Cationic Surfactants

Analytical and Biological Evaluation

John Cross, Edward J. Singer

• Publisher: CRC Press
• Year: 2019
• Print ISBN:
  9780824791773
• eBook ISBN:
  9781000063622
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

This work focuses on the environmental availability and effects, toxicological properties and numerous applications of cationic surfactants, detaling the modern analytical processes by which this important class of compounds may be studied. It discusses the types of microorganisms that are susceptible or refractory to the actions of cationic agents.

III
Analytical Evaluation

JOHN CROSS Faculty of Sciences, University of Southern Queensland, Toowoomba, Queensland, Australia

The analytical evaluation section of this book is devoted almost entirely to a detailed examination of cationic surfactants themselves rather than to the somewhat complex mixtures that are often marketed under that heading. There are many parameters in the quality and process control of quats and of the materials from which they are synthesized that are measured routinely. These include such determinations as water content; total, primary, secondary, and tertiary amine content; acid or amine value (i.e., the amount of excess acid or amine that may be present); ash content; iodine value (i.e., unsaturation in fatty chains); nonvolatile matter; and pH.

There are standard test methods available for each of the parameters above, and these are constantly under review. Perhaps the most comprehensive collection is to be found in the Annual Book of ASTM Standards, published in the United States by the American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103. Many of them, however, are to be found in a companion volume to this book, Analysis of Surfactants, Thomas Schmitt, 1992 (Volume 40 of Marcel Dekker’s Surfactant Science Series). Readers with an interest in surfactant analysis in general will find in it much useful complementary material to that presented in the more specialized volumes in the series.

5
Volumetrie Analysis of Cationic Surfactants

John Cross Faculty of Sciences, University of Southern Queensland, Toowoomba, Queensland, Australia

• I. Introduction
• II. The Surfactant Ion: Indicator Reaction
• III. Titration with Anionic Surfactants/ Visual Indicators
  • A. Methylene blue
  • B. Mixed indicators
  • C. Indicators without phase transfer
• IV. Titrations with Sodium Tetraphenylborate/Visual Indicators
  • A. Single-phase titrations
  • B. Diphasic titrations
  • C. Cationic and anionic surfactants in admixture
• V. Alternative Antagonists
• VI. Instrumental Detection of Endpoints
  • A. Self-contained apparatus
  • B. Photometric titrations
  • C. Potentiometric titrations
• VII. Nonaqueous Titrations
• VIII. Selected Reagents: Practical Details
  • A. Hyamine 1622
  • B. Sodium lauryl sulfate
  • C. Sodium tetraphenylborate
• IX. Concluding Remarks
• References

I. INTRODUCTION

For routine determination of a wide range of materials, the traditional combination of burette and pipette has withstood the test of time and the relentless competition of a wide range of instrumental techniques: ionic surfactants provide no exception.

By way of summary, the volumetric analysis of cationic surfactants has largely evolved alongside the development of procedures for determination of the more common anionic counterparts, in which the well-known “antagonistic” relationship between these two types:

R4N(aq)++R′SO3(aq)−→R4NR′SO3(aq)orR4NR′SO3(org)

has been used to full advantage. The reaction between these two species is followed by detection of either the disappearance of the last trace of titrand or the appearance of the first trace of excess titrant.

Many other ions have been shown to react quantitatively with quats, the most popular of these being the tetraphenylborate ion. Originally marketed as Kalignost, sodium tetraphenylborate (STPB) was introduced as a welcome alternative to perchloric acid for the gravimetric determination of potassium. Since ammonium and potassium ions are readily exchangeable in many crystalline structures, the reaction of STPB with ammonium and then other amines were soon investigated. Natural product chemists found it to be an ideal reagent for precipitating alkaloids from solution. Schall [1] was one of the first to suggest its application to the analysis of cationic surfactants.

Although these two titrants have dominated the scene for some decades, the style of titration has varied: a two-phase system involving acid-base indicators has given way to single-phase potentiometrie techniques, although most standard methods continue to be based on the former.

One factor that may surprise even experienced analytical chemists meeting surfactant titrations for the first time is the extreme sensitivity of these methods. The majority of traditional volumetric analyses involve solutions with concentrations around 0.1 M for acid-base reactions: in some areas, such as water quality work, more dilute solutions are common [e.g., 0.01 M ethylenediaminetetraacetic acid (EDTA) for hardness and even 0.0125 N(sodium thiosulfate for dissolved oxygen]. Solution concentrations for surfactant titrations, however, vary typically from 10^-2 M down to a remarkable 10^-6 M, far below that found in any other common types of titration. Indeed, attempts to titrate 0.1 M surfactant solutions would be thwarted by foams and emulsions.

One essential requirement for the development of any analytical method is ready access to a supply of reliable primary standards. In surfactant analysis this has been a particular problem, due to the mixture of homologs that comprise the fatty chain when derived from natural sources. By way of example, a commercial sample of benzalkonium (alkyldimethylbenzyl-ammonium) chloride was shown to possess a C₁₂,₁₄,₁₆,₁₈ alkyl distribution of 39.3, 46.45,11.2, and 3.0%, respectively [2]. Particular credence, therefore, should be paid to work in which the purity of the materials was carefully controlled. Of particular current use are “specially purified” sodium lauryl sulfate (SLS) (1) in which the fatty alcohol has been carefully fractionated prior to sulfation to ensure that only the C₁₂ component remains, and p(diisobutylphenoxyethoxyethyl)dimethylbenzylammonium chloride monohydrate (2) (mercifully more commonly known as benzethonium chloride or Hyamine 1622), which is entirely synthetic. Most practitioners regard both of these materials as primary standards.

(1)

(2)

The various options available are examined in some detail in the ensuing pages. Tackling the task on a chronological basis, the cationic versus anionic systems using visual indicators are treated first. It is appropriate to begin with a few observations on the indicators.

II. THE SURFACTANT ION: INDICATOR REACTION

It has been known for many years that addition of surfactants to acid-base indicators of opposing ionic charge gives rise to color changes [3]. Mostly, these changes occur at surfactant concentrations orders of magnitude less than the critical micelle concentration. For example, Hiskey and Downey reported that the normal adsorption maxima of the acidic and basic forms of methyl orange (507 and 426 nm, respectively) were replaced by a new maximum (463 nm) that was stable over an extremely wide pH range (0 to 12) and for cationic surfactant concentrations as low as 3 × 10^-6 M [4].

These colored complexes can conveniently be regarded as stoichiom-etrically composed salts formed from the surfactant and indicator ions. For example, Mukerjee and Mysels isolated two compounds...

Table of contents

• Cover
• Halftitle Page
• Series Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributors
• I Introduction
• II  Biological Evaluation
• III Analytical Evaluation
• Index