco® Inc., an innovative, solutions-focused natural ingredient manufacturer based in Calgary. He is current-ly located in Botaneco’s personal care site in beautiful Lambertville, NJ. Previously, John worked as Innovation and Formulation Lab Manager at Vantage Specialty Chemicals for a number of years. He holds a number of patents utilizing natural emulsifiers in novel ways. His passion for cosmetic chemistry is only surpassed by his love of his family.

RSVP by Monday, April 6th

[email protected]://tinyurl.com/flsccapril2020

Florida Chapter Members FREE!!! Non-Florida Chapter Members & Non-SCC Members - $45.00

Cocktail Hour Sponsored by


FLSCC Meeting on February 6th, 2020


J. Cosmet. Sci., 67, 59–70 (March/April 2016)

59

Streaming potential measurements to understand the rheological properties of surfactant formulations containing anionic and zwitterionic surfactant

JOCHEN KLEINEN and JOACHIM VENZMER, Evonik Nutrition & Care GmbH, 45127 Essen, Germany.

Accepted for publication April 13, 2016.

Synopsis

Surfactant formulations are often based on an anionic primary surfactant combined with an amphoteric secondary surfactant. One popular option is the combination of lauryl ether sulfate and cocamidopropyl betaine, because such formulations are not only mild but also easy to thicken. Changes in the molecular structure of the betaine in terms of alkyl chain length distribution and headgroup structure do have dramatic effects on the viscosity of these formulations, as can be explained in terms of properties of rod-like micelles and exchange kinetics by oscillatory rheological measurements. The root cause of the effect of the different betaine derivatives on the micellar structure, however, remains unclear when considering rheology only. Although the streaming potential of colloidal objects is typically determined to forecast the stability of dispersions, we have used the streaming potential to characterize micellar solutions of different betaine surfactant structures. It could be shown that (a) the hydrophilicity of the surfactants can be nicely probed by this method and (b) there is a good correlation of these values with the rheological properties of binary mixtures of the betaines with anionic surfactant. Also, the chemical structure of the headgroups has a signifi cant infl uence on both the isoelectric point and the magnitude of the streaming potential of the zwitterionic surfactants. These effects have again a dramatic infl uence on the interaction with anionic surfactants, as becomes obvious when looking at the rheology of such mixtures. Therefore, the fi ndings obtained can be utilized to better understand and design surfactant formulations of a desired viscosity profi le.

INTRODUCTION

Aqueous surfactant formulations such as shampoos, shower gels, or hand dishwashing liquids are often based on a combination of an anionic primary surfactant with a zwit-terionic secondary surfactant. The goal is to obtain formulations having Brookfi eld vis-cosities in the range of 1000–5000 mPa·s; this is typically achieved by the use of rheological additives. The fi rst option is hydrophobic thickeners, i.e., cosurfactants, which cause transition of the spherical micelles of the primary surfactant to mixed

Presented at the 70th Annual Scientifi c Meeting and Technology Showcase, New York, December 10–11, 2015.Address all correspondence to Joachim Venzmer at [email protected].

Technical ArticleProvided by the FLSCC Secretary, Krupa Koestline


JOURNAL OF COSMETIC SCIENCE60

rod-like micelles. The second option is hydrophilic, associative thickeners, which are able to bridge the surfactant assemblies—or a combination of both (1). While Brook-fi eld viscosity is just an indication of the viscosity under shear, oscillatory rheological measurements can provide a lot more information, not only on the behavior at rest (e.g., existence of a yield point) but also on the type and properties of the surfactant micelles present. A network of rod-like micelles can be described by the Maxwell model; the plot of the storage modulus G′, the loss modulus G″ and the complex viscosity as a function of frequency yields information on the zero shear viscosity, the network den-sity, and the structural relaxation time, which is a measure of the exchange kinetics of the surfactants (2).

The combination of the anionic sodium lauryl ether sulfate (SLES) with the zwitterionic cocamidopropyl betaine (CAPB) is quite popular not only because of its mildness, but also because it is easy to thicken (3,4). In general, the origin of the viscosity behavior is known; the pH of personal care formulations is typically set to about 5.5 to match the pH of human skin. At these slightly acidic conditions, the zwitterionic CAPB is par-tially protonated and thus carries some positive charge, leading to a strong interaction with the anionic SLES. While SLES forms spherical micelles of only low viscosity, the combination with the protonated CAPB leads to a sphere-to-rod micelle transition (5–7) and thus to an increase in viscosity of the mixture (8). The resulting viscosity of the surfactant mixtures depends on the mixing ratio (9), the amount of minor components such as sodium chloride (NaCl) and free fatty acids (6), and the chain length of the used betaines (10). The variation of the chain lengths of the betaine has two consequences: on the one hand, longer chains lead to an increase in packing parameter (11), i.e., to a more effi cient sphere-to-rod micelle transition. On the other hand, the exchange kinetics of aggregated surfactants, and hence the viscosity, scales with the chain length of the molecules forming the aggregate (12–15). In general, in the system SLES/CAPB there are only minor adjustments necessary to achieve the desired rheological profi le of the formulation.

However, the question remains how the rheological properties of the formulations are related to the molecular structure (chain length distribution, headgroup structure) of the secondary surfactant. Because oscillatory rheological measurements can only show the rheological effects of the different betaine structures, we have used streaming potential measurements to study and understand the root cause of the rheological effects. The streaming potential is a measure of the surface charges of, e.g., colloidal objects (16); it is typically used to determine the isoelectric point (IEP) of proteins or to forecast the stabil-ity of dispersions. We have used the streaming potential of micellar solutions of different betaine surfactant structures as a measure of the hydrophilicity of the surfactants, and to correlate these values with the rheological properties of binary mixtures of the betaines with anionic surfactant. Also, the chemical structure of the betaine headgroup varied, and a signifi cant infl uence on both the IEP and the magnitude of the streaming potential of the zwitterionic surfactants could be shown. These effects have again a dramatic infl uence on the interaction with anionic surfactants, as becomes obvious when looking at the rhe-ology of such mixtures.

The alkyl amidopropyl betaines (APBs) used in the fi rst part of the paper for studying the effect of alkyl chain length and its distribution were all based on dimethyl aminopropyl amine (DMAPA), fatty acids or fatty acid esters (triglycerides), and sodium monochloro-acetate. In the second part, the alkyl chain distribution is kept constant, but the structure


RHEOLOGICAL PROPERTIES OF SURFACTANT FORMULATIONS 61

of the zwitterionic headgroup is modifi ed: on the one hand, the distance between the amide and the quaternary nitrogen was reduced by using dimethyl aminoethyl amine instead of DMAPA in the synthesis of an alkyl amidoethyl betaine (AEB). On the other hand, an alkyl betaine (AB) was investigated, which can be obtained by reaction of alkyldimethyl amine with sodium monochloroacetate and hence does not contain an amide group at all.

MATERIALS AND METHODS

MATERIALS

SLES was obtained from BASF (Ludwigshafen, Germany) (Texapon® LS 35, 30%) and used as received. The amidoalkyl betaines were synthesized according to standard procedures used industrially (17) via a two-step process. At fi rst, a fatty acid (mixture) or hydrogenated coconut oil was reacted with dimethylaminoalkyl amine at elevated temperatures. After-ward, the respective amidoamine was carboxymethylated with sodium monochloroacteate in an aqueous solution to yield the respective betaine. The APBcoco is commercially avail-able from Evonik Nutrition & Care GmbH (Essen, Germany) under the trade name TEGO® Betain F 50. The APB8/10 + 12/18 is a mixture of TEGO® Betain 810 with TEGO® Betain CK. The AB12/14 is also an Evonik product with the trade name TEGO® Betain AB 1214. Figure 1 shows the chemical structures of the different betaines used in these studies, and Table I provides an overview of the alkyl chain length distributions of the betaines.

METHODS

Streaming potential. A Charge Analyzing System (CAS; emtec papertest, Leipzig, Germany) was used, and 20 mL of 0.5 wt% surfactant solution, which is above the critical micelle concentration (CMC) in all cases, was added to the cuvette. Aqueous hydrogen chloride solution (0.5%) was added until a pH value of 4 was reached. Then the solution was ti-trated to pH 9 with diluted sodium hydroxide solution (0.5%).

Figure 1. Structures of the zwitterionic surfactants used: alkyl APB, alkyl AEB, and AB.



Figure 2. Viscosity as a function of shear rate of formulations of 9 wt% SLES/3 wt% APB/2 wt% NaCl (pH 5.5) using different alkyl APBs.

Formulations. For the rheological characterization of the surfactant mixtures, a simple model formulation consisting of 9 wt% SLES, 3 wt% betaine, and 2 wt% NaCl was used. The pH was adjusted to 5.5 using citric acid.

Rheology. The shear viscosity of the formulations was measured using a MCR 302 rheom-eter (Anton Paar, Ostfi ldern, Germany) with a plate–plate geometry. The upper plate had a diameter of 50 mm. Oscillatory measurements as a function of frequency were con-ducted on a StressTech rheometer by Rheologica (formerly Lund, Sweden; now available at TS RheoSystems, State College, PA) with a plate–plate geometry, with a diameter of 40 mm; the applied stress was 0.2 Pa. All measurements were conducted at 25°C.

RESULTS AND DISCUSSION

INFLUENCE OF THE ALKYL CHAIN LENGTH DISTRIBUTION IN ALKYL APBs

The alkyl APBs used in the fi rst part of this study were all a variation of CAPB, as can be seen in Table I: fi rst the pure lauryl APB12, then two variants with an average of 12.7 carbon atoms in the fatty acid chain, APB12/14 with a narrow distribution and APB8/10 + 12/18 with a broad distribution, were obtained by using a mixture of the corresponding fatty acids, and fi nally the “real” triglyceride–based CAPB with a typical alkyl chain distribution

Table IAlkyl Chain Length Distributions of the Betaines Used (n = narrow; b = broad)

NameAverage alkyl chain length C8 (%) C10 (%) C12 (%) C14 (%) C16 (%) C18 (%)

APB12 12.0 100APB12/14 12.7 (n) 70 30APB8/10 + 12/18 12.7 (b) 10 8 47 17 9 9APBcoco 12.8 8 8 48 17 8 11AEBcoco 12.8 8 8 48 17 8 11AB12/14 12.7 (n) 70 30



Figure 3. Plot of storage modulus G′ and loss modulus G″ versus frequency of a formulation of 9 wt% SLES/3 wt% APB12/2 wt% NaCl (pH 5.5); the inset shows same data as Cole–Cole plot (the semicircle is a guide for the eyes).

of hydrogenated coconut oil was obtained. The differences in the chain length might be small, but they have a signifi cant infl uence on the shear viscosity of the model formula-tions SLES/APB/NaCl as shown in Figure 2. In all cases, the behavior is typical of rod-like micelles: a plateau at low shear rates, followed by shear thinning at higher shear rates. The level of the plateau, however, is quite different, for the broad alkyl chain length distribution being the lowest. Considering there is no additional thickener in these model formulations, all their viscosities were remarkably high.

To study the origin of these viscosities, oscillatory rheological measurements were per-formed to determine the storage and loss moduli G′ and G″ as a function of frequency. In case of a network of rod-like micelles, the slopes G′ and G″ in a plot versus frequency should be 2 and 1, respectively. Figure 3 shows such a Maxwell plot exemplarily for the model formulation with APB12. At low frequencies, the slopes of G′ and G″ are just as expected for rod-like micelles. The crossover point of G′ and G″—aka structural relax-ation time, a measure of the exchange kinetics of the surfactants—was 0.5 Hz. The initial shear modulus, the plateau value of the storage modulus at high frequencies, which is a measure of the network density, is about 190 Pa. The inset in Figure 3 shows the same data as Cole–Cole plot; the data in the accessible sensitivity range are all on a semicircle, confi rming the interpretation given above (2).

For the other alkyl APBs, these plots are in general quite similar, but there are quantita-tive differences concerning the specifi c values (Table II). The addition of C14-APB leads

Table IIResults of Oscillatory Rheological Measurements of Formulations Containing 9 wt% SLES/3 wt% of

APB/2 wt% NaCl (pH 5.5) for Different Alkyl APBs

Name Structural relaxation time (Hz) Initial shear modulus G′ (Pa)

APB12 0.50 190APB12/14 0.35 220APB8/10 + 12/18 0.72 145APBcoco 0.50 160



Table III Streaming Potential and Isoelectric Point of Micellar Solutions of Different Alkyl APBs

Name Streaming potential at pH 5.5 (mV) Isoelectric point (pH)

APB12 129 6.57APB12/14 400 6.78APB8/10 + 12/18 62 6.08APBcoco 232 6.25

to slower exchange kinetics and higher initial modulus as compared to the pure C12-APB. The fastest relaxation time of 0.72 Hz was found for APB8/10 + 12/18, the APB with the broadest alkyl chain distribution, i.e., the APB with the highest content of C8- and C10-APB; this is also the material leading to the lowest low-shear viscosity (Figure 2). It seems to be reasonable that this is caused by the presence of the short-chain APB, since a chain—as well as rod-like micelle—can only be as strong as the weakest link. Accord-ingly, both the APBs containing the short-chain APB (APB8/10 + 12/18 and APBcoco) have lower initial moduli.

The changes in exchange kinetics, however, are not suffi cient to explain all the differences in rheological properties of the model formulations. To study their root cause, we mea-sured the streaming potential of micellar solutions of the betaines as a function of pH. The results are given in Figure 4 and Table III. All four betaines show the typical behav-ior of zwitterionic surfactants like CAPB; the surfactants have a negative streaming po-tential at basic pH values and become positively charged at low pH values due to the protonation of the carboxylate group. There is, however, an effect of the chain length distribution on the IEPs: APB12/14 has the highest IEP, and the two betaines with a rather broad alkyl chain distribution have the lowest tendency to become protonated as indi-cated by the low IEPs.

The infl uence of the chain length on the pKa of fatty acids is already known. Kanicky et al. (18) have reported that pKa increases as chain length increases, from 6.4 for octanoic acid to 8.7 for palmitic acid. The behavior for betaines, however, is signifi cantly shifted to lower pH values due to the presence of the quaternized nitrogen and its strong inductive (−I) effect. The pKa of the natural betaine trimethylglycine (CH3)3N

+CH2COO− is 2.351

Figure 4. Streaming potential of micellar solutions of different alkyl APBs as a function of pH.



Figure 5. Correlation of the streaming potential of the different alkyl APBs and the zero shear viscosities of the corresponding formulations with anionic surfactant.

(19), indicating that the infl uence of the quaternized nitrogen on the protonation equi-librium is immense. This effect of the quat group has also been shown earlier by Weers et al. (20). A more detailed discussion on the infl uence of the headgroup architecture on the streaming potential is the topic of the second part of this paper; here, only the effect of the alkyl chain length and its distribution are considered.

The streaming potential values of the different betaines are not only shifted along the pH axis but also different concerning their slopes and thus their absolute values. The data in Table III show that a narrow distribution in the composition of the alkyl chain leads to signifi cantly higher streaming potentials as compared to the broad distribution having the same average molecular weight. Thus, it seems that the effect of chain length and its distribution on the hydrophilicity of the betaines can be probed by measuring the streaming potential.

The differences between these APBs are also obvious when it comes to their ability to build viscosity in combination with SLES. Again, the betaine with the narrow chain length distribution yields the highest viscosity, whereas the broad distribution gives the lowest viscosity when combined with the anionic surfactant. A plot of the zero shear vis-cosities of the formulations of the betaines with SLES and NaCl versus the streaming potential of micellar solutions of the corresponding betaines at pH 5.5 is shown in Figure 5. There is an excellent correlation, indicating that the streaming potential is able to probe the average polarity/packing parameter of betaines with same headgroups but dif-ferent alkyl chain length distributions. Thus, the streaming potential of different beta-ines based on DMAPA can be used to predict their ability to thicken mixtures with anionic surfactants. This is an attractive option, since measuring the streaming potential requires considerably less effort than other methods, such as the determination of CMC either by measuring surface tension or by using fl uorescence probes (21,22).

INFLUENCE OF THE STRUCTURE OF THE HEADGROUP OF THE BETAINES

The infl uence of the spacer group between the two charged sites in ABs has already been investigated by several authors (23–25). A strong interaction of the two charged groups in betaines via back folding was only observed if the spacing between the two charged



sites was longer than 10 methylene groups (20,26). However, the interaction of an amide group with the cationic group has not been investigated yet. Therefore, we have modifi ed the headgroups of the betaines to study the infl uence of the amide group and its position. In addition to the APBs based on DMAPA discussed so far, betaines with an ethyl instead of a propyl spacer between the amide and the quaternary nitrogen (AEB) as well as beta-ines without the amide group (AB) were synthesized (see Figure 1). The alkyl chain lengths were chosen to allow direct comparisons with the APBs used in the fi rst part of this study (see Table I).

As it becomes obvious when looking at Figure 6, the structure of the hydrophilic head-group has an enormous effect on the viscosity of formulations with anionic surfactant. All formulations again behave like it is typical for networks of rod-like micelles. However, reducing the spacer length between the two nitrogen atoms by one methylene unit leads to a signifi cant drop in viscosity, whereas leaving out the amide group leads to dramatic increase in the plateau value of viscosity. Oscillatory measurements (Table IV) confi rm these results. The AB12/14 has a slightly higher structural relaxation time and initial modulus as compared to the APB of the same alkyl chain length APB12/14. Even more signifi cant are the differences for the betaine with an ethyl instead of a propyl spacer be-tween the two nitrogen atoms; the AEBcoco has the lowest initial modulus and the highest structural relaxation time of all the betaines studied.

To explain these differences in rheology, measurements of the streaming potential of micellar solutions of the betaines were performed again; the results shown in Figure 7 and

Table IVResults of Oscillatory Rheological Measurements of Formulations Containing 9 wt% SLES, 3 wt% of

APB/2 wt% NaCl (pH 5.5) for Betaines with Different Headgroup Structures

Name Structural relaxation time (Hz) Initial shear modulus G′ (Pa)

APBcoco 0.50 160AEBcoco 1.15 110APB12/14 0.35 220AB12/14 0.46 245

Figure 6. Viscosity as a function of shear rate of formulations of 9 wt% SLES/3 wt% betaine/2 wt% NaCl (pH 5.5) using betaines with different headgroup structures.



Table V Streaming Potential and Isoelectric Point of Micellar Solutions of Different Betaines

Name Streaming potential at pH 5.5 (mV) Isoelectric point (pH)

APBcoco 232 6.25AEBcoco -887 <3.5APB12/14 400 6.78AB12/14 510 >9

Figure 7. Streaming potential of micellar solutions of different betaines as a function of pH.

Table V reveal signifi cant and surprising differences between these on the fi rst sight sim-ilar zwitterionic surfactants.

Surprisingly, the two betaines APB and AEB, which differ only by one methylene unit in the spacer, behave quite differently in terms of streaming potential. While APB has its IEP at pH 6.25, the AEB yields micelles that are negatively charged over the entire in-vestigated pH range; extrapolation to lower pH values suggests that the IEP must be somewhere below pH 3.5. In contrast, removing the amide group, i.e., transforming the APB to an AB, yields micelles that are positively charged over the entire pH range, mean-ing that the IEP of AB must be >9.

The challenge is now to understand this infl uence of the chemical structure on the stream-ing potential. One useful approach is to envision how this electrochemical property is achieved. The IEP is defi ned as the pH at which the numbers of positive and negative charges within a molecular assembly are equal; each quaternized nitrogen is “equalized” or “neutralized” by one carboxylate group. In a situation with an identical number of quat and carboxylate groups to begin with, there will be an excess of cationic charges, i.e., a positive streaming potential, as soon as the fi rst carboxylate group gets protonated. Obviously, this is the case for AB12/14, which has a positive streaming potential over the entire pH range studied, because the pKa of the carboxylate is reported to be as low as 1.8 [for pure AB12 (19)].

The presence of the amide group close to the hydrophilic headgroup makes the situation much less straightforward. In case of the APBs, the streaming potential above the IEP at



about pH 6.5 is negative, meaning that there must be an excess of negative charges at the surface of the surfactant micelles. Since the presence of an additional signifi cant number of anionic groups is not a reasonable option, the only way to explain the negative stream-ing potential at pH of >7 is that the number of effective cationic groups must be reduced, for whatever reason. This reduction is even more pronounced in the case of AEB, leading to an IEP <3.5. Obviously, the interaction of the amide group with the quaternized ni-trogen is infl uenced by the distance between these functionalities within the molecule. Interestingly enough, there is a chance for a six-membered ring formation as shown in Figure 8, which could be one possibility for the carbonyl group of the amide to reduce the cationic character of the quaternary nitrogen. As a consequence, in case of AEB, more carboxylic acid functions need to become protonated to obtain a positive streaming potential as compared to APB.

Coming back to the starting point of our study, the viscosity of formulations of the dif-ferent betaines with SLES, considering that the streaming potential of AB12/14 is strongly positive basically at all pH values relevant for personal care formulations, it is to be ex-pected that this betaine exhibits the strongest interaction with the anionic SLES, thus leading to the most effi cient transformation to rod-like micelles, i.e., the highest viscosity (see Figure 6).

Somewhat surprising is that there still seems to be an interaction of the AEB exhibiting a negative streaming potential at pH 5.5 and the anionic SLES. However, one should keep in mind that the negative streaming potential refers to micelles of the pure AEB; in a mixed system SLES/AEB, there is still the possibility for the anionic SLES to interact with the quaternary nitrogen, and hence to reduce the average packing parameter. The extent of interaction and accordingly the viscosity of the formulation is, however, signifi -cantly smaller.

CONCLUSION

Measurements of the streaming potential of micellar solutions of zwitterionic surfactants were used for the fi rst time to differentiate between betaines with different chain lengths and chain length distributions. The values of the streaming potential at pH 5.5, which is the pH value of the surfactant formulations of the betaines with SLES, provide a measure of the infl uence of the chain length on the average polarity or hydrophilicity of alkyl APBs. These values can be used to predict their ability to thicken mixtures with anionic surfactants.

Figure 8. A six-membered ring formed between the quaternary nitrogen and the amide carbonyl in alkyl AEB.



Also, the infl uence of the headgroup structure of the betaines on the streaming potential can provide valuable insight into the intra or intermolecular interaction of the betaines, which in turn determines their ability to interact with anionic surfactants in formula-tions. Although AEB shows only a negative streaming potential due to the strong inter-action of the amide group with the quaternary nitrogen, there is still some—but limited—interaction with SLES, thus leading to the lowest viscosity of all betaines stud-ied. Increasing the distance between the quaternary nitrogen and the amide group re-duces their interaction, and hence yields both a more positive streaming potential and a higher viscosity in the mixture with SLES. In the AB without an amide group, the cat-ionic charge of the quaternary nitrogen is unaffected, and hence, the streaming potential is positive at all relevant pH values. Therefore, the interaction with SLES is most effi -cient, resulting in the highest viscosity of all systems studied.

ACKNOWLEDGMENTS

The authors are thankful to Dominik Schuch and Uwe Begoihn for their help in the syn-thesis of different betaines and for fruitful discussions and also acknowledge the help of Karin Fürch, Luca Supovec, and Meike Buchholz for performing the physicochemical measurements.

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More articles could be found at:http://journal.scconline.org/contents/cc2002/cc053n01.html


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