Perfume formulation is a science that combines fine chemical control with sensory art. For readers with a science and engineering background, understanding the molecular structure, reaction dynamics and skin interactions behind these seemingly contradictory odor phenomena can reveal how certain ingredients in perfume transform into “abnormal” odors at different concentrations or in different environments, even reminiscent of the smell of urine.

香水配方是一門結合精細化學調控與感官藝術的科學。對於理工科背景的讀者而言，了解這些看似矛盾的氣味現象背後的分子結構、反應動力學與皮膚相互作用，可以揭示香水中某些成分如何在不同濃度或環境下轉變成“異常”氣味，甚至讓人聯想到尿騷氣味。

1. Key Chemical Components in Perfumes 香水中關鍵的化學成分

1.1 The Dual Role of Indole 吲哚 (Indole) 的雙重角色

Indole (C₈H₇N) is a heterocyclic compound formed by the fusion of a pyrrole and a benzene ring. It is naturally found in floral scents such as jasmine and orange blossom.

吲哚（C₈H₇N）是一種由吡咯和苯環融合而成的雜環化合物。在自然界中，它存在於茉莉花、橙花等花香中

• At low concentrations: Indole adds depth and complexity to floral notes, making the fragrance rich and layered.
  低濃度時：吲哚可增添花香的深度和複雜性，使香氛顯得濃郁而有層次。
• At high concentrations: Due to its molecular structure, when the concentration is too high, it can emit odors similar to feces or animal waste. This phenomenon can be explained by the “threshold effect,” where once the molecular concentration surpasses a certain critical value, human perception of the scent shifts from “sweet” to “pungent.”
  高濃度時：由於其本身的分子結構特性，濃度過高會呈現出類似糞尿或動物排泄物的氣味。這種現象可用“閾值效應”來解釋，即當分子濃度超過某一臨界值後，人類嗅覺會從“甜美”轉變為“刺鼻”。

1.2 Synthetic Musk and Other Animalic Ingredients 合成麝香與其他動物性調料

Traditionally, natural musk was extracted from animal secretions, with a strong and lasting scent.

傳統上，天然麝香是從動物分泌物中提煉而來，其氣味濃烈且持久

• Synthetic Musk: Modern perfumery often uses synthetically produced musk. However, some synthetic musks (e.g., nitro musk) can, if improperly blended, produce odors reminiscent of animal secretions.
  合成麝香：現代製香技術多採用人工合成麝香，然而，某些合成麝香（如硝基麝香）在使用時如果配比不當，也可能產生類似動物分泌物的異味。
• Other Components: For example, compounds found in some leather accords (such as isobutyraldehyde) or fermentation by-products can, during chemical reactions or aging processes, generate odor molecules that resemble urine-like scents.
  其他成分：例如某些皮革調中的化合物（如異丁醛）或發酵副產物，亦可能在化學反應或老化過程中產生尿騷類的氣味分子。

2. Fragrance Formulation and Physical-Chemical Dynamics 調香配方與物理化學動態

2.1 The Balance of Volatile Organic Compounds (VOCs) 揮發性有機化合物 (VOCs) 的平衡

The ingredients in perfumes are primarily VOCs, whose evaporation rate and scent release are influenced by the following factors:

香水中的成分主要屬於VOCs，其蒸發速率和氣味釋放受以下因素影響：

• Molecular weight and polarity: Lighter, more volatile molecules evaporate more easily, while polar molecules interact with water molecules, altering their evaporation behavior.
  分子量與極性：輕質、高揮發性的分子較易蒸發，而極性分子與水分子間的相互作用會改變揮發行為。
• Environmental conditions and skin interaction: When perfume is sprayed on the skin, the sebum and skin’s pH level affect the conversion and evaporation rate of certain ingredients, leading to changes in scent. This is one reason why the same perfume smells different on different people.
  環境條件與皮膚相互作用：當香水噴於皮膚上，皮脂和皮膚pH值會影響某些成分的轉化和蒸發速率，導致氣味變化。這也是為什麼同一款香水在不同人身上呈現不同氣息的原因之一。

2.2 Molecular Interactions and Threshold Effects 分子間相互作用與閾值效應

In theory, molecular interactions can be described by mechanical models such as van der Waals forces and hydrogen bonding.

理論上，分子間的相互作用可以用范德瓦耳斯力和氫鍵等力學模型來描述

• When certain odor molecules (e.g., indole) are present in excessive amounts in a perfume, the cumulative effect between molecules can lead to significant changes in the scent profile.
  當某些氣味分子（如吲哚）在香水中比例過高時，分子間的疊加效應會導致氣味特性顯著變化。
• Additionally, perfumers use dilution and blending techniques to ensure the components complement each other when their concentrations are below the sensory threshold, achieving an ideal overall effect. However, if the proportions become imbalanced, unpleasant, discordant odors such as “urine-like” smells may emerge.
  此外，調香師會利用稀釋和混合技術，使各組分在低於各自的嗅覺閾值時互補，達到理想的綜合效果，但一旦比例失衡，則可能引發“尿騷味”這類不協調現象。

3. Challenges in Perfumery from a Scientific and Engineering Perspective 理工科視角下的調香挑戰

3.1 Precise Measurement and Reaction Kinetics 精準計量與反應動力學

• Proportion Control: When creating a perfume, precise calculation of the molar concentrations of each ingredient is crucial. Excessive indole concentration may exceed the olfactory threshold, resulting in a pungent odor.
  比例控制：在調製香水時，精確計算各成分的摩爾濃度非常重要。過高的吲哚濃度可能突破嗅覺臨界值，從而產生刺鼻氣味。
• Kinetics and Aging Effects: During storage, chemical reactions (such as oxidation or polymerization) may alter the proportions of ingredients, thus changing the scent. This requires the use of kinetic models to predict how the formula will evolve over different time scales.
  動力學與老化效應：香水在存放過程中，化學反應（如氧化或聚合反應）可能改變成分比例，進而改變氣味。這需要透過動力學模型預測配方在不同時間尺度內的變化。

3.2 Olfactory Sensing and Molecular Recognition Technologies 嗅覺感測與分子識別技術

• Electronic Nose Technology: Modern electronic nose devices can quantitatively analyze the VOCs in perfumes, helping perfumers precisely control the release of odor molecules during the development phase, avoiding concentration imbalances.
  電子鼻技術：現代的電子鼻設備可以對香水中的VOCs進行定量分析，幫助調香師在研發階段精確調控氣味分子的釋放，避免濃度失衡。
• Molecular Dynamics Simulations: Using computer simulations, scientists can predict the molecular interactions of different mixtures under varying environmental conditions, providing data support for perfumery.
  分子動力模擬：利用電腦模擬，科學家可以預測不同混合物在不同環境下的分子交互行為，為調香提供數據支持。

The change in a perfume’s scent is a multi-layered issue involving organic chemistry, physical chemistry, and engineering.

香水氣味的變化是一個涉及有機化學、物理化學和工程學的多層次問題。

• The concentration effects of molecules like indole and the characteristics of synthetic musk are primary factors.
  吲哚等分子的濃度效應和合成麝香的特性是主要原因。
• The physical interactions between molecules, environmental factors, and chemical reactions with the skin collectively shape the final olfactory experience.
  分子間的物理作用、環境因素以及與皮膚化學反應共同塑造了最終的嗅覺體驗。
• By using precise measurements, kinetic models, and modern molecular analysis technologies, perfumers can strike a delicate balance between art and science, satisfying both sensory enjoyment and chemical principles.
  利用精確計量、動力學模型和現代分子分析技術，調香師能夠在藝術與科學之間達到微妙平衡，既滿足感官享受，又符合化學原理。

This cross-disciplinary challenge is what makes the perfume industry so fascinating, and it offers us a perfect case study for exploring the aesthetics of life at a molecular level.

這種跨領域的挑戰正是香水工業魅力所在，也為我們提供了從分子層面探討生活美學的絕佳案例。