Chapter 1: The Power of Scent

In our everyday lives, we encounter various fragrances that significantly influence our mood, stress levels, and productivity. The aroma of freshly brewed coffee kickstarts our mornings, the scent of jasmine heralds the arrival of summer, and the crisp ocean breeze evokes cherished vacation memories. In Marcel Proust’s work, Remembrance of Things Past, the flavor and fragrance of a madeleine trigger a flood of childhood memories.

The experience of smell encompasses not just the odors we encounter but also the emotions and memories they evoke. As Nietzsche once stated, "Every word has its scent." The delightful aroma of laundry dried in the open air is a scent deeply embedded in our olfactory memories, conjuring feelings of nostalgia and cleanliness.

While many detergent brands strive to replicate the beloved scent of "clean laundry," the drying process is crucial in shaping the final fragrance. You might use the most fragrant detergent available, but if clothes are left too long in the washing machine, they won't smell fresh!

Recent research led by Silvia Pugliese, a promising Ph.D. student in Chemical Sciences at the University of Copenhagen, delves into the chemistry behind the scent of line-dried laundry. Her team explored the physical and chemical changes that occur on damp fabric during the drying process, uncovering the origins of this distinctive aroma.

In this video, The Truth About Origins, discover the chemical reactions that contribute to the unique scent of line-dried laundry.

Section 1.1: Why the Interest in Laundry Scents?

You may wonder why chemists are intrigued by laundry fragrances. The answer lies in the fact that scent is fundamentally a chemical phenomenon. Odors result from small molecules (with a molecular weight of less than 300 Da) that humans detect through their olfactory system. For a molecule to be perceived as a scent, it must evaporate if it's a liquid or sublimate if it's a solid, such as menthol or camphor. These volatile compounds belong to the class of Volatile Organic Compounds (VOCs).

I had the opportunity to speak with Ms. Pugliese, and she shared insights about her captivating study. Initially, it began as a side project influenced by her supervisor, Dr. Matthew Johnson, who was intrigued by the scent of laundry dried outdoors. Together, they designed an experimental approach to identify the molecules responsible for this fragrance.

Subsection 1.1.1: The Experimental Setup

To investigate, they washed 100% cotton towels from Ikea three times using ultra-purified water—without detergent—and dried them under three different conditions:

1. Outdoors in sunlight on a balcony at the Department of Chemistry.
2. Outdoors in the shade on the same balcony, covered to block sunlight.
3. Inside a dark office.

After drying, the towels were sealed in specific sampling bags for 15 hours, and the air inside was analyzed for odorous molecules. To provide a comprehensive study, they also analyzed:

• Fresh, unwashed towels to understand the role of moisture in scent production.
• An unused sampling bag to check for potential contamination.
• The air from the drying environments (balcony and office) as a control.

All air samples were examined using Gas Chromatography coupled with Mass Spectrometry (GC/MS), a preferred method for analyzing small, volatile molecules due to its capacity to separate complex mixtures and quantify analytes, even at trace levels.

Section 1.2: What Did the Analysis Reveal?

Are you eager to learn about the findings? The GC/MS results indicated that only towels dried in sunlight displayed detectable levels of oxidized VOCs, including carboxylic acids, aldehydes, and ketones.

These compounds are significant because many possess low odor thresholds, making them easily recognizable, and they are commonly used in the perfume industry for their pleasant fragrances. Specifically, line-drying generated methacrolein (floral scent), pentanal (fruity aroma), and 2-hexenal (almond-like smell), among others, creating a beautiful bouquet of scents.

Ms. Pugliese noted, "Once we identified the components of the clean laundry scent, our next goal was to understand how these oxidized VOCs—responsible for the fresh fragrance—formed." Since these compounds weren't found in the other drying conditions or in the fresh towels, the researchers could eliminate various sources.

The outdoor air samples showed only low levels of un-oxidized VOCs, with no oxidized VOCs detected.

So, how are these oxidized VOCs produced? The answer is captivating: both sunlight and the presence of liquid water are essential for forming these aromatic molecules. The layer of water on a damp towel is particularly crucial.

As towels dry, un-oxidized VOCs from the surrounding air dissolve into the water layer. Here, they interact with radicals—unstable species produced by UV light reacting with towel dyes and brighteners—leading to oxidation reactions that create the odoriferous compounds.

Chapter 2: The Longevity of Fresh Scents

Now that we understand both the components and their origins, it seems the mission is accomplished. However, Ms. Pugliese and her colleagues revealed another intriguing aspect regarding the longevity of the clean laundry scent.

Oxidized VOCs can bind to cotton fibers, which are linear cellulose polymers, through hydrogen bonds, allowing them to linger on the fabric for several days. The interaction between the water on the cotton and the oxidized VOCs creates a “sink” that retains the fresh aroma.

Thus, the combination of fabric material (like cotton), the washing process (presence of water), and sunlight (UV radiation) are all vital for generating the delightful scent of line-dried laundry.

Despite transitioning to her role as an early-stage researcher in the eSCALED projects, focusing on encapsulating synthetic metal complexes in nanostructured electrodes for CO2 reduction, Ms. Pugliese remains curious about the scent of line-dried laundry. She mentioned, “I would love to repeat the experiments in different locations, considering that air quality can significantly affect the compounds that fabrics can absorb.” Additionally, she expressed interest in exploring whether UV light lamps could replicate the effect of sunlight.

Perhaps one day, we will see another publication from Ms. Pugliese addressing these intriguing questions.

In The Truth About Fresh, dive deeper into the chemistry that makes line-dried laundry smell so refreshing and how environmental factors play a role.