I think there is some confusion over the concept of moisturizer versus emollient. To moisturize tends to imply ‘to put water into the skin’ or to hydrate. An emollient, on the other hand, softens and soothes the skin.
Vegetable oils are more emollient than moisturizing as they do not deliver moisture to the skin but rather prevent the skin from becoming dehydrated as well as they are able to ‘trap’ moisture on/in the skin (e.g. applying a vegetable carrier after a shower while skin is still wet or damp). So, they are not directly ‘moisturizing’ in the same way as a cream or lotion would be. Vegetable oils do, however, play an important role in maintaining moisure in the skin as they are able to prevent transepidermal water loss (TEWL). From my Aromatic Applications for the Skin certificate course (www.theida.com): How to protect the skin from loss of moisture and support lipids.
The stratum corneum plays a key role in maintaining the water level of the skin below and in regulating the natural moisture flow out from the deeper layers to be lost eventually by evaporation from the skin surface. This flow is known as the transepidermal water loss (TEWL). The stratum corneum plays a vital role in controlling and reducing TEWL. With the brick-and-mortar design (discussed earlier), the cells in the stratum corneum (the corneocytes) form a water-retaining barrier embedded in a lipid matrix. As the living cells from the basal layer move upwards, they lose their central nucleus, and start to produce skin proteins (keratins) and fats called lipids. The principal lipids found in the stratum corneum include “ceramides (approx. 40%), cholesterol (20%), fatty acids (25%)” (Howard, 2005) and phospholipids and waxes.. It is this mortar of lipids that serves to prevent water loss through the stratum corneum.
The lipids and the natural moisturizing factor (NMF) of the stratum corneum are crucial in maintaining the water level of the skin as well as reducing transepidermal water loss. The NMF is a collection of water soluble compounds (such as amino acids, urea, lactic acid, sugars, peptides, etc) that are only found in this layer. These compounds are responsible for keeping the skin moist and pliable by attracting and holding water. NMF components are hydrophilic, that is, they attract water. They can hold large amounts of water in the skin cells and are also capable of absorbing water from the atmosphere and/or products applied to the skin. The lipids serve to prevent water loss from occurring in the NMF.
Emollients such as honey, glycerin and seaweed or algae serve as humectants. Humectants are substances which attract water. Substances such as beeswax, squalene, lanolin, shea butter, avocado oil, and other vegetable oils, not only provide valuable nutrients to the skin, they also serve as emollients that are considered slightly occlusive.
Occlusive substances have a ‘hydrating effect on the skin because they form a barrier on the skin’s surface, which helps to reduce the evaporation of water from the skin” (Howard, 2005).
On a slightly different note: A few years back Bob Harris included a brief research report in the International Journal of Essential Oil Research (or it could have been the IJA) on how some essential oils, specifically German chamomile (Matricaria chamomilla) can enhance the absorption of water through the skin. This could be incredibly valuable for using German chamomile in serums along with hydrosols in moisturizing the skin. Of course, attending to other variables which may be contributing to dryness (e.g. air quality, water intake, diet, etc. etc.)