Application of Quaternary Ammonium and Sulfonium Compounds

Dear Colleagues,

This Special Issue of Molecules encompasses the preparation and application of quaternary ammonium compounds (QACs). Quaternary ammonium compounds are a versatile group of compounds, which are broadly applicable. They are, in general, easy to prepare, making them very attractive from an economical point of view. Although at first glance, QACs look like very simple compounds, their possible structural compositions are nearly unlimited. The alkyl moiety connected to the nitrogen atom can be linear, aromatic, or cyclic, as well as a have varying number of nitrogen atoms per molecule. A general feature of useful QACs is that they carry a positive charge and a hydrophobic moiety. Amines are in an aqueous environment, highly protonated at a pH of ≤7. However, these protonated QACs and compounds with only short alkyl chains do not, in general, have the special properties that are desired. The interesting properties for nearly all of the applications appear when at least one of the alkyl chains is C6 or longer. In some respects, phosphonium and sulfonium compounds may have advantages over QACs, but they are more difficult (expensive) to prepare in the same abundant amounts of structural compositions as can be achieved with QACs.

QACs have a long history and have already been for about 100 years (first introduced in 1917). The main areas of use are in medical applications, cosmetics and disinfectants, and as surfactants and (ionic) solvents. QACs are very potent biocidal compounds, effective against bacteria, viruses, and fungi, while other disinfectants (e.g., phenolics, iodophors) are often less operative. QACs kill bacterial by destroying the cytoplasmic membrane, but do not interfere in the proliferation process, as antibiotics do. This makes them less susceptible to build-up resistance. Although QACs are not toxic chemicals, they are not harmless for the human body, but are nevertheless indispensable for combating infections. Because of the increasing resistance of bacteria against antibiotics (e.g., MRSA and VRSA) this biocidal behaviour becomes even more important. In addition, antibiotics are, unfortunately, not considered as blockbusters by the pharmaceutical industry. Phase transfer catalysts and surfactants make abundant use of the intrinsic amphiphilic character of QACs, in order to bridge the difference of the surface energy of water and organic compounds. Interestingly, some other QACs in a neat state have (such as salts) such a low melting that they are liquids at room temperature, and are applicable as highly polar solvents. One of the advantages is that their low vapor pressure (high boiling point) makes them suitable solvents under a high vacuum. In summary, quaternary ammonium compounds have excellent properties in many different fields.

This Special Issue aims to cover the recent progress and trends in the utilization of quaternary ammonium compounds and quaternary ammonium comprising polymers. Submissions are welcome to the list below, but are not limited to these topics. Types of contributions to this Special Issue can be full research articles, short communications, and reviews focusing on the preparation of quaternary ammonium compounds and quaternary ammonium comprising polymers, and their use in a variety of applications.

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

• Antibacterial materials
• Disinfectants
• Cleaning
• Cosmetics
• Medicines
• Drug carriers
• Phospholipids (cell composition)
• Phase transfer catalyst
• Ionic liquids
• Surfactant and emulsifier
• Natural occurring compounds (e.g., lecitin)
• Phosphonium and sulfonium compounds